Power catamaran's chine design

Discussion in 'Boat Design' started by Kastally80, Mar 12, 2021.

  1. Kastally80
    Joined: Mar 2007
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    Kastally80 Chargui Yacht Design

    Hi guys,
    I didn't find any thread or research publication about flat chine position and shape design on a semi-planing to planing power catamaran (speed between 20 and 30 kn, Fn between1 to 1.4).
    what is the effect of flat chine immersion or position on the hull's efficiency.
    Here are three configurations of chine position.
    I think that Shapes A and B are more efficient for planing regime but will create more drag at lower speeds. Shape C will have softer ride and will be more efficient at semi-planing regime.
    Thanks in advance
    Hull shapes.jpg
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Shape A will have more lift at speed, and be less inclined to porpoising, the "flats" on the other two won't really be in play at speed. Most designs in use would be a variant of A, I would think.
     
  3. messabout
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    messabout Senior Member

    All of those sections show a lot of deadrise. If operating cost is of interest then you will do better with less deadrise. On the other hand if ride quality in a seaway is the primary objective then the sections as drawn are more appropriate.

    I agree with Mr. E. Section A will be more efficient for planing. The overall weight of the boat will also play into the section decision making process and the dimensions of the section elements.
     
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  4. Mr Efficiency
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    Mr Efficiency Senior Member

    That amount of vee lends itself to a "delta" pad, which widens going toward the stern, less wetted area, little change in buoyancy, and dynamic lift aft probably more than before, the lesser angle of attack compensated by the flat vs the deep vee. And draft is reduced.
     
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  5. Walid
    Joined: Aug 2020
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    Walid Junior Member

    the position of the flats is determined in function of the deadrise angle,taking in considiration that the chine must be under the waterline to produce an additional lift"shape_A"
    shape_B:it will work for the three modes,providing some lift at low speed and waterflow control at high speed.
    shape_C:above the WL,the flats will be a Spray rails I guess

    just an oppinion
     
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  6. fallguy
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    fallguy Senior Member

    Is there another option, though?
     
  7. DCockey
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    DCockey Senior Member

    At higher speeds with B and C sheets of spray will separate at the chine, travel up and out and miss the flat. The spray will be blown back and may hit the upper portion of the hull or deck. If the flat is intended to control spray then it needs to be located at or slightly above the chine.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    What do you have in mind ? All sorts of "funny" shapes are possible with the demi-hulls of cats, because they not an independent hull, and don't have to take into account factors that would render a single hull based on a similar shape, impractical, as for instance the deep-vee shape here of 35-40 degrees would, on a monohull.
     
  9. fallguy
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    fallguy Senior Member

    Less vee, actually.

    The comparison above assumes the vee is the ideal to discuss chine placements, but to be fair, all chine and no vee is worthy of consideration on the furthest end of the logic. Think version AA.
     
  10. Alexander Peter Bromley
    Joined: Apr 2020
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    Alexander Peter Bromley Junior Member

    They built a boat here in Cape Town with shape "B" chine configuration, characteristics were the following:

    Into the sea, boat was one of the softest riding boats I have been on to date, mono's and cats combined, very wet anywhere but the in the cabin and you would be soaked even in a mild sea state.

    At high speed and flat water the boat would chine walk

    Following sea see the boat would broach quite a bit and struggle to stay on a course with autopilot working overtime.

    Beam and quartering seas the boat would still tend to broach and roll quite a bit.

    Bearing in mind this particular boat was intended on the fully planning cat regime, and it is an asymmetric hull. For semi planning with the same chine configuration you might have a different result.

    upload_2021-3-13_11-44-1.png
     
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  11. Mr Efficiency
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    Mr Efficiency Senior Member

    With slender hulled planing cats, the deadrise angle probably makes less difference than people imagine, the width of the bottom is a bigger determinant of lift, and in a lot of cases it is undesirable to have a very deep vee on outboard powered cats, as even a 25" leg engine is in danger of drowning.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    As you might have seen here, a dead flat demi-hull ( at the transom) can work, but naturally it doesn't extend forward, or it will result in slamming.
     
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  13. Mr Efficiency
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    Mr Efficiency Senior Member

    This is an early 23' Shark Cat from around 1974, when there were no V6 outboards, or extra long legs, note the demi-hulls are almost dead flat right aft.
    Sharkie.jpg
     
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