Planing Trimarans

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

  1. Doug Lord
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    Doug Lord Flight Ready

    A displacement hull can be faster or as fast as a planing hull depending on its L/B ratio. Farrier's point always has been that the planing main hull allows the wider, roomier planing configuration to be at least as fast as the skinny high L/B ratio hulls of other tris(of the same weight).
    Saying that a hull planes is not , necessarily, a claim for the speed of a particular hull. The fact the the Farrier planing main hull allows speed close to equal of a skinny high L/B hull of the same weight is all that counts.

    From Farriers site( http://www.f-boat.com/pages/introduction/features.html ) :
    The planing ability of the low rocker trimaran center hull also allows a slightly wider hull section to be used, for more room inside, but without any detrimental effect on speed.

    -------------
    Parliers research showed that a planing hull can have considerably less drag than a high L/B hull at and above a certain speed(20k in L'hydraplaneur's case). Parliers cat performed poorly at speeds much below 20 knots because of the step drag-a problem that has since been solved.
    See http://www.boatdesign.net/forums/multihulls/planing-trimarans-ii-real-thing-33046.html
     
  2. Chris Ostlind

    Chris Ostlind Previous Member


    Would you be so kind, then, to produce Farrier literature quotes that address, specifically, the points you are trying to make?

    Keep in mind before you do so, that there is a ton of stuff from Farrier's own site in which he looks to make the exact opposite point.





    His research, along with that of others, also shows that in every operating speed outside of the selected window, the boat is a near complete dog compared to a less dedicated design.
     
  3. cavalier mk2
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    cavalier mk2 Senior Member

    Good observations Dennis . Boat hulls that need pseudonyms huh Gary ? The names have been changed to protect the innocent....How much heavier are the wider hulls? If the wider hulls are providing more lift ( its hard to see how they couldn't) the ama would depress less when the vaka rides up on a "whatever". If the ama is immersed the same amount the wider hull is lifting enough to compensate for the added weight. If the vaka is just being "levered" the ama loading will be a little deeper because of the added weight of the widened hull and the ama would be immersed to a displacement of the whole boat weight. Wide or skinny if the ama isn't immersed to the boats full displacement something else is providing lift. Or whatever.....
     
  4. Gary Baigent
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    Gary Baigent Senior Member

    Doug, you're still repeating yourself, repeating does not create a reality. Parlier's stepped hull ORMA was a cat, not a tri. Also its length to beam, even though it had lifting, stepped planing hull, was still composed of two very slim hulls, it was not a broad arsed fatty like Farrier's. Also the forward section underwater shapes were very flying boat configuration ... and they made an ugly fuss going through seas, (see prototype) especially when travelling fast. No wonder it was a dog in the light.
     

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  5. Doug Lord
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    Doug Lord Flight Ready

    ===================
    Gary,thanks for pointing out that I said "tri" in the above post-I sure meant to say cat! Now corrected.
    The length/beam ratio of Parliers cat hull has to be looked at in two segments-overall(each hull) and the area from the step forward(each hull). The overall(each hull) L/B ratio has very little, if any, meaning on this type of hull because regardless of what the overall L/B is the step ruins any low resistance implied by the relatively high hull L/B.
    If you read Parliers stuff you'll see that above 20 knots the thing had a sharp drop in drag and that would be partially related to the L/B of the planing surface. The part of the hull aft of the step was designed to be mostly clear of the water with the AOA of the planing surface(pitch) controlled by a rudder t-foil. I'm convinced that a stepped planing hull like Parliers design could work as an excellent ama for a trimaran -assuming that you have a solution to the drag problem below the "magic" speed. And I do.....

    Pix- rudder t-foils, RC model ,hull bow
     

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  6. Gary Baigent
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    Gary Baigent Senior Member

    You do, eh? By having a canvas bottom with a Hyfield lever that racks up and down, to have or have not, the step. Yes, something like that, or a slit below waterline base that opens and closes. But Sod's law will reign Doug. That is very tricky thing to do, having a variable hull shape.
     
  7. Doug Lord
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    Doug Lord Flight Ready

    =================
    Much simpler-the whole hull rotates. As I've said before: if the leadbellies can rotate a keel why can't a tri rotate a hull-especially when it appears to result in a viable, all condition, boat that can take advantage of the apparent drag reduction of a stepped hull starting at "X" speed .....
     
  8. Capn Mud
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    Capn Mud Junior Member

    Doug,

    This must be a new definition of the word simple I was previously unaware of...

    How would the ama hull be rotated? What would provide the motive force to do so? If it requires a motor running below decks for any decent sized boat like the leadbelly movable keels then it just lost my interest.....

    I know many don't agree (and it has been argued ad-infinitum elsewhere) but to me that isnt sailing.
     
  9. Doug Lord
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    Doug Lord Flight Ready

    ------------------------------------
    Capn Mud, the "much simpler" was used in reference to Gary's "variable geometry" comment. The rotating hull is much simpler than that.
    The original conception was for the ama of an 18' tri. The model was made using independently rotating components. It has been improved to a single one piece hull restrained just behind the step with a flush carbon collar and a rotating attachment at the transom. It is very simple considering what it accomplishes. On an 18, the hull would be rotated by hand-there is no load on the hull when it is being rotated.I would imagine that almost any hull size could be rotated w/o requiring engine power.
    The advantage is that you have a high L/B hull from zero to rotation speed ,then you have a stepped planing hull. The rotation speed is the speed at which the stepped planing hull begins to have less drag than the high L/B displacement hull.
     
  10. cavalier mk2
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    cavalier mk2 Senior Member

    Its a neat idea, I still think having a underneath section of the stern step drop/lower would be easier, stronger and lighter. Visually it would lack the gatling gun effect which is sure to mesmerize the competition. It would also work with the ama in the loaded/immersed state.
     
  11. pogo
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    pogo ingenious dilletante

    Please see pix of Nielsen 35 here:
    http://www.boatdesign.net/forums/multihulls/planing-catamaran-30415-4.html

    The boat has indeed pretty sharp amas and “flat” bottoms for planing.
    Originally designed for taking part in Carlscrona speedweek, making at least 30Kn average over the 500m strip, she´s now mainly used for cruising.
    With a cruising deplacement of 2.2m3 she´s still able tor reach peaks of 25, while making cruises with averages of 16kn.
    On a beam reach she`s able to "plane" on her ama alone, which deplaces than about 700liters, that means dynamic lift is about 1500kg.
    On a broad-reach she`s really planing on ama and mainhull. One can really feel it when she`s "free". `Cause of her sharp forward sections (comin`close to a wavepiercer) she`s going thru a chop of up to about 8% of her mainhull lwl (10m/33`) then.
    Very interesting is planing in a swell, she is faster climbing up a wave than surfing down ! It must be the resonance of the planing hull with the orbital wavepattern, ground-effect.
    Sailing pretty often in shallow waters (less than 1m) , we also use the ground-effect. While making in the beginning at least hullspeed, it can speed u up to 60%. It`s possible to sail with a speed of 16-18kn over water with a depths of less than hulldraft. Note my special rudder , i called it MARS- Multi Adjustable Rudder System.
    The boat was really ahead of her time. A lot of her features u find today in different modern designs.
    She´s as fast as F28Rs , F9A, Df920, Df800 and other Micros. Coming always in under the first three. In a blow and in a wave she`s faster than her competitors. On a run, when the circumstances allow planing, she`s unbeatable .
    By the way, she`s not built in ply ! Dyvinicell Sandwich gave her a designed empty weight of 1200kg. Together with the prebended alloy beams (coming only from the amas`weight) and the bridle -shrouds going to both beams, held by struts of stainless steel bars, she`s a very stiff boat. Torsion of the whole structure is nearly unknown.

    shallow water planing:
    http://www.youtube.com/watch?v=cVwYPCprPDw&feature=related

    pogo
     
  12. pogo
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    pogo ingenious dilletante

    Interesting vid of a mainhull, struggling along with bow-up attitude at humpspeed, partly breaking thru this barrier sailing in neutral longitudal trim, at the end sailing slower at the beginning of humpspeed.
    Because of lwl/bwl ratio, difference in speed between humpspeed and planing mode is little. One can judge the mode only in trim-angle , but, only with a ratio up to 10. With a higher ratio, there is no change in longitudal trim angle (no humpspeed) , the boat goes from fast planing mode into planing mode (having of course flat surfaces).
    Farrier boats, with their mainhull ratio between 8 and 9, sail very often with a bow up attitude, speed then is between 1,2 to 1,4 times hullspeed---classical range of humpspeed.
    For having the bow-up attitude as a safety feature at higher speeds, Farrier gave the F27 very much rocker at the stern. Using the bernoulli-effect, which sucked the Stern down, the dynamic lift in the forward sections also lift the bow.
    http://www.youtube.com/watch?v=ckVRRhTEqJ0

    It ain`t no question.
    U-spaped/ flat bottomed hulls with ratios of more than 10 are able to plane.
    The transition from fast deplacement to planing mode is hard to notice.
    Sometimes something "in-between"
    One should judge/calculate it by the actual draft of the hull how much dynamic lift the bottom generates.
    When the boat is really planing (sailing with nearly as much dynamic lift as the boat displaces---or more) u gonna feel it. No limit ! More wind--more speed--more dynamic lift--less wetted surface--more speed--more dynamic lift ---a.s.o.
    Screamin` for more wind, praying for less ,or better, no waves.

    pogo
     
  13. Blacky
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    Blacky Junior Member

    @pogo:
    where can i find more infos in the internet about the Nielsen 35?
    Are there plans available for self-building?
    thx!
    blacky
     
  14. pogo
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    pogo ingenious dilletante


  15. Tom.151
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    Tom.151 Senior Member

    Pogo,
    Thanks for that link - looks a winner to me!

    I'd really like to see more pics of the hull shapes - can you help there?

    Looks like a family resemblance to the Pinta 33 - VBG - I always liked that boat!

    Jim Brown's newest design, the Seaclipper 20, has a hint of the angled flat ama bottom -- but Jim's are angled the opposite way, he says the angled flat is there just to stop slapping/pounding of the amas.
     
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