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Thread: Advice on a PAD

  1. #11
    Team Member Smokin' Joe's Avatar
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    Not necessarily. If you stop and think you'll realize there must be a limit,
    because a pad V is faster than a flat bottom, all other things being equal.
    On the 13' Allison with OMC 75 an 8" pad was too wide, the boat ran deeper at the
    transom. With a 7" pad the boat gained speed and turning ability. The original pad width was
    5.5". The problem with an 8" pad is that it was too deep, that caused the extra drag that allowed
    the boat to sink at the transom. The pad was too deep because the Allison is built with just
    the right degree of deadrise to allow the boat to bank and turn well. Look at an old 14' Allison
    'with a 7" pad. You don't want the pad to be any deeper than that. Otherwise, you're dragging
    extra water.

  2. #12
    Team Member Smokin' Joe's Avatar
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    My 14.5' Allison weighs about 350 lb and runs about 42 mph with a 1983 Johnson 35,
    after a lot of triclky prop work. the pad width is 8". A wider pad would not be faster,
    the boat already runs too wet because 35 hp is not enough to get it on the pad. I
    would not widen the pad, I'd add lift strakes off the pad designed to make the flow more two
    dimensional. that would lift the boat at the pad.

    I think that there is no rule from hydrodynamics that tells us what is an optimal pad
    width. Were there such a rule then I would have been able to derive it mathematically.

    The Professor

  3. #13
    Team Member Smokin' Joe's Avatar
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    I'd hate to think of trying to straightedge a steel or val pad. Putting a pad
    on a flat bottom won't work, and it won't be of much use on a semi-V with small deadrise.

  4. #14
    AeroMarine Research Jimboat's Avatar
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    Every hull design/setup has an optimum pad design (width, length, depth) for the desired performance expectations. The tricky part of the design is that the characteristics change at each velocity throughout the full operating velocity range of the hull.

    So the design of the pad will always be a 'compromise' that performs "best" at one speed and condition. Never-the-less, the performance of any pad design can be predicted by analysis.

  5. #15
    Team Member Smokin' Joe's Avatar
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    Quote Originally Posted by Jimboat View Post
    Every hull design/setup has an optimum pad design (width, length, depth) for the desired performance expectations. The tricky part of the design is that the characteristics change at each velocity throughout the full operating velocity range of the hull. So the design of the pad will always be a 'compromise' that performs "best" at one speed. Never-the-less, the performance of any pad design can be predicted by analysis.
    Hi Jim, I could agree with you if there were a way to calculate that, but the problem is highly
    non-unique, is ill-posed. E,g., what pad width and depth would I need to run an 840 lb rig 65 mph with a
    Johnson 75? There are a lot of different answers to that question, in practice. If I add the constraint of
    wanting to turn and run rough water well then I may narrow the field (as I know from experience, but
    not from calculation), but there is still no single answer. Best, Joe

  6. #16
    AeroMarine Research Jimboat's Avatar
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    Quote Originally Posted by Smokin' Joe View Post
    E,g., what pad width and depth would I need to run an 840 lb rig 65 mph with a
    Johnson 75? If I add the constraint of wanting to turn and run rough water well then I may narrow the field, but there is still no single answer.
    Joe - true enough. As i suggested, nearly every hull design is a "compromise" of many performance desires/expectations. Given an existing vee hull design of weight/size/power - the best pad design is a compromise of the design for ultimate speed and the design for handling/turning. Acceleration could be a consideration too, which also would be part of the 'compromise'. Each of these concepts have a pad design that can be calculated and performance predicted. The designers job is to select the "compromise" design that best "balances" the needs for ultimate speed and handling/turning that will meet the overall intended uses of the hull.

    As you will know, the vee-pad design that does well overall on a closed course (with turning important) will likely be different than the vee-pad design that does well on a straight-line kilo course intended to set top speed mark.

    The result can be complicated further if there is a need for 'comfortable planing' at intermediate speeds; or stable speed control at, say, waterskiing speeds.

    It's all very 'calculatable', though. All the parameters can be considered, and there's a method to calculate all the loads, lifts, drags, stability and performance. But in the end it's still a compromise - and the compromise is a single design.

    Luckily, it can all be fun to do!

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