basic multihull knoweledge

Discussion in 'Multihulls' started by neptunkryssare, Oct 23, 2012.

  1. neptunkryssare
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    neptunkryssare Peter

    I would to learn about the relations between theese komponents.

    To begin with:
    Amas and akas. (shape, position etcetera)
    The placement of the mast.
    Sideboards placement.
    Foils placement,

    I understand that even though the questions are simple enoungh - the answers can be many different depending what I want out of the boat.

    But what I really want to learn is this: how can I see that x amas combined with y hull gives z result.

    I understnad theer will be mthematics involved, plese point me in the right direction to gain knoweledge.
     
  2. dougfrolich
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    dougfrolich Senior Member

    You might find this paper helpful
     

    Attached Files:

  3. Richard Woods
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    Richard Woods Woods Designs

    Judging from your previous posts I assume you are interested in small daysailing trimarans. But maybe not.

    For of course the criteria needed for an offshore trimaran that may have to lie a hull in large beam seas is very different from a small boat where the crew play an active part in stability

    So to avoid a too vague, meandering discussion I suggest you make your questions more specific to a certain boat type and sailing conditions

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  4. warwick
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    warwick Senior Member

    Thanks Doug Frolich For the article.

    One aspect I am thinking is size the float as a percentage to use with foils in the future. Start small while learning ( earlier warning signs) then lifting once more experienced. I am think of dagger boards in the float, from the start.
     
  5. dougfrolich
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    dougfrolich Senior Member

    Float volume as a percentage of total displacement would likely depend on RM and inshore or offshore use (more researve volume offshore) Amas in the 250% range not uncommon, but if foiling is the ultimate goal then much less researve is volume needed and weight of the structure is more important. there is a good article in PBB No. 139 Oct/Nov. 2012 by Jim Brown "Takeoff Window" a sort of a primer on foiling multihulls
     
  6. warwick
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    warwick Senior Member

    Does any one know what the float size for the early Lock Crowther trimarans were?
    I have thought they may have been about 150 Percent, but on reflection they may have been smaller (80%).

    My Idea is for a easily driven trimaran with foil assistance in the end.

    As Gary Baigent says efficiency over brute force.
     
  7. neptunkryssare
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    neptunkryssare Peter

    dougfrolic , thankyou for the post. That's that one of the kind of litterature I need to read.

    Richard Woods , my main interest for now is small dayssailing 3-hulls, and I would like to have a basic understanding how a boat-designer think when he construct such a multihull. How he calculate forses, weight etcetera.
    (edit)
    ----
    Regarding sailing conditions, There is often light winds where I use to sail, But I am that kind of persons that can go out in half storm just for the heck of it.
    ---

    warwick, thats very good input. I will remember that. Is there a proved way to add daggerboards / foils on the floats for testing purposes - in such a a way that you can change position and angle.?

    Else I have the idea that one can have a plate where the daggerboard is mounted at the side of the float. Then it's could be possble to adjust the angle.
    [​IMG]
    Of course the construction in the example can not stand the forces, but it suits as an view for what I have in mind.

    I read somewhere here on boatdesign.net that a multi which have the daggerboard in the middle becomes more 'wet'.
    How come.? Which forces are in question.?
     
  8. neptunkryssare
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    neptunkryssare Peter

    I also have to add, I am not asking for you to eduacate me to be a boatbuilder

    (edit. I mean - a boat designer).

    I simple would like to gain the knoweledge needed to combine part x with part y.

    For example this -> http://www.blocket.se/vi/42593586.htm If it is is worth the price of 5500 sek (about 420 usd). It is a homebuilt, oneoff. 3-5 mm ply, 4.4 m length and 2 m width. Two adults is to much weight for it. No jib. And obviusly, it's v-style.
    The owner have lost connection with the builder.

    It took me a quite long time to understand that it is a piece of crap. At least if you want speed, and later want to add an aka.

    I almost was going to buy that piece of c.r.a.p, but I am glad I decided to not do that.

    So what I am really asking for - it's a way to build up a good judgement regarding multis.
     
    Last edited: Oct 24, 2012
  9. neptunkryssare
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    neptunkryssare Peter

    RM, what does that mean.?

    250% is that 125 per ama.? The most tris I have seen have smaller amas, so I suppose I missunderstand what you wrote.?

    To me - foiling is not the ultimate goal, but I wqould like to have that as an choice.
    Is there any boat that's have been done with detachable foils.?
     
  10. Petros
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    Petros Senior Member

    study the details on the most modern and fastest boats out there and you can learn a lot. However, as Richard Woods points out, consider their intended use, off shore vs. inland waters, etc.
     
  11. warwick
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    warwick Senior Member

    From what i understand A float with 250% would be 2 1/2 times the main hull displacement.

    A further question I have wondered about lately, what main hull displacement should be considered, The normal pay load or a maximum pay load?
     
  12. tspeer
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    tspeer Senior Member

    The 250% would be for each ama. That may sound excessive, since an ama cannot be loaded with much more than 100% of the weight of the boat. (There will be some down-force from the rig when heeled.)

    But the question is, "Which 100%?" When the boat is pitched bow-down and heeled, the aft half of the ama may be out of the water. When going to windward, the ama will be more evenly loaded, and it will be the upper half of the ama that is out of the water.

    So you need enough buoyancy in the right places to handle the extreme conditions. Buoyancy elsewhere in the ama is useless for a given loading condition. When you do that, you will find the ama has enough volume that it would displace much more than 100% when fully submerged.

    I think something like 180% or more would be necessary for a modern cruising tri, and over 200% for a racing tri. But the most important thing is to have enough buoyancy in the right places.

    With regard to foiling, you won't be able to foil all the time, and in the most extreme conditions, you don't want the boat to be going fast enough to fly. So foiling doesn't necessarily allow you to make the ama that much smaller than for a pure displacement boat. And the foiler has to be able to handle crash conditions, when it can develop a more extreme bow-down attitude and pitch rate when the hulls enter the water. This could call for more reserve buoyancy than the non-foiler.

    You might want to look at an energy balance to size the reserve buoyancy for foiling. At the point where the boat comes to a stop and starts to recover, the potential energy from the buoyant submergence would just equal the kinetic energy of the nose-diving boat, plus allowance for the continuing driving force of the rig.
     
  13. warwick
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    warwick Senior Member

    Thanks Tom.

    For putting some perspective on float volume, for various boat uses.

    One persons idea of foiling would probably be different from another persons such as hulls clear of the water or for added stability.

    Could foiling be considered for a safety margin or not?
     
  14. neptunkryssare
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    neptunkryssare Peter

    Thankyou tspeer, its exactly that kind of informationn I need.

    I understand that the foils try to lift the hull even if there is not speed enough to get it off water.
    What I would like to know is if that process produce a lot of drag.?

    Thats why I earlier in the thread asked about detachable foils.
     

  15. tspeer
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    tspeer Senior Member

    Yes, foiling can provide additional safety, but it's not guaranteed, either. If you're partially lifting the hull with a foil, then you have more buoyancy in reserve. If you're fully flying, then you may have less disturbance from waves, but you get no stability assistance from the hulls.

    But a lifting foil can also have unstable pitch-heave coupling and produce some wild motion. The buoyancy of the hull can help stability, as with reserve buoyancy in the bow. Or it can hurt stability, as with a boat that drags its transom, leading to unstable behavior as it pivots about the stern.

    Everything is coupled together in a sailboat. The sideways force from the sail rig produces pitching moment as well as rolling moment. The leeway angle necessary to counter the side force influences the vertical force from a foil. What you do with one part of the boat affects how it behaves other directions from what you might expect.

    You either need to develop an understanding of the interplay of forces and dynamics by the seat of your pants by a lot of sailing and flying (and crashing), or you need to work out the dynamics as part of the design process. When Laird Hamilton got a ride on the AC45, his comments afterward showed him to be an astute observer of the former school. If you don't have that experience, then you need to dig into the dynamics more quantitatively.
     
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