Efficiency: Hull interference vs length on catamarans

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Jun 3, 2021.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Correct.

    Unless the boat is ostensibly siting on top of the water at speed... i.e planning, it will always be semi -"something". In this case semi-displacement.

    But dig deeper and you'll open up a major can of worms of semi-arguments that go on and on... :D
     
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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    What you described with the round hull sections will end up like a sailboat banana shape, a further invitation to excessive pitching, you go find a displacement motor cat that looks like that, very hard to find
     
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  3. dustman
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    dustman Senior Member

    I may not have a nautical engineering degree, but achieving high speed in full displacement mode simply requires a smooth transition for the water, gently parted, gently put back together. This means long, thin hulls. I don't know why this myth of having to plane to achieve greater speeds than "hull speed" is so persistent. With a hull long and thin enough wave making is practically nonexistent and "hull speed" becomes practically irrelevant, what is relevant is friction. The only speed limitation is really power application. Of course this is assuming proper hull shape and calm water.
     
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  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Planing isn't part of the equation here, my concern would be a hobby-horsing boat on a heading you have to take, which could become very wearing
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Correct on all counts :cool:
     
  6. dustman
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    dustman Senior Member

    It will have very little rocker, not banana-like at all. It's hard to describe.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    OK that sounds better, but assymmetry between the ends is desirable
     
  8. dustman
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    dustman Senior Member

    The hull I would create would hobby horse very little since it would have little rocker, plenty of buoyancy towards the ends, the majority of the weight will be in the middle third of the boat relative to it's longitudinal center of buoyancy. Additionally, the hull bottoms would be round for the most part which I imagine is fairly resistant to plunging, unlike a deep v, like a Wharram(even though I think he uses this to his advantage). I am however concerned about pitchpoling with my design.
     
  9. Dejay
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    Dejay Senior Newbie

    Ok so looking at the 1.8m long "Wigley" hull catamaran series in prelimina (which matches experimental data until Fn=1) you clearly see this hump. And you also see that hull separation between 0.3 and 0.5 width/length ratio matters relatively little:

    prelimina wiggley catamaran test.png

    These are hulls that don't have rocker so shouldn't get "sucked down". They have a canoe stern that should "regenerate" some of the power expended by the water when it's "rushing back together aft" and creating force forward. Still you get the hump.
    EDIT: Never mind, even without rocker they do have a hull shape aft that bends upward and gets "sucked down".
     
  10. dustman
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    dustman Senior Member

    Talk about asymmetry in relation to hobby horsing was one reason I started exploring a submerged transom. What would happen if I just chopped the end off a symmetric hull, a little bit, a lot...
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Pitch-poling is a sail cat problem, but I like the sound of your hull better now, just think 20:1 is too much
     
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  12. dustman
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    dustman Senior Member

    Yes, you have to have some upsweep if your hull bottom isn't perfectly flat. Minimizing upsweep seems like a good idea, especially at higher speeds.

    The vertical axis on this graph is resistance, and the horizontal is froude number(speed), and the lines each represent aspect ratios?
     
  13. dustman
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    dustman Senior Member

    I will definitely take your concern into consideration, put more thought into it and do more research.
     
  14. Dejay
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    Dejay Senior Newbie

    The vertical lines represent the froude numbers. The speed range I ran prelimina ran from 0 to 10 m/s (= 19.4 knots or Fn 2.38) in 21 steps.

    EDIT: Ah, yes, the 3 lines represent different ratios of BOW / LWL
     
    Last edited: Jun 4, 2021

  15. Dejay
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    Dejay Senior Newbie

    Not sure how relevant for or how accurate this is, but I done the same for my idea of a simple to build "harry proa" style hull catamaran. It's 15m with 1m wide hulls and 4t displacement. The hulls are box shape with no flare and no rocker so there should be no suckage. This is also between 0 and 10 m/s.

    The hull separation hardly matters here either between 4, 6 and 7m (BOA/LWL ratio of 0.26, 0.4, 0.46) except for specific points.

    That there is no hump could either be explained by there not being any vertical surfaces that pull the stern down, or that this hull is inefficient before the hump.

    prelimina 15m catamaran hull shape.jpg prelimina 15m catamaran test.png
     
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