Planing Trimarans

Discussion in 'Multihulls' started by Doug Lord, Sep 30, 2006.

  1. Dan S
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    Dan S Junior Member

    Yea thats it...
     
  2. Willallison
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    Willallison Senior Member

    Perhaps this is the root of the problem - it is just one od the definitions of planing. The fact that there are so many makes it difficult to prove it one way or the other...
     
  3. ianfarrier
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    ianfarrier Junior Member

    The preceding sure is a lot of talk on planing multihulls, but I find sailing the actual boats answers these questions better than any discussion. The video entitled 'F-9R Sailing at both Low and High Speed' may also help, and this is at:

    http://www.f-boat.com/pages/introduction/videos.html

    When sailing, what I (and my owners) call planing usually always occurs as the boat heels and some of the weight is taken by one of the floats, which are displacement hulls. There is then a definite transition point between the planing and displacement mode of the center hull. I have also never claimed that any of my center hulls will plane just on their own with the boat level.

    However, in fact, they can be made to do this and I have been on an F-27 (without sail) planing at over 15 knots, with a 40HP motor on the back, and completely level. The F-27 is not designed as a power boat, and the stern rocker needed for a sailboat sucks the stern down, to where the bow lifts very high and the boat struggles, as does a power boat just before getting on the plane. However, the owner found that by putting a horizontal lifting foil on the rudder, this would lift the stern, and the boat would then get up on the plane. Power boat style trim tabs may also have the same effect, but, as stated, the F-27 was never designed for such a purpose.

    However, for a sailboat, the planing center hull is a definite advantage as the flat low rocker allows a much wider and roomier center hull, but one which is too wide to perform according to many experts, who probably don't get out much. However, being able to plane (or skim across the top of the water - whichever you prefer) changes all this, which makes the boat a great all round roomy cruiser, yet one that can frequently embarrass the racers.

    Of course there are other multihulls that are faster, but they always have much higher power to weight ratios, skinny coffin like hulls with much less room, and are usually very impractical boats. It is easy to make a multihull fast, just pile on sail, while eliminating the room and such convenient features like easy folding. But such boats usually end up as marketing flops, and worse, some can threaten the very viability of the multihull market. See:

    http://www.f-boat.com/pages/news/safetyaspects.html

    Designing a boat that is roomy, fast and practical is a lot harder, but I find the effort worthwhile, and it seems the market agrees with me.

    Ian Farrier

    Farrier Marine (NZ) Ltd
    Farrier Marine, Inc.
     
    1 person likes this.
  4. RHough
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    RHough Retro Dude

    I agree completely.

    If someone was to say, "The planing hullform of the Farrier Trimarans provides dynamic lift that further reduces drag as the main hull is lifted by the forces acting on the sails. The reduction due to the planing hullform's dynamic lift is greater than the reduction due to lower wetted surface alone."

    Would anyone contest that statement?

    Remainder of post deleted, the designer has spoken.
     
    Last edited: Oct 13, 2006
  5. Doug Lord

    Doug Lord Guest

    F Boats

    Thank you, Mr. Farrier!!
     
  6. yipster
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    yipster designer

    i second that :)
     
  7. RHough
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    RHough Retro Dude

    Ian, Thank you for taking time to read and post in this thread.

    Nice video, but it does not show the main hull planing.


    Thank you.


    I suspected this would be the case from looking at the lines.


    This is the centre of the debate. Any number of sailing multihulls are able to get the weather hull to "skim across the top of the water". Would you consider a Hobie 16's weather hull to be "planing" when this happens?

    Thank you again for participating in this thread.
     
  8. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    I think we all agree, except for detailed definitions of "planing" :)
     
  9. Doug Lord

    Doug Lord Guest

    planing tri

    --------------
    To me the above quote sums it up fairly well; I don't think it's definitions(semantics?) that are the problem: it's getting a handle on the above concept. It's realizing that- as shown in the very first post- when the ama is partially loaded the main hull is still supporting weight. And it supports the weight thru a combination of buoyancy and dynamic lift(planing) as speed increases. The dynamic lift is made possible by the partial unloading of the main hull due to heeling moment(and a planing hull shape) and the ama taking some of the weight(first post).
    Two hulls operating at the same speed, at the same time in different modes: one displacement, one planing.
     
  10. RHough
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    RHough Retro Dude

    "Combination of displacement and dynamic lift"? Is it planing or not?

    "The dynamic lift is made possible by the partial unloading of the main hull"? NO, the dynamic lift is there or it isn't. It is created by forward motion and hull shape, it does not depend on partial unloading of the hull (otherwise boats that don't get partially unloaded by outside forces would not create dynamic lift).

    The concept of one hull planing while the ama slices through the water in displacement mode is not hard to grasp at all. You just haven't provided any data that would lead to that assumption.

    What happens when you drive the main hull with enough power for it to plane?

    Like many hulls, the dynamic lift pitches the bow up. Just as the rocker aft indicates it would.

    Have you done the math to see if planing is possible?

    According to Bethwaite it takes about 10% of the total weight of the boat and crew to provide enough power for free planing (upwind and down). In terms of SCP (Sail Carrying Power) 27% SCP/Total Weight is the minimum needed for marginal upwind planing. The F-22 weighs 1300-1500#. With 2 crew at 200# each we have 1700-1900#. Let's say that the F-22 has a rotten planing shape and needs drive equal to 15% of total weight (255#). If a 10% of total weight hull needs 30% SCP/Weight, the 15% power would be 45% SCP/Weight. Using the light model (1700#) the boat needs 765 pounds of SCP to plane.

    If the sail arm is 12ft we get 9180 lb/ft of Heel Moment to plane.

    Using 17 ft as the beam (between the CB's of the amas) we have 17 x .5 x 1700 or 14450 lb/ft of RM if the weight of the main hull is adding to RM. More than enough to plane even a poor shape.

    However, if the main hull is planing, it is supported by dynamic lift and is not adding to RM.

    If the main hull is planing (and supported by dynamic lift) do we still have enough power? If the main hull weighs 900# and the ama's 150# each we have 150# x 17 = 2550 lb/ft of RM. We need 9180. To get our 9180 lb/ft of RM to plane we need 390# on the windward ama. We have 400# of crew, so with the crew on the weather ama, the main hull does not have to add any RM for the boat to produce enough power to plane (my previous assumption was wrong). Of course as the main hull is lifted further out of the water from it's planing attitude it does contribute to RM. Therefore it is safe to use 14450 lb/ft as the RM. This gives the F-22 1204 pounds of SCP for a SCP/Weight ratio of 70%.

    With a SCP/Weight ratio of 70% it becomes very hard to think of a case where the F-22 would not plane.

    The numbers were there all the time. The driving force of 255 (15% of 1700)pounds creates 3060 lb/ft of pitch down moment to help overcome the hull's tendency to lift it's bow up and struggle.

    I asked easy to answer questions (the above took about 30min to figure out) and no one stepped up to answer them. Instead Doug quotes himself several times and provides no substance. In other words, I think he's right but he can't prove it to himself or anyone else.

    I think CT 249 said it pretty well:
    There is no doubt in my mind that the Farrier boats can and most probably do sail upwind with the main hull planing.

    Here's a photo of Redshift with the bow pitched up in breeze that looks too light to get the boat planing.

    [​IMG]
     
  11. Doug Lord

    Doug Lord Guest

    planing tri

    Good work, Mr. Hough. Congratulations!
    I must add, however, that in the very first post of this thread I was able to show -correctly- that the F22 would have to have the ama support part of the weight in order to plane with it's upwind SA.
    =================================
    Using Bethwaites formula with the original assumption(1st post of the thread) that in 1lb. per sq.ft. pressure the boat would require 600 pounds buoyancy from the ama. All up 1800 pounds minus 600=1200 pounds(main hull load).
    RM= 600 X 8.5=5100. 5100 divided by 12.75(CE-CLR)=400=SCP. 400(SCP) divided by 1200(load on main hull)=33% . More than enough for upwind planing in 1lb. per sq. ft. pressure with the crew* in the center of the boat.
    Great idea to use this formula-had not occured to me!
    Thanks, Mr. Hough!
    ===============================================
    References (3)
    ===============================================
    1) FROM POST #115- RHOUGH-excerpts:
    ========================================

    2) FROM POST #108 Ian Farrier:
    =============================================
    3) FROM POST # 1:
    ==========================================





    * edit
     
    Last edited by a moderator: Oct 16, 2006
  12. ActionPotential
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    ActionPotential Junior Member

    OK if planing and skimming across the top of the water are interchangeable the sure these trimaran centre hulls are planing, and so is the windward hull of my catamaran when it is skimming across the top of the water. Even long skinny hulls will produce some lifting through hydrodynamics when lifted far enough by other forces. When my windward hull is lifted such that the tip of the daggerboard is skimming across the top of the water is it planing on the tip of the daggerboard?
    It really has to be all or nothing. Planing must be (for me) when a hull is lifted only by hydrodynamic force.
    I have very little doubt that the Farrier centre hull shapes will plane (according to my definition) if used as the leeward hull of a bimaran (single ended proa).
     
  13. ActionPotential
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    ActionPotential Junior Member

    Quote:One definition of planing is that the weight of the boat is supported by dynamic lift. Since this is not the case, how can the sailing trimaran be planing?End Quote.
    Qoute:perhaps this is the root of the problem - it is just one of the definitions of planing. The fact that there are so many makes it difficult to prove it one way or the other...End Quote.

    Yes, correct if your definition is supported by dynamic lift, then it is not planing if it is supported by something else. If your definition is skimming across the top of the water, then it is planing, whatever is supporting it.

    Quote:Again, the mental block seems to be conceiving of a high beam to length ama operating in a displacement mode AT THE SAME TIME and AT THE SAME SPEED a relatively low beam to length main hull is planing.End Quote.

    No mental block here at all. I conceive of a high beam to length ama operating in a displacement mode with, AT THE SAME TIME and AT THE SAME SPEED a relatively low beam to length leeward hull is planing (lifted hydrodynamically, not just skimming across the top of the water).
     
  14. Willallison
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    Willallison Senior Member

    I have to say that after reading Mr. Farriers post, I think you guys might be getting a bit too excitied about this!;) No way out claims about the worlds fastest boats, no ascertions that his boats leap onto the plane like an 18ft skiff.....
    The simple reality appears to be that the centre hull is indeed in the planing conditon, partially supported by 1. the ama's, 2. it's displacement, and 3. hydrodynamic lift. Not necessarily in that order.
    No controversy in mind at all....
     

  15. RHough
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    RHough Retro Dude

    Damn ... why do you shoot yourself in the foot?

    What started this whole pointless argument is the idea that a hull can be planing while supported in part by other means.

    Your example of the SCP/the partial weight of the boat equaling 33% is voodoo. You cannot have the main hull supported by dynamic lift and use it's weight to create SCP. If it is adding to SCP it is not planing. All those numbers prove nothing, they only point out the poor logic that has a hull planing and adding RM at the same time.

    Ian Farrier points out that the centre hull only planes when one of the ama's is carrying weight. That is due to the righting moment needed to produce enough power to make the centre hull plane. Not because the ama and sail are lifting the hull out of the water (as I so *incorrectly* assumed).

    Using the simple concept of the two ama's as a catamaran, it is easy to show that the weight of the main hull is not needed to produce enough power to plane a 1700# boat. Since the main hull does not have to contribute to RM to produce power, and there is more than enough power to plane a 1700# boat, the only thing that would keep the main hull from planing would be an extremely poor hull shape. I have too much respect for Ian's designs to even consider that the hull shape could be that bad.

    The only way to "prove" that the Farrier Tri's can plane is to show that they have enough power to do so with nothing but dynamic lift supporting the main hull. Your posts and quoting yourself, owners, and the designer have never shown anything of the sort. You keep trying to sell the "partially supported is still planing" idea.

    A hull with outside support is NOT PLANING.

    It is like the kid in math class that comes up with the right answer and tries to support it using the wrong theorems. He doesn't get a passing grade for stumbling upon the right answer since the logic is wrong. As you said, proving it logically with Bethwaite's (pretty well proven AFAIK) formulas never occurred to you, so don't site the flawed logic in your first post as proof that the Farrier Tri's plane. Describing the flawed logic of your first post (the one that started the debate) as correct is pure vanity. You have been unable to prove any such thing.

    There may be more going on here that I cannot grasp. It may well be that the dynamic lift required to get the main hull on plane is less than it would appear and the main hull will plane at lower wind speeds than are required to produce 10-15% of the total weight as drive. I am the first to admit that I do not have enough knowledge to evaluate that possibility. I am, however, convinced that the F-22's main hull can plane, in fact it is damned likely that they do.

    Action: When I made the statement " ... the weight of the boat is supported by dynamic lift. Since this is not the case, how can the sailing trimaran be planing?" I was still trying to figure it out using Doug's "partial support" numbers.

    I no longer think the case of the weather hull of a catamaran skimming the water and the main hull of the Farrier Tri's planing are the same (I was mistaken). It can be shown that the Farrier Tri's do not need the weight of the main hull to add righting moment to produce the power they need. If a cat could produce the power it needs without the weight of the windward ama, the windward ama could plane (support all of it's weight with dynamic lift).
     
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