Will it plane?

Discussion in 'Multihulls' started by F3M4, May 3, 2011.

  1. Spiv
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    Spiv Ancient Mariner

    The Manta was the only cat with asymmetric strakes.
    Planing strakes are there to lift the bows; what I found is that (at high speed) as you hit a wave, the strakes out of the water come into play.
    If you have 4 strakes (2 on each bow) that hit the water at the same time, you get a sort of a jerk up.

    I'd suggest to have them staggered so that only two hit at the same time, then as the bows keep penetrating the wave, 2 more provide lift and so on.

    On a 6m boat I would have this asymmetry only in the first 1.5m, then all symmetric, with the V gently opening till, at the transom is nearly flat.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    ------------------
    Thanks-makes sense.
     
  3. F3M4
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    F3M4 Junior Member

    [​IMG]
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    As you can see from the transom pic, the strakes are asymmetrical, the inner strake is 2" lower than the outer.

    I tried to use as much advice and borrowed techniques from other designs as I could. The length comes in at exactly 20'.
     
  4. DaveJ
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    DaveJ Senior Member

    I think also that the center "wave-breaker" also redirected the waves back down to give extra lift at very little extra drag (like floating on a cushion of water).
     
  5. Spiv
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    Spiv Ancient Mariner

    Dave is correct, the wave-breaker should redirect the waves downward to increase lift, hence I think it is important to:
    • make the tunnels round, see this pic of another Manta.
    • Make sure the wave-breaker is out of the water at rest,
    • that the tunnels are parallels and do not get smaller aft
    • that you have a reasonable bridgedeck clearance (250~300mm)
    Have a close look at the planing strakes at how they curve and how they are staggered.

    Try and make the boat width half the length, it seems to be the best performing ratio.

    Lastly, I would definitely flatten the aft section more, perhaps bring the chines down, so you get shallower draft and more buoyancy for your motor and passengers.

    I am not sure about the asymmetric aft sections, the Manta is only asymmetric forward.
     

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  6. F3M4
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    F3M4 Junior Member

    I will attempt to round out the tunnels more, my concern is the limitation of using the stitch and glue method. I am unsure if that compound curve will be achievable.

    Currently as it sits, if the draft comes out to be 1 foot, then there will be 458mm of clearance before the wave breaker hits the water.

    By flattening the aft section, do you mean to make it more squared at the bottom?
     
  7. Spiv
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    Spiv Ancient Mariner

    I will attempt to round out the tunnels more, my concern is the limitation of using the stitch and glue method. I am unsure if that compound curve will be achievable.
    Double compound curves with stitch and glue plywood (I suppose) will be very hard, but you can find some sort of compromise.


    Currently as it sits, if the draft comes out to be 1 foot, then there will be 458mm of clearance before the wave breaker hits the water.

    That seems way too much, your centre of gravity would be too high and you will feel it when you take a turn at high speed....
    I meant 250~300 from the top of the tunnels to the water, but keep the wave-breaker 100mm out of the water.


    By flattening the aft section, do you mean to make it more squared at the bottom?

    Yes, you can achieve this by lifting the keel or lowering the chines, I'd suggest the latter for buoyancy reasons.

    Don't take all I say by gospel, I had 4 power cats from 6 to 12m and they all behaved differently (one behaved like a dog, one like a pig.....)

     
  8. F3M4
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    F3M4 Junior Member

  9. PAR
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    PAR Yacht Designer/Builder

    Is this a design study exercise or are you really attempting to design a cat, without an understanding of the fundamentals of hydrodynamics, not to mention engineering?
     
  10. F3M4
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    F3M4 Junior Member

    It will be built. All of the materials, save the wood, will be surplus from an industrial connection I have and will be received at no cost to me.

    And I am a student in chemical engineering, though that applies very little to this scenario.

    I plan to document the entire process, and make all of my models and research available to the public.
     
  11. F3M4
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    F3M4 Junior Member

    That post came off wrong...

    I'm very motivated to accomplish this, and truly want to see it happen.
    I will be building models to test the design.
    I've come to what I've surmised to be the best possible source for help in this project.
    If I learn nothing other than that I can't design a boat, then I've still learned.


    I've already changed what kind of boat I want to build and have decided on this, with which I hope to be able to rekindle the tournament fishing tradition I shared with my Dad prior to katrina.

    Looking at the first two designs, it's evident that I've already learned a great deal from this experience...
     
  12. upchurchmr
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    upchurchmr Senior Member

    F3M4

    I have always wondered what would happen if you used old Tornado hulls for a power boat. They are not classic flat bottomed hulls, but they have been clocked in excess of 25 kts with just a sail and limited by the righting moment of 2 bodies. I bet a modest motor could hit 30 mph, so long as everyone doesn't insist on discussing "true" planing. The Tornado has always had a very high displacement hull compared to the contemporary catamarans of the time. Limiting the total weight would be necessary to have a chance, or just start at that shape.

    Marc
     
  13. Doug Lord
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    Doug Lord Flight Ready

    ==============
    Here is Gary Dierkings's power cat using Tornado hulls:

    a video: http://exposureroom.com/members/vakaman/419485058b6647a7b148c2497e2ab986/
     

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  14. F3M4
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    F3M4 Junior Member

    That is interesting....

    I wonder about it handling the chop with a load....
     

  15. upchurchmr
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    upchurchmr Senior Member

    Doug,

    Thanks very much. I'm going to look for any comments on the performance. Looks like it would have gone better with the CG more forward. I would never have sailed those hulls with that much of the stern depressed.
    As a sailboat the Tornado was quite safe in a chop, the hulls are more boyant than Hobies of any type (70's and 80's boats). Actually we would sail with the leeward bow depressed to within 6" of the deck to keep the stern from dragging.

    The load as a sailboat was not very much - 350# + the pressure from the sails at about 13 to 14 ft height.

    The other thing was that the bows are quite tall as you can see, so a more normal attitude would have lots of bow in the water, not a typical power boat attitude.

    Marc
     
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