Planing Trimarans

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

  1. Doug Lord
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    Doug Lord Flight Ready

    Its interesting: in Bethwaites High Performance Sailing book he describes the "humpless" planing hull. No hump but planes like a bat out of hell. Might be a clue......
    And quite a few WETA owners descibe the main hull as planing.
    Trimarans most certainly can plane if they are designed to.....
     
  2. Capn Mud
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    Capn Mud Junior Member

    The Weta may plane - but the F27 sure doesn't

    Doug,

    Have you sailed the Weta and F27?

    Two very good examples as far as I am concerned becasue until recently I owned 1 of each and sailed both regularly (well 1/3 of the F27 but who is counting?)

    The Weta can possibly plane because the is the possibility of using the crew weight to sail the boat flat, amas basically free of the water, such that the main hull can potentially get up on a plane like a flat bottomed dinghy. In my experience it doesn't in fact readily plane because at the higher winds you need to hike out pretty hard to sail her flat - the leeward ama is usually very much in the water using its bouyancy to keep the boat upright. And that is what you would expect because with a trimaran rig should be big enough to cause that to happen. That is the point of a trimaran after all. Otherwise it is just a skiff with "training wheels". The fact that you seem to not understand this has me scratching my head as it just seems so fundamentally obvious to the design of a trimaran.

    The Weta main hull itself is relatively long and narrow and not easily planing even if it were sailed flat.

    The F27 always has the leeward ama in the water providing buoyancy. That can and does help lift the main hull up in the water to the point that it can be just skimming the water much like planing. Perhaps one could even say that the main hull is "planing" - questionable but perhaps. But the BOAT is not planing because it has an ama taking the bulk of the boat weight by its dosplacement and bouyancy. In fact there is a great pic in the F27 manual shoing the boat sailing with the main hull almost completely out of the water (but the leeward ama amost buried). By your logic that would mean the boat was flying because the main hull was not touching the water.

    And by the asme logic a catamaran would "plane" with one hull acting in displacement floating the boat and the other skimming the surface. Obviously it doesn't plane. The analolgy is, in fact, quite exact. With the windward ama out of the ater the boat is behaving like a catamaran (albeit one with 2 different sized hulls and an off centre mast).

    Simple physics. Draw some force diagrams. The physics is completely different to a planing mono hull.

    I used to have a trimaran dinghy that really did plane - the Tri-Fly. The amas were very small and short, the connection to the main hull not really very rigid, and the main hull relatively flat bottomed almost like a sail board. It could be sailed relatively flat with the leeward ama basically skipping the water providing minimal, if any, bouyancy due to the low rigidity of the connection. It was really fun to sail but it didn't achieve the best it could have because of the small size of ama, and for that reason wasnt exactly the best example of trimaran design. As an aside it was a pig to go anything downwind of a beam reach in a breeze because she would bury the whole main hull nose - not enough bouyancy and perhaps the mast too far forward.

    Bat On (for another 20 pages perhaps?),
    Andrew
     

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  3. Capn Mud
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    Capn Mud Junior Member

    To plane or not to plane

    Doug,

    Where in that video does the Weta plane?

    She is sailing fast sure and doing all the fun things the Weta does - and I know from lots of first hand experience it feels exhilarating. But it isnt planing. The leeward hull is always in the water and even the main hull doesn't seem to get up as much as you expect to see with a planing dinghy.

    http://www.youtube.com/watch?v=6TSdXQNPjIA&feature=related
     
  4. Gary Baigent
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    Gary Baigent Senior Member

    Weta is basically a small, narrow, planing dinghy/vaka plus amas and akas to keep it upright, or to lean on when beating/hard reaching. Offwind that hull will plane .... but Weta is not your average multihull, like Farriers, Tennants, Newicks etc. Put beams and floats on a Lazer, or Javelin or any 14-15 foot planing sailing dinghy and yes, by the Messiah and St. George - there we have it, planing trimaran, eh Doug? Yes, it's a revolution alright. Why in hell did I reply to this turkey?
     
  5. Capn Mud
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    Capn Mud Junior Member

    Doug,

    What he (Gary) said.....

    What I was trying to say but far far more succinct. "Laser with amas" - perfect example Gary
     
  6. Doug Lord
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    Doug Lord Flight Ready

    ===========================
    Andrew, I haven't sailed either boat but I've read a lot about both. I know tha Ian Farrier(a man I greatly respect) says the boat planes. I know a lot of owners say the Weta planes as well. But I did design, build and sail for sometime two planing tris-a14 and a 20. You did watch the video of the WETA planing,right?
    The simple physics and the crux of the whole debate is that the NP's(non-planers) say that because some of the weight is supported by the ama it is impossible for the main hull to plane. All that happens is that the main hull is effectively lightened up. The main hull still has weight(!) and there is absolutely no reason that the hull wouldn't plane IF it was designed to do so.
    The physics of planing aren't magically suspended just because the ama holds some of the weight!
    Another great mental block for some is that the ama is a displacement hull(on some tris) so how can the ama behave like a displacement hull and the main hull behave like a planing hull? That can be answered by looking at the speed length ratio of both hulls: if the ama is approx 14/1 or better then it probably has a speed length ratio of around 4-6. If the main hull is, in fact, a planing hull it will begin to plane sometime above 2-higher the narrower it is. And clearly both the displacement hull and planing hull can do their thing at the same exact speed.
     
  7. Capn Mud
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    Capn Mud Junior Member

    Doug,

    The main hull may "plane".

    The boat as whole does not. The displacement of the ama is always going to stop that being the case.

    Maybe we actually agree then?
     
  8. Capn Mud
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    Capn Mud Junior Member

    Doug,

    Re the Weta planing video - I refer you to my post #303
     
  9. Doug Lord
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    Doug Lord Flight Ready

    ==========================
    You may have hit another nail on the head: every time I refer to a plaining trimaran I'm usually refering to the main hull plaining, not the amas. Same with Ian Farrier.
    So you say it's not a planing tri if the ama's don't plane but the main hull does?
    PS-I googled the WETA with something like: "Do WETA trimaran's plane?"
    A lot of customers refer to the boat planing.
    And when the Weta is planing the ama is touching the water! How, in the name of all thats holy, can that be??? Huh? It's just physics......
     
  10. Chris Ostlind

    Chris Ostlind Previous Member

    It matters not, Doug, if the main hull is shaped as a planing hull variant if the complete trimaran form achieved its state of plane through mechanically leveraged lift due to transference of the boat's weight to the leeward ama. Planing hulls can get to the Promised Land all by themselves through dynamic lift.

    Of all the boats in this discussion, the Weta is the one craft that could accomplish this feat. Due to its small size, the crew weight placement plays a very big part in being able to sail the boat in a manner in which the amas are not holding the boat up. I have seen them sailed this way and it is possible to hold the boat on plane with minimal water contact on the two amas for support. Takes a good bit of skill, but it is possible.

    The bigger and more powerful the boat gets, the more difficult it is to seriously influence the heeling function. You simply do not see F-boats being sailed like some big, ama equipped dinghy. The forces are far too great. I have asked, repeatedly, for someone, anyone, to produce a video clip showing an F-boat being sailed in that fashion while it is up on its claimed planing state, amas clear of the surface. To date, nobody has done so. Not Ian, not any of the hotrod F-boaters, not any casual owners who get such a rush talking about how their boat gets up on plane... not anyone.

    Yet, this magical act is expressed as being a commonplace event.

    When that leeward ama goes in the water and the weight shifts from the vaka hull over to the ama in displacement mode, it is not the same thing as planing as it is described by Savitsky, or any of the other very smart guys who have studied this process quite seriously over the years. Savitsky's planing hulls are very definitely achieving their lift from dynamic pressure on the hull that is generated from hull form, power applied and angle of attack. They also very specifically depend on a transom form that provides a neat departure from the surface. The transom shape issue is very clearly outlined in the collection of Savitsky's papers on the subject.

    Virtually none of the described planing hull characteristics exist on the typical trimaran main hull. It's not just about water plane area. It's about how all of the elements come together when coupled with available power to weight capabilities.

    Capn Mud is correct when he observes that just like hull flying cats, trimarans achieve a state that may look like it is planing when the main hull is lifted to a point where it is skimming the surface. Trouble is, that windward cat, or trimaran main hull did not get there because it followed the lessons taught by Dr. Savitsky. It got there because of mechanical leverage, with but the slightest of assistance from the dynamic lift being generated by any hull form as it moves through the water. That process is not planing.

    Unless one is prepared to completely refer to all hull lifting multihulls as planing boats, which they are very clearly not, the claims are simply unfounded in the honored fashion of recognizing what the planing state is all about. Perhaps someone can produce a serious set of technically researched papers that moves the planing goalposts into a new paradigm. I'd be open to that process if it can be shown that the issues are worthy and repeatable. Science and design will continue on to other interesting areas of study.

    Still, there's that nagging bit about the weight being transferred to the non-planing leeward hull that sits there like a 500 pound elephant in the room. You, Doug, have not made it go away, Ian has not, nor have any of the folks who wish this planing thing to be so, as they tool about on their "planing trimarans".

    So, there are lots of issues to address, lots of physics that stand in the way and a whole warehouse full of simple logic that says it isn't so. Still, I'm not capable of changing a mind that does not want to be open to change. You are open to change, aren't you, Doug?
     
  11. cavalier mk2
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    cavalier mk2 Senior Member

    A planing main hull will lift to the surface reducing wetted surface far sooner than one that depends on the lever arm for all its lift. Combining hull shapes in one craft for a better all round performance envelope is a good idea. Anyone remember water skis? Long thin shapes that plane on the surface with a gradual rise....The longer the shape the less of a hump there is. Dinghy sailors with wider hulls will have a more pronounced hump than a F-boat main hull. If you make a model of 2 F-boat main hulls linked as a catamaran and tow them Iyou can easily see if they plane. If only one hull will do tow with 2 levers angled in to the bow. A simple test that can work with any hull form. A sharp sterned boat can plane if the run aft is kept flat, ask your powerboat friends. I'm with Capn Mud on this one and find no problem in identifying craft with planing main hulls as such. To argue for a whole boat definition of planing as Mr. Ostlind is doing misses the point of the design. A long thin hull can pierce its bow wave, leaving the stern wave behind. But if it develops lift it is planing by the dynamic lift definition. I identify my old Nicol as having a planing main hull, it has a flat run, wide stern and definite wake separation. Mr Farrier's upswept stern might affect planing performance but keeping the bows up lets you drive harder. Any design is a blend of features to cope with the wide range of performance and conditions. This thread seems to be more about ego and definitions than design. Ah well, jousting with hobby horses is entertaining......
     
  12. Chris Ostlind

    Chris Ostlind Previous Member

    So, who said, "all its lift"? I very clearly said that any hull gets some dynamic lift by simply moving through the water. The salient point here being that the moment the sail is loaded and the boat moving, the leeward ama is depressed. When the sail is loaded to the point where the ama immersed buoyancy equals the weight of the rest of the boat, the main hull has been lifted from the surface. I'd suggest doing the math and evaluate the relative relationships.

    As it is in your quote above, you provide no math at all to support your argument. It sounds more like a ballpark estimate called good.


    Longer shape, less hump... Sure, OK. But what's the point since none of the hulls resembles a waterski in any way, shape or form. Trouble is, Ian emphasizes that his hulls are no longer long and thin. In fact, he goes to great lengths to call that kind of hull "a coffin", to borrow his unfortunate term.



    So, feel free to build the models and prove your point.


    I would guess that you are referring to the Bartender, type of hull when you say this about sharp sterned boats. Look carefully at the photos below and you will see the magic of how the Bartender gets up on plane with its sharp stern. Recognize the form needed on each side of the hull?

    I also agree with Capn Mud about identifying planing main hulls. I further agree with Capn when he makes his points about trimarans being complete assemblies and if one wishes to suggest that they are planing tris then all the hulls in the water must be on plane. Sail it with the amas out of the water and get it on plane? By all means and do send us a video clip showing you doing it.


    It is not planing while it is running in displacement mode. Please check your definitions for various states of operating form and hull design parameters


    Lovely Cav-man. We'd all like to see what you describe as a planing main hull in operation. We'd also like to see you sailing her with both amas out of the water while doing so.


    So, which color have you selected for your toy horse, my friend?

    Every conversation on this planet is about ego. Definitions, by their very, ah... definition, are precisely what ah, defines the design. I suggest you try making a boat design to completion without definitions.

    Next I suppose that you'll be claiming that your Nacra beach cat, should you have one, gets up on plane. We all await that bit of uniqueness as it will cleanly set you apart from the rest of the worldwide beach cat racing fraternity.
     

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  13. cavalier mk2
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    cavalier mk2 Senior Member

    Ah a sport of splitting hairs.....head in the sand mr. ostrich man. I suspect you are unable to evaluate any information without bias. Now my pony is named Don Quixote and it appears we have tilted at a hot air wind bag er windmilll that is.........
     
  14. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Shouldn't the pony be Rozinante??
     
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  15. Chris Ostlind

    Chris Ostlind Previous Member

    No, actually, Raggi, I think he has himself confused with Sancho Panza who was promised an island and wound-up with nothing but some funky quips. ;-)

    The funny part about this is that by identifying himself as the Don, he is apparently heading directly for the region of fools who run off looking for conflicts when there are none to be had.

    A lovely Freudian slip in anybody's book.
     
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