First impressions of a Goat

[CCBB]

Last night I took a trip out to see a Goat being built in my area here in Maine. I had been wanting to see one in the flesh after looking at so many photos and drawings. I was nervous that I wouldn't like what I saw as far as proportions, looks, feel, etc.

The trip made me quite excited to build this boat as soon as possible. I was first struck by how large it felt for its LOA and beam...I think the depth/high freeboard is key to that effect. I immediately thought that it'd be a safe boat to load kids into and definitely provide the room for adults that I want in an easily, quickly built boat.

My second impression was regarding scantlings. I sat inside and felt the bottom was very flexible. I also thought about the weight of kids inside and pushing over a rock or grounding atop a rock (many of these things here on the Maine coast). I left thinking I might use 7 mm ply for the bottom and a light cloth sheathing, 4 oz. at most. My current dinghy took quite a gouge in the bottom regardless of the keel strip and my care...went clear through 6 oz cloth into the wood. I'd like to not worry about the bottom during the season.

I also wondered about creating a spot for some lead ballast as a good way to get an easier boat to handle for when going solo. I wondered if a little 50 pound lead pig either side the D/B would be good for use when going solo.

The fine entry and very sharp bow was striking. Really handsome. This is a design observation: the bottom rocker increases quickly toward the bow such that the stem is out of the water when sitting on her lines: why? I had learned that to minimize pounding in a flat bottom boat to immerse the stem below the LWL.

The boat I saw was relatively heavily built for a Goat and I wondered WHERE if anywhere Michael would advise beefing up, the high priority areas, if the customer knew the boat would get used harshly. Would it be bottom scantlings, sheathing, or framing...what be the first few places.

Lastly, for off the wind or downwind stretches when solo, do people sit in the bottom...where do you find youself sitting to trim the boat when on a broad reach or a run?

Oh, one more thing...if rowing a lot would a small skeg be a handy additon?

I guess this was more about questions than impressions. Great design. Solid yet light, handsome, and back-to-basics without losing sophistication. Can't wait.

Cheers,
Clint

[Boatmik]

ADDITIONAL EDIT ... the Goat that Clint was looking at was not fitted with the bottom skids. So all the discussion below does not relate to a finished Goat. So the observation that the bottom "oilcans" is premature. Good discussion below though

Good and interesting observations Clint ...

I will get back to you when I have a bit more time. Probably this evening.

MIK

[arbordg]

Good and interesting observations Clint ...

I will get back to you when I have a bit more time. Probably this evening.

MIK

Mik - As usual, I have some thoughts, but I'll give you first go, then elaborate or argue :wink:


"Wisdom is what's left after we've run out of personal opinions" -- Cullen Hightower

[Boatmik]

Last night I took a trip out to see a Goat being built in my area here in Maine. I had been wanting to see one in the flesh after looking at so many photos and drawings. I was nervous that I wouldn't like what I saw as far as proportions, looks, feel, etc.

The trip made me quite excited to build this boat as soon as possible. I was first struck by how large it felt for its LOA and beam...I think the depth/high freeboard is key to that effect. I immediately thought that it'd be a safe boat to load kids into and definitely provide the room for adults that I want in an easily, quickly built boat.

OK .. now lets get into the nitty gritty of design ... the Goat has a narrow bottom and is a very light boat. These are things that reduce the stability.

But they improve rowing, motoring and also improve the sailing ability particularly in terms of the ease of reaching higher speeds easily as well as getting maximum performance upwind ... even when there are waves.

This has been balanced by giving the goat way more freeboard (hull height above the water), in fact MUCH more than most boats of this type, but cunningly disguising it using the sheerline and the dropped deck.

Actually I can't take toooo much credit ... because some of this was a lucky mistake. When I saw the first photos of the Goat I realised just how extraordinary it was - so much volume. And then sailing it in mixed fleet racing ... and realising how much faster it was than Beth. BEth is still the winner in terms of thrill and outright speed. But the Goat just seems to be sailing along happily, but closes on Beth most of the time.

The other aspect is that you can load it up without losing much average performance because of the way it sinks down. And that every extra person you stick aboard increases the stability a huge amount. Single handed it is flighty, two up it is more relaxed.

Remember that it can be reefed too without going out of balance. So if you want to tame it down to the normal level USA boat for this size you can put a reef in and sail around on 75 square feet of sail and still move well.


My second impression was regarding scantlings. I sat inside and felt the bottom was very flexible. I also thought about the weight of kids inside and pushing over a rock or grounding atop a rock (many of these things here on the Maine coast). I left thinking I might use 7 mm ply for the bottom and a light cloth sheathing, 4 oz. at most. My current dinghy took quite a gouge in the bottom regardless of the keel strip and my care...went clear through 6 oz cloth into the wood. I'd like to not worry about the bottom during the season.First question is whether the bottom had the external skids fitted to the bottom. They are an essential part of the structure. They mean that the bottom is subdivided into panels - the biggest one is about 1.3ft by 2.5 ft and all of that framing is indestructible it is so huge relative to the size of the boat.

It will feel a bit flexy out of the water because the framing of the bottom will flex between the areas of support and the pressure. However when the whole bottom is supported by water the feeling is somewhat less.

Clearly because of all that framing the bottom cannot move much at all.

And the bottom cannot break because all the joins are as strong as the plywood.

HOLES ... as far as holes ... my philosophy ... backed up by some experience is that you can always hit something hard enough to break it. Any structure that does not exhibit this characteristic is too heavy and too expensive.

STIFFNESS - As far as stiffness compared to strength .. this is a whole interesting area itself. Furniture and houses (normal woodworking) is stiff - really stiff, but it has no strength to speak of ... you can pull the legs off a table or chair so easy.

But try that even with a lightly built boat. Walk up to it and try to tear it apart. Totally different. Providing it is designed and built properly. When we started building the OZ PDRacers some of the Americans said that we were crazy. That they were having oilcanning problems with half inch bottoms and the hulls were twisting if built of 3/4" ply.

They thought we were going too far with 1/4" bottoms and everything else of thinner ply still. But we have had no problems with either "oilcanning" or twisting of the hulls because the design is quite sophisticated. Geez it was hard to get it to fit on the three ply sheets we bought!

So some had problems still even though the panels were approximately 27 times stiffer!!!!

So you could say the thicker hulled boats were less susceptible to damage.. but ... ???

OK ... now ... most Oz raceboats of this size will use 4mm plywood and are quite durable within the context of use. The 6mm is much better in several ways.


I also wondered about creating a spot for some lead ballast as a good way to get an easier boat to handle for when going solo. I wondered if a little 50 pound lead pig either side the D/B would be good for use when going solo.David Graybeal started with sandbags under the mid seat ... perfect place when going solo. He used them a bit at the beginning but then started leaving them behind. Also be aware that reefing has the same function.


The fine entry and very sharp bow was striking. Really handsome. This is a design observation: the bottom rocker increases quickly toward the bow such that the stem is out of the water when sitting on her lines: why? I had learned that to minimize pounding in a flat bottom boat to immerse the stem below the LWL.

http://farm2.static.flickr.com/1100/875545453_cb1a2d913d.jpg

This might well be true of boats with fatter bows (which is the main reason they can't sail to help themselves by the way). There is no way a bow this narrow will slap around much at all. By the way ... it is very different if we are talking yachts which might be moored with ppl aboard at night. Every flat surface .. slap .. slap. Beware of designer's trying to sell you cruising yachts based on raceboats for just this reason!!!


The boat I saw was relatively heavily built for a Goat and I wondered WHERE if anywhere Michael would advise beefing up, the high priority areas, if the customer knew the boat would get used harshly. Would it be bottom scantlings, sheathing, or framing...what be the first few places.So if you really want to increase the strength ...

My feeling about this question followed up with the Jarcat people is that light glass is just about as effective as heavy glass for their 20ft plus cruising catamarans. So if you want to "beef it up" just use 2oz (75gsm) glass before the bottom skids go on and only do the bottom and go round the chine by 1" (25mm) There is no justification for doing the sides.


Lastly, for off the wind or downwind stretches when solo, do people sit in the bottom...where do you find youself sitting to trim the boat when on a broad reach or a run?Have a look
YouTube - Goat Island Skiff - 3 Men In a Boat

If there is picnic involved I would probably have an icebox in the back section and be sitting on it. Sliding it from side to side and there might be a swimming pool beanbag in the front section for someone to sit on the floor.


Oh, one more thing...if rowing a lot would a small skeg be a handy additon?If just rowing occasionally ... even if it is quite long distances the goat does fine. Sometimes I have put the rudderblade in the centrecase to reduce sideways drift (the goat is higher sided than a proper rowboat). But the skeg won't help much with that anyhow. Give me a yell if you want to try and I will work out how it will have the least detriment to the sailing performance and also to prevent it from puncturing the hull at the front end.


I guess this was more about questions than impressions. Great design. Solid yet light, handsome, and back-to-basics without losing sophistication. Can't wait.

Cheers,
ClintCheers to you too Clint ... great bunch of questions!

MIK

[arbordg]

Clint,

Now that Mik has pretty well covered the topic, I'll go ahead and add a slightly different perspective. First, my background, so you'll know the slant my comments come from. I'm a professional woodworker with 30+ years experience: cabinetry, furniture, gates, gazebos, and other exterior wood/expoxy composite projects. Boatbuilding only for the last 5 years or so. 6 boats in that period. I've now added boatbuilding and boaty bits (spars, foils, tillers, companionway doors, etc.) to my professional offerings. Been boating since I was tiny. Grew up in, on and around the water in Astoria, Oregon - where the mighty Columbia meets the blue Pacific. Summer home on Puget Sound. Waterskiing, kayaking, hydroplanes, diving, surfing, and cruising were all on the menu as I grew up. Small boat sailing, though - I'm a real beginner. I've been sailing for 3 summers now, and am slowly learning how.

First - I know you're interested in building boats with kids. I built our GIS with friend Jerry and my two sons, who were 11 & 16 at the time. Very simple, quick build. Great design for kids to tackle.

Second - Rowing. She's not a whitehall. She doesn't have the weight and long, straight keel to "carry" far/straight. On the flip side, she has great maneuverability. Wonderful for a slalom course, or for jockeying around tight moorages and exploring small waters. At times, she reminds me of handling a McKenzie River Dory. Mik & I talked about adding a skeg to help her track a bit straighter. My impression was that she'd lose more in sailing than she'd gain in rowing. My plan at this time is to use the Huntington Helm Impeder - that I plan on rigging for sailing - to also keep the tiller centered and put the rudder blade partway down (how far will take some experimenting) for rowing when I want to go some distance (say straight across a lake or bay). It's not a huge problem, even without, but I'm always looking for ways to improve/optimize. I use the boat more than any of the 4 of us, and I actually don't row her that much. I mostly sail. Sometimes I row. On rare occasion I motor. Jerry really likes to motor first, then row - because he's still a very raw beginner when it comes to sailing. He really likes the way she rows.

Third - Ballast. As a beginning sailor, I very much appreciate the extra stability afforded by the two 70# bags of sand I lay flanking the daggerboard case. I leave them in there loose - the theory being that if I ever did capsize, they'd just slither overboard. I've thought about strapping them into place with shock cord, but I'm not sure it's even desirable. I chose them because: they come in a long, skinny bag (as traction aids from the local tire store); at $7.50 each, they're cheap enough to lose overboard if it came to that; I'd rather have something soft in the boat than something hard like a lead pig, or even water jugs. Less like to damage the boat (or me) if they did shift. I haven't used them since early last summer, but I will again, if conditions warrant, and I can't find a second bod to serve as ballast.

Finally - Scantlings. The lightness of the material has caused us 3 problems.

Earliest problem: jockeying it from the top of the van one day, we dropped it on the port gunwale, and cracked a cedar rib. I'm pretty sure if the rib had been douglas fir, it wouldn't have fractured. However, I don't think I'd change the ribs - just be more careful handling the boat. We have her on a trailer now, so it's much less of an issue.

Next problem: backing the trailer up at my shop after a very long weekend of camping, sailing, drinking, swimming, and late nights around the campfire (i.e. I was quite tired), I forgot how far the mast was sticking out over the transom (I don't usually carry it that way) and kissed the mast tip against the overhead door. This, in turn, pushed it against the anchor box in the bow, forcing both lower corners to bust out the 6mm ply at either side of the bow. If it had been built stouter, maybe it would have taken that abuse without creating an unwelcome repair job. Again, I wouldn't think about making the plywood sides thicker - just be more careful.

Final problem: punched two holes (cracks, really) in the bottom, near the case. This occurred after an outing on a High Cascades lake in 25-30 mph winds. Trying to beach on the windward shore of our campground. Shoreline strewn with rocks, boulders, and stumps - stirred up so murky, one couldn't see what was just below the surface. I hit numerous things, but two of them (I later discovered) had cracked the bottom. Easily repaired. If you know what a Payson Butt Joint is, I did what I call a Payson Patch. Ground a slight hollow around the areas and infilled the hollow with epoxy and light fiberglass cloth. Sanded fair, and repainted.

That final problem points up the only change in scantlings I'd consider making if I were building this same boat again for my personal use. I would seriously consider making the bottom of 9mm plywood. Building for others, it would depend on the circumstances. I wouldn't do it casually. As it stands, we plan a Fall project, stripping the bottom and up around the chines. When it's wooded, we'll apply some light xynole cloth set in epoxy, fill, fair, sand, and repaint. It will add some weight and strength to the bottom. We're willing to trade the weight for added strength. I actually have felt the 6mm bottom oilcan a bit (even with slightly over-spec. runners on the bottom) when sitting on my bum on the bottom of the boat in rough conditions. Maybe Mik hasn't noticed it because he sails standing up, or perched on the gunwale.

Keep in mind that part of the reason I'd consider changing the bottom to 9mm is the way we use the boat. Jerry had never handled any boat before we built this one. Colin & Devin are teenagers - what can I say. My personal learning style is to jump in and try it. This sometimes leads to mishaps or disasters, and the boat suffers. When I raced motorcycles, one of my more flattering nicknames was, "FullTilt". So I'm not sure beefing up the bottom should be considered for most folks.

The other unknown is how much the added cloth/resin on the bottom (or changing the bottom to 9mm) will affect the speed, handling, and stability of this little gem. For me, it's an experiment. I certainly don't know for sure. If Mik knows, he's been too polite to tell me what a bloody idjut I'm being. As is, she's fast and nimble. I sincerely hope I don't degrade her performance much. We'll find out.

OOPS -- Editing to add the fourth problem that we had with 6mm ply. A large friend (325# ?) lost his balance climbing out of the boat one time. He came down hard on one knee in the middle of the center thwart and punctured it. We peeled off the 6mm and replaced it with some 9mm. Feels bulletproof now.

[Boatmik]

I was hoping you would appear David, because you have actually run up against a lot of the areas that the questions centre on.

Really .... I don't know why people get so het up about "oilcanning" as if it is a major problem.

I do feel it, but it is right in the normal range despite the lack of obvious framing.

As long as the structure remains together and the deflections are small then there is no problem at all. If the whole bottom is lolloping in and out then there is a serious problem.

A test for adequate structure is a bit paradoxical ... if you can hole the boat rather than break a join then it is definitely strong enough. And I am so happy you have done a bit of this David! Any damage has been very localised and has been caused by fairly extreme circumstances.

Stiffness is a completely different matter and most small boat structures that don't deflect are way too heavy and usually quite weak because the "designer" doesn't have a lot of experience of these types of structures. Happily I have my background in plywood raceboats which means I tap into the thousands of boats as the whole Australian ply boatbuilding scene developed in the 60s, 70s and 80s. I got sucked in by composites for some time in the mid 80s, but got driven out of racing by the expense. Never own a boat that is more than 4 times the price of the car you drive!!!

Then I found the whole wooden boat scene and ... a niche! Otherwise I would have been a fairly useless person!!! Wandering the earth Nicheless!

A 9mm bottom would be an alternative to glass ... but my question is what is being achieved?

The boat sails well and doesn't break over lots of sailing. There are some deflections - I can feel them too - but is a small deflection a problem? Particularly considering that there is little correlation between stiffness and strength in structures.

The advantage is the light weight of the boat ... it just brings it into the range where a couple of adults can lug the hull around on shore. Considering that most boats this size and function are about 50% heavier and many are 100% heavier a large part of its advantage in the marketplace is that people like the lower weight.

So that is why I am so very reluctant to talk about beefing it up.

If routinely landing on rocky shores I would probably go for either the 9mm ply bottom or the 2oz glass. The weight addition is probably the same. But also be aware that some brass half round on the runners can make it easy to haul the Goat up on concrete launching ramps and the like - with horrific noise that makes all the "stander's by" wince and scowl as they think you should be taking better care of the beautiful boat.

http://farm4.static.flickr.com/3195/2718854795_a728a4c4f6.jpg

People will always build the boat that they want ... but I will also apply some pressure to keep it simple and straightforward.

That is a great creative tension for me and the body of clients! Improves all of our work!

Best wishes
Michael

[arbordg]

I was hoping you would appear David, because you have actually run up against a lot of the areas that the questions centre on.

<SNIP>

That is a great creative tension for me and the body of clients! Improves all of our work!

Best wishes
Michael

Mik,

I hope my monologues are helpful to you and/or other builders.

In regard to beefing up the bottom and what would be accomplished: what I hope to accomplish is to make the boat better able to stand up to the shenanigans of my wild, rowdy, clumsy, enthusiastic Harbor Woodworks crew esp. myself :rolleyes:

What I hope to avoid is degrading the performance noticeably (or at least substantially) by adding the additional weight.

Will it work? Time will tell. Worst case, we can always grind it back off :doh: Sounds like fun, eh?

If it doesn't work, you can say, I told you. If it does seem to work, you'll have to come to Oregon and try it yourself :wink:

[Boatmik]

Timely I suppose for some more Goat News .. found this on the Duckworks site

http://www.duckworksbbs.com/sails/storer/gis/GIS.jpg


Chuck:

Got the Goat Island Skiff launched Saturday. Light wind, but otherwise a good day. I was impressed with the good shape of the Duckworks sail, especially since this was the initial use and the wind never got over 6-8 and was usually less. Didn't seem to need much adjustment. I've attached a picture that shows the sail and the codes from a video I uploaded to YouTube.

Olivier was quite the proud papa as he sailed Tiffany around.

Gary Blankenship

YouTube - Launching day for the Goat Island Skiff

Another beaut looking machine. Classic boats almost call for being in white!

Congratulations on a successful launch!!
_______________________________________________

The other side of looking at the video ... the weight is designed to centre around that mid seat. That will bring the boat nicely onto its lines. It also allows the crew to chat more easily.

And gradually I will win my struggle to get everyone to REALLY tighten that downhaul. Not too bad in the vid ... because it is such light wind .. but more downhaul will give everyone 10% performance on all points of sail.

But really .... who cares ... the function of launching day is to get people smiling!!!

MIK

[keyhavenpotter]

Michael, discounting cost and effort, if the bottom 6mm ply was replaced by 4mm 5 ply top class plywood with a light kevlar layer inside and a light carbon layer outside where would we be interns of stiffness and puncture resistance?
If we were at effectively 7mm ply with greater stiffness and better puncture resistance, with regular offcuts of both items on eBay at reasonable cost, might it be a choice for certain parts of the world?
Brian.

[CCBB]

I can take a stab at the last post and follow up a bit on my initial post. Don't drop to below 6mil only to start building up with very difficult-to-work-with materials like Kevlar! You would regret that. If you want a slighly thicker bottom, add some glass 2oz or 4 at most, OR you can get Okoume by Shelman in 7mm and 8mm. I will probably go 7mil and maybe a very light glassing, certainly on the chines.

Regarding a skeg to help tracking, I look forward to hearing how it worked to let the rudder down a bit/or the CB down a bit...I agree that one of these ways should be tried first!

Mik, thanks for you responses...was very interesting. I think if anyone out there uses a boat as hard as some folks have on this post you have to reaize that things will wear out or break naturally, and that is OK...and it isn't a design flaw...the designer was quite aware of any "slight flexing of the bottom" type things and if he let it go then we should trust that, esp. given years of dinghy sailing experience in different ply dinghy boats.

With that said, I build boats for a living and my work is on the line so if I blindly follow every plan then perhaps a mistake would be made and that would not be good either. So for first timers out there, this couldn't be a better boat or a better forum! Yet, for all the help you can get, you might also get a variety of answers from different people, and at the end of the day you need to have done your homework, listened to the architect, and make your own judgement call.

Can't think of any more questions...about where things have broke...I tend to really look at the areas that break and figure out Was it the glue joint/the epoxy that failed or the wood around it that failed? How well the boat is glued together is totally up to the builder, no matter how light the scantling. And the better quality the fillets the more strength too.

CHEERS,
CLINT

[BobWes]

Clint, Michael, David, and everyone else:

I for one appreciate your thoughts, observations, and comments. Learning from the experiences of those who have gone before is so valuable . Even though I'm a couple of months from beginning my GIS, I find plenty in these threads to think about.

Thank you all.

Bob

[CCBB]

Brian,

I could have explained my reasoning better to the answer I gave for your last Question about using 4mm ply and laminating carbon or kevlar. In these situations you are essentially making a composite panel with the core being a thin 4mm ply...that is a very thin core to get what we want in the bottom of the Goat. I don't see any gain in strength of that bottom panel. The 6mm ply spec'd on the plans with some abrasion resistant sheating of 2-4 oz glass would be as strong or if it wasn't as strong you could bump up the thickness of the plywood. For your method to work, you'd need a thicker core...I'd still want to use 6mil ply. Carbon and Kevlar are a pain to wet out, a real pain.

http://www.shelman.gr/en/proionta_pages/plywood/shelmarine.htm

That is the link for Shelmarine Okoume...you can see that they make sheets in every mil thickness one would want. When not wanting the go all the way up to 9.5 mil on the garboards to my faering, I ordered the 8 mil...easier to bend to for a garboard! In the case of the Goat bottom, if one is a little leary about using 6mil for the bottom in a harsh use environment, you could get 7mil or 8 if you wanted. The weight difference is very small...probably less than ifwe use 6mil and a heavier cloth....that is a hypothesis.

I wouldn't go highher than 7 mil on the bottom and I certainly wouldn't go higher than 6mil for sides. Stick as close to the plans as possible...these are suggestions.

Cheers,
Clint

[DALukens]

Mik, All:

Clint was out to see the GIS that I am building. First and foremost, I want to thank him for his tips about glassing etc.

He is correct that the boat is relatively heavily built, as I used native hardwoods (black cherry and some walnut for gunwales and knees) for framing etc. and added some extra framing under the mid- and aft-seats and some positive flotation under fore and aft seats. Cherry is still relatively light, I believe. Both Clint and I observed some flex in the bottom, but the skeds are not yet mounted (I am not at that point in the construction process). I think I will use some lightweight glass on the bottom; where we live (Maine) there are a lot of rocks and pointy things, especially in the lake where I expect primarily to use the GIS.

I am excited to finish the boat. It is the first boat I've built; thanks to the simple design and clear plans everything seems to be going well. Since the sail won't be ready until mid-September or later, my plan is to get it on the water by the end of August and then build the spars and blades. I started on June 4th, so its been pretty quick and easy.

Thanks,

Dana

[Boatmik]

Howdy Brian,

First ... this is an excessively elaborate discussion for the problem above ... because the boat that prompted the discussion had not been fitted with the bottom skids which are a prime factor in the stiffness of the structure.

The thing is the Goat as designed is fine. David Graybeal has some useful observations ... however most of that is from his learning curve. My experiences with most of the things that SISU (his goat) has faced are long in the past ... most experienced sailors would be saying .... why did that happen? Be aware that they are examples of difficult situations.

His approach has been to strengthen various parts of the boat .... but if most boats don't have problems in those areas ... then it probably means the boat is OK.

The thing is that with SISU the type of damage was related to the situation. Whether a reinforcement for a specific situation makes sense is whether it will happen again in the same place with the same amount of force.

What would have happened to a car if it had faced similar forces? Would it be right to be asking Ford or Fiat to beef up the doors and the front and the roof?

If deeply concerned about this ... I would follow the Jarcat people. They have 20 and 22ft cabin trailerable catamarans and use the same thickness of plywood as the Goat. If you want performance from a multi it has to be LIGHT. But the boats are durable in every sense. The designer there stipulates 2oz (75gsm) glass over the 6mm ply.

If you are concerned .. really concerned .. then use 2oz glass over the outside of the bottom only, bring it around the chine and onto the side panel by 25mm which allows you to eliminate the glass tape.

Alternative is make the bottom panel thicker - 9mm or under.

Both of these are with the reluctant support of the designer who thinks it is going way too far for normal use.

BUT, BUT, BUT ... I think that David's damage has been entirely consistent with each of the circumstances. It has been localised in each case (despite the extremity of each situation) showing that there is no structural problem with the Goat.

(The internet is really bad at reflecting emotions so please be aware I am making the points here strongly, but there is not anger at all ... maybe a bit of tension ... but you need that to make a good argument!!!)

NOW for one of my favourite activities ... a bit of debunking of stuff that circulates on the net!
______________________________________________________

The best way to get stiffness with composites efficiently is to use a thicker core.

To use a thin core is dumb, dumb, dumb. (not aimed at you Brian ... but there is a lot of discussion on this point on the net ... so it is aimed at that).

I just about SHRIEK when I read such dopey stuff.

So first ... let's be a little generous...

Imagine we just have a good old fashioned glass sandwich on a 6mm plywood core...
For the purposes of this demo I will ignore the stiffness of the plywood.
Stiffness = x
Cost = y

Now if we replace the glass with a similar amount of carbon fibre the panel will be three times stiffer and much the same weight.
Stiffness = 3x
Cost =

Now lets reduce the thickness of the core from 6mm to 4mm. This has a big effect because the effect of stiffness FOLLOWS THE SQUARE.
So the change in stiffness is going to be 4 squared divided by 6 square ...
or REDUCED by a factor of 2.25

So the stiffness was increased by a factor of 3 by changing to carbon from glass but now it has been reduced by 2.25!!!!

Result is that stiffness is slightly increased
Stiffness = 1.33x

This with all the extra expense and hassle of the fancy materials ... NO net weight saving (because you have removed 2mm of the lightest component ... the timber.

I would just save the money and accept the extra pound every square metre from the extra wood thickness.

SO RULE 1 ... DON'T EVER CONSIDER REDUCING THE CORE THICKNESS IN COMPOSITES UNLESS THERE IS A COMPELLING REASON.

and RULE 1A ... TO MAKE COMPOSITES MORE EFFICIENT INCREASE THE CORE THICKNESS.

So ... this is rather abstracted from comparing plywood alone with a composite.

I will go into that in the next post. But another place I want to clip the bozos that are promoting these methods on the net is in weight.

Gaboon ply weighs in at around 320kg per cubic metre
Glass weighs around 2500kg per cubic metre
Carbon weighs around 2500kg per cubic metre
Epoxy/polyester and other resins group around a bit over 1000kg per cubic metre.

Just impress that on your minds for a moment.

There are lots of canoe sites that say that glass, "somehow" ... "does not add much weight to plywood"

This is clearly rubbish. Because the glass and the resin is so much more dense it adds truly significant amounts of weight very quickly.

I will look at that in the next post.
__________________________________________

But to answer the question...

Following up with the Jarcat people, their

[keyhavenpotter]

Thanks Michael and Clint. An answer in red ink! Wow.

Just to explain where I am coming from.

1- I would only ever build a boat to the designer's spec. Enough said.

2- Hence I only asked a theoretical question. My interest was based on Hugh Horton's Bufflehead using 4mm and carbon and glass. He used 4mm to get the panels to conform to the moulds, even then some sections were cold moulded where the ply split. The thin 4mm ply needs extra reinforcing for the corel heads he sails amongst.

3 - I sail in a very rough area both sea wise and beach landing wise. I really am hoping to build Solo when available and feel that driving up an unknown gravel/pebble beach will need as tough a bottom as possible - that is 6mm and 2oz glass I now know. Thanks for clarifying things for me.

4- I agree that over examining Goat's bottom stiffness is unfair to her (him?). I have two Oughtred designs both built by woodworking professionals and both have problems with panel splits in the floor areas where the crossmembers are glued to the floor after very little use from new. Hence my interest in floors I can trust to cope with my local conditions.

I love the Goat, it's just that I sail singlehanded all the time. Hence looking forward to Solo so much. Brian.

[Boatmik]

Howdy Brian,

I am always writing for two audiences here. There are us lot ... who have followed the context of the whole discussion. Red ink is not necessary for any of them/us.

But there will be people skimming through from now until eternity. I was worried that some of those souls would read some of the comments and think there was a problem without noting the context.

Thus the redness!!!

MIK

[arbordg]

Howdy Brian,

I am always writing for two audiences here. There are us lot ... who have followed the context of the whole discussion. Red ink is not necessary for any of them/us.

But there will be people skimming through from now until eternity. I was worried that some of those souls would read some of the comments and think there was a problem without noting the context.

Thus the redness!!!

MIK

And I just figured your printer was low on black ink :p

Seriously - I just wanted to reinforce what Mik said about beefing up Sisu's bottom. If we weren't such neophytes and knuckleheads, it really wouldn't be under consideration. Building for someone else, I think it's very unlikely I'd do so. I'll know more after we do ours and sail it a bit.


"Honesty may be the best policy, but it's important to remember that apparently, by elimination, dishonesty is the second best policy" -- George Carlin

[RossL]

Mik,

I understand your frustration with some of the uninformed comment which gets splashed around the net.

I am not keen on glass sheathing as a method of increasing stiffness in a panel - the only time I use it is when there is a compelling reason for extra abrasion resistance. Even then, I feel that it is better to leave the panels un-sheathed and just fix up the dings. If you want more panel stiffness, add thickness to the timber panels (or cores in composite construction).

One point I need to question is your assertion that panel stiffness varies acording to the square of thickness. My understanding of the calculations indicates that panel stiffness varies according to the CUBE of the panel thickness, not the square. I've tried to follow calculations involving moment of inertia (more properly called 'second moment of area') but my understanding of mathematics let me down, so I checked with Jim Michalak, and he confirmed that panel stiffness varies acording to the cube of thickness.

A reading of 'The Gougeon Brothers on Boat Construction' will demonstrate the dubious value of glass sheathing. Go for thicker ply if anything.

Ross Lillistone

[Boatmik]

Howdy Ross,

Michelak is right but wrong as well, or is being applied to the wrong problem.

But first ... I take delight in (rather than being frustrated in) the uninformed stuff that gets washed around and around the net. It makes it a lot of fun to try and re-orient people to better simpler ways of doing things! And it gives me a chance to spread the reaches of my evil empire.

I've seen you do your share of that too!

OK ... to the Batcave, Robin ...

It is a cube relationship to the thickness for any solid
It is a square relationship to the core thickness for any fixed thickness laminate over a core.
(this assumes the laminate is thin compared to the core)

For those who are interested the long explanation is below.
______________________________________________________

There are two calculations that work together for composites to work out the stiffness of the final panel.

CORE STIFFNESS

One is for the core ... which you are exactly right on ... it goes with the cube.
The full calculation for a rectangular body (most sandwiches look rectangular
1/ I = breadth x depthcubed / 12

That is it in the normal case .. nothing more to be worked out.
_____________________________________________________

LAMINATE STIFFNESS

But for a fixed thickness laminate there are two parts to the equation.
One is the stiffness of the face or faces operating exactly as above.
2/ Ic = breadth x depthcubed / 12

But also there is an effect from the two faces being separated by a core - usually called the "parallel axis theorem".
look down this page for that heading
http://en.wikipedia.org/wiki/Second_moment_of_area#Parallel_axis_theorem

The effect of the faces being separated by a core is related to the cross sectional areas of the face laminate which I will call "area" and the distance between the centres of the areas of the laminate and the neutral (bending) axis of the total sandwich panel "distance"
3/ Ig = Area x Distancesquared.

So to get the total stiffness of the faces you add 2/ and 3/

And to get the total stiffness of the sandwich you add 1/ and 2/ and 3/

BUT ...

____________________________________________

HOWEVER ... some interesting thing happens with normal foam composites.

One is the core has little stiffness ... so you can almost neglect the core.
so 1/ is close to zero

Also the face laminates are so thin that their stiffness from equation 2/ is tiny and their greatest effect is from equation 3/ which is the stiffness effect of the separated face laminates.

UPSHOT is that you often can ignore the stiffness of the core and just worry about the cross sectional areas of teh face laminates.

Equation 3/ carries the day by itself
This is a relationship to the square.
____________________________________________

With my example in the post above ... showing how stupid it is to use a thinnner core with expensive materials ... it makes the thin ply and carbon kevlar look better than it actually is because I am ignoring the effect of the core completely.

So the effect of the thinner ply with the carbon and kevlar on the faces will be LESS than 33% better than just using plain glass.

Dumb dumb dumb

Considering that I would build the same boats out of 6mm with zero reinforcement (rounded shapes excepted - but I don't believe in rounded shapes - remember I am a lazy boatbuilder and would not foist anything on my customers that I am not prepared to do myself!)
____________________________________________

The other comment is that you should include the core in the full calculation if the core has good mechanical properties ... such as timber strip or plywood. But you need to remember that the stiffness offered depends on the direction of the strip. As it also depends on the direction of the fibres in the laminate.

Best wishes
MIK

[RossL]

Mik,

Thanks for the detailed reply.

I know that the matter changes when dealing with a core of limited stiffness, but my only concern is with plywood and natural timber planking/panels. For me, the cubic relationship is all that matters.

I think that people are generally surprised about the (solid) panel stiffness varying acording to the cube of thickness, and it is something that is worth meditating on - particularly for those situations where someone thinks that they will save a bit of weight by using 4mm ply instead of 6mm, for example. The 4mm is nearly 71% less stiff than the 6mm. (I have done a number of repair jobs on people's Herons with 4mm bottoms).

My advice to those who want to second-guess the designer is to get hold of a decent set of scantling rules and do the calculations - it isn't difficult - and then they won't be firing blind.

Extra panel thickness on the bottom puts the extra weight where it helps in providing ballast, as well as improving strength and stiffness. Also, if the panel thickness is added to the outside, the displacement of the boat will increase more than the weight of the hull increases - therefore, the boat will float higher in the water, even though the bottom is heavier and stronger. An added benefit is that the panels will (within reason) take a fairer curve.

Ross Lillistone www.baysidewoodenboats.com.au (http://www.baysidewoodenboats.com.au)

[arbordg]

Mik,

Thanks for the detailed reply.

<SNIP>

Extra panel thickness on the bottom puts the extra weight where it helps in providing ballast, as well as improving strength and stiffness. Also, if the panel thickness is added to the outside, the displacement of the boat will increase more than the weight of the hull increases - therefore, the boat will float higher in the water, even though the bottom is heavier and stronger. An added benefit is that the panels will (within reason) take a fairer curve.

Ross Lillistone www.baysidewoodenboats.com.au (http://www.baysidewoodenboats.com.au)


Ross - can you slow down a bit and repeat that part? In small words that an old woodworker can grasp? You're saying that by adding a layer of glass set in epoxy to the bottom of my boat it'll make it sit higher above her lines in spite of having added more weight? Or are you saying that effect would only be obtained by replacing a 6mm plywood or solid plank bottom with a thicker ply or solid plank bottom? Or are you saying something else, entirely? I'm confused... or perhaps just too ignorant to wrap my mind around the concept.


"If confusion is the first step to knowledge, I must be a genius" -- Larry Leissner

[RossL]

Arbordg,

No, I'm not advocating the use of glass. If someone wants to improve the strength and stiffness of the bottom of their boat, I believe that it is far better to increase the thickness of the ply (or solid timber). If, as an example, one was to use 9mm plywood on the bottom of a boat designed to use 6mm, the increase in hull volume (ie the additional 3mm of bottom planking) would make the boat float higher in the water if all other things remained unchanged. This is because the extra wood in the thicker bottom panel might weigh 600kg/cu.m but the water it displaces weighs between 1000 and 1025 kg/cu.m.

The benefits of that extra 3mm of thickness are substantial. By changing from 6mm to 9mm, the stiffness of the bottom panel is increased by 337%! The mathemeticians out there may be able to correct me, but my understanding is that the stiffness of a solid panel increases (and decreases) according to the cube of the thickness.

So, in my example, the boat floats higher, the bottom is stronger, the bottom is more than three times stiffer, and the centre-of-gravity of the hull is lowered. All for a tiny increase in overall weight.

Having said all of that, I am not suggesting that a person should disregard the designer's recommendation for planking thickness. What I am saying is that if you have a serious need to increase the strength of the bottom of a boat, increased plank thickness is a superbly efficient way of doing it. Adding glass is a very inefficient way of doing it.

If I was determined to use some sort of fabric set in epoxy to add serious strength, I would put Kevlar on the inside of the hull - that is where it would do most good.

Ross Lillistone

[arbordg]

Arbordg,

No, I'm not advocating the use of glass. If someone wants to improve the strength and stiffness of the bottom of their boat, I believe that it is far better to increase the thickness of the ply (or solid timber). If, as an example, one was to use 9mm plywood on the bottom of a boat designed to use 6mm, the increase in hull volume (ie the additional 3mm of bottom planking) would make the boat float higher in the water if all other things remained unchanged. This is because the extra wood in the thicker bottom panel might weigh 600kg/cu.m but the water it displaces weighs between 1000 and 1025 kg/cu.m.

The benefits of that extra 3mm of thickness are substantial. By changing from 6mm to 9mm, the stiffness of the bottom panel is increased by 337%! The mathemeticians out there may be able to correct me, but my understanding is that the stiffness of a solid panel increases (and decreases) according to the cube of the thickness.

So, in my example, the boat floats higher, the bottom is stronger, the bottom is more than three times stiffer, and the centre-of-gravity of the hull is lowered. All for a tiny increase in overall weight.

Having said all of that, I am not suggesting that a person should disregard the designer's recommendation for planking thickness. What I am saying is that if you have a serious need to increase the strength of the bottom of a boat, increased plank thickness is a superbly efficient way of doing it. Adding glass is a very inefficient way of doing it.

If I was determined to use some sort of fabric set in epoxy to add serious strength, I would put Kevlar on the inside of the hull - that is where it would do most good.

Ross Lillistone

Ross,

Gotcha.

Another question arises, however, from your final comment. It has been my understanding that fiberglass cloth was actually stronger on the inside of a plywood hull, and kevlar, and the polyester fabrics were most effective on the outside of plywood. Do I have it turned around?

Another Confused Step Toward Genius

[RossL]

Arbordg,

My use of glass or polyester would normally be for abrasion resistance and would therefore be placed on the outer surface. The only thing that I would consider using Kevlar for would be as a local impact barrier, in which case it would go on the inner surface. You can read more in the 5th edition of The Gougeon Brothers on Boat Construction. Glass can be used as an impact barrier, but it is very heavy compared with Kevlar or increased panel thickness. I hope this helps a bit:)

Ross Lillistone www.baysidewoodenboats.com.au (http://www.baysidewoodenboats.com.au)

[Boatmik]

OK All,

Making the bottom thicker - stiffness
About making the bottom thicker. Moving from 6 to 9mm does have a big effect, however the 3.37 times increase in stiffness is for the ply only.

The effect of the seat structure in combination with the bottom runners has by far the dominant effect in making the boat stiff.

Which is why with 6mm you only get very small deflections between these framing items.

Even with 9mm ply the framing is still going to have a dominant effect that the overall stiffness will be increased ... but nothing like 300%

Making the bottom thicker - buoyancy
In general if you increase the thickness of the ply bottom of the goat from 6mm to 9mm, because of the construction ... the boat will float higher.

But if you build of gaboon it will be floating higher by a bit less than 2mm.

If you build of a mid range ply it will be a bit less than 1.5mm

Negligible effect.

Making the bottom thicker - stability
Basically for small boats getting some extra weight down low is negligible too.

Making the bottom out of 9mm on the Goat will add about 3.5kg (9lbs) if gaboon is used or about 50% more than that if other plywoods are used. The reason this is important for the GOAT is it is just at the point where it is OK for two adults to pick up the hull as designed. It is still not EASY. But it can be done OK. I don't want it to go too far past this point as a small increase will make a little bit of difference.

But as far as altering stability - that 9lbs won't make much difference because
1/ it is so close to the vertical centre of gravity of the boat
2/ it is such a tiny proportion of the overall weight (with two people aboard it is 2% of the weight)
3/ it is proportional to the sine of the angle of heel. So if the boat is over at 30 degrees then this negligible effect has to multiplied by 0.5 (halved) at the same time the boat is about to fill up anyhow.

Actually the good thing is because of those high topsides the goat can go over a lot further than most boats and still recover and it still steers quite reliably at high angles of heel unlike many modern boats.

Cloth location ... inside or outside

Another question arises, however, from your final comment. It has been my understanding that fiberglass cloth was actually stronger on the inside of a plywood hull, and kevlar, and the polyester fabrics were most effective on the outside of plywood. Do I have it turned around?

David ... it depends what you are trying to achieve.

For abrasion - Like Ross says ... we quite agree that the effective part of the structure is wood and glass can be used for abrasion resistance where needed. So has to be on the outside.

For stiffness ... it makes no difference at all whether it is on the inside or outside. The panel is the panel is the panel - it is not going to be stiffer if you have it "right way up" or turn it over.

Energy Absorption, Kevlar an' stuff
This is perhaps the most interesting area of engineering at this stage - and a bulk of research is looking from this angle. It is by far the most accurate method when calculating strength of materials and allows a close study of the mechanism of failure ... why materials actually fail, why they are reliable or unreliable.

Now stiffness and strength are two completely different things. Generally you are designing most structures to be stiff enough. Generally if they are "stiff enough" they will be strong enough too. Aerospace applications will push both boundaries simultaneously ... but boats only really have to be stiff enough in the types we are interested in.

Energy absorption is the thing that links stiffness and strength into one useful entity.

NO CLOTH
I have added this for completeness. Ross, PAR and I all know the sense of this because ... remember those weights from a few posts ago.
GLASS, CARBON, KEVLAR are waaaaay heavier than wood (5 to 8 x heavier)
Resin - needed to hold them in place is double the weight of most common woods (almost 3x for gaboon) and doesn't contribute to stiffness and is along for the ride.

The resin has some beneficial effects for timber structures though as we know.

POLYESTER CLOTH
First .. polyester cloths are only any good for abrasion. They are not very stiff materials so will still be stretching happily as the ply underneath them is breaking up by the moment. They might stop water from getting in. So including polyester cloth makes little sense to me as it is just going along for the ride ... because its density is less than resin it has an unpleasant tendency to "float" if too much resin is added.

GLASS CLOTH
Glass cloths have a nice compatibility with timber as they are roughly similar stiffnesses and elongate about the same amount before they fail. The timber part and the glass around it will share the load very effectively. it is also hugely abrasion resistant and there is evidence that the abrasion resistance and some useful increase of impact strength is there from very light cloth weights. In the case of thicker hulls, good design timber/glass composites can allow the decrease of the timber component because of the use of glass so there is not too large an weight penalty.

However for thin wooden hulls it makes no structural sense because glass is so much heavier than timber.

CARBON CLOTH
Carbon is WAAAAY stiffer than timber, so if you put it around timber then the carbon will prevent the structure from deflecting much so the timber will get little load. So almost all the load will be in the carbon. Then when the carbon fails the load will be transferred to the timber which will fail too usually. This is why carbon boats generally gravitate toward foam or cellular cores - there is no point of carrying the extra weight of the timber along when it is not doing much at all in terms of stiffness or strength.

THE SPECIAL CASE OF KEVLAR
Kevlar is also a stiff fibre so has a similar incompatibility with wood as Carbon - so it makes much greater sense to move to a foam core as a wood core will be "just along for the ride".
However when engineering a composite sandwich structure kevlar is an interesting material.

It has huge energy absorption compared to the others - bullet proof vests (and is terrible to cut, sand and wet out by hand all which correlate to its ability to absorb energy).

However whereas almost every other material has pretty similar tensile and compression strengths kevlar has a high tensile strength but its compression strength is pretty similar to "S-glass" which is way cheaper.

S-glass was originally developed for the aircraft industry and is a bit more expensive than the normal E-glass (the E means it was an electricity industry standard) that we all know and love.

Carbon doesn't care too much about whether it is tensile or compression load and can be made in lots of different flavours (high tensile or high strength) so it has edged Kevlar out of the picture. But the carbon doesn't absorb energy very well.

So that is why kevlar tends to gravitate to the inside of the hull. Because as the hull tends to bow inwards under water pressure, or an impact with a wave or something more serious the kevlar will be in tension rather than its weak compression mode. Put Kevlar on the outside and it has to be twice the weight compared to the inside if you want both sides to fail at the same time.

So ... Carbon doesn't behave well in absorbing energy - put that on the outside because it can handle tensile and compressive loads equally

The structure is a bit prone to impact damage ... so put the kevlar on the inside where it can increase the energy absorption and also only really get tensile loads to make best use of its uneven strength.

FINALLY

So this might reinforce the idea why good wooden boat designers are reluctant to use lots of fibers in their thinner skinned structures and why they focus on glass/timber sandwich for the thicker ones.

There are ways of getting some compatibility between carbon and timber if wanted, particularly if direction of the fibres is considered. But all the points above do need to be addressed to design effectively in them.

A cookbook approach of a bit of this and a bit of that is bound to be expensive in materials and labour as well as way too strong or stiff for the required application.

This is a bit longer than necessary for the question being asked but covers the main points.

Best wishes
Michael Storer

[arbordg]

Mik,

Your posts aren't at all "longer than necessary for the question being asked. Just enough info to help me weigh pluses, minuses and relationships - and to make distinctions. The more of the theory I understand, the better builder I'll be. Plus, I love arcane detail :U

But back to the here & now. For Sisu's bottom, would you say we'd be better off to add a 1/8" or 1/4" layer of ocoume (bonded with epoxy) than add a light layer of cloth in epoxy?


"I was brought up to believe that the only thing worth doing was to add to the sum of accurate information in the world" -- Margaret Mead

[Boatmik]

Howdy David,

I think it might be a lot easier and simpler to use 2oz glass. Because of the risk of voids you would have to use a lot of 'pox between the layers of ply.

So I would go with the glass. Take it around the chines by 25mm approx. I think that is the easiest way out of the two.

MIK

[arbordg]

Arbordg,

My use of glass or polyester would normally be for abrasion resistance and would therefore be placed on the outer surface. The only thing that I would consider using Kevlar for would be as a local impact barrier, in which case it would go on the inner surface. You can read more in the 5th edition of The Gougeon Brothers on Boat Construction. Glass can be used as an impact barrier, but it is very heavy compared with Kevlar or increased panel thickness. I hope this helps a bit:)

Ross Lillistone www.baysidewoodenboats.com.au (http://www.baysidewoodenboats.com.au)

Ross - Thanks. I'll take a peek at the Gougeon book when I get to the shop today.

[arbordg]

Howdy David,

I think it might be a lot easier and simpler to use 2oz glass. Because of the risk of voids you would have to use a lot of 'pox between the layers of ply.

So I would go with the glass. Take it around the chines by 25mm approx. I think that is the easiest way out of the two.

MIK

Mik - OK, so I'm back to the original plan. Cool Beans (as Andyrew would say). :D And just when I was starting to noodle on the challenge of a clean glueup for skinning the bottom. I figured if I did it right, I'd end up having the fairest bottom on the land (not that I don't already :p

[Daddles]

I just going to continue being a grumpy, old fashioned traditionalist and slap paint all over my timber, but an enlightened one because I'll slap poxy on it first.

Richard