Kerdyn Green Foam - trimaran / multipurpose

Hi

After a looooong hiatus I am looking to build my trimaran again. Kids and house move meant the stitch and glue never got finished, and the bottom oil canned whilst sitting too long. So the intention is to use the existing hull as a buck, and lay over Kerdyn Green foam (80) on it in strips, and strip foam build it, glass the outside, throw out the wooden original, then glass inside. I had bought 20mm foam way back, but it is very stiff, and working it around chines will not be easy, it likes to crack. You can see some on it from years ago, but I didn't have the tools I needed, and 20mm is overkill. Thermoforming it is possible, but would take dozens of hours with a heat gun. So I am thinking 5mm or 10 mm foam to better contour to the existing hull. Which hull? The Windrush by Selway-Fisher. (If you google it you will see it in stitch and glue and a much younger me). It is (of course) a touch more complex than this... the intention is to have it as a rowing boat or motor canoe, but have a frame on top to add outriggers (from a 14ft Nomad initially) and a freestanding mast (or two - Solway Dory Osprey style). So it will be multi purpose. The question is what foam thickness to use. Naturally much will turn on cloth used - so I am thinking two layers 200gram either side. It will have a false bottom for boyancy and aft and for sections (see the finished Windrush on the Selway Fisher site). So this will add rigidity. I am not too worried about stress on the hull from the extra hulls as the frame will sit on top and bear much of this to some extent. Leeboards off the frame. I do want to keep the weight down so I don't want to overkill the build. Reinforcing around mast step will be needed. It will be going into some rough waters from time to time, bit of offshore sailing from our harbour and beach - the idea is it can be assembled and launched from the beach - one carries each hull down and assembles there.

In terms of foam thickness, at this small scale I think some of the larger standard calcs not really play out. And I understand 5 mm with foam on both sides is extremely strong. Thoughts? Anyone built a small foam strip dinghy? Ply is more the norm on weight and strength, but rebuilding this from scratch is going to be a right pain, I have the buck, and the foam adds a long term durability that is incredible. I have the use of a 50 year old fibre glass canoe that is still going, and having that durability is important, as well as the low maintenance aspect.

 

Second question is whether this will be worth it as a trimaran. Intention disclosed to the designer, so freeboard increased (but may increase a touch more with another foam plank) and breadth increased, and this will be intended for fast cruising. Flying hulls etc not in the picture - hence free standing mast or two. Very much like Solway Dory Osprey use. But I am wondering about making a set of simple stations with a much simply 3 chine boat with a flatter stern - super stretched GIS if you will, but add a slight V in foam on the flat bottom so it doesn't slap too much - and strip building on that. But at hull speed this might cause drag, whereas the current wineglass might work better. It is also a seriously pretty boat in current form, and whilst looks aren't everything, it is nice to sail something nice and aesthetic.

 

That is a really pretty boat. If you want a trimaran, that is not an ideal hull. It is too beamy, particularly if you don't want to fly at least one hull. By the time you make a hull on top of that, it will cost more in time and materials than if you build a new stitch and glue. Take into account that a trimaran cost at least three times more than a monohull. The time to build is also more than tripled: three hulls and connecting beams.

 

No. Bad idea..

The foam will be too flimsy over the top of the existing hull. When you go to flip it; she’ll lose shape, and with jigs or bulkheads, not so. Better off building over a jig.

Let me mull over other ideas a bit, bit for now, try to not cause another failure..

I just read Gonzo and he adds a completely different, but valid point.

 

Thanks Gonzo and fallguy for the answers.

So having the hull anyway, and actually the bulkheads that go in it, I am experimenting with 10mm Kerdyn green laid over it and then glassing. Its not a huge effort, and I can insert the bulkheads to keep it sturdy as I glass the inside. Lets see how it goes. I will do a lay up in 10 mm as a flat panel and see how stiff it is. 400 gm either side for starting. So will experiment a bit. I have the foam in any event. I may have to add a lot of glass and chase my tail a bit. I did find scantling details in Gerr's book which is really useful. I had forgotten I bought this years ago when I started the project. But rebuilding from stitch and glue is not something I can do right now.

Lets see how we go. Will test hull etc before spending too much time on other components. If all else fails I end up with a pretty row boat.

Gonzo - yes - not ideal as a centre hull, but I want to use her as a row boat when not in trimaran mode potentially, or if she is a little heavy, then add on 5 hp motor for trolling up rivers. But one can't have one's cake and eat it, so some compromise. She will hopefully sail nicely with speed of a dinghy or better, but with stability, but she will not be a racing beast.

So a bit like the Eureka canoe in the video, but a bit longer at 19 feet, and bigger outriggers to carry a larger sail plan as balanced lug.



Of course a bigger sail plan means more reinforcing in the hull making it heavier etc, but the plan is to step the mast through and into the aka frame in a truss structure.

 

The BH spacings will be insufficient for foam and you’ll likely end up with the same oil can issues unless you glass the panel insides first.

 

I've built three foam strip canoes over the years. The first was over a male mold and I can't in good conscience recommend the procedure. The other two were just like a wood stripper but used 1/2" foam instead of cedar strips. The first one was klegecell (pvc) foam and the last one was Gurit Gpet foam both about 5-6 pcf density.

The issue with the first boat was all the effort it took (in hindsight) foam strippers far easier and quicker now days. In your case the mold is already there which makes a difference. My question is how do you hold the foam to the mold? My first canoe it was, build themold, lay the inner skin on the mold, vacuum bag the foam in sections, fair it all out and then glass the outside. I get wore out just thinking about it and that was almost 40 years ago, I don't recommend it.

With the mold already at hand if you can find a way to attach the foam to the mold and then reliably release it once the outside is glassed you are almost home. A canoe hull glassed just on the outside is really flexible but holds its shape really well. You can play around with pinching the gunnels in a bit and watch rocker decrease and visa versa while the hull remains fair.

The only two ways I can see at the moment requires two people. One way is to stitch the foam to the mold with U shaped 'staples' of thin gage wire tiedon the inside.The other is to screw the foam down from the inside which a thicker foam and careful selection of screw type and length.

Reading back through the posts It may well be easier to strip over a new mold based on the existing hull. If you have enough foam use the 20mm extra thickness weighs little and makes a far stiffer hull and alot of floatation. My strippers are done with station spacing of 8-10" for 12mm foam. For 20mm you could easily go to 14-16" spacing. I usually use pegboard for stationd and hold strips down with rubber bands.

Best wishes

 

I don't understand the idea of strip planking over a multichine hull. It is designed and built using flat panels.

 

Hi Skip, Gonzo

Skip

Thanks for that insight. The first thing I have done is wrap the boat in cling film so I can get the foam off it later. Epoxy doesn't stick to cling film I have discovered, so it acts as a release if any epoxy seeps through. I have found I can do large panels in foam on the top, but under the main chine I have to add it in smaller sections - see the photos. I have screwed these in. This process has been really quick thus far, but trimming and fairing required, and I will need to steam a nice bow stem again, but I quite enjoyed doing the last one. Rest of the foam should go faster.

Once all the foam is on I will undo the screws on some of the panels one at a time and glue the foam together with epoxy, re screw and wait for it to set. The foam will be clamped at the bottom and then screws removed and it will just be sitting on the hull. Then it is trimming and fairing with epoxy. I will then glass over hull and add some rub rails for stiffness and to keep the boat in shape once flipped.

You are right, the next bit is tricky. She could, and doubtless will, oil can if she sits on her hull, which is what happened to the plywood boat - a parent fell ill and it lay on the porch for a long time. So once glassed I will flip her and suspend for from the rafters of the pool area which is shelters, and tie between the rub rails to keep her tight. And then glass inside. After adding the seat at the back, front bulkhead, and some stringers she should stiffen up nicely.

On the amas - this will definitely be stations and 20mm as you suggest, but less strip planking and more planking. Going to build these slightly over size as I suspect I will want a more purpose built tri hull at some point. I have an old hull from a catamaran called a Nomad that might make an interim test bed. About the right length at 14 feet. But I suspect I will make amas quite rapidly, just working out presently volume, shape etc.

Gonzo

Why over a chine? Because sometimes terrible things happen to pretty boats. And some men just want to watch the world burn.... And I will send you pictures so you can see the horror unfold....

Seriously though - it is that or throw the hull away, and I have room for foam out in the weather to build, but I can't do plywood through winter, and it is seriously pricey where I live at the moment - the foam is actually tremendously cheaper. More so if you have it in the garage. I have the hull, I have the foam. etc. My only cost is glass and resin at this point, so worth a crack. Additionally for stitch and glue one has to cut extremely accurately on the ply and transpose accurately. And it will still be a pretty boat, though I will miss the more prominent chines which will be somewhat lost in the foam. It will be less pretty, but it should be sound and still a good looking craft. Its okay if the perfectionist in you rolls yours eyes. And when I make a more suitable tri hull I will reach out for your thoughts which will be appreciated.

 

I think you are doing a simple thing the hard way.

 

This has gone relatively well as the hull makes a nice 'buck' and I have been able to get large sections over it. So I have gotten lucky and a quick process so far. Flipping her and cradling her once the outside glass and rub rails are on the outside (so I can glass inside) will be interesting, but I have done this before with thin ply. It is also light enough to suspend easily. I am relieved she still looks quite pretty despite most of her chines being smoothed out by the foam. Not as pretty as before, but she will be a good looking motor/rowing canoe if nothing else.

I am planning the amas now and have a question on buoyancy volume for a smallish boat like this. Buoyancy various quite a bit on design, and is given as % of the displacement. This is where it gets a bit uncertain on this size. It is a smallish boat at 18 feet and very light. So pure boat displacement is going to be quite minimal. A considerable portion of displacement will be crew. But given crew can hike out on the windward ama this somewhat balances the boat and dramatically decreases bouyancy requirements on the leeward hull preventing it (to a degree) from burying. Additionally she has a wide beam for a vaka for a trimaran, so l/b is 6.2:1 at full displacement, but on waterline should be around 1:6.7 ish. Not a racer then. But again, she will have different roles. And again, no flying hulls. (Not sure where Doug Lord is, haven't been here in a while, but no foils please.)

Looking at some older trimarans like the Cross 18 etc it is quite hard to get a gauge for bouyancy %s and what this accounts for in terms of crew. The Weta is not a great comparison as this is fun adrenaline sports machine. What I am building is a fastish (/er than dinghy) displacement boat. But not a performance racer. Just quick(ish) in light winds. Sail area 10m square, which is also plays a role - planning on using something very similar to the Goat Island Skiff balanced lug. So much smaller and less sophisticated sails than W17 and Strikes. This is much more Selway Dory Osprey, which heavily inspired the idea, but I wanted a main hull a bit more accommodating. The Cat Ketch of the Solway dory is not appealing. Would rather have one sail given will be sailing with my kids, particularly my son, and on sailing upwind the latter sail may be more redundant. In short though, a smallish sail area for boat length

But back to bouyancy - one must account for the crew, because otherwise the buoyancy requirement would be ridiculously low. But how much - they can sit out somewhat of the main hull on the windward side? And how much righting angle does a 10m2 lug sail add like that on the lug island? And I don't want to be burying amas thank you, so a degree of conservatism. I know some use burying the ama as a safety indicator to reef in, but the sail will be moderate for its size and set back somewhat, so opportunity to pitchpole will be quite limited. I also don't intend to sail on its limit. Having looked at a lot of boats one sort of gets a feel at this size for what 'looks right'. Any thoughts from anyone who has sailed 18 foot open trimarans? Adding 4.8 metre amas roughly 30cm by 30cm adds about 200 kgs of buoyancy on rough maths, after its own weight. Long an relatively thin. Any (constructive) thoughts on this by someone who has sailed 18 footer or sailing canoes? Or is this more of a 'at this size if it looks right, it probably is' thing.

 

Rough plan - long and skinny. And yes, aesthetics play a role. Like racing stripes on a car, she will go faster purely on looks of course But I get a rough 200kgs of buoyancy. Might fatten them out a bit and add buoyancy forward, at some point one on ratios is likely just adding drag above a 12:1 ratio.... But looking for a feel on buoyancy.

 

To work out bouyancy a bit roughly (and very roughly) I stumbled on this website where one can calculate wind load. Naturally a bit simplistic, one must factor in load and righting moment vs heeling, plus wind spill is a factor (I think 30% at 45 degree heel......?) etc But still interesting. Simplistically I presume one can assume the height and shape co-efficiencies of 1 for a dinghy, and work mast area into sail for simplisitic and conservatism.

Wind Load vs. Wind Speed https://www.engineeringtoolbox.com/wind-load-d_1775.html


Or one can use the formula and tables here if one wanted, but I imagine at some point it is just more data into what is probably a roughish guide in any event?

3.2 Righting moment and heeling moment curves https://www.imorules.com/GUID-418A1974-228D-4A70-AA75-AF8A1C532E78.html

 

Some years ago on the Texas coast Charlie Jones built a ~19' cat ketch called Travellor which performed well but was overpowered often when the wind picked up. He added a couple of outrigger floats (I don't care for the word ama). The boat performed well and he and his lady friend Laura passed me easily early one morning in the 2009 Texas 200 when the wind was light but I did catch up to him later in the day ;-) .

 

Gorgeous!! Thanks for sharing that! Very pretty boat.

I was thinking of the GIS balanced type lug set up in terms of easier and quick reefing. But two unstayed masts with boomless mainsails could furl reef? I think that is the set up on the Solway Osprey. I think I will err on more volume in the outriggers.

Thanks again.