Thread: GIS - Rocky beaches: graphite, glass, or...?

Mik will probably reach right thru the computer and smack me when he sees this reply. Coming factory-equipped, as I do, with a large ration of both ignorance and irreverence... I see nothing wrong with glassing the bottom in your circumstance. You probably have - in addition to the rocks - mussels and barnacles... and gravel. In that case, I'd be inclined to sheath the bottom - up to the waterline (or ever so slightly above it) - with the lightest polyester cloth I could find (xynole, dynel, etc.). I say the lightest, for three reasons: polyester cloth is more protection from impact than fiberglass cloth, so you don't need much for that type of protection; poly cloth is far more protection from abrasion than fiberglass cloth... etc.; and finally - and most importantly - poly cloth soaks up significantly more resin than fiberglass cloth - adding weight (not a good thing) - which you'll want to minimize. Some minimize by vacuum-bagging. Not a bad idea, if you want to gear up for it, and learn the procedure. Keep in mind that you'll be trading some of the trademark GIS light = fast character for this protection.

Graybeal might be thinking he is about to die ... but I am a gentler soul than he thinks.

There have been a number of quite large boats built in OZ with very light glass on their ply hulls.

As you probably realise ... a larger boat often makes it difficult to choose the stoneless spot on a normal beach ... or it will make contact with unseen rocks on an otherwise sandy bottom coming into the shallows.

Some such boats, bigger and heavier get away with 2oz (75gsm) woven glass, so I can't see why that wouldn't be enough for most plywood boats of trailerable size as well.

I suspect that having SOME glass is almost as effective as having a lot for normal practical use.

With the GIS ... I would suggest glassing before the bottom skids go on and wrapping the glass just 25mm onto the side panels so the glass tape on the chine can be eliminated.

As far as the graphite junkies go ... show me the science! My experience with epoxy graphite is sometimes i put it on the inside of a centrecase and sand it a bit so the graphite surface is exposed.

It is a while since I have done it, but it doesn't seem particularly different in terms of sanding compared to other things - ie you can cut through it much the same as epoxy - this indicates that it is not particularly abrasion resistant.

The other side is that there is an argument that it will slide off a rock more easily without doing any damage. Well .. maybe ... but only maybe.

As far as it making the boat faster ... there will categorically be no difference at all. It is based on a misunderstanding how water "flows" around an object. Actually the first "layer" of water is 100% pulled along by the surface then the next layer out can move a little bit relative to the surface ... then the one further out can move a bit faster.

The general phenomenon is called the "Boundary Layer" and may be a fraction of a millimetre thick at the front of the boat to a quarter inch thick at the back for our little boats through to 200mm thick at the back of a tanker. It is taken to be the thickness for the water to be flowing past at 95% or 99% of the boat's velocity relative to the water - different definitions in different places.

So graphite might lubricate a hand or centreboard that rubs against it as it makes the flat graphite plates slide over each other. But the water never rubs against it ...

Best wishes
MIK

David Graybeal ... SURVIVOR!

I spoke to the technical guru at SP Systems recently and he mentioned how they use graphite on their local Cowes boats.

They use the graphite only on the centreboard to make them easier to raise and lower for sailors with special needs.

To make the centre board inside case surfaces 3 times more abrasive resistant they use the powder that the grit blasters use in the mix.

The beach where Trim will be landing is a very rough gravel ( ie from local gravel extraction not rolled pebbles which are fine) so she has a light glass covering.



My understanding is that, whilst abrasion is aided by external glass, to protect from possible rocks punching through the hull, the glass should be on the inside of the skin, not the outside.

Brian

Brian,

I've been studying, but have no first-hand experience with polyester cloth (yet). But here's what I've learned.

Having the fiberglass on the inside of the boat for impact protection is correct. With polyester cloth, it's the opposite. One is stronger in compression, and one in tension. Since one - presumably - would want the cloth/resin matrix to pay for it's added weight by also serving as abrasion protection, you'd want it on the outside. Thus my recommendation for xynole, or the like.

There are two drawbacks to the polyester cloth. First, they tend to actually absorb the resin - which is good in a way, but means more resin (weight) builds up. That's why I suggested the lightest cloth available. I gather that Mik's correct that just having the cloth on is a big step, but adding thickness/weight doesn't add strength/abrasion resistance in a linear fashion. Diminishing Returns comes into play. The other caveat is that it also tends to float in the resin, rather than clinging to the hull. As long as you know that, and take steps, that issue is manageable. The steps taken could range from vacuum bagging to extra staples or tape to simply extra care with a Bubble Roller as the resin cures.

Cheers,
The Survivor
(Sometimes I think it's a Good Thing that Mik's in OZ)

"Every man is wise when attacked by a mad dog, fewer when pursued by a mad woman, and only the wisest survive when attacked by a mad notion" -- Robertson Davies

I seem to remember a thread from somewhere, probably the WB forum, discussing this. Are the xynoles just for abrasion, but do not bring strength as the glass fibres do? Will see if I can find the thread.

Brian

I'm neither a physicist nor a mechanical engineer so I'll give you my (in)expert opinion on this.

I suspect the polyester weaves, being heavier due to more resin, only APPEAR to work best in compression. They are resisting blows--from test hammers in the lab and rock hammers in the river. Their extra thickness and weight gives them a natural advantage when it comes to fending off a direct attack.

Conversely, lightweight fiberglass cloth on the outside succumbs to the hammer and the rock because it is a relatively hard (but thinner) membrane protecting a much softer core (the plywood). The hammer dents it, the sharp rock penetrates and/or rips it. But on the inside the initial impact is softened by the relatively soft plywood. Impact stresses are dispersed so when they arrive at the inside fiberglass layer they are weaker per square inch and the fiberglass survives.

I would suggest if you just want abrasion resistance for dragging up on a coarse beach use what MIK suggests--a thin flberglass layer on the outside. If you are going to be sailing around sharp rocks that rise from the depths and lurk beneath the surface (DON'T DO THIS!), put one of the polyesters on the outside and heavy glass on the inside (DON'T DO THIS!).

---You pays your money and takes your chances...--M. Twain

Howdy Chaps,

I don't like anything other than glass because I hate adding weight without getting some structural benefit. Nice thing about even light glass if needed is it moves the neutral axis of the ply ... so 3 ply plywood becomes fabulously stiff as the middle veneer finally has something to work against. Makes the world of difference.

I think people overlooks the obvious when talking about preventing damage. Personally I would go for the next ply size up and if I hadn't already made the decision, do it in Gaboon.

It will be way cheaper and probably lighter than all the glass/plastic fabric and resin.

Basically I think it is a huge waste of materials and effort to glass the inside of a plywood boat and does emphasise a strong lack of analysis.

MAM is right about the right places for stuff - unless you are asking the cloth to carry load, then it is glass outside too ... but I always like to go back further in the process and ask "what is the attempt intended to achieve?"

When a largeish trailer catamaran with a cabin that is built in large numbers over many years can be built of 4mm ply with 2oz glass on the outside ... I think it forces those advocating glassing the inside and outside of a simple and very much lighter sea kayak with quite heavy cloth into some sort of reassessment. That's what it did to me when I realised!

You can see the Eureka (not the canoe - the catamaran) moment here.
https://www.woodworkforums.com/f35/jarcat-vs-kitten-18-trailerable-catamarans-64457
It was only a short time ago. Yap .. that large cat routinely uses 4mm ply and 2oz glass on the outside only.

But there seems to be some sort of race in Sea Kayaks as to whose boats are the most durable and claim the others are underbuilt in some way.

I just can't figure it at all.

Also there were some guys building a PROA with a 4mm hull and carbon skins inside and out ... that is another thing I cannot figure. I can make the structure work with 6mm ply and no cloth and some seriously loaded high performance cats are built of 4mm and no glass at all ... so they really should do some basic calcs and work out the relative weights of what they are doing (not to mention the cost).

4mm ply and serious loads ... couldn't find the pics of the Tornado in wood but here is a Unicorn A-class in 4mm, with glass taping only and intelligent design of the internals


If you want to ponder these types of lightweight but hugely strong structures you can download a building guide for the Tornado here. One of the truly interesting things is how it changed from quite a delicate structure when designed by Rodney March in 1966 approx and became a very durable structure by the 1980s with no increase in weight. Actually there was a possible reduction that enabled builders to centralise weights in the hull more for better rough water speed.
http://www.thebeachcats.com/modules....download&cid=6

Same problem with the guys that advocate building strip plank canoes and cut down the planking thickness to "save weight" They are removing the lightest and most structurally efficient part of the structure - the heavy glass is the first thing that should go - one light layer all over then a second over the top where it is really needed. Maybe shift to Paulownia if it is available too. Keep the core thickness ... reducing the core from 6mm to 4mm more than halves the stiffness of the structure.

There is a weird mindset out there about using composites with wood.

I think they just don't trust or understand wood at all!

Hmmm ... looks like I needed a rant ... sorry chaps!

Best wishes
MIK

Mik,

I can't help but agree with your rant. Partly because of some things I know (beam strength, monocoque construction techniques, etc.), but mostly because of all the things I don't know, but know that you do - you bloody wizard, you

But think back. This discussion started with Zippy wondering how to protect his beautiful GIS, once she's built, from the depradations of a rocky, gravely, beach with barnacles and mussell shells. Having grown up on Puget Sound, I'm all too familiar with such beaches. Until one sees it, it's hard to imagine how much of a shredding a patch of barnacles can inflict upon a plywood hull. As a child, many of our boats were heavily, and traditionally built. If you ground 6mm off the keel every season, it really made no big difference. With a GIS, that's putting a new bottom on every season. No Thanks.

Which is why I suggested the fabric epoxy matrix on the bottom. And, since the xynole is supposed to be superior to fiberglass in protecting from these sort of Extreme Abrasion conditions, I suggested the lightest xynole he could find. Now... maybe the extra abrasion protection of xynole is not worth the tradeoff of added weight penalty it carries. I can't speak to that one from first-hand experience

Watcha think?

Thanks for dragging me out of rant mode, David!

OK .. The mix of what I want for the Goat is a bottom that resists scratching and gouging.

But also want to cover the eventuality of the bottom resting on some rock and then somebody hops aboard without realising.

So the first is abrasion resistance, the second requires strength.

So I would be suggesting the light glass on the outside.

If it was a REALLY horrendous place I would probably be saying thicker ply and the light glass.

The reality is it is always possible to break any structure in some circumstance. The thing is to find a reasonable solution that will prevent it from being a common occurence for the way the boat is used or what it was designed to be used for.

MIK

I have barnacle rich beaches here in Maine. I plan to sheath the bottom in 4oz. We're doing the same on the kids' rowboat, but actually we're using 6oz b/c it is free.

Thanks all, I appreciate your thoughts (and rants). I'll think about it for a while, but at the moment am leaning towards light glass on the bottom. I'd consider thicker ply but I've already bought the ply...

Jeff

I think the thicker ply is a last resort. I added it to complete the picture. The glass will make a HUGE difference.

MIK