stepped hulls for trimarans

Discussion in 'Boat Design' started by tugboat, Mar 11, 2009.

  1. tugboat

    tugboat Previous Member

    stepped hulls seem like a great idea to me as a vaka- I just looked at the type used for airplane floats(spoon shaped) and i cannot see why a large sail area trimaran shouldn't be able to incorporate a stepped hull. I have never sailed a tri (only a cat) but i have viewed how fast the farrier designs are on youtube and other similar designs and they look like they have planed off when doing 13-20 knots! this gives a C ratio of about 3.5!! " classifying a trimaran as a "fast speed" hull. which means stepped hulls could be usable- yet the present tri hulls to my knowledge are not true planing hulls- it seems to me that a stepped hull would greatly increase performance- they may not be suitable for light airs however, as the bow configuration has a plough entry(like a spoon) and would need a lot of energy to overcome the bow wave at the beginning. but with a large rig it should, once the hull started to get up on plane, add a few knots by reducing wetted surface. a stepped hull needs to have most weight centered at the step i.e inboard engines etc- but a mast would act like the weight of an inboard with the forces acting on it- what concerns me is since most tri's heel over slightly- and some in fact even fly on one ama (ala the infamous "Moxie"), i wonder if a stepped hull would make any difference on a reach or close hauled? but if the tri could be made to run off the main hull slightly more i can see it being viable. i am not sure how it would repond to heavy weather but on bushplanes and bellylanders this type handles chop very well- what r peoples ideas on this?
  2. Chris Ostlind

    Chris Ostlind Previous Member


    I think that what you are angling for here, is likely to turnout as a one trick pony. You MAY see some benefit at the higher wind speeds necessary to generate the boat speed you'll need, but what's the point?

    Tris don't need to have stepped main hull forms because they can unweight the hull through heeling leverage and reduce wetted surface area quite easily. Take some time and study the vaka hulls of the fastest trimarans out there... the ORMA 60's and the big, maxi-tris used for round the world record attempts. They are slender, have generous rocker to maintain displacement while keeping that slender form and quickly move through normal sailing speeds to the point where they are flying the main hull, or just slightly maintaining surface contact. They do not plane and do not need to plane. The leeward ama is in full displacement mode save for the examples with ama mounted lifting foils.

    The very high end of multihull naval architecture firms design these boats and they have access to all sorts of testing regimes and computerized simulators in order to produce the trimaran with the best possible speed potential. These guys have not seen fit to draw a fast trimaran with a stepped hull.

    You'll be much better off with a boat that can work really well in a wide variety of sailing conditions and wind speeds, than one that is potentially super hot in a very narrow range of conditions.... and an absolute cow all the rest of the time.

    I mean, how embarrassing would it be to have a monoslug blow by you in light air while you are being hounded by excessive drag from the stepped hull form?
    1 person likes this.
  3. tugboat

    tugboat Previous Member

    Chris I had to laugh at that one...yep cant have those mono's beating us...i agree with you after further thought it seems like a nice idea but the extra work in the design of it and the gains wouldnt be worth ill stick to the standard radius hullform. i do like your designs - what are you asking for your plans??
  4. tugboat

    tugboat Previous Member

    Chris i love the weta design - i wish i could just resize that design for a 28 ft' that'd fly and be awesome..sigh....
  5. Chris Ostlind

    Chris Ostlind Previous Member


    Perhaps you could write me offlist for a discussion about plans and such?

    I'd prefer to keep the business potential as a separate thing from what goes on here at BD.

    You can reach me at: or
  6. Chris Ostlind

    Chris Ostlind Previous Member

    Perhaps the character of the boat would change once it got to that length and horsepower. Right now, it's all about the movable crew weight relative to the sail area while trying to keep the amas as small as possible for overall weight concerns.

    The bigger boat with the much bigger rig would have less crew weight influence by comparison unless you started looking at a gang of typical sheet monkeys... and they would need to be pretty quick... That, or the amas could be sized up in the fast trimaran range of volume and then the skiffy-like hull would look like a wetted surface penalty generator.

    I have a feeling that this design would be kinda topped-out at something like 20 feet LOA, or so, as it is more or less a recreational boat for families and such. I'd prefer to keep the boat as simple as possible within the genre, as I'm not a proponent of more complicated boats just because it is technically possible.
  7. Fanie
    Joined: Oct 2007
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    Fanie Fanie

    I agree Tugboat. Come on Chris, 8m500 is a nice size for a trailable beach tri. It's what I've been saying all along :D
    1 person likes this.
  8. Chris Ostlind

    Chris Ostlind Previous Member

    Fanie, you devil.

    OK, OK, you guys win... ;-) let's make an 8.5 m Montage and flat-out go for it.

    Yes, it will be fast in strong air. About the same as a slender hulled tri, I would guess. But when the air is light, the skinny ******* will flat walk away.

    You guys are making me run for the coffee pot with all these demands ;-)
  9. Fanie
    Joined: Oct 2007
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    Fanie Fanie

    It is rather tempting, isn't it ?

    Slightly larger than the run of the mill pocket size always makes the heads turn.
    Who that wants to have a beach tri wouldn't seriously consider it first...
  10. RHP
    Joined: Nov 2005
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    RHP Senior Member

    Whats taking Chris so long ?? :p
  11. tugboat

    tugboat Previous Member

    Ok- after some months (400 days to be exact)- I designed a trimaran but similar to the transat class 65 footers as in the type used for round the world races-but 32 ft loa ...the vessel has a slight concave rather than a v shape hull., the beam is 12 ft in order to carry two rigs- and this is what makes the difference. the parallel rigs!...There would be no weighted counter blanced swing keel- instead- two small amas would provide heeling resistance. or a counter weight system that a single hander could use- by using a block tackle and tracks and a water filled weight and tracks- the weight is counterbalanced to the opposite of the heeling side. It could cross the deck to counter the weight of the heel, the idea or concept is that the wieght is like a person hiking over the side to keep the boat planed off-heel.. the hull would have a step about 1/3 the length of about 4 inches. there would be a small fin for leeway resistance. or the amas could be finned. the idea again is to lose the swing keel. and have only 6 to 8 inch fin keels to reduce friction. the twin masts and the lack of a huge dragging keel would offer great friction savings.
    i can offer more detail if i wasn't clear on this- i could scan in my preliminary sketches? the whole boat is basically stitch and glued 3/8ths ply then made into a cored hull with epoxy and glass on both sides. and some light stiffeners. or it could be strip planked. the masts would be rotating of course. the dual rig would give massive amounts of sail.
  12. tugboat

    tugboat Previous Member

    oh yeah and the hull is stepped...
  13. dskira

    dskira Previous Member


    I PM you on something perhaps will interrest you

  14. Stephen Ditmore
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    Stephen Ditmore Senior Member

  15. kach22i
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    kach22i Architect

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