Planing Trimarans

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

  1. ActionPotential
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    ActionPotential Junior Member

    I pretty much agree with all of the above.
    I find it interesting that many associate planing with performance and hence the need to claim planing as an indicator of such.
     
  2. Vega
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    Vega Senior Member

    This discussion seems quite meaningless to me. It is obvious that in the absolute sense of the word, trimarans will have difficulty to plane, I mean, all the boat. They don't need it, to be performance boats and that is what matters in the end.

    I believe that people talk about planing trimarans to mean that most of the boat is lifted out of the water and that permits a speed, several times over hull speed. Of course, this doesn't happen with all trimarans, and I am referring to relatively small cruising trimarans. For instance, while the 920 Dragonfly will not have any difficulty to go at several times hull speed, the heavier 1200 will not go a lot faster than hull speed.

    Even if incorrect, in the absolute sense, I find a lot easier to think at the 920 as an easily planing trimaran and the 1200 as a semi-displacement boat, and I suspect that it is not only me.;)
     

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  3. ActionPotential
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    ActionPotential Junior Member

    We have established by reference to speedsailing sailboards that planing does not equal performance(speed) and non-planing does not equal non-performance. The problem is that many people associate planing with performance and thus have to say if I am going really fast I must be planing. I think we all agree that a performance trimaran will be fastest with 2 hulls flying, regardless of the shape of the centre hull.
    The ones with a 'planing shape' centre hull will go a lot faster as the centre hull rises with heeling force and thus reduces wetted surface and the associated drag. This will feel like planing to those who associate speed with planing.
    My objection is to the proposition that the boat is going faster because it is planing when in fact it is going faster because heeling force has reduced wetted surface and the associated drag.
    The main hull is not 'climbing up onto its bow wave' (traditional description of planing) but rather lifting due to heeling force and thus making less bow wave to climb up onto.
    Regarding the 'heavier 1200', perhaps it just needs more buoyant amas to allow the centre hull to rise under heeling force and reduce drag.
    Perhaps the amas should be 'planing shape' so that once the centre hull is up it could plane on the ama.
    I suspect that a longer narrower ama with sufficient buoyancy to support the whole boat would result in a faster result.
    It all comes back to the fact that a long thin hull, thin enough to 'cheat' hull speed will be a better overall performance proposition.
    I would prefer a trimaran with all 3 hulls long and thin at the waterline, with the accomodation width in the main hull above the waterline.
    Then the next step is to add the foils.
     
  4. Chris Ostlind

    Chris Ostlind Previous Member

    OK, Yeah, Why Not?

    Quite simply an evil dude
     
  5. Dan S
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    Dan S Junior Member

    I agree, He is an evil evil man :D
     
  6. RHough
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    RHough Retro Dude

    Insignificant change?

    Assume 60% of the weight is in the main hull and 20% in each ama. The boat weighs 1000 pounds and has a beam of 10 feet. The RM would be about 600 x 5 + 200 x 10 = 5000 pound/feet.

    If the main hull planes wouldn't that reduce the RM by 3000 lb/ft? Then as the main hull is lifted further it adds to the RM again? So you might have a RM curve that has points like:
    5 deg = 5000 lb/ft
    7.5 deg = 2000 lb/ft
    10 deg = 5000 lb/ft

    The boat would heel linearly until the main hull starts to plane, then the boat would pop up to a heel angle such that the weight of the main hull is adding to RM again and be linear again from there.

    I've never read a description of a sailing trimaran that exhibits this characteristic.

    Would a planing main hull have less drag than one that is lifted from the water by the rig? Does the drag actually go down when the main hull planes? Is the drag of the reduced wetted surface due to rig induced heel higher than the induced drag to create dynamic lift on the main hull? If so, there should be a polar plot that would reflect this change of state.

    Looking at trimarans like the F-22, are they designed to pitch up as they heel (to provide positive angle to a "planing" surface)? The driving force is providing a pitch down moment. Are the bows of the ama's full enough to counter both the pitch down moment from the rig and provide a pitch up moment for planing? How far forward would the CB have to move for this to happen?

    Just because some easy ratio's say the boat has enough power to plane means little. There are many examples of boats with good numbers on paper that fail to perform as the numbers would predict.

    As far as what the owners say ... well. I deal with boat owners and sailors every day. What some of them believe is so far from reality or possibility that intelligent conversation is impossible. I'm sure that the owners of the F boats believe they are planing, that doesn't make it so.
     
  7. Dan S
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    Dan S Junior Member

    You know, I said almost the exact same thing back in posts 14 and 21 of this thread.
     
  8. ActionPotential
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    ActionPotential Junior Member

    Easy enough to prove, take the Fwhatever and fill the windward ama with water. Fold the leeward ama so that it can't contribute to support Then sail and when the centre hull pops up it is planing.
    Or maybe unbolt the leeward ama and load it on top of the windward one, thus adding no weight to the total.
    I imagine that it won't happen but rather the double weight windward ama will fly first.
    Alternatively bolt a 200hp outboard on the back and demonstrate that it is a 'planing hull'
     
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  9. CT 249
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    CT 249 Senior Member

    In your early post, you wrote "A rudimentry analysis of the F22 sailing in 1lb. per sq.ft. pressure shows that it requires approximately 600 pounds of bouyancy(max buoyancy around 3500 pounds) from the ama leaving the main hull still needing to support 1200 pounds. Since planing requires about 500 sq.ft. per long ton(2200lb.) and the F22 has 294 sq.ft. not including a screecher or spinnaker-2200 divided by 1200 gives 1.83 multiplied by 294 = 539-easily enough SA/ton for planing. Because of the high beam to length ratio of the displacement ama's they wouldn't retard planing of the main hull significantly."

    Formula, ratio, whatever. It doesn't include beam, and hydrodynamics teaches us just what Alan and Doug mentioned - that a narrow beam shape does not generate as much planing lift as a short fat shape, whatever its many other virtues.

    Whether the greater non-planing speed of the narrow hull is enough to generate so much more speed that it actually planes faster is an open question. However, surely the interesting information you obtained from Alan and Doug C. underlines that the effect of narrow beam must be taken into consideration when discussing planing. Since your ratio doesn't include this very important effect, it's not very persuasive and therefore those who say it doesn't prove Farriers plane surely deserve to have their views respected.

    I will say again; I'm not saying you're wrong on the basic point of the thread. I merely wish to discuss the "proof". You may very well be able to find a formula that proves what you want to prove, but the one you're using won't.
     
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  10. Doug Lord

    Doug Lord Guest

    Ct

    CT, I discussed the beam to length ratio of the main hull at least three times in subsequent posts. I have pointed out numerous times the critical role beam to length plays in WHEN a hull would plane as well as showing that much narrower hulls than the main hull of the Farrier F22 do plane.
    It was reinforced again by Mr Ostlind that the biggest block to understanding what happens with the Farrier is the inability to comprehend that at the SAME SPEED(roughly 2-4 times the sq.rt. of the waterline length): the ama is a displacement hull exceeding hull speed because of it's high
    beam to length ratio and the main hull is planing because of it's relatively low beam to length ratio coupled with the planing hull shape.
    This misconception is codified in "Ostlinds Law":
    --------------------------
    "In order for this one unit to plane, it must be completely supported on that plane by one or more of the hulls at the same time, all of the hulls in question planing in concert."
    -------------------------
    The above misconception is fed by the misunderstanding that the main hull is still supporting weight even with the ama partially loaded. And that means that the main hull- quite independently of the ama- and dependent on the factors mentioned previously could plane supporting that weight with dynamic lift and/or a combination of dynamic lift and buoyancy depending on speed.
    It seems that it is impossible for some people to conceive of the two hulls operating in different modes at the same speed . But that is the genius of the Farrier design with the planing main hull allowing much greater interior room than would a high beam to length ratio hull w/o too great a performance penalty(because of extra wetted surface in light air).
    ==========================================
    RHOUGH:Let's assume the main hull rises 3" when it starts to plane. For convenience and purposes of illustration assume the CLR stays in the same position and just the CE rises-it doesn't happen this way ,of course, but it serves to illustrate a worst case scenario of the change to HM and RM due to the hull rising 3". I don't remember the actual F22 SA and beam so I've used artificial figues: 200sq.ft. at 1 lb. pressure at a CE 12.75 ft. above the CLR before planing with the CLR-CE distance increasing 3" to 13 ft. for purposes of illustration.
    That results in 2550 ft. lb. HM before planing and 2600 at the onset of planing. That's a 50 ft.lb. difference. Divide that by 10'(approx. 1/2 beam) and the increase in buoyancy required in the ama to offset the onset of planing is 5lb..
    Significantly less in practice and insignificant by any measure.
    =======================================
     
    Last edited by a moderator: Oct 12, 2006
  11. ActionPotential
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    ActionPotential Junior Member

    I don't know that anyone has demonstrated that inability.
    I for one am quite happy to conceive of 2 hulls, attached to each other operating in different modes at the same speed. If they are being driven by a powerful motor the long skinny hull will travel at whatever speed in displacement mode and the short fat hull will plane at that same speed.
    However when the combination is driven by sail power with the apparent wind on the beam or thereabouts, assuming the long skinny hull is to leeward the short fat hull will not be planin but be lifted by the heeling force. If the long skinny hull is to windward and the short fat hull lifts, then it will be planing.
    If we take a short fat monohull and attach it to a hot air ballon such that the hull rises up in the water, is it planing?
     
  12. Doug Lord

    Doug Lord Guest

    planng tri

    AP, re-read my post ; I was still working on it when you apparently posted.
    Your hang up appears to be an "all or nothing " view of what happens when the ama is loaded by heeling. The fact is that in the earliest planing
    conditions the main hull still supports weight and does so as described above.

    You've almost got it; just take the motor & balloon off....
     
  13. CT 249
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    CT 249 Senior Member

    Jeepers Doug, I'm pretty much agreeing that the F boat planes, but you're a hard man to have on the same side.

    To restate - it's not whether you have recently mentioned beam/length ratios, since you have provided no detail of the effect of L/B on the F boat ratios. It's the fact that the ratio you used at the start of this thread is obviously incomplete when it comes to very slim hulls, which are less effective planing surfaces due to basic physics.

    Therefore people are quite right NOT to be convinced by your ratios that you called "irrefutable evidence", because they are wrong in this case.

    To make it simple before bowing out;

    1- For what it's worth, I think F boats MAY plane the main hull as you say.
    2- However, the ratio on which your case partly relied leaves out a highly significant factor, a simple but well known matter of basic planing theory AS THE PEOPLE YOU BROUGHT IN TO BOLSTER YOUR CASE AGREE, and it is therefore unreliable.
    3- Since there is a major flaw in the ratio you used as "irrefutable evidence", surely you must agree the other side is reasonable to disagree until you can find a more accurate bunch of numbers?
     
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  14. Doug Lord

    Doug Lord Guest

    planing tri

    CT, sorry to be dense but could you be more specific about the significant factor you say I've left out-and just to save me some time which post did I use the words "irrefutable evidence" in? Thanks.
     

  15. CT 249
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    CT 249 Senior Member

    The specific factor not accounted in your formula/ratios in posts 1 and 14 etc is beam as I have said repeatedly. Rules of thumb for fat boats which develop more dynamic lift for their LOA than skinny boats (all else being equal) are surely very poor evidence for your case.

    I realise that the "irrefutable evidence" line came in relation not to the ratio in post 1, but to some reasoning in post 3 which I don't personally all that convincing. I'd have thought that if the ama was taking enough of the displacement the main hull could possibly exceed S/L or 2 even if it's not a planing shape. Certainly, your claim lacks any resistance curves or figures to prove your case that a non-planing hull of similar beam couldn't exceed a S/L of 2. It is therefore not "irrefutable evidence". I apologise for applying "irrefutable evidence" to the wrong piece of refutable evidence.

    The way your post 3 read it seemed as if it was referring to post 1's formula. Mea culpa

    Maybe I can switch in your call "So we have the Science, the Designer, and the Sailors all saying that F boats plane. There is NO DOUBT WHATSOEVER". The science as presented here doesn't say F boats plane. A bunch of rules of thumb for fat monos don't provide absolute evidence for the way a multi hull works. So there IS doubt.

    As I said, I feel your side may be correct. All I'm saying is that there's no real proof so far.

    No, I don't call what sailors say is "proof". There are experienced sailors who argue against you in other matters. If you don't accept their words as gospel, how can you expect the other side here to accept what F Boat sailors say?

    I agree Ian's case is very persuasive, especially since I've seen F Boats look and feel as if they are planing. However, we know that designers like Dovell and Bethwaite say that planing is very hard to define. The Moth example (where some designers say the boat planes and others don't) is yet more proof that presenting the writings of a designer (no matter how skilled) is not conclusive evidence.
     
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