Semi-displacement hull, HP at displacement/hull speeds

Discussion in 'Boat Design' started by sean27, Nov 10, 2011.

  1. Easy Rider
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    Easy Rider Senior Member

    I had no idea you were going to drive the boat w a 10hp OB all the time. I thought you were going to have a 60hp engine (or so). You need another boat. And the Scooter will be too light in the stern to be safe w only a 10hp engine. Should broach easily unless you put a great deal of fixed weight aft like batteries. Go to Atkin Boat Plans and look at the many designs that are not planing hulls that would be excellent for what I think you want to do. See the boat "Little Silver". It's actually designed for 10hp ....a 10hp inboard however. But I think many of Atkins inboard designs are suitable for OB power especially w small engines. But build it as designed w a 10hp inboard but I'll bet the OB would be easier to live with. Or look at Ketewomoke. With 10 or 20hp it should go 7 or possibly over 8 knots. You'd have to design and build your own cabin on her but I'll bet if tou kept the weight and windage low she'd be an extremely seaworthy boat. Much much better for you than Scooter.
  2. DCockey
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    DCockey Senior Member

    So you don't have confidence in the designer's assessment reported in post #8?
  3. Easy Rider
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    Easy Rider Senior Member

    No David I do'nt. I can't believe he recommended using the Scooter w a 10hp engine and 150lbs less on the stern that is wide and obviously designed for planing speeds and much bigger engines. I know ...I am just an amateur and he is a very successful designer but the obvious is obvious me but if you'd like to show me I'm wrong ...I stand waiting. If the boat was already built I would place much more credit on his answer and especially if it came w instructions about maintaining proper trim with ballast. I'm actually hoping there's more to it but with what we were given I stand by my post. But the big issue is that the Scooter is not suited for full displacement work. It's the wrong boat.
  4. eyschulman
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    eyschulman Senior Member

    Since you are dealing with OB power here I would try different size motors. You could probably get your hands on some motors for a day or two especialy the smaller ones. There is theory and then there is actual feild testing.
  5. sean27
    Joined: Oct 2011
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    sean27 Junior Member

    The V-berth could fit one 6' person and one smaller person. For a boat of this size with pilothouse, the Vberth is actually a pretty good size. The boat is just small.

    I can modify the design so that the Vberth can have a temp. extension on one side into the pilothouse to make it work.

    I believe the Redwings are flat bottomed boats, which is something I don't want.

    I'll look into the launch cruiser 24, but that is starting to get to be more boat than I wanted to build. I am trying to keep it 22' or less.

  6. Joakim
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    Joakim Senior Member

    I think the problem is your rather high speed requirement with OB. 6-6.5 is at or even a bit over hull speed and it will require quite much thrust, which is not good for the small and high rpm OB propeller. The propulsion efficiency will be low for both semi-displacement and displacement hulls at that speed.

    I don't think you will reach 6.5 knots with 10 hp OB, but certainly you will reach 6.0 knots. I had a similar size and displacement sailboat with 4 hp OB (Mariner two stroke sail version). The cruising speed was 5-5.2 knots with about 1 l/h consumption. Top speed was 5.6 knots with 2.2 l/h consumption and a lot of noise and vibration.

    If you are fine with about 5.5 knots cruising speed 6-10 hp is probably OK. If you want to cruise over 6 knots, you'll need more power.
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    FAST FRED Senior Member

    For "pure" displacement (low fuel bill) cruising most is done at a SL of between .9 and 1.15.

    A SL of 1 is simply the sq rt of the lwl.

    The advice to slow your desired cruising speed is good!

    2hp per ton (2240lbs =ton ) is an OK rule of thumb.

    Gas engines operate very well at part throttle , so if you need 10 hp a 15 pr 20 would be just fine. The 4 strokes in 20 hp are heavier than the 2 strokes but the fuel burn is usually better.

    A 10 hp diesel run at WOT would be lowest in fuel burn , but I don't know of an engine source.
  8. Willallison
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    Willallison Senior Member

    In general terms, I'd tend to back Easy here... semi-displacement and planing hulls tend to have big, fat, flat aft sections that make them prone to broaching in following seas. They also tend to have relatively deep sections forward to smooth the ride and small keels to reduce wetted surface, both of which exacerbate the problem. Now I'm not familiar with the OP's area of operations, so this may or may not be a concern... again another reason for a well written SOR.
    Again, none of this is a criticism of the designer - it's just an inherent aspect of the shape that must be considered in the design choice process.

    I too would suggest that fitting a bigger o/b would make sense. Unlike the bad old days where big engines used to throw 1/2 their fuel out the exhaust at lower revs, modern direct injection 2-strokes and 4-strokes are remarkably economical throughout the range. Undersized engines that are under heavy load will more often than not use more fuel than a lightly loaded bigger one.
    If the Surf Scooter is your vessel of choice, then fit the engine that it was designed for: you may only use the speed it is capable of on the odd occaision, but you will better off on IMHO
  9. eyschulman
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    eyschulman Senior Member

    If you become a member of you can use their prop calculator and power calulator service. You just plug in the info about boat and different motors and you get lots of good approximations which you can modify and play with.

  10. MCDenny
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    MCDenny Junior Member

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