Thread: First impressions of a Goat

Originally Posted by 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

And I just figured your printer was low on black ink

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

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

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_...l_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

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

Originally Posted by RossL

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

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

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

Originally Posted by 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

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

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

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

Originally Posted by arbordg

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

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

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

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

Originally Posted by 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

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

Originally Posted by 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

Mik - OK, so I'm back to the original plan. Cool Beans (as Andyrew would say). 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

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