Using SOF as part of the boat design

Hey guys, I'm new here but have lurked over the years. I have designed and built a few 15-16ft flats skiffs using stitch and glue, and a cedar strip panel kayak. Now I'm thinking about making a small plug to play around with actually making a mold/mould.

I'll ask about the materials in another thread, but I wanted to know if anyone has used, or knows where I can find some info on using skin-on-frame to create a plug? I understand it can be tricky and will naturally require faring but I would still like to see if anyone has done it or has more info.

I know it has been done in the past, cause the sock boat is my inspiration for doing this. I just love the shape it takes naturally. Thoughts?

http://www.britishpathe.com/video/stills/do-it-yourself-boat

 

I have built a number of skin-on-frame hulls, mostly sea kayaks and small sailing dingys, though the methond coud be used for larger hulls was well. I have not done it, but I have seen some fiberglass sea kayaks made by pulling a mold off a skin-on-frame kayak. So I know it has been done.

It appeared to work okay, but the performance and behavior of the two nearly identical hulls was very different in the water, the original skin on frame hull was light and flexible, while the heavier fiberglass one was very rigid. This should not be a problem if the design was intended to be a "hard shell" hull from the beginning.

It should work okay as a plug except you want to use a heavy and stiff fabric, and pull it very tight, and do not have any large unsupported fabric panels on it. the Weight of the wet glass might tend to allow the fabric to stag so you end up getting a hollows in between the supporting stringers. If your intent is only to make it as a plug, than there is no reason not to make the frame very rigid with a heavy lumber frame. And fully fill the weave of the fabric, and sand it smooth, before you attempt to make a mold or you will never get it separated off the plug.

 

This was the process I was thinking of.

Build a fairly heavy frame and stretch some kind of fabric over it (I was thinking about polyester fleece like they use for making glass speaker boxes).

The frame will be well supported and the stretched fabric will be stapled at each main line of the hull since it will be faired later (keel, chines, shear...).

Once it is where I like it I will mix up a thinned resin and spray it on in very thin coats, maybe 2. and let it cure and harden. I might even flip the whole thing and spray it from the inside first.

When it's all hardened I would add several layers of glass, light weight at first, a few layers of .75-1oz mat. Followed by a layer or 2 or cloth, then more mat before fairing.

Would this work? I'm only talking about maybe a 13-14ft skiff, Maybe 48" max at the chines?

 

I'm toying with the idea of recovering my SOF boat with a composite skin (here's a link to photos of it when I was building it: http://www.boatdesign.net/forums/boat-design/beautiful-skeleton-37456-4.html ).

My thoughts are to rip off the heavy duty aircraft polyester skin, re-skin the frame in the ultralight polyester (I happen to have a lot of it left over from covering aircraft) and then lay up a thin Kevlar/glass/epoxy outer skin (one layer of 175g/m² Kevlar followed by a couple of layers of thin glass cloth). I'll probably peel ply the outer surface, to be able to squeegee down to a reasonable reinforcement/resin ratio and give a pretty fair surface that shouldn't then need much fairing. A couple of coats of decent two pack PU paint and I reckon I'll still have a fairly light boat but with much better abrasion resistance.

SOF is great, the only real weakness is the need to protect any hard point that will get rubbed, as it's easy to get abrasion damage. I've been using PU "helicopter tape" along the keel and gunwales, but this ends up getting tatty and sometimes stones cut through to the cloth underneath. The cloth between the frames is really tough, and not really at risk of damage (short of someone knifing it).

It'll be a while before I get around to doing this though, as I'm in the middle of building us a new home.

 

It will be a while before I can get to it as well, but I look forward to the results. Where do you buy the polyester skin?

 

In the US you can by it from home built aircraft suppliers, like Aircraft Spruce (see here: http://www.aircraftspruce.com/menus/cs/ceconite.html ).

This stuff is easy to use, as you glue it on to the frame with the cloth just laid over it, then heat shrink it with a iron to get it nice and taught.

If you search for instructions on aircraft covering using Ceconite or Stits you'll find some useful info that may help.

 

As a sidelines observer with no experience in SOF construction, I wonder about all of the effort taken to build up a reasonable strong mold from multiple layers of glass and mat. Could you not build a SOF type jig and cover it with rigid ply panels or something similar?

 

Rigid panels can't make the same compound curves I am trying to achieve, at least not without a lot of scoring and a few hundred more man hours. Stretching a fabric over a frame lets the frame dictate the final shape and can be easily adjusted.

 

Sag is an issue but it is likely manageable. Pucker and asymmetry would be the flaws I fear.

One point I would make is that you only need to pull the fabric over rails were it is convenient -you are free to support wide sections with flat or convex panels. You could use some large plastic pipe to make rounded chines. The shorter the unsupported span of fabric the lower the tension force to maintain shape.

 

Using heat shrinkable fabric allows a degree of compound curvature and does away with the sag risk. Ceconite/Stits aircraft covering fabric shrinks so that it's as tight as a drum skin. Having covered a few aircraft with the stuff, and used the heavy grade stuff to cover the rowing boat I linked to above, I'm very confident that there'd be no problem in using this as a form for laying up a composite skin.

My only concern is with getting a good bond between the polyester form and the composite, as I intend to retain the alloy tube frame and inner skin as a part of the boat (in my case the composite skin will be pretty thin, and not really structural). I may well go over the polyester skin with some sort of spiked roller to create a lot of small holes to get a better mechanical key with the composite layers.

 

You might consider actually saving a lot of materials and cost by just laying up directly on the SOF mold, rather than make a female mold.

I would not use felt, you want woven fabric, much less stretch. And using the aircraft type heat shrink fabric would allow you to eliminate all puckers and create compound curves.

But this is what I might try: build your heavy frame, with a design that allows to dismantle it with a skin on it. Make sure you sand smooth all the places the frame will touch the skin. Install over it the aircraft polyester (buy the uncertificated fabric, much cheaper and you do not need the certification, it all comes off the same machine), paint it to fill the weave and sand smooth, apply parting wax. Than lay out your composite skin on the outside as you would your finished hull, than dismantle the wood frame and remove it, and than add all the stiffeners and supports to the inside of the skin. You will want to make a frame to support the skin, or even have some stringers or something else to stiffen the skin that goes on your skin-on-frame mold that stays with the skin to help keep its shape.

Seems that would save having to make a female mold. If you are making more than one hull, than the extra effort of a full female mold is likely worth doing.

 

I would do it that way, but I am thinking of making a few of them. I suppose I could always do it that way, and then splash a hull off of it later right?

The ceconite looks like a great option, at 9 bucks a yard for 72" wide it's pretty affordable too. Do you have to use an iron? Or does a heat gun work on the fabric too?

 

I did some layup on a fabric 'skin' on two occasions

Both times, the polyester warped the skin slightly concave as it cured.

You cant have compound curves on a SOF, as it will make the ribs stick through - eg the panels will need to be develop-able to a large degree.

So I would think using some stiff 'board', like they use for temporary signage, that cant warp at all , would be the best solution.

Even better, use proper 4mm thick high density foam, and layup the outside with glass. If you put a few ribs inside before you laid up the interior, you would end up with an extremely strong, lightweight craft.

I was astounded how stiff 4mm foam with 6 0z glass on both sides on a test panel I tried..

 

A heat gun works on the aircraft fabric, but it is not used on a boat. I have built a number of panels using this fabric, use an ordinary clothing iron (you can buy one at a second hand store pretty cheap). Do not over heat the fabric, it will damage it. Most instructions have you testing your iron with a thermometer to verify heat. You use the low setting on first pass, and than set the iron hotter (I do not remember the max recommended temp setting), and do the final shrinking. It makes a smooth and tough skin. just buy a few yards of the stuff and play around with it on a wood frame.

 

Thanks guys, I have a lot to consider before I start the project. If I go ahead with it the goal will be to make a utility/technical poling skiff about 13-14 feet long, and light weight enough that it can be hand loaded by 2 guys, and pushed well with a 5-6hp outboard. A challenge no doubt, but I think it is possible.

I still think the SOF method will work perfectly for the shape I want, I'll just need to develop the frame and jig accordingly and may need to incorporate the flat panels as mentioned here.

I'll have to get a good iron and test it out, but I have to tell you this video got me excited about the ceconite and the shape possibilities with it and a heat gun.
http://www.youtube.com/watch?v=oiUbFR21-6s

I think it I make the jig solid enough to rotate I can put the ceconite on and shrink it. Then flip the jig and mist the inside with thinned resin, just enough to add a little rigidity before working on the outside to control the cupping a bit. I'll do a small scale test or 2 before actually doing it though.