Small trimarans under 20'

The flex is mostly twist, but I can see the 2x4 beams bend up and down a bit, expecially when the loads on the shrouds are high, like if I pull on the forestay hard.
The mast was 1' EMT conduit, but it has since been replaced with about 2.5 inch diameter thin-walled aluminum with 316 stainless steel shrouds and forestay. I put a bigger jib(about 110% genoa) on and it promptly bent the 1' EMT mast right over about 10 degrees before I even got it on the water.

The sails are Tyvek, housewrap. I since have taken the boom out of the sleeve so it is loose-footed. It seems to produce a lot more power now. The sails are completely flat so there was no curvature. I also replaced the battens so they are a lot softer, but the pockets are a bit short so I need to make them longer.
I moved the floats up 1 inch on each side since they were getting flooded a bit too much. I want to try it with the stronger mast and re-positioned floats, but I haven't gotten to it, since I just did that last week.

There is a centerboard in the middle of the boat in a trunk. I think it might be a bit small, but it passes catalinas racing to windward like they are sitting still. Still doesn't keep up with a hobie cat yet.

 

The thread upchurchmr referred to is here. http://www.boatdesign.net/forums/multihulls/trimaran-sail-area-39935.html

I too have been thinking about a sof trimaran.
The forces involved as you are finding are significant and the open canoe shape and sof construction are poor to resist torsion.

My idea was to ignore the hull and have a separate space frame to resist the forces of the mast, rigging, cross beams and centre board etc., then attach the sof hull to it.
It would probably involve expensive materials.

Your ambition to fly 2 hulls would make the stresses huge, which would need an engineered design.

Also thinking of sof floats for a sailing canoe, a concern I had was if sealed it would baloon in high temperature and go concave at low temperature. My solution was to seal a plastic tube to the float and lead the other end to somewhere on the main hull where it is unlikely to be submerged.

Well done for going ahead with this project.

 

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Congratulations on a good looking boat! To fly the main hull will require more beam -probably. You can lay it out on paper assuming the bigger ama and see what angle the boat will sail with when flying. There's a thread here about that sort of thing. Ideally you would want the smallest angle possible. The range is something like 10 degrees to 26 degrees(too much!). You'll have to get some help, if you need it, with the engineering to make sure you don't go too wide for the strength/weight of your beams. You should be very proud of yourself for completing a project like that. Don't hesitate to experiment-just be careful! I think the main could be improved by being taller and narrower, but that starts to add more stress as well. I'd suggest you keep sailing the boat as is and make notes and save the money you'll need for the mods. Eric
Sponberg is a member here and a well known and respect naval architect and marine engineer. He does consulting work by the hour and if I were you I'd talk to him to see what he thinks it would cost for his help in deciding on the longer beams. He is very approachable and a damn nice guy.
Again ,congratulations! Is this your first boat project?
Here is a link to the thread I mentioned: http://www.boatdesign.net/forums/multihulls/trimarans-angle-heel-main-hull-takeoff-39553.html

Here is Eric's member profile-you can PM him or email him from there: http://www.boatdesign.net/forums/profile/eric-sponberg.html

 

rsnesneg,

Twist in the hull between the crossarms will need some diagonal bracing in the hull and probably increased stiffness in the Gunwale.
The SOF construction that just has vertical ribs and horizontal stringers doesn't have strength against twisting, the joints between the ribs and stringers just get twisted, so there is little material to resist. If you were to laminate "ribs at a 45 Degree angle on the inside of the current ribs, then attach them there would be material to directly take the load. Of course that puts all the load in these new parts, so they get loaded up quickly.
This is not a normal feature of SOF.

Crossarm stiffness can be improved quickly by making hollow box beams instead of solid boards. The top and bottom of the beam can and should be solid wood. The sides can be plywood, but to make it most efficient (lightest weight) you need to cut the wood at a 45 degree angle. This works ok if you have 5 ply plywood, but most is 3 ply. With 3 ply the inner ply runs across the board. So what you end up with is 2 plys going +45 degrees and 1 ply going -45 degrees. For this to work well you need to add 1 more -45 degree ply. Putting on the same thickness at the right angle with epoxy will work.

All that "equal ply" business is just to get the "best" design, you probably can just use a little thicker plywood (1/4"?). Solid wood sides will also work well, just a little heavier than the "best" design. If you really want to know how this works, look into bending of beams

Once you get everything stiffer you will be able to go faster and will rapidly generate bigger loads - resulting in the next weaker thing requiring a fix. My guess at the next thing is the structure holding the mast foot (very high load pushing down) and attachment of the crossarm to the hull, and particularly the structure between the front crossarm and the mast foot (large twisting/ crushing load).

The next thing you may find is that the bow is being pushed underwater, which will generate large loads trying to rip the bow off by being pushed upwards from where the mast is.

Don't let me ruin your enjoyment of your boat. After all I am just arm chair sailing instead of actually doing it, like you are. All these things are just the next issues which come up when you power up a boat.

About the sail, flat doesn't generate much power. Tom Bradshaw makes sails and he provided a simplified sail making instruction which was aimed at people making Tyvek type sails. If you will send me an email at upchurchmr@yahoo.com I'll send it to you. Its too big to post here. Bradshaw is a professional and well respected.

Last suggestion: if you want to fly the main hull, and don't want to fix everything above all at once, don't increase the beam. Narrower beam will allow the sail force to tip the boat over easier and get you flying without even higher loads caused by wider beam. Of course as you have seen you need bigger floats (amas).

 

Agreed. It seems to do just fine with the amount of flex, it is very flexible and it absorbs the motion rather well. The other option in reducing flex might be to run shrouds to both the front and rear crossbeams, so it would limit the amount it can twist. As far as going a lot faster, I don't really want to put the money into it at this point, and I would need to basically replace everything except the shrouds and forestay, so there really isn't much point to that. I do want to make it more seaworthy though, and that includes finding a way to keep water out of the floats.

Edit:

This is not my first boat, I made a 14 foot canoe out of paneling when I was 12.
I also made a 14.5 foot canoe from Geodesic Airolite in Maine. It weighs 26 pounds.
I am currently working on a 16 foot keel boat (looks like the J class yachts) and an 8 foot mini-yacht. Looks like a baby Catalina. Both of them are waiting on lots of $$$ for fiberglass and epoxy.

 

You might just try tape over the zippers. Some of the Aluminum airconditioning tape might work well, if you tape it while the skin is dry.
The other option would be to use some skin sewn down on one edge, making a flap to cover the zipper. put velcro on the other three edges and the mating pieces on the float. I would suggest using Goop to glue down the velcro - not the stick on type.

Sounds like you understand your situation a lot more than you first let on.

 

I thought about filling the hulls with expanding foam, but it is expensive and I can't fill it too much or it will just make me two big balloons. I've also thought about filling it with Peanuts. I've tried duct tape, it works OK, but not as well as I would like. It still lets water in.

 

Have we considered canting masts? They could allow higher main hull takeoff angles and still power up the rig, so more freeboard when things get rough, and you can easily keep an ama out in light wind. Seems logical.

 

I still think you can't go past the Piver Frolic.
I built one for my teenage son in the mid 1960s.
He had the urgent need to visit his GF who lived on the West shore of Lake Simcoe and would sail day or night and in any weather to get there.
I built it with vertical daggers on the amas which didn't extend below the line of the vaka keel. With a pivoting rudder this allowed the boat to sit upright on the beach and left the main hull clear enough to sleep in when covered with a tarp.
Very fast and Lots of fun.

 

Rchesneg,

I have never see a setup for a canting mast.
Would anyone have an illustration?

 

Canting Mast

Several big tris have used canting masts as well as at least one dinghy. I'll see what I can find pictures of. The middle two pictures are the Merlin Dinghy-the inside picture is hard to make out anything. The last two are both Groupama one an ORMA tri the other their most recent C Class foiler.

 

Doug,

What I really want to see is the mechanism/ equipment to cant the mast.
Those pictures clearly show there are some that cant.
I want some details.

Thanks.

 

Canting Masts

If you go to the 2013 Little America's Cup thread I think there is a video showing how Groupamas system works. Seems like it would be pretty easy to rig up especially with high strength line instead of wire though I've never done it. Have you googled it?

 

It is very simple. You have ball and socket bearing at mast base to allow the mast to cant and 3 part block and tackle on the shrouds - setup like a main sheet system. Here is Sid's. You can enlarge to see. There are cam cleats on the lower blocks and fail/safes in the cockpit.

 

The handful of multis in Auckland set up to cant use the system Gary describes above. Another option that could probably work quite well on a smaller size boat is to have the mast step mounted on an athwart-ship mounted track. You then pull the mast base from side to side to get the cant without the need to alter the rigging tension. Newick tried this on Ocean Surfer, but I've never read anywhere how it worked out. You can make out the track in the attached pic.