Twiggy 32 construction

Discussion in 'Multihulls' started by Twiggyman, Oct 15, 2014.

  1. Twiggyman
    Joined: Jan 2013
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    Twiggyman Junior Member

    This thread is for those who are interested in the Lock Crowther 32 foot "Twiggy" trimaran design and construction.

    The Mark 1 version was in multi chine plywood.
    The Mark 2 is cold molded in plywood, wood veneer, or foam sandwich.
    The cabin and beams are in plywood/timber construction.

    The design has an option to be made demountable for transport, although there are about 80 bolts to remove to disassemble the hulls and beams entirely.

    Here is my Twiggy Mk2 fx under construction: :p

    I plan to make a folding version that works with Twiggy's extreme beam of 30 feet. The original design has water stays which I think will be wise to retain.
    This makes the beam a simple compression member, and the beam integrity will not rely on folding mechanism strength.
    The original full width beams drop into saddles spanning the main hull deck.
    Instead of full width, I will build 1/2 beams which will drop into the same saddles. Similar to Farrier's Command 10 design.
    Where the beams meet at the center of the main hull, they will lock together with large removable pins.
    This is in addition to the original design eight 1/2" stainless bolts which pass all the way thru the beams and main hull bulkheads which sandwich them.

    Being able to launch while folded at busy launch ramps is desirable.
    A Twiggy will be more welcome at the marina when folded too.
    Storage folded on a trailer on the hard would be possible provided there are no overhanging wires between the launch ramp and the storage. You could leave the mast up.

    It will not be possible to road trailer when folded since the top of the folded beams will be about 17 feet off the ground, which is higher than overhanging wires or normal bridge clearance. For road trailering the hulls and beams will have to be disassembled, but then can meet the 8' 6" road width requirement.

    Admittedly, this method of folding will not be as quick or easy as on an F boat, yet some desirable features of folding can still be achieved.
    The Twiggy's extreme beam, which enhances performance and stability, makes folding more complicated.

    Attached Files:

  2. bruceb
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    bruceb Senior Member

    nice work

    It looks like you are well along on a nice project. I know several other people on the forum have either owned or built twiggys and thought they were good boats, it will be nice to see a new one.

  3. Marmoset
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    Marmoset Senior Member

    This will be cool to see, nice project!

  4. John Perry
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    John Perry Senior Member

    A fascinating project. I am very impressed. Over the past few months I have been thinking about something similar as a 'development platform' for some ideas I have for hydrofoils, but just thinking and doodling with CAD, not cutting timber as you have been doing.

    You refer to 'waterstays', do you mean stays in the sense of wire ropes, or possibly modern high strength polymeer ropes? Or do you mean ridgid struts as the Farrier system? If you want to fold the boat afloat to come into a marina, I would have thought that at least the upper link of the four bar linkage needs to be a strut capable of taking compression, rather than a bendible stay, otherwise the boat would be likely to flop over when the float is in the intermediate positions in your series of folding diagrams. However, I can see that it might just be possible to use a ridgid strut for the upper link and a flexible stay for the lower link, a novel thought perhaps. Such an arrangement would save weight and water drag when the beams hit wave tops, but I think it would also make folding more difficult than with the standard Farrier system.

    I am sure you have already thought through all this, but having the beams both ridgidly locked (encastre) to the main hull and also supported by a Farrier style linkage, leads to potential uncertainty as to which of these two structural solutions will actually carry the load. Unless you carefully design to avoid it, the Farrier style struts will likely end up being the stiffer of the two systems, so will carry nearly all the load, making the extensions of the beams into the hull centre just extra weight and complication (although they could act as a back up system should the Farrier struts fail in some way). Maybe you could make the lower Farrier struts slightly elastic(!!), designing them in just such a way that the bending moment is correctly shared by the cantilevered float beams and the lower links of the Farrier system - but that would perhaps be an over clever design approach, prone to disaster if it doesent quite work out as planned!

    Looking at your diagrams, I cant help wondering why you dont just copy Ian Farrier's brilliant and well proven system. Ian's patent has expired and in any case he might not be too upset about a one off project by an individual enthusiast. If you copied the Farrier system, I guess that the length of your float beams would be reduced by something like 4 feet, so your air draft on the trailer would be reduced from 17 feet to not more than 14 feet, which although still a lot, is just within the 'loading gauge' for the road systems in most countries.

    Just another thought looking at your folding diagrams - have you checked the lateral hydrostatic stability with the boat fully folded, assuming that is how it is to be in a marina? Comparing it to a typical F-boat, the folded floats as drawn are even less immersed, the overal folded beam may be a bit less and you have those long cross beams sticking up and raising the CoG. It may well be OK, but I would want to be sure if it were my design.

    Anyway, great project, good luck with it.
  5. Twiggyman
    Joined: Jan 2013
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    Twiggyman Junior Member

    Folding choices

    Thanks to all for your encouraging remarks!

    I really like the Twiggy design concept and decided to try to retain the original design as much as possible yet add folding. For me it has some desirable options compared to being just demountable. Thus I plan to use 4 folding struts very similar to Farrier folding, but these will not figure into the structural integrity of the boat when ready to sail. The struts will contribute some extra strength, but that will be a bonus over the original beam design.
    For example, in case a water stay were to break unexpectedly.

    Also, I prefer to retain the original overall beam of the boat, preserving the original design concept. The geometry necessary to fold the floats into the correct position will need to attach about 7 feet from the outboard end of the beam. If the strut was used to replace the original water stays, the beam would no longer be a simple compression member, but would become a significant cantilever from the strut attachment point. This would need some redesign of the beam with significant strengthening. I'm sure it could be done with the appropriate engineering and application of carbon fiber - but I prefer not to move that far away from the original structural design which has proven reliable.

    The joint where the beam ends pin together at the center of the main hull will call for some strengthening, but this is fairly easy to calculate so as to retain the original strength of the beam, and when thus pinned in place retains the original beam design concept.

    John, you have a good point about folded stability when floating. I have thought about that too. But I should do up an Acad drawing and analyze the roll stability. I'll get on it and post something soon.
  6. Marmoset
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    Marmoset Senior Member

    I would think the some of the wide beam would be offset by them being mounted so high. Kinda always thought anyway they just have to make it more compact, they don't have to tuck all the way in right? Unless your really gonna trailer it.

  7. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Good on you

    Hello Twiggyman

    Good luck on your project. I would love to see it succeed and see a folding Twiggy around sometime. I lived on my Twiggy for a few years about 20 years ago. I have some thoughts on your idea.

    I would be cautious in cutting down the amount of bury in the beams. As they are extraordinarily wide the Twiggy floats will put large fore and aft loads on the beam ends. Farrier gets around this by having cross wires under his beams but the geometry would make this less efiicient. Burying the beams would assist in this.

    I did some reverse engineering of the Twiggy beams and would recommend you keep the waterstays as they are designed. It would be good if the folding mechanism went sloppy at full extension so the waterstays, beam ends and hold down bolts took the whole load. I would make the beam ends with some room to fit, wax the ends and then put epoxy glue in the ends and replace the beam ends in to the socket to make a precise fit.

    Twiggys have been known to break their underwires. Usually at about 20 years old. The beams in both cases I know of did not break so there is some redundancy in low load conditions. I like the redundancy in such a wide boat. The underwires will need some way of being fully tensioned. They need to be tightened tighter than the rig or the boat will flop about in waves. My Twiggy had loose underwires at first and after sailing JLS and finding out how tight underwires meant the boat moved as one unit rather than three loosely held kayaks I did the same. The top folding strut will have to do a lot of work or you will have to tension them after extension.

    All the above being said you could possible do a few changes to the design. I liked our aft cabin, which a number of Twiggys built on. I also liked tiller steering rather than the wheel which I really didn't like on the aft bulkhead. If you plan to spend most of your time on autopilot then the wheel is okay but for helming the vertical tiller is much better. We also put a small permanent dodger on our boat to protect the companionway which helped.

    you will also need to include some chainplates for lowers to hold the mast up when folded. Some Twiggys like Jonathon Livingstone Seagull and the original had an inner forestay to stop the mast inverting when reefed. You could have some sort of runners system to go to this point for folding.

    One thing to think about. Most of the Farriers (almost all) that I see in marinas are not folded. I would think that you will really only fold the boat for transport, slipping and storage. It could be worthwhile. I did have friends who had a non folding Command 10 and they were happy with it.

    Are you cold moulding the hulls?


    Phil Thompson
  8. Twiggyman
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    Twiggyman Junior Member

    Cold Molding

    Thanks Phil, I appreciate your input with your first hand experience.

    I plan to implement all your good suggestions.

    The folding struts will have a small amount of "clearance" at the pins which will not interfere with the folding process, but will mean that the bolted beams and under wire water stays will take the load under sail. The folding struts would still help out somewhat if a water stay broke and the beam was flexing enough to reduce the clearance and then stress the struts.

    All eight 3/8" ss wire water stays will be tensioned with turnbuckles after the beam bolts are in place. I agree that having a stiff hull and rig structure is very desirable so a little time added to the unfolding in order to tension will be worth while.

    Launching folded will be a big advantage at our busy launch ramp. Although the side stays when folded will still prevent the mast from coming down, they will be quite slack and it would flop around. I will install chain plates on the main hull near the rear beam for lowers which will keep the mast steady when folded, launching, or in the marina. Lock's plans show a chain plate for an optional heavy weather inner fore stay, so attaching the lowers to the same point on the mast makes sense.

    I wrote to Lock back in 1987 about using western red cedar veneer instead of plywood for the hull skins. He agreed that this would be OK, but that I should use 3 layers of 1/8" cedar veneer in place of the plan specified 2 layers of 1/8" Okoume plywood. There is a slight weight penalty in doing this, but cedar grows everywhere in my area and cost savings are significant compared to plywood all the way from Africa or China.

    total skin area all 3 hulls ~ 440 sq. ft. x 3/8" cedar @ 22 lb/cu ft -> 300 lbs
    or 1/4" okoume @ 28 lb/cu ft -> 257 lbs
    so the weight penalty for the whole boat is around 45 lbs.
    Not insignificant, but I will save more than that amount by using red cedar in many other places instead of Douglas fir, such as for the frame edging and the timber in the beams. I will use Okoume ply for the cabin, underwing, and decks.

    I have enjoyed the cold molding process even if it is a little time consuming.
    The hull shapes come out just gorgeous.
    Here are some photos of the cold molding in progress (below):

    There are around 100 more photos at my build blog:

    I should be setting up the main hull on the strongback soon.


    Attached Files:

  9. Marmoset
    Joined: Aug 2014
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    Marmoset Senior Member

    Wow just got real interesting for me! Haha I'm starting the kraken soon and plan to do same exact swap as you for western cedar! Haha I'm in Northern California, cedar and redwood are literally all over the place! How's it working so far for you, bending ok? and although I haven't decided which way I'll go yet did you consider strip plank method?

  10. Tom.151
    Joined: Jul 2009
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    Tom.151 Senior Member

    Great job you're doing there -- love the triple veneer job.

    Curious about the the type of metal used for the reinforcing plates on the crossbeam bulkheads? It looks like it has quite a bit of "shine" in the pics is why I ask.

    The material (incorrect aluminum alloy) used on mine caused lots of work when they all corroded badly and had to be repalced. Be cautious that your supplier actually provides the material as Lock specified. The original builder properly specified the material, but the supplier delivered a different (unknown) alloy and it was never caught.

    Thanks for sharing your build, it's great to see the birthing of another Twiggy.

  11. Twiggyman
    Joined: Jan 2013
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    Twiggyman Junior Member

    Marmoset -
    I found the 1/8" cedar veneer easy to work with. It will bend around a fairly tight curve, maybe radius of 18". For tighter curves I would plane it thinner. I ended up with some 1/16" veneer and it will bend much tighter. Make sure you find a discount supply for staples. So far I have used about 10,000 for the 2 floats.

    Using strip planking instead of the stringer frame method could be done, but I suspect that a thicker skin would be necessary for strength. Most catamarans done in strip planking in the mid 30 foot size have 1/2" cedar planking with epoxy glass cloth on both sides. Chris White's book, and the Simpson Design manual both indicate that strip planking will be heavier than stringer frame. Such a substitution really calls for a re-design and consult with the designer.

    I debated retaining the stringer frame method, but using strip planking for the skin. This would use the cedar more economically than veneer. But panels would have to be removed from the frames and stringers after the strips were edge glued into a large panel over them, to apply glass cloth on the inside. Then the fiberglassed large panels glued back onto the frame/stringers. I decided not to do this since it would be difficult for 1 person to handle large panels, and the necessary large gluing operation. The beauty of veneering is all the pieces are small and you glue them a little at a time.

    Lock's plans call for 5083-H321 marine aluminum for the float frame reinforcement plates. However, this alloy was not normally stocked by suppliers in our area, and they would only bring in a large order. So I substituted 5052-H32 marine aluminum which is reported to have "very good corrosion resistance" in salt water. The aluminum delivered was marked with a 5052 sticker so I think I will be all right on corrosion problems.
  12. Marmoset
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    Marmoset Senior Member

    Very true given the size of the build, hasn't thought about that, as you said you'd be having to lift a large hull off molds!

  13. rogerf
    Joined: Apr 2012
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    rogerf Junior Member

    G'day Twiggyman

    IMO the corrosion on the alloy plates is caused mainly by the interaction with the s/s fasteners - you need to ensure that there is no direct contact between the metals.
  14. Marmoset
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    Marmoset Senior Member

    Oh on staples, did you need Monel or are standard holding up?


  15. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    Nice bit of cold moulding, very neat arrangement. It looks like about a 30 deg from vertical offset for your 'Layer 2'? . Reminds me of building the rowing shells, that I used to do. Your right, the thin veneers bend much more easily, we were using 4 layers of around 0.7mm (+/- 0.2) thick veneers on far tighter radii, and still ended up lighter than carbon sandwich....

    Great looking build, best of luck with her. As for the corrosion issues with staples, it depends if you leave in or out, or use polymer ones. Mostly the moulding I've done has been vacuum or sand bagged so no staples, a big plus.
    Where staples have been needed, generally run'em through tack strips which are ripped off later leaving only the pinholes which fill later.

    Look forward to future updates, best wishes, I'm sure she will be a good boat. I don't think the small weight penalty will hamper her much. Maybe payback a little in extra stiffness and insulation, probably a worthwhile tradeoff. It's only a couple of buckets of water (weight wise) at the end of the day.
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