Jet boat hull chine/strakes questions

Discussion in 'Boat Design' started by YTPaul, Apr 5, 2015.

  1. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Hi Kevin
    1) re the existing proprietary extrusion shown on the OP Picts
    There was a book that I read maybe 20 years ago that showed this extrusion and there was a reference made within this book that the idea of the extrusion was to give the manufacturer some room for error when fitting up the side of the boat to the bottom.
    It might have been the Pollard/Colvin Mrtal Boat Building book
    Most jet boat builders in the Pacific NW take the time to fit this joint up tightly and do not use an extrusion in this area. I believe from some of your other posts you do the same.
    We run our side down past the bottom of the hull, about 1 3/4 prox,weld inside and out continuously, the add in a 1/4 inch chine flat to give us the 12 degree down angle of the chine flat. We end up with a sharp edge after grinding
    2) due to the nature of gel coat and fibreglass, any manufacturer of glass boats with chine flats and lift strikes have a small radius at the out side edge of these pieces with little apparent deficiency in performance.
    3) as we progressed from 6 foot chine widths to 7 foot chine widths and no lift strikes, to narrow to wider lift strakes and from horizontal angles to downturned angles, we always were rewarded with better steer ability and performance.
    There is a point as the boat speeds up where the boat becomes significantly more responsive, ie small movements in the wheel gives you a quicker response in the tracking.
    While this is qualitative, it was a tangible and recognizable point.
    Additionally, our fuel consumption decreased from 3 mpg to 4 mpg at 30 mph
    4) while Adhoc makes reference to contributors who take a feel of a boats performance, qualitative, and misinterprets the actual "hydro dynamic" forces in play, I am not one of these.
    The same "qualitative" comparison could be made between driving a car with 4 summer tires compared to 4 studded winter tires on your Alaskan ice roads. You feel the difference, but could not quantify that the studded set up is 11% better than the summer set
    As part of our curriculum many years ago we were involved in designing vane shapes and stator profiles, with multiple stages to optimize flow, thrust and power requirements in Fluid Mechanics for compressible and incompressible fluids.

    Fairing the inlet edge or the start of the lift strake/chine flat minimizes turbulence that would be caused by a square edge/stagnation point of an unfaired edge
  2. YTPaul
    Joined: Apr 2015
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    Location: Yukon

    YTPaul Junior Member

    Great information. Thanks very much for all the input.
    One handling trait I am trying to get rid of or minimize is when I turn hard, it still wants to grab or drop off a bit. I am assuming this is due to having no lift off the chine. When I cut the old strakes off, my hull speed did increase.
    As this boat has a delta pad, I am assuming that while in step, it's using the delta pad as the lift point? If I weld the angle over the chine, would I be better off keeping the bottom leg flat with respect to the hull (12 deg. dead rise) or as shown? I'm reading that I want to keep the turbulence down, so I am guessing a flat bottom on the angle Al cap would be preferable. Would larger angle than shown, which is 1", be preferable?

  3. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    Basically yes. As sharp as possible, for a clean flow separation. There has been much research in previous decades on the where is the cut-off between sharp v a small rad. I don't have the info to hand right now (im still out of the office). But bottom line is, if you can obtain a nice hard sharp corner, why buy one with a small rad? Thus I tend to ignore the papers/research that show a 0.00000001% increase in speed by changing from Rad A to Rad B, when the effects are minor and only measurable with decimal places on a computer program.
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