jet power on warped bottom hull?

Discussion in 'Jet Drives' started by nbehlman, Oct 18, 2011.

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  1. nbehlman
    Joined: Aug 2011
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    Location: CT

    nbehlman Junior Member

    I am planning to build a 14 foot runabout in the style of the early 30's and 40's chris crafts. The power plant is a 70hp TigerShark jet ski motor and pump. I am designing the hull from scratch and I wanted to see what the experts on this forum have to say. If you have any comments or advice on what types of design features I should incorporate into my hull, please share them! Here is some basic info:

    http://nbehlman.com/hullprofile.png
    length: 14ft
    beam: 4.8ft
    weight: 650lb (jet ski weighs 476lb)
    deadrise: 6.5 deg
    draft: 6.5 inches (one person aboard)

    Are there any special considerations I should take into account when designing a hull for a jet drive? These hulls are meant to take an inboard prop. I have noticed that jet skis have a much more rounded hull, while my boat will have sharper chines and a low deadrise vee bottom.

    I have looked around on the internet for similar projects and there seem to be quite a few. Most of them are simple "what happens if I do this?" type projects. I came across one site that shows a very strange profile below the waterline:

    http://www.sponbergyachtdesign.com/Designs/IC20 Profile single small.jpg

    It looks to me like it would create massive amounts of pressure drag because of the keel line near the intake.

    I found a video of a chris craft converted to a jet drive:

    http://www.youtube.com/watch?v=uBUUeNfGjTg

    At the 10 second mark, you see it go into a drift turn characteristic of jet boats. I am concerned that these kinds of turns could cause the hull to "trip" over the nearly square chines.

    I expect that my hull will provide a great deal more lift than the jet ski hull, but will of course weigh more. I have done quite a bit of work with the Savitzky power prediction method, but I suspect it is not very accurate for jet boats since the kind of suction onto the water. My target speed is 30 mph. The jet ski currently does 40 mph.
     
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