porpoising hull

Discussion in 'Boat Design' started by 640 blazer, Jan 28, 2009.

  1. 640 blazer
    Joined: Oct 2008
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    Location: New Zealand

    640 blazer Junior Member

    If a 21ft planning hull is balanced corectly and the motor trimed correctly but has a tendancy to porpoise in any given situation (if thats how you spell it )whats going on with the hull design , is it concave or convex and would trim tabs help ?
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    How do you know your 21'er is trimmed properly? 90% of the time, small boat trim (engine and weight distribution) will sort out proposing. If it's a manufactured boat, it's very likely not a design issue. If it's a home made boat, then Lord knows what you've got going on.
  3. 640 blazer
    Joined: Oct 2008
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    640 blazer Junior Member

    Lordy knows its a manufactured hull ! it runs fine I was just interested to know what the reason for a senario like this would be . My boat hates being tail heavy but when its wt and ballance is the best for planning it seems nose heavy when just bobbing around in a semi rough sea .
  4. Guest625101138

    Guest625101138 Previous Member

  5. PortTacker
    Joined: Nov 2008
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    Location: Oregon USA

    PortTacker Junior Member

    Look closely at the bottom of the hull, especially the aft third.
    Is there concavity or convexity? If there is either, it will probably porpoise.

    Also, could you be trying to outperform its design? (over powering?)

    We put a 55hp jet (from a PWC) in one of those 8' mini boats once - it wouldn't go any faster, it just porpoised horribly. We added a trimmable nozzle which helped a little - ended up with huge trim tabs, stopped the porpoising but netted very little extra speed because of the drag. It was simply not a design that could support the speed, and it preferred the weight of the motor off the transom, not inside the stern. (We got to about 30mph.)

    The shape/design of the hull makes a difference. Lots of variables.
  6. Guest625101138

    Guest625101138 Previous Member

    Attached shows what Savitsky predicts for my guesstimate of data matching your mini boat.

    Rick W

    Attached Files:

  7. Nick F
    Joined: Aug 2008
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    Location: Auckland

    Nick F Junior Member

    Thats a great site Rick

    This may be a silly question but how do you read the out put on the graph? how dose one tell if porposing is going to be a problem?

  8. Guest625101138

    Guest625101138 Previous Member

    The red line gives the lift relationship and trim angle for the particular hull being examined. If this line is above the limit line for the deadrise of the hull shown in the various green lines then the hull will have propensity to porpoise.

    There are two common solutions to porpoising. One is to shift weight forward. This increases the lcg. You can alter the lcg and see what it does. With a small boat the pilot and any passengers weight will have a large impact on the location of the lcg.

    The other is to fit trim tabs. This reduces the trim angle and you will see from the plot this is stabilising as you get the same lift at lower trim angles.

    With outboards and some jets you can alter the thrust line as well and this can be used to reduce tendency to porpoise but Savitsky indicates it has little affect.

    Rick W
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