A few trimaran design questions

Discussion in 'Multihulls' started by FantasyTrimaran, Dec 20, 2019.

  1. FantasyTrimaran
    Joined: Aug 2019
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    FantasyTrimaran Junior Member

    On trimarans...

    Why are the floats usually shallower at the aft ends than the fore? The floats of many, but not all, trimarans that I've seen appear to be narrower and taller at the front ends and shallower - and sometimes wider, sometimes not wider, at the aft ends. Is this the best design for a specific reason? Is it to reducd the "wetted area" of the boat overall by "slicing through" the water at the front ends and then transitioning to "floating upon" the water towards the back ends?

    Some of the floats seem very thin and shallow at the back, or far too wide with a sort of bulkhead transom at the back. Are such design choices losing something by way of flotation/bouyancy volume? Also the transomed floats would, I think, be worse with a wave pushback - thw transoms tending to "catch" and "dig-into" the water if pushed backwards in rough seas or something, like someone catching their heel on a loose rug behind them - therefore presumably being more likely to picthpole, backwards. Compared to floats that are narrow at both ends, which would presumably be more likely to slice through the water rather than catch on it.

    Is this just a matter of Make your choices and take your chances? Both designs being valid but just with different strengths and weaknesses? Blunt and shallow, versus tall and narrow, symmetrical with the front?
     
  2. fishwics
    Joined: Mar 2004
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    fishwics Quiet member

    It depends, surely, where you want the centre of buoyancy of the float.
    The sail force acts somewhat forward of abeam, depending upon the point of sailing. Ideally the CB of the float should be on the same line, but as always there is a compromise required. Boats likely to sail a lot at low apparent wind angles (i.e apparent wind well forward of the beam) can use floats (or foils) with a CB/Centre of Lift further aft than slower boats (that may at times have the apparent wind aft of abeam).
     
  3. Niclas Vestman
    Joined: Aug 2017
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    Niclas Vestman Senior Member

    Conclusions from my own research as a complete amateur: 1) yes, float bows are almost always designed to slice, more or less. Mainly depending on float volume vs boat displacement. A 150% volume vs displacement makes it possible to sail on one float, lifting the main hull. Like the SeaCart30. Around 100% will be burried enough to act as a break, reducing apparent wind speed and also sail forces thus reducing risk of capsize. 2) load transfered to the floats vary a lot depending on wind and point of sail. Although the forces often act to burry the float bows. If the bow is shaped like an axe, the shape is quite neutral as to how deep the bow is forced down by the sail forces. 3) the forces acting aft on the floats vary less. Meaning it will lie quite flat on the water no mater the conditions. 4) early tri designs had canoe shape floats, pointy at both ends, as it was assumed to be a optimal shape to reduce resistance. But later it was found to be false for most cases. 5) Floats are design specific. An extreme racer will have different design compared to a cruiser. But both will probably have flater stearns. This often gives better speed, pitchpole resistance but most importantly significantly reduces hobby horsing.
    Still there are many more design considerations for floats. Eg, how many degrees they lean inward while the boat is level at anchor. Also, are the trimed downward back to front. And are they shorter than the main hull or maybe even protruding, like in the new Dragonfly 25?(forgot the name. Maybe 26). Among many more design aspects. As for tripping backwards, i would say that this scenario is almost non existent as to the sort of movement, and also the float stearn being way to small to offer enough resistance making a backflip impossible. .... Still just my amateur opinion. Best regards
     
  4. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    Take a look at this article.
     
  5. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Tom. Thanks for Shuttleworth's Great Brittany Ferries article. Modern OMRA tri's now sail more like catamarans with only one hull in the water and 2 hulls in the air. Result outer hulls more shaped like cat hulls. For an older style tri or pure cruiser the article is very valid.
     
  6. tspeer
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    tspeer Senior Member

    Yes, the article is a bit dated, but it goes to the OP's question as to why trimaran amas are deeper forward. Diagonal stability is typically the trimaran's weak point, and they need lots of reserve buoyancy in the ama bow to prevent digging in the ama.

    Been there, done that, spent the night on the overturned hull.
     
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  7. W17 designer
    Joined: Oct 2010
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    W17 designer Senior Member

    Greetings,
    Should Fantasy Trimaran not already be aware of the Trimaran Design Info shared via my website www.SmallTrimaranDesign.com, perhaps he and others will find this helpful.

    Although written originally for the W17 hull design, the aspects discussed affect all trimarans and should help to answer the original question.

    Review of W17 Hull Forms https://smalltridesign.com/W17/w17-design-approach.html

    If I remember correctly, I think this article links to two others which also address related aspects, but in case not, here are the links and hope readers will also find this of interest.

    DiagonaL Stability: Multihull Diagonal Stability https://smalltridesign.com/Trimaran-Articles/Diagonal-Stability-of-Multihulls.html

    Efficiency of Simple Shapes: https://smalltridesign.com/pdfs/W17ProBoatOctNov2017.pdf
    I will add that although I think you will find my personal design thoughts are totally in line with those of John (Shuttleworth), the execution of them is significantly affected by boat size, as it's often easier to get closer to the ideal on a larger boat, whereas more compromise is necessary when the boat is short.

    Mike/ .. and a great 2020 to all.
     
  8. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    For information only and only applicable to larger racing tris the following add shows the float hull lines for a 60 ft racing tri's.
     

    Attached Files:

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  9. W17 designer
    Joined: Oct 2010
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    W17 designer Senior Member

    Thanks for sharing that ... we don't often get to actually see the lines ;) I'd call this 'a good compromise shape' taking into account that one shape will not perfectly serve all operating conditions. My own W22 has a very similar form that can be summarized like this from bow to stern. Deep forefoot for fine, early water separation and longest waterline; semi-circular underwater section for lowest wetted surface; deep chest with most buoyancy forward (like a fish) that helps to move the CB well forward of the CB of the main hull. This is recommended (by me anyway) so that the two are not in line. This offset dampens pitching in my experience, so all my amas have this feature. The tail is as clean and 'fish-like' as is possible ... but particularly on the smaller boats, the stern has to be at least Vee'd in section, if not somewhat U'd, to retain helm and stability balance when heeled. If you don't plan to sail with it depressed very much (good luck!), you might get away with a broader stern for better downwind performance (as Kurt Hughes often does), but there are compromises with that. Too much buoyancy aft not only defeats the forward CB attempt, but can lift out a central hull rudder and you lose steering control.
    I personally think that its often forgotten that the ama is NOT just a buoyancy pod ..its a hull that you are sailing on .. and its immersed volume actually replaces much of the main hull volume when sailed with a heel. So the more efficient you make the ama, the better the overall performance can be. I personally felt that early Farriers were too banana shaped and could use more bow in the water with an earlier immersion of buoyancy. Especially after a buddy pitchpoled his F25 on a small, flat lake!
    Then additionally, there are other compromises needed that have nothing to do with hydrodynamics .. such as having a decent deck area to stand on (to fend off) or to give a good structural connection for the aft beam that may well be very near the aft end. Yes, amas can have much variance for many different reasons, but my advice is to ask the designer why he did what he did. If he doesn't have a justified reason, then just forget you saw it ;) ..... Mike
    W22 ama.jpg
     
    Last edited: Dec 14, 2020
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