36 foot, $300,000 boat porpoises to no end

Discussion in 'Boat Design' started by gofastguy, Mar 26, 2019.

  1. Angélique
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    Angélique aka Angel (only by name)

    The B&B custom Rough Point 37 doesn't have a pod on the B&B web page, but in the below hull turning video there turns out to be a pod, which seems to be of the same type as in post #29, if it's the same pod type, then they're quite consistent about it over the years. Here's the build on Facebook.

     
    Last edited: Mar 29, 2019
  2. rwatson
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    Location: Tasmania,Australia

    rwatson Senior Member

    Good pictures.
    Those show a clear water stream hitting the props.

    I put a few diagrams together for comparison. To my eye, you have gotten excessive upwards angle on the pod chines creating "downsuck" at high speed, from the hydro flow, creating the porpoising.

    That flat spot is still unusual, but I can't "picture it" causing major concerns.

    I won't volunteer tank testing to validate my hypothesis, but you could just build some "cheekpads" and glue them to the existing chines, and see if that helps.
    You may not even need to move the motors.

    PorpBoat08.png

    PorpBoat06.png
     
  3. rwatson
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    rwatson Senior Member

    Lots of scope for "infill" on the cheeks.
    PorpBoat09.png
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I really think this is an example of heavy boat, heavy engines, and crucially, narrow for a 36 footer. ( any news about the chine width ?) People have been driven to near insanity by porpoising podded boats, and they usually have those attributes. I think bigger would be better with the tabs, small tabs being over-applied cause a lot of drag.
     
  5. rwatson
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    rwatson Senior Member

    The only reason they have a "reverse slope", is that they have deliver water to the prop, which is higher than the keel line.
    The ideal is a continuous flat keel line, with an abrupt edge.
    Pods are a design kludge.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    They don't have to have a reverse slope, some are just like a step, but as I mentioned, a good way to get a porpoising oscillation going. I don't like pods, I'd rather have more boat length in the water, for the same overall length. But one can see they do have legitimate uses, if the LCG is not going to be radically altered, and that can be the case in sterndrive to outboard conversions. But a lot has changed since speed demons dreamt up the idea.
     
  7. rwatson
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    rwatson Senior Member

    The pod chines are almost parallel to the boat chines

    PorpBoat10.png
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    A bit hard to tell off the photo, but isn't much slope to it.
     
  9. Angélique
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    Angélique aka Angel (only by name)

    Yes on a second look the B&B Rough Point 37 pod chines seem to have only a very little upward slope compared to the hull chines, the pod's V bottom seems to be even parallel to the hull's V bottom.

    B and B Yacht Designs Rough Point Boatworks Rough Point 37 hull turning pod view upside down.jpg

    B and B Yacht Designs Rough Point Boatworks Rough Point 37 hull turning pod view right side up.jpg

    P.S.

    I've updated the last picture with a screen shot of about ½ a second earlier, now like the pod's chines, the pod's V bottom seems to have just a little upward slope.
     
    Last edited: Mar 28, 2019
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    If the step is sufficiently high, it won't be a problem, to have that configuration, and looking at the picture, it is hard to imagine much contact occurring. But a few inches lower, you might get a bucking bronco.
     
  11. Angélique
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    Angélique aka Angel (only by name)

    Below a 2013 picture of some huge anti Squat Boards (aka Hobbles) on an ancient motorised Shad Boat on the parking lot of the Manteo, NC, USA, Roanoke Island Maritime Museum in the Albemarle Sound.

    [​IMG]
    (large)

    [​IMG]
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Yeah, not sure how effective that would have been.
     
  13. Angélique
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    Angélique aka Angel (only by name)

    I suppose it helped against squat with the very little engine power of the time.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Must have been working well enough if they warranted a name, but "hobbles", what goes better with hobbles on ? :rolleyes:
     

  15. fallguy
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    fallguy Senior Member

    If he fails to vent in a hard turn, I'd go up.

    Still has to reprop for that 30 hole shot....abysmal.
     
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