Lengthening front lifting strakes.

Discussion in 'Boat Design' started by ICBlowboaters, Oct 13, 2016.

  1. ICBlowboaters
    Joined: May 2014
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    ICBlowboaters Junior Member

    I have recently had a hook removed from the rear of the boat and at the same time we shortened up the rear lifting strakes for more speed.
    The issue that popped up was a more pronounced porpoise at cruise and a hop at full speed.
    I think what happened is the hook was keeping to bow down in the water (water used to break forward of the cockpit) giving a better ride but scrubbing speed. Now the bow is trying to stay up but can`t. It`s a heavy flat bottom boat but i do have teh drives on boxes.

    My idea ( I could be completely wrong) is to extend the strakes closer to the keel a little further up hoping to stop the porpoise and keep the bow up and possibly gain back a little transom lift that was lost by shortening teh rear strakes.
    The fiberglass guy recommends trying different props but with cleaver props that is an expensive undertaking.

    My question is would making the strakes longer help??

    Any thoughts would be appreciated.


    Rear strake shortening;

    [​IMG]


    the same bottom as mine:

    [​IMG]


    At the end of this vid you can kinda see how the water hits the end of the strakes ant it falls.

    https://www.youtube.com/watch?v=DkFZo9x-vWg&feature=youtu.be
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    I would say restoring the outer strakes to their former state will reduce porpoising, but your tabs would also do that. I think it would be a brave(?) man who would tinker with the underwater shape of such a well proven, rigorously tested classic. If it has somehow gone out of shape, it arouses suspicions of structural issues, the original shape I would leave well alone.
     
  3. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    Try putting a bit of weight in the bow to see if that eliminates the porpoising. Personally I don't recall seeing any strakes left 'cut off' shy of the transom unless properly faired back into the hull. The arrangement you have looks like it would produce aeration/separation at the strake short of the transom, which I doubt is good. Like Mr E says the tabs should let you restore the trim effectively producing a changeable hook. I'm sure PAR will give you some good advice on this particular hull. Have you changed engines? (or prior owner?), or increased the weight balance rearwards?

    I don't notice any hook on the second photo, but realise it can be very subtle. Also the second shot shows the keel side short forward strake well faired in very far forward, very different to your arrangement.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I thought those Cigarette hulls had a little inbuilt rocker.
     
  5. ICBlowboaters
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    ICBlowboaters Junior Member

    Originally the boat came with some TRS drives and I`m sure much lighter motors. Now there`s blower motors, SSM drives on boxes and the gas tanks are shorter moving the weight rearwards. Weight in the front does not seem to make much difference.

    I don`t believe they ever had a built in rocker.
     
  6. ICBlowboaters
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    ICBlowboaters Junior Member

    [​IMG]

    This much material was removed and the boat does faster and you`are correct the tabs do add transom lift. Running the drives in a bit more also gets rid of the porpoise.
    I was hoping to run the drives closer to neutral along with the drives but that just may not be possible till I get over 65mph and she settles down.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Truncating that outer strake can be safely categorized as not such a great idea, if the intent was to calm porpoising.
     
  8. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Indeed. Bad trim is the general culprit of porpoising. So if you can reduce the general running trim, that's help matters.
     

  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Your particular boat possibly not, others did, the main idea being to lessen the chances of "stuffing" the boat, but also probably gives an increase in speed as well.
     
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