Wide Gap Strip Plank - what problems could this building method cause?

rfnk wrote:

>I think there'd be three problems with a `skin on frame' approach to building a cruising yacht as opposed to a skiff or kayak. Firstly, the design provides little resistance to impact, especially from anything sharp, unless some sort of bulletproof membrane could be made.


This is incorrect, I can see you have no experience with skin-on-frame construction. It is much more resistant to impacts or punctures than conventional fiberglass construction (note that fiberglass nor wood planks are "bulletproof"). I have actually participated in a destructive skin test of a kayak skinned in 12oz nylon sealed with polyurethane finish, it was dragged across a gravel parking lot with a person it in, dragged over broken concrete and broken oyster shells, it only scratched the paint, it was still seaworthy. It was then flipped over and pounded with bricks, rocks, oyster shells and stabbed with a piece of roughly cut off 1/2 inch steel re-bar and pounded with a claw hammer. It took repeated and sustained heavy blows in the same spot before the skin failed, way more than a fiberglass hull would have taken. because the structure is redundant and "fail-safe" even with broken ribs or stringers, it still would have been seaworthy. The skin flexes with impacts, but stays water tight. The only thing that could puncture it was a sharp knife or a screwdriver. see this out (and this is mild compared to what we did): http://www.youtube.com/watch?v=gYyvIExWkqU
A well designed skin-on-frame hull with rub strips in the right places using 28 oz fabric would be very tough indeed.


>Secondly, assuming there's a skin on the inside as well, you'd have a whole lot of voids. The expansion and contraction of the air in these voids from temperature changes and the everyday pressures of sailing would introduce huge amounts of water into the structure, causing all kinds of problems.


All your objections to this are wrong. Never, never never! You would never put a skin on the inside, you are confusing this construction method with conventional fiberglass. It would only add weight and cost, and prevent access to the frame for inspection and maintenance. On a large hull you could have wall and floor panels that are easy to remove, but they would not be sealed. You show your lack of familiarity with the construction method.

>Thirdly, wouldn't the hull be too flexible if constructed in this way (affecting performance)?

Totally irrelevant, the stiffness of the hull would come from the design of the frame. It can be as stiff or flexible as the designer wants, it is not related tot he type of construction. The skin is a non-structural element that is there only to keep the water out.

>If the skin is so thick that it's impact resistant, so strong that it can't allow water ingress from `pumping' and so stiff that it can't flex too much, isn't it then just a GRP boat? In other words, a neat way to create a mold. You could peel the strip planks out altogether afterwards.

A tough flexible skin is MORE impact resistant than foam and fiberglass construction, the skin gives with impacts reducing the impact loads. I do not know what "pumping" you are talking about, the skin is never stiff enough to add any structural support to the hull except perhaps in tension. It would not be skin-on-frame if it had a stiff skin.

It appears you, and many others too are stuck on the wrong paradigm about skin-on-frame construction, no doubt you are thinking about fiberglass or similar cloth "glued" together with a brittle resin. You have to think more objectively about a separate structural frame to that takes the hull loads, with a tough but flexible skin that just keeps the water out. Larger examples of up to 40 feet long have been used successfully for centuries in the past, there is no reason it could not be employed in modern designs, using modern materials taking advantage of its many virtues, and using good design to over come its few disadvantage.

None of your objections here are valid.

 

I once had an engineering professor that said that one simple test is worth a 1000 expert opinions! So true.

You might want to make a few panels of convention fiberglass and foam panels to run through the same tests, and you can compare their weight too. Then you get a pretty good comparison of the properties relative to each other on something you built (not a lab test sample).

Good luck.

 

Yes - the testing is the key.

I will also do a panel of 12 oz nylon panel stretched over a frame, and attack it with a steel r-bar.

It wont make a hole you know!!!

Can you imagine 2 tonne of boat with 12oz nylon skin, resting on a rough rock as the tide goes out?

 

RWatson: I've moved to a different computer, and stay inside the same house as my wife - now I can continue to be online! PS: like the idea to involve pre"star wars" engineer to my project?

 

use something more like 20 oz ballistics nylon, yes it would hold it just fine, remember we are only talking 1-2 inches between stringers.

What would a 2 tonne boat of foam and fiberglass do to the hull resting on rough rocks? Or even a steel or aluminum one for that matter?

 

Sub Project

I think you could start a new thread under "Projects and Proposals" forum - that way you could update it as you go. I think the design challenges, the testing, refining etc would make a great story.

Doesnt matter what you call the thread ... I am sure a lot of people would follow it.

Keep sanding

 

Petros
You're absolutely right that I have no experience with skin on frame construction. All I've tried to do is raise was seem to be to be logical issues - if they're not `valid', then that's good. I do wonder why, though, I don't see any racing yacht designers (who'll do anything to save weight) using this method and why none of my whitewater kayaking friends use this method. I did assume you'd have to have an internal skin in this construction (if only to avoid a requirement for a massive number of frames?) hence my comment about having large voids which, through expansion and contraction, would pump in water - but you say there's no internal skin. Two questions then, as, as you point out I'm not familiar with this kind of construction for a large vessel - stiffness, flotation, strength and watertight`ness' in a hull are sought through use of materials that combine to achieve this - they all do their part. In skin-on-frame aren't you relying on a lot of framing to provide strength and stiffness (so where's the saving?) and how are the diagonal loads borne (I'm assuming the skin doesn't provide this if it can be easily removed)?

 

Well, I can tell you the answer to *that* question. It gouges a half inch deep hole through the gelcoat on a fibreglass boat, it puts a small dent in an aluminium boat, barely disturbs the paint on a steel boat, and it puts a 6 inch scratch about half an inch deep in a wooden boat - and none of them sink, and all are repaired with a minimum of fuss.

20 oz ballistic nylon - are you kidding? For a start, I bet its not UV stable, nor constant immersion approved - and over $120 per square metre.
Lets not get into how to sew or glue it into the shape of the hull, and sew the joins etc. You probably need a laser to cut it into sections.

I have had a lot of experience with flexible skin canoes - and I know exactly what is involved with using them in boats.

I dont know why you insist on pushing such an impractical idea.

 

As usual you guys are way over thinking this. The cost of the epoxy and foam fillers, fairing them far out weight solid bead and cove strip plank hull.
If your wood working skills are that poor consider hiring a boat builder or go back to straight grp.

 

Hmm. somehow in Chandler's world anyone interested in exploring alternative approaches is doing so because their woordworking skills are not up to, presumably, his standards. That logic would take us back to where? Carving sailing canoes from solid trunks or rafts of bamboo and coconuts? I think any ideas that may lead to greater simplicity, efficiency, cost-savings etc., should be explored. If they happen to lead to something worthwhile, and they sometimes do, then that's great. If not, where's the loss? There are certainly plenty of people who think that strip-planking sheathed in glass is not REAL wooden boat building! Personally, having watched my brother build a kayak using this method, I'd consider it for a larger boat but would go with the stitch and glue approach as I think it's really much more efficient for that kind of craft. But that's just my view. I've also seen plenty of people with awful woodworking skills create beautiful boats and, more frequently, seen people with excellent woodworking skills create horrors!

 

My point was that it would not be cost saving.
Stitch and glue is an excellent alternative for chined boats but has no use in round bottom boats.

 

The point wasnt that it would be COST saving - rather EFFORT saving.
I did the financial analysis early on in the piece, and it doesnt have any siginificant financial downsides. Besides, saved effort = saved cost.

Sure, I know woodworking skills come into the strip plank equation - (and I have built in strip plank so I know EXACTLY what is involved) The point is, that the fine woodworking skills may be *obsolete*!!

If you are going to encapsulate the planks in FG, and not do a clear finish, then why bother with all the finicky joins and plank fitting????? I have been told by my naval architect that the mechanical sheer strength for strip plank is hardly taken into account for scantling calculations. This is because the sheer strength of glued edges is so variable and minimal anyway. So why bother with all the clever fitting and joining?

If I was using foam, the close fitting problem is done away with. BUT - I would have to buy a lot of expensive oil based foam.

If I could utilise the self-fairing strip planks to get a good shape (like they had to do for the vertical foam technique in the first post), replace the finicky joins and gap resolution with easy to apply "goo", and still get a solid hull, why shouldnt I ??

 

Sounds fair enough to me (pun intended)! Just another question - and please forgive the ignorance! - would you need to use expensive foam to fill the gaps? Why couldn't you use polystyrene since the strips and sheathing are giving you the stiffness you want? Actually, thinking it through, I guess it would be just too weak and with the hull flexing, it would tear away from the sheath and the strips, would it?

 

No - not expensive foam. In the first post I envisaged just epoxy mixed with some type of filler - say microballoons or wood flour.

Some of the other comments were about flexing, and I do see problems if just loose unfastened "fill material" were used. I think epoxy should provide enough toughness.

In my mind, the stiffeness needed is a function of incompressability. Most foams (and say balsa) have little else going for them. If you think about it, end grain balsa is just a variation of this concept - blocks of light material held loosely together until the exterior glass is applied. With balsa though, you dont get long, self fairing strips, you have to have the mould constructed to lay it on.

Back in the "olden days", they used to nail strip planking together to provide some sheer strength, but nowadays, no-one does that - it just isnt required.

The test panels I plan to build should provide enough proof that the exterior FG layers will provide lateral stiffness.

The *dream* is to lay all the planks unglued over the forms, stapled to the mould formers, then just mix big pots of "bog" and fill in all the gaps, just like grouting tiles.

Initially in my mind I saw half inch gaps between planks, but now I see the planks barely touching, but with big bevels facing outwards towards the "bogger". Where the gaps are substantial due to hull curvature, I would simply back the gap with that clear stuff that epoxy doesnt stick to, held in place with packing tape.

Being able to do the fill in one step does away with having to do the messy glue between planks, and waiting till it dries to do the next plank and those crazy joins where the planks "run out" over each other. Hell, I am not even going to do the traditional "bow" construction. (see attached)

I will do the bow section as a fibreglass module, glue it to the first frame, and epoxy the outer skin to it. This does away with all that rubbish about trying to force epoxy and cloth down in the bottom of the bow with a 3 ft stick and rubber gloves. I went through it with a canoe - what waste of time that was!

Must start on those test panels soon.

 

I guess endgrain blocks of balsa won't compress the way foam would, soo, over time, there'd be a lower risk of delamination from the foam compressing. The Schionning catamaran design company is located near us at Lemon Tree Passage and I've watched some of the builders using sheathed sheets of endgrain balsa on their cats. Some of the builders have used strip planking when they haven't been able to get the balsa sheets.

A librarian gave me a large roll of clear plastic a few years ago that they were throwing out. They cover books with some adhesive these days, apparently. Anyway, this stuff is not much thicker than Glad Wrap but quite a bit stronger. Whenever I'm making up laminations etc. with epoxy I always use this stuff to prevent the glue getting onto clamps, clean timber etc. It peels off the cured epoxy really easily so I think it would be perfect for your backing requirements - you'd just need to clamp in a batten to hold it in place, I guess. What sort of boat are you intending to build by the way? Rick