Designing a 360cm(11ft) powerboat hull, suggestions?

Discussion in 'Hydrodynamics and Aerodynamics' started by Glattnos, Jul 14, 2018.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    The disadvantage of a cathedral hull is a bumpy ride, the advantages are easy planing and good stability. It is really the ride that has seen them almost vanish.
  2. fallguy
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    fallguy Senior Member

    B9289248-EBF4-47EF-A10F-D46FB1DEB542.jpeg Boston Whaler 13' Sport is the most famous cathedral hull in that size.

    Mine needs paint.
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Efforts to reduce the crash, bang, wallop, have caused the tri-hull to morph into a vee hull with vestigial traces of the old form. But in some cases, a good ride has been obtained while preserving much of the old appearance. bp4825421643162271148.jpg
  4. Yellowjacket
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    Yellowjacket Senior Member

    The 13 foot whaler has significant hook in the bottom near the transom. The Whaler was designed in the mid 50's when the biggest production outboard was the 40 hp Mercury. More power and trying to push a whaler in to the mid 40's mph range and the hull gets unstable, but for low power and moderate speeds it works well.... The hook makes it possible to plane and perform well with very small motors, even as low as 15 hp. The cathedral hull has nothing to do with why the Whaler has good low power performance, it's the hook that does all of that. You can even see the hook the bottom in the pic posted above. If you're only going to have about 10 hp, then some means of adding hook effectively, like planing flaps at the transom might be a good idea, since it lets you plane heavier loads, and if you don't have a heavy load in the boat you can pull the flaps up and get more speed.
    fallguy likes this.
  5. Will Fraser
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    Will Fraser Senior Member

    The boat in the video belongs to a friend, I just used it to try out my 5hp for the first time. That is unfortunately the extent of my experience on it. Still got to build my own hull for the little motor.
    The friend also uses a 9.8hp and often goes fishing two-up (i.e. total displacement around 300kg). Careful weight distribution at such high loads apparently helps retain reasonable performance.

    Jurgen Sass has developed some powerboats with a preference for lower deadrise but reduced beam and increased length. Tests show better ride quality than even some shorter hulls with deeper V's. He also employs the use of interceptors which add similar advantages as an aft hook.

    If you have not done so already, have a look at Ross Lillistone's Fleet for some idea of what is possible with a fine entry and low horsepower.
  6. HJS
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    HJS Member

    Remember to calculate the g-number, how hard the boat goes in waves. Bottom width is much more crucial than deadrise.
    Why limit the length of the boat? With 9.8 horsepower, it is possible to reach a much higher speed while maintaining seaworthiness. Just these engines usually give more than what is written on them.

    0175 Rosen persp 4.jpg

    Attached Files:

  7. Mr Efficiency
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    Mr Efficiency Senior Member

    The specification for self-righting makes it a tough assignment. :eek:
  8. Glattnos
    Joined: Jul 2018
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    Glattnos Junior Member

    Thanks for all inspiration, there are obviously many low powered designs out there :)
    My design is also going forward and I have soon got everything in the correct place. The self-righting ability is saved even with 1,1 m width on the planing part of the hull. Length is increased a bit to 372cm.
    I have decided to first build the lower part of the hull and give it some test-drives and then make temporary modifications to it if I think it can give higher performance or better handling.
    I will have a build-thread on Swedish on a Swedish forum and will post a link here when I start the build :)

  9. fishwics
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    fishwics Quiet member

    FWIW Atlantic College (the place that invented the RIB) built a series of mini-RIBs in the late 1960s/early 1970s.

    The rigid hulls were about 10 ft (3.0m) long, and designed to fit under an Avon production tube (so a fairly blunt bow). At the transom they were 4 ft (1.2m) beam, truncated deep vee section, about 9 inches (240mm) deep (IIRC), and the flat on the bottom was about 16 inches (400mm) wide, and carried forward to the bow. Made of fir plywood, and easy lift (without engine) for two teenagers.

    Loaded with one person, and using a 10hp Mercury redband 2-stroke outboard, they'd happily keep up with an RNLI C-Class (which was supposed to be capable of 20 knots) on flat water.
    A similar (slightly larger) design is now being used by the Atlantic-Pacific rescue organisation.
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