Planing Trimarans

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

  1. Capn Mud
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    Capn Mud Junior Member

    Does it give much advantage?

    I tend to agree with Chris by and large (Though I am not sure Cav's opinion warranted such a sharp response?)

    The main hull being of planing form may help it lift up in the water quicker as the leeward ama depresses but how much advantage doe the boat actually get from it given that with the leeward ama acting is displacement mode, limiting the benefit. You are not going to get the big performance acceleration "hump" that you do once a planing mono sits up will you?

    Tri-marans go fast and accelerate swiftly anyway.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    =======================
    You know, one of the most significant benefits of a planing main hull is that it has more room than does a displacement(High L/B ratio) hull. From my 14 and 20 to the WETA and to Farrier hulls this is true.
    When I first designed and built my planing(main hull) tris I was convinced that was the way to go from a performance standpoint-it sure is from a room standpoint.
    The "hump" you refer to may or may not be there-it wasn't on either of my boats. And there are high performance monohull skiffs that have no "hump" in the transition from displacement to planing.
    One thing to understand, and I've tried to explain it before, is that an ama with a high length to beam ratio(14/1 or above) will be a "displacement" hull with a high speed range-in the vicinity of 4-6 times the square root of the wl. A planing hull with a L/B ratio around 8 will plane at around 2-3. So, and this is very approximate:
    --A 24' ama with an L/B of 14 or over will have aa approximate low resistance speed range of 4-6 times the square root of the waterline or 4-6 X 4.9= 19.6-29knots,
    --A 24' main hull designed to plane(L/B approx 8/1) will start to plane at between 9.8knots and 14.7 knots depending on its L/B ratio.As an example, everybody knows that the Moth foiler is fast but before it was a foiler it was sailed throughout the wind range at a 10-11/1 L/B ratio. It had an 11' hull but didn't start planing until 12 -15 knots of boat speed and even w/o foils was one of the fastest dinghies. Planing starts later in narrower hulls-but it DOES start -if the hull is designed to plane.
    So, it is a complex subject but the High L/B hull can operate very effectively right alongside the Low L/B planing hull. I hope that makes sense and is of some help....
    One major disadvantage of ,at least, my planing(main hull) tris was that they were(esp. the 20 footer) slow in very light air because of the wetted surface of the main hull.
    ------
    Since I designed those boats I've decided that,for me and for best performance, the main hull should be a high L/B hull with low wetted surface
    and ,maybe the ama should be a planing hull if you can make it work in light air.
    This is slightly off topic but check this out if you're interested:

    http://www.boatdesign.net/forums/multihulls/ultimate-18-tri-17644-2.html

    Posts 1 & 29 best describe my idea.......
     
  3. Capn Mud
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    Capn Mud Junior Member

    Well I agree with the latter part of the statement thats for sure.

    Sorry you have lost me with your argument here - and the moth example didn't make it any clearer for me..... Care to clarify?

    Thanks,
    Andrew
     
  4. Doug Lord
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    Doug Lord Flight Ready

    ===========================

    Andrew, by trying to explain that the ama is operating in a very low resistance mode I was trying to show that it did not limit any benefit gained from planing. In fact, the two "modes" are complimentary rather than in conflict.
    The nature of the planing main hull is that the dynamic lift reduces main hull wetted surface over and above the reduction in main hull wetted surface achieved as a result of heel(and partial ama loading). Any better?
    PS-just because the main hull is technically planing does NOT mean that it is faster than a main hull of equivalent weight carrying ability that has a High L/B ratio. The High L/B ratio hull might wind up having to be a bit longer because it probably wouldn't be able to carry equivalent weight without excessive rocker.
    So the planing main hull can have more room at the cost of,perhaps, poor very light air performance.
     
  5. cavalier mk2
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    cavalier mk2 Senior Member

    I looked at the Weta and the main hull is close to the shape of the Nicol. I do feel a bit of a hump off wind and a speed jump when the boat is light. I should have asked Gary what kind of keel they had as mine currently has a wide fairing of dubious section that actually makes it harder to lift (but it still will in the strong stuff) downwind because it is busy digging a hole under the lifting surface. It seems politic in the current situation to use a term for the lifting that is not such a trigger. Perhaps a enhanced dynamic lift hull ? I talked to a Nicol Wanderer owner in Australia who also reported his main hull experiencing enhanced dynamic lift when lightly loaded. The room is nice regardless of what the hull is perceived as doing. The computer simulations of Martin seem great (they are probably getting lift by breaking that bow wave) but are a bit out of my budget hence the model suggestion. Unfortunately nothing can satisfy everybody which is why a couple of pages back I offered the boat as a test platform if someone wanted to supply a new suit of sails ;) We need to get another year or 2 out of the old ones. Now lets see a main, two or three jibs and a bigger spinnaker :0 The ama speed length ratio performance coupled with the wide main hull matches my observations, which were unbiased and made to satisfy my curiosity on a design I initially new nothing about except that it suited my needs and budget and had some interesting thought put into it.
     
  6. Gary Baigent
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    Gary Baigent Senior Member

    Cav, When I was on Drumbeat it had a pivoting centreboard - but after it was sold, the new owner, a few years later discarded the board and put a long keel on the boat instead.
     
  7. cavalier mk2
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    cavalier mk2 Senior Member

    No skegs then ? When I first got my boat there was no rake to the mast and the forestay was led far forward. This kept the bows down and kept the boat from rising. When I added the designed rake and moved the fore stay back to location it became a whole different boat, lifting and able to carry sail far longer. If you don't pinch it the speed keeps the ama up far better than you would expect. Offwind the amas don't appear to be under much load at all sailing with the genoa. The wide stern tends to tow a transverse wave but leaves it behind when breakout/lifting velocities are obtained. The hump seems to be around 10 knots, the designer reported 4 inches of lift at 15 knots and 8 inches at 20 sailing flat downwind in strong conditions. A lot of those tests were filmed on 16mm film, maybe Ian Nicol and family still have them archived?
     
  8. Doug Lord
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    Doug Lord Flight Ready

    --------------------------------------------
    Don't you think it is a shame to change the language rather than use the correct description for "breakout/lifting velocities"? Don't be intimidated....
     
  9. cavalier mk2
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    cavalier mk2 Senior Member

    I'm not intimidated you rabble rouser but polite :) OK I'll say it (with the theme of Young Frankenstein playing in the background) The main hull PLANES. This is all pretty silly. I also think the direct downwind test requests are a bit unfair, if you look at the polar curves of planing boats the action is supposed to happen a bit before 8 and after 12 points off the wind. Then your sails are acting as airfoils instead of drag devices and it takes much less wind. Incidentally the direct downwind approach can keep the bows down and keep you from planing. We tack downwind. The part about speed length ratios might be news to some readers who never learned the square root of waterline length times 1.35 = displacement hull speed equation. My books all said planing starts after speeds of 1.5 - 2 in hulls designed to plane. Yes lift speed is dependent on hull width, weight etc.....but we're preaching to the choir . Now if the trend is too break the bow wave, and I've long thought that added wetted surface is a major source of drag, are the boats Martin is working on forced dynamic lift hulls ?
     
  10. cavalier mk2
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    cavalier mk2 Senior Member

    Here is a definition of planing from Juan Baader's classic tome "The Sailing Yacht". "Planing is a state of sailing with partial dynamic lift in which the weight of water displaced is LESS than the weight of the hull. Furthermore, the hull detaches itself from its wave system , leaving its stern wave far behind and beginning to ride on its bow wave." ......." Light boats have even been made to sail with TOTAL dynamic lift by fitting hydrofoils under the hull." If you wanted to be technical you could say a trimaran was planing as a whole (even if it is just the main hull) if its displacement was lessened as a whole. So a tri with a planing main hull that is able to be lifted clear of the water and supported by the ama would start at displacement speeds, proceed through the state of planing and finish as a displacement craft again when all the load is taken by the ama unless it too is a planing hull. Of course some hydrofoils are used to supply partial dynamic lift and there are wings and kite sails that provide aerodynamic lift.......but it is all sailing.
     
  11. Gary Baigent
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    Gary Baigent Senior Member

    You can't say,
    "a trimaran was planing as a whole (even if it is just the main hull) if its displacement was lessened ..."
    - that doesn't make sense, firstly, "as a whole" means just that, not some partial "whole" definition
    and secondly, the displacement is not "lessened" - because it has been transferred from vaka to the aka .... it is still the same displacement, it's just being carried more on the depressed aka. Perhaps there is a microscopic change because there will be fractional dynamic lift on the vaka ... but bugger all really - the poor aka is carrying damn near the whole boat and the extra wetted surface from the depressed float will nullify any small gain the vaka makes from a flattened *** end hull configuration.
    Damn, I thought this topic was decayed leaf mould - remind me not to reply again.
     
  12. Doug Lord
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    Doug Lord Flight Ready

    ==================
    Geez, Gary you keep saying that,thread after thread and yet.......well, nevermind.
    But you really have to improve your ability to have a technical discussion because you don't make sense-especially in light of the facts that have been presented. Word tripping,obfuscation, innuendo, mischaracterization of other contributors is unfortunate-and just doesn't cut it. I really mean well-it's just a shame,really.
     
  13. Gary Baigent
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    Gary Baigent Senior Member

    Everyone is bored with your smug pomposity Doug - you have about as much contact with reality as my old climbing boots - in fact the boots would be more aware, and certainly more experienced, maybe even more intelligent, than your nonsensical utterances.
     
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  14. Doug Lord
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    Doug Lord Flight Ready

    ====================
    Ouch!(2) Ah, but should I do this now?? Just curious....
     

  15. Doug Lord
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    Doug Lord Flight Ready

    And now back to your regularly scheduled program:
    ----
    I wrote to Martin Fischer about planing multihulls-this is his reply (posted with permission) :

    " Hi Doug,

    The catamarans I designed plane partially at least under certain conditions. As of the main hull of a trimaran, I am not sure that it is really necessary that it planes. On racing trimarans the main hull is in general not heavily loaded and for me the main goal for the central hull is to create little drag for waves that hit the (flying) main hull and/or to provide at least some additional longitudinal stability when sailing downwind in very strong winds. In very rough conditions downwind it is not possible to fly the main hull for safety reasons and in these conditions you want some longitudinal stability."

    Cheers

    Martin
    ================================
    Martin is one of the premier beachcat and multihull designers out there and his comments about planing are most interesting to me.
    If you want to know about Martin check out this interview: http://catsailingnews.blogspot.com/2010/05/cs-interview-martin-fischer.html
    From the interviewer:
    " You can say Martin Fischer is one of the fathers of modern racing beachcats, but he also has worked in some key big multi projects like Groupama II-III and BOR90.
    Many catsailors have heard his name, but there is not much info available on him. Martin was involved on the first A-class wave piercer, the F18 Capricorn and the latest Hobie racing extreme machine, the Wildcat, among many others."
     
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