Keel rocker aft for LDL powerboat

Discussion in 'Boat Design' started by huibes, Nov 11, 2020.

  1. huibes
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    huibes Sittin' on the dock of the bay

    Hi all,

    Not so long ago, I ran into this interesting article:

    Ross Lillistone Wooden Boats: Flint and Fleet - Different Hulls for Different Jobs http://rosslillistonewoodenboat.blogspot.com/2019/02/flint-and-fleet-different-hulls-for.html?m=1

    It's a nice real life comparison of 2 similar boat that differ in aft keel rocker.

    Now, I'm trying to design a 7,5 by 2 meter lightweight (D/L=80) inland power launch for my dad, an (ex)sailor. We want her to run 8 knots with minimal power and wake, and about 12 knots max (S/L=2.5). I'm trying to figure out the ideal keel rocker aft and if the aft buttocks/diagonals need to have any curvature at all.

    I assume that the flat buttocks with immersed transom of full planing is not efficient around 8 knots. And Flints shape will not like 12 knots.

    My guess is it should be shaped somewhere between Fleet and Flint, could that be right?

    Most studies and calculations are for either displacement or full planing though...

    Does anybody have any suggestions about how the find the ideal aft keel rocker for this application? Any suggested literature is welcome too!

    I'm also not sure if I'm going to let the BWL decrease significantly from the midsection aft. Same story really...

    Hope anyone can help!

    Many thanks,
    Huib
     
  2. jehardiman
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    jehardiman Senior Member

    It will depend on how much deadrise you want to have. Less deadrise, flatter run.
     
  3. huibes
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    huibes Sittin' on the dock of the bay

    Almost flat at the transom, pls see image
     

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  4. jehardiman
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    jehardiman Senior Member

    How "inland" is inland? Take a look at the Thames slipper launches for how flat a hull designed for low wash can be.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    What type and size power do you have in mind, in other words, quantify "minimal".
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    The old Atkins boats were in some instances suitable for the semi-planing speed ranges, but designed mainly for inboard engines. There may be one or two for outboards.
     
  7. Tad
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    Tad Boat Designer

  8. huibes
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    huibes Sittin' on the dock of the bay

    Jehardiman, with inland I'd aim for the CE-D guidelines.

    The slipper launches were great, I especially like the way the stern wake separates the transom at pretty low speeds already, while the bow is still in displacement mode.

    But due to their weight they still pull quite a wake.
     
  9. huibes
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    huibes Sittin' on the dock of the bay

    Mr Efficiency,

    We want to install a Yamaha F25, only weighing 62 kg.

    With 2 people on board it would be ideal to reach 8 knots at 1/3 throttle. Something like that?
     
  10. huibes
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    huibes Sittin' on the dock of the bay

    Yes, the Atkins boats are inspirational. And indeed, the nicest shapes are inboard. Even with a prop tunnel, which is a bit too extreme for me.

    The sgt Faunce setup would be nice for electric inboard. I've tried to draw it, but get confused how the diagonals would flow at the area where the aft part of the box keel meets the planing area...
     
  11. huibes
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    huibes Sittin' on the dock of the bay

    Wow Tad, that's great info!

    I've seen your article before, but didn't really have enough background back then to understand this graph.

    Unfortunately the resolution is low. Looks like the top curves are for S/L=3 and 2,2. Can you please state what the others are?

    Many thanks!
     
  12. kerosene
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    kerosene Senior Member

  13. huibes
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    huibes Sittin' on the dock of the bay

    Nigels boats are truly amazing. They run level and seem to have a lineair power to speed curve.

    But I want to build from developed surface aluminium. Perhaps sacrificing some efficiency.

    So I'm aiming at minimally submerged transom, however preventing too much keel rocker aft, to run level.

    But how to determine the sweet spot....?
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    You are going to have to build very light to get 12 knots out of a 25hp outboard, in your specified length. I doubt it is possible.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    If 12 knots is the maximum cruise speed you want, I would say the high-thrust variant outboard is mandatory. Your common 25hp motor is really optimised for speeds higher than that, on smaller boats/
     
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