Low speed planing hull theory

Discussion in 'Boat Design' started by Doh, Jan 5, 2020.

  1. BrissoDamo
    Joined: Dec 2019
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    Location: Brisbane

    BrissoDamo Junior Member

    hook also helps low speed planning, but with increased powers bogs down the bow
    Doh likes this.
  2. DogCavalry
    Joined: Sep 2019
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    Location: Vancouver bc

    DogCavalry Senior Member

    Well, the SOR precludes more power.
  3. Yellowjacket
    Joined: May 2009
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    Location: Landlocked...

    Yellowjacket Senior Member

    When I was a kid I had a small 8 foot boat that was home built out of plywood. If you can imagine a miniature airboat with a motor board sticking up to hold an outboard this is what it was. It was 4 feet wide and made from 1/4 inch plywood with 1x2 stringers (one on each edge and two about 16 inches on center apart) and it had two intermediate frames made of 1x2's. The sides were exactly 1 foot high and it was cleverly designed to be made from exactly two sheets of 1/4 plywood and one piece of thicker plywood for the transom. We used this boat with everything from a 5 hp to a 18 hp Merc. The biggest addition to the boat was a pair of flaps at the transom that extended about 10 inches from the transom and other than a notch for the motor in the middle and were adjustable with turnbuckles so we could set the flaps at whatever angle we needed to get the boat to trim with the smallest motor and the highest loads. With small motors we'd get the boat to plane a lot easier by angling the flaps down a couple of inches. For big motors we ran the flaps level with the hull and if we put the flaps up higher we'd end up swimming with a big motor since the boat would flip if it got air under it. (that happened more than once). The one issue we had with heavier motors was we had to get weight forward to get the boat to plane off. This was hard to do with tiller steering and a twist throttle but if you used your bare foot on the tiller/throttle you could do it if you were careful.. A wider boat like this is harder to plane than a narrower hull, but it will plane a heavier load. We often had 3 kids (probably 300 pounds total) in that boat with a 5 hp and it would plane easily. It wasn't that fast, but it wasn't a dog with a 5 hp outboard.
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  4. alan craig
    Joined: Jul 2012
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    Location: s.e. england

    alan craig Senior Member

    The reply above copied from another thread sums up neatly.

    I have seen my son, when he was a boy weighing about 30kg/70lb planing a 9ft flat bottom round transom inflatable with a 3.3hp outboard. You would expect the water flowing off the bottom to run up the inflatable round transom but it didn't, maybe the glued joint between flat bottom and tube was enough to disrupt that "coanda" effect.
    Doh likes this.
  5. Doh
    Joined: Jan 2020
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    Location: Germany

    Doh Junior Member

    Lots of thanks for all your inputs! My c240 dinghy is currently drying (had to glue the floor back) and it takes time.. because ******* Germany is badass cold right now
    The electric system is currently reaching it's it's stage as soon as I have to finalise my propellerS. Yes "S", I want to test different pitch, different stators and diameters (also have a RC variable pitch hub which need to be adapted).
    Again, I am very greatful for your experiences and time. I know that lot of reply question the "meaning" of this. But again, it's something I want, I won't navigate on open waters, don't want to make a transatlantic and also don't want to slam any harbor or habited areas :) just a small project I work on, when my wife is getting to hard on me .
    Have a nice week and thank you all again!
    BrissoDamo likes this.
  6. HJS
    Joined: Nov 2008
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    Location: 59 45 51 N 019 02 15 E

    HJS Member

    Here comes a proposal for a 2.5 meter planing power boat.
    The calculations are based on a total weight of 150 kilos. The bottom width is 0.8 meters, neither more nor less. The efficiency of the driveline is assumed to be 65%.
    The aftship interceptor penetrates from 19 to 5 millimeters. Dry transom at 4.5 knots.

    Power Trim.jpg Mini planing powerboat pers.jpg
    Last edited: Jan 16, 2020
    fallguy likes this.
  7. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Is there a power callout?
  8. yades
    Joined: Aug 2006
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    Location: italy

    yades Senior Member

    If you like you could have a look at the pdf file of feb 2008 regarding steps on calculaing power and resistance for planning hull
    Doh likes this.
  9. yades
    Joined: Aug 2006
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    Location: italy

    yades Senior Member

    Doc which i have elaborated and inserted in my thread
  10. Doh
    Joined: Jan 2020
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    Location: Germany

    Doh Junior Member

    Thanks mate! I'm still alive. just bought a boat actual. a small etap 20 (sailing 6m) and waiting my dinghy to dry before put in on the water :)
    Would you be kind enough to link it for me? I searched your files but couldn't find it :)
  11. Heimfried
    Joined: Apr 2015
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    Location: Berlin, Germany

    Heimfried Senior Member

  12. Doh
    Joined: Jan 2020
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    Location: Germany

    Doh Junior Member

  13. Doh
    Joined: Jan 2020
    Posts: 16
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    Location: Germany

    Doh Junior Member

    My build is still ongoing :)
    In the mean time, I landed on this video on youtube:

    This is really impressive to me. The 6x "775" motors are in the best case sucking 200W/unit and DC motors are known to be really inefficient. I'm really impressed by the kind of speed he gets.
    I though some of you here would like it ;)
    Greetings and thanks again for all your help!
    clmanges likes this.

  14. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    That's pretty impressive video.
    Liquid cooling.
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