Convert Mono Hull to Tri?

Discussion in 'Multihulls' started by Kalagan, May 8, 2009.

  1. Kalagan
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    Kalagan Junior Member

    Hi,

    I was wondering if anyone has done or heard of anyone converting a 40ft Mono Hull Sail Boat to a Trimaran?

    I just thought it would be an interesting project and if done correctly, could be a nice boat when finished.

    What are your thoughts?

    Thanks
    Kalagan
     
  2. alan white
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    alan white Senior Member

    Tri main hulls are very narrow even if they appear wide from above. A forty foot tri might have a waterline beam of three to five feet, while few monohulls of forty feel are less than ten feet wide.
    Speed in multihulls comes (besides a stable hull platform) from light weight and wave-piercing hull designs. A forty foot mono won't pierce waves well, but will ride over them, and few forty footers outside of all-out racers will get up and plane. Bottom line, a mono hull is designed to carry a large displacement and to have a high initial stability, so it has to have a pretty wide waterline beam. That wide beam usually limits speed to a certain formula that is based on water wave frequencies.
    A multi, with its narrow knife-like hulls, defeats the limiting factor of wave frequencies by simply slicing through waves. Monos, on the other hand, become stuck in the common trough of their own bow and stern waves.
    I suppose one could use some mono hulls as the basis of a tri if the hull originally had a very marrow beam (which would have required a very deep keel ballast or a very low rig height). Some such monos have been built, and they sail on their ear even with a deep high ratioo ballast.
    However, once that ballast is removed the hull lines are nothing like the original. The hull rises a lot and windage increases, the rudder can become essentially smaller, the waterline becomes shorter, and so forth.
    Unfortunately, monos and multis are two different animals operating on different principles. Few monos could be converted and still be useful boats.
     
  3. Kalagan
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    Kalagan Junior Member

    Hi Alan,

    Thanks for the reply, sounds like you know what you are talking about, so I figured I would ask two more probably basic questions.

    1) It seems like there are some displacement hull trimarans, like some of the Hortsmans I have seen. From your description, it sounds like they are not going to be fast boats, which to me is ok. I guess I am looking for stability and space and not so much speed. If speed is not my primary concern, would your suggestion still be the same?

    2) Just about every Tri I see has the Yamas much shorter than the Center, why is that? It seems like you loose a lot of potential deck space. Would it be feasible to have the Yamas start behind the Center (like most Tri's), but be longer then the Center (by maybe 5ft) and have integrated Stairs like many Cats do? This way you have the stability of a Tri, the deck space and easy water access of a Cat. Seems like the best of all worlds to me. Am I missing something?

    Thanks
    Kalagan
     
  4. alan white
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    alan white Senior Member

    Kalagan, answering the first question, I don't know much about displacement trimarans. Obviously, you'll get more stabil;ity by making the boat wider and putting the bouyancy out at the extreme beam. However, I think paying such a price for initial stability is a bad comprimise. The boat would be slow, balky to manuever, and very expensive to maintain due to so much skin surface.
    Regarding the second question, a tri has so much deck space as it is, I wonder why ten times the space of a mono isn't enough already. A multi designer isn't usually attempting to maximize deck space. There's enough deck. The length of the floats is related to performance, like tacking the boat. If the floats are too long the boat won't come about. Shorter and they would be too fat, since they need to carry a volume of space approximately equal to the whole vessel's displacement.
    You can assemble a trimaran from a monohull. It's not impossible to do, though the whole thing will be a nightmare to design properly due to the structural changes.
    But issues like stairs to board and deck space to lounge on, or lack of tippiness are all an attempt to overcome initial reactions to sea boats in general. After familiarity with sailboats, those issues disappear and others take their place, like performance, economy, and usability.
    There are many people who can't be coaxed aboard a sailboat due to heeling, even though heeling is only psychologically dangerous. In reality, it's completely safe if the boat is designed right. A boat should never be designed to assuage the fears of a novice. Instead, the novice should become familiar with the time-tested designs available. It seems as if a person who hasridden a bicycle for five minutes is asking why not a tricycle?
    But given a bit more time, that person would realize how much more handy a two-wheeler is.
     
  5. multihullsailor
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    multihullsailor Junior Member

    Kalagan,
    Have a look at wahat Erik Lerouge has done to this mono:
    www.yachtworld.com/boats/1976/Sapo-Tri-45'-Tilikum-1738768/Bretagne/France
    There is also a Finnish built tri in the UK called Amphytrite (spelling?) based on a monohull design, currently back in the water after catastrophic cross-beam failure some years back.

    Roger
     
  6. Chris Ostlind

    Chris Ostlind Previous Member

    Kalagan,

    Over the years, I've been collecting images of all sorts of multihulls. The images shown below represent a working versions of a slender monohull conversion to very functional trimarans.

    If the base hull is selected carefully, you are capable and willing to make rather extensive mods to the original hull, and have more than a passing understanding of how to balance and properly engineer a boat of this type, it can be done.

    The boat shown has been used to cruise, I think, in the Med. It was modified by a French couple, as I remember it. There is an article somewhere out there on the Net about the boat. The owners talk at length about the conversion and the subsequent sailing. You'll have to do some hunting if you want to do a follow-up as I do not have the link at hand.

    In any event, you can use the images for dreaming and maybe, you too, will be able to cook-up a boat of your own along these lines.

    Where are you located in L.A.? I'm originally from the South Bay area.
     

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  7. Kalagan
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    Kalagan Junior Member

    Hi Chris and everyone else,

    Chris, I currently living in Redondo Beach, but have lived in Hermosa Beach and Manhattan Beach, love the South Bay, couldn't live anywhere else in LA.

    Alan, some Tri's have deck space, but my main problem with the space they give you is that it is spread out all over the place, I would like to have one big open multifunctional space (like many Cats have) for hanging out and entertaining. Just about every Tri I have ever seen (including the huge 58ft Hortsman that's on Yachtworld at the moment) is not designed that way. The Yamas are for storage only, they are not useful for people space, the Pilot House is closed off and only connected to two MINI Cockpits on either side (functional for nothing really, except a lookout) and the back deck (if you want to call it that) is virtually non existent, broken up by the Dog House for the Aft Cabin. So, in reality, Tri's don't have much functional space, they are beamy, but the Decks are broken up in small chunks spread out over the boat, the interior is cramped at best in the Center Pontoon and the Yamas are not used for much except Storage. Which leaves you with a big beamy boat, that you feel cramped on. In my humble opinion, not the best use of space.

    Alternate option, what about basically building a Cat with a Center Pontoon, laid out like a Cat, but with extra space for Captain's Berth, Toys and storage in the Center Pontoon? In this approach, the Center Pontoon would be shorter than both Yamas? For example, the Yamas would be 50ft and the Center Pontoon would be 40ft, the Beam would be 30ft. To me, this configuration would give you a lot of space on both the interior and exterior and give you space to store heavy items centrally located in the middle of the Center Pontoon. Like a couple of Jet Skis, Kayaks, dive gear, compressor, you know all of the fun water toys.

    What do you guys think?

    Thanks
    Kalagan
     
  8. Chris Ostlind

    Chris Ostlind Previous Member

    Don't get too screwed down on that 30 foot beam number. It is going to be about your sail area, how its apportioned (perhaps fore/aft in a ketch configuration and the weight of the boat when it is completed.

    You have lots of stuff to blend together here and by starting with a fixed vaka hull size and likely weight, you will have fewer elements with which to play in order to get that magical partnership you seek.

    I do not wish to sound harsh in this, but you are probably going to be starting out with a vaka hull that is much heavier than the corresponding, purpose built trimaran vaka would be. You will also likely have a hull form that was designed for a different sailing behavior, so that will have to factored into the mix right at the outset.

    The mounting of the aka beams for a boat that size is no trivial matter and a serious look at the structure of the boat that will serve as the vaka hull is also paramount.

    It would be very prudent to consult with a very experienced large multihull designer and get yourself rolling on a reasoned path that has a stringently engineered design process. I can recommend Jim Antrim in San Francisco and Kurt Hughes in Seattle as two gentlemen with proper credentials. Do not be surprised if they do not wish to get involved with this kind of project, though. This kind of makeover is loaded with possible problems and at 50' LOA, it is not a small chunk of change.

    There may be some real wisdom in first taking on a smaller project of this sort to see how it all works and what kinds of things will be of great concern. There are a lot more small monohulls, which might make for a candidate than there are bigger boats and you get to spend a lot less money and time doing it.

    My 50 cents on the matter without boring you with more details than this forum deserves right now.
     
  9. Richard Woods
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    Richard Woods Woods Designs

    I think you've got some good answers there already.

    However one thing you haven't said is Why???

    I assume that cost has a lot to do with it. In which case you'd probably find it cost effective to use the rig, engine, interior fittings etc from a 40ft monohull and fit them to a already part finished trimaran.

    Then you'd get a boat that was designed from the outset to have three hulls, not a monohull with training wheels.

    I'm sure you can think of a comparison, like why not convert a biplane into a single wing aircraft, or a motorbike into a pickup truck.

    After you've spent a couple of years on the project and spent twice what you hoped (and you will, don't worry) you will then be risking your life (and possibly your family's) on it.

    And probably you won't be able to sell it again.

    If you do proceed remember that monohulls are designed to heel and have a limited top speed so the hull shape is very different from a multihull. Possibly the weight saved when you take the keel off will go back into the outriggers and beams, but maybe the new all up weight will be less, in which case the hull will look and behave quite differently.

    And then you have to decide how and where to fit the crossbeams. These are the highest loaded parts of a multihull. The real strength on a monohull is in the keel area, not the gunwales. And the accommodation/deck layout probably won't work well either. You'd probably have to make a new rudder and certainly something to replace the keel. The "lead" is very different on a multihull than a monohull so the converted boat probably won't be balanced under sail.

    A lot to think about, but I wouldn't attempt it myself, even for my own use, and certainly as a professional multihull designer I would advise against it were you to ask me to help you

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
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  10. mydauphin
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    mydauphin Senior Member

    Do not underestimate the amount of strength require at the attachment point of the hulls. We are talking massive forces and massive structures to do it properly. Much cheaper to buy a present trimaran and refit or a cat for that matter.
     
  11. alan white
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    alan white Senior Member

    It seems to me that a monohull such as a Choy Lee, Valient, or any number of beamy and heavy monohull cruisers would suit your need to store things and to have decent deck space. These boats might be over 40 ft and a 14-15 ft beam and displace upwards of 15 tons. Built for comfort, they have a lot of room belowdecks and extra weight bothers them little.
    You will find that a converted mono will (if properly designed and built) be far more expensive than a used large wholesome mono cruiser and yet far less capable of carrying extra gear (you can't load up the floats too much, and shouldn't at all). A pilothouse model mono is quite commodious. The engine is below the pilothouse.
    While the others are correct, that you can make the conversion if the candidate mono is the right one, it's not going to have a precedent. At least that would be very unlikely. It will be an experiment. Even doing everything right, you won't get anyone to guarantee it will work as planned.
    To me, tris are worth it only because they are super fast. They don't have much room and if you found a 40 ft mono that fit the bill, it would not be one with loads of room either. Since your idea is to gain room, realize that weight out on a lever is inertia that works against you, requiring much beefier beams than a tri would normally need. You can't win. your best bet is still a big displacement mono.
     
  12. rayaldridge
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    rayaldridge Senior Member

    Kalagan, can you tell us why you want a tri rather than a cat?

    By the way, to me the great advantage of multihulls over monohulls is not speed, but comfort. However, this is comfort underway rather than the kind of comfort that you get from big gensets, freezers, air conditioning, and so forth. You really have to decide what you want from a boat, and the best way to do that is to try out as many as you can. Chartering may seem expensive, for example, but a week aboard the kind of boat you're considering is only a tiny fraction of the cost of building or buying that boat.
     
  13. sailsocal
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    sailsocal Junior Member

    I'm developing a tri design that solves this issue in a very effective and innovative way. Nothing built yet, but I'm actively seeking investment to finance construction. I think the design will be a major milestone in multi-hull design.
     
  14. mydauphin
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    mydauphin Senior Member

    I look for years at advantages of Mono, Cat or Tri...
    And it boil down to this ... In a storm... a real storm... Which would rather be in a Mono, Cat or Tri.
    Which is the only one that can be self righting, and doesn't need a hatch or hatchet in bottom... Otherwise a well designed mono, cat or tri are equal in all regards .
     

  15. rayaldridge
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    rayaldridge Senior Member

    So, it's better to sink than to capsize?

    I don't know if I'd agree with that. You don't even need to be in a storm to start sinking. At best, you end up in a tiny liferaft with most of your food and water on the bottom of the ocean. At worst, you get to find out how long you can tread water.

    I can't see that any boat is equal to any other boat "in all regards." Even if they're both popped out of the same mold, everyone equips their boat a little differently.

    Cats and tris take massively more energy to capsize than a monohull of equivalent size, so it's going to be a lot easier to find conditions capable of capsizing the monohull. Any mono that capsizes violently may come back up missing hatches, or even a deck house. When that happens, you'll need to be very good and very lucky to keep the boat from sinking.

    Maybe I'm weird, but I think it's very comforting to know that no matter what, the boat isn't going away. Even disabled via capsize, you still have your gear, your food, and your water. If a ship with a launch responds to your Mayday, you even have a chance at salvaging your boat.

    One last thing: If anyone has really thought about the relative safety of monohulls vs. multihulls, it would be marine insurers. As far as I know, they do not charge an added hazard premium for insuring multihulls.
     
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