converting a monohull into a trimaran

Discussion in 'Boat Design' started by tomthebikeman, Mar 5, 2008.

  1. tomthebikeman
    Joined: Mar 2008
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    Location: key west

    tomthebikeman New Member

    Why not? I have a Beneteau 235 2'9" draft that has been a great boat but has a small cockpit. I sail in skinny water 3-4 feet, live in key west warm weather so sleeping on tramps instead of a hot boat would be ideal. And snorkeling and hanging out on the tramps would be great fun. I did design and build a small 18 foot trimaran years ago before I even knew how to sail. I had a blast with it until its demise 5 years later. So now I was thinking about an f boat or a Telstar but jeeze thats a lot of money! Why not unbolt the keel, buy a couple of used cat hulls, build some cross beams( the hard part), rework the rudder and keys trimaran that draws less and is easier to to get on and off of and easier to get off the sand bars than my wing keel. OK so of course I am an expert but I was just wondering if any of you experts think that the hull shape of the 235 would work and if so what type of cross beam, alluminum, plywood, glass over ply, wood, solid wood, glass over foam, Whatever!!!!! I wont need to trailer it and to keep it simple it wont need to come apart but that would be nice. Obviously it wont be raced against a corsair. Thanks for your time and you cant hurt my feelings much. Tom
  2. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    Hi Tom, welcome aboard the forum!

    Not sure how well it would work from a performance perspective, although I wouldn't be too optimistic. I suspect you'd get into trouble with the rigging, though. That mast and its stays are engineered for the righting moment and typical load conditions of the original monohull. The tri conversion would have vastly greater righting moment and would lose the ability to reduce rigging loads by heeling in a gust- thus you'd very likely overstress the rigging, with a high chance of losing the mast. Same goes for the moments that would be applied to the new crossbeams and their mounts- vastly greater than the hull was originally engineered for.

    By the time you work through all the re-engineering, the new rig, the extra hull bracing and reinforcements, etc., not to mention the resale value which would be minimal at best.... does something designed as a tri from day one still sound expensive?
  3. terhohalme
    Joined: Jun 2003
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    Location: Kotka, Finland

    terhohalme BEng Boat Technology

    Just sell your Beneteau and build a new multihull from tested plan!
  4. kach22i
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    Location: Michigan

    kach22i Architect

  5. charmc
    Joined: Jan 2007
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    Location: FL, USA

    charmc Senior Member

    Hi, Tom,

    Welcome aboard!

    Intriguing idea ... Matt raises probably the most important point, and terhohalme has a relevant suggestion.

    You will need some sound reengineering to have a safe rigging, and you will be taking a well known boat with a recognized value and turning it into a homebuilt. Selling it and building a tri from sound plans might give you what you want with an even better result. :)
  6. Frosty

    Frosty Previous Member

    Oh these guys have no sence of adventure into something new. I think its a good idea and you got the right donar boat for the job.

    I think the rigging thing is a problem but beefing it up is not a big job in comaparison to what you intend to do, and you could put some additional shrouds on rather than move the originals.

    Who ever said your end result wont be worth anything is right, but if you dont care about that!!!

    You could make bolt ons , then if it dont work or you dont like it you could remove them and re fare the hull and paint --gone.

    You could start a new craze with that idea.
  7. ted655
    Joined: May 2003
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    Location: Butte La Rose, LA.

    ted655 Senior Member

    I'm with frosty, BUT, what a can of worms you will spill.
    Hull stress from the outriggers would be the first concern. Feeling good about that and then you get into performance issues. That boat was designed to heel and carry a certain amount of sail. Now you change all that. How big will the sponcins be? Will they submerge at optimum wind load? The drag factor? Angle of attack?
    Last but not least, You now have a "wide body" boat to keep somewhere.
    Go for it, send pictures.
    1 person likes this.
  8. tomthebikeman
    Joined: Mar 2008
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    Location: key west

    tomthebikeman New Member

    being ignorant was more fun

    When I built my first tri,( no plans) I had no clue about performance, tacking, boom vangs, dangers of sailing at night, etc. The crossbeam broke after about a year( something stupid i did) and of course the mast came down at the same time. I rebuilt the mast and added a couple of feet to the stern and continued with my wandering. I broke the boom when I added a vang in the wrong spot. Things went well for a few years until I accidentally burned down a big island off of Key West. The island burned for a week and made the front page. No structures or people were on the island but I took the boat apart( held together with lashings) and hid it for a few years and the fresh water rot got to it. I never got caught. All in all lots of great fun and lessons learned.#1 why build your own boat when there are so many cheap ones out there? But if I did do the conversion, it would be a lot of fun and a good story. The rig was the one thing I never even considered and I'm not sure you guys are right about the stress involved. What if I had 4 big dude Kahunas hanging off the side of my boat? They would keep it flat and stress the rig. I think I'll leave the rig and add a couple of shrouds to the amas. Now, back to those pesky crossbeams. Tom
  9. Frosty

    Frosty Previous Member

    My brother works in Key West as a fire investigator. He investigates arson and prosecutes careless fire starters. Apparantly he stays on the case for years till he gets his man. Ille be seeing him this weekend.
  10. tomthebikeman
    Joined: Mar 2008
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    Location: key west

    tomthebikeman New Member


    Ask brother if he thinks I should use ply and lashings over the top of the deck or cut holes in the boat and use alluminum cross beams. Also would 22 ft amas made from cat hulls work or should i build custom? getting his man should be no problem for him in key west.

  11. poldi
    Joined: Apr 2008
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    Location: Thailand

    poldi New Member

    monohull to trimaran

    Hi Tom,
    I was just searching for the same issue.
    My boat a Beneteau Oceanis 390 is quite comfortable but slow (max 8 knots).
    My idea was to convert it into a trimaran using pre-manufactured parts. Got to the f-boat site with their folding beams. I'm at the moment checking out that posibility! Hope it works but if not I going to build my own folding system (already designed).
    Crucial point with the whole convertion thing is to do it easy, cheap and with less stress to the hull as possible. Hence I'm going to use marine shock absorbers to keep the stress low! Most problematic the connection points to the main hull! Design the beams flexible (shock absorbers and adjustable) and find the best adjustment through careful trials!
    For the rigg, no changes are planned since the rigg got more stress on a monohull in sailing close to the wind as with a trimaran which can't sail that close windwards! A trimaran sails like a catamaran best with the wind not close to it! If you do so it will pitch or capsize before the rigg breaks!
    Anyway, if you feel uneasy about it just add a few more stays.
    I'm grateful for experience exchanges please contact me by email!
    All the best
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