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

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

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    Then you have the problem of maybe not having enough ability to lower the engines. I think the first thing to do is scope out this issue of whether there is flow attachment.
     
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  2. Angélique
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    Angélique aka Angel (only by name)

    When the hobbles need to be that long then they may need slots for the outboard legs, but indeed best check the easiest things on the current pod first.

    P.S. - I've added an altered sketch to post #104, keeping the added plate within the current pod length.
     
    Last edited: Apr 6, 2019
  3. Angélique
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    Angélique aka Angel (only by name)


    [​IMG]
    Note the full hull width trim tab supported by gussets, just visible in the water aft of the underside of the transom, a bit below the water surface.

    Muskoka Seaflea forum thread: Mini Most XL Inaugural Test Run and Prop SelectionReply #14 = trim tab issue of an Mini Most XL, note the red note in the sketch, and the below quote !

    [​IMG]

    — September 02, 2013 —
    ‘‘ . . . .
    And Rob (aka Cousin XL) may get the final laugh. When he was helping me launch the boat yesterday he asked if I cut off the "built-in" trim tab called for in the original plans. If you recall, the original Mini Most plans call for the hull planking to extend beyond the transom about 3". In retrospect, that built-in trim tab would have allowed me to tilt the motor at a higher angle with no porpoising...I feel that if I tilt the motor up, (just like you have suggested) then the water splash over the transom will stop.

    This winter, I think I need to fabricate some aluminum trim tabs........this will hopefully allow me to angle up the motor, stop the water splash and prevent porpoising.
    . . . . ’’

    the same boat as in the first above pic, and the above quoted reply #14 I think, below a 2014 video.
    I believe this is the boat that originally was built without the part of the bottom that protrudes behind the transom as a kinda "trim tab", which according to the first above picture was restored later.

    the Mini Most XL of Rob (aka Cousin XL), himself mentioned in the above reply #14 quote, and his daughter in 2007.

    another Mini Most XL in 2012, note the underside of the outboard is trimmed so far forward that the bow hardly lifts from the water.
     
    Last edited: Apr 7, 2019
  4. gofastguy
    Joined: Mar 2019
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    gofastguy Junior Member

    Yes, totally agree! His sketch was rough and I would not have considered and upward sloping extension.

    Thank you!
     
  5. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Post #23 photos mean little because I don't understand the framing or context to them.
    But, you say the pod has no attached flow BUT there is no porpoising at 30 knots when the photo was taken.
    So, it is kind of useless.
    Do you have any photos when it IS porpoising?
    Get some good-angle GoPro and let us know.
    I'm starting to lose hope in this rescue...
     
    Last edited: Apr 8, 2019
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  6. Angélique
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    Angélique aka Angel (only by name)

    Post #33 ? - Perhaps you've meant the photos in post #23 ?
     
  7. Angélique
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    Angélique aka Angel (only by name)

    When any in line attachments to the original hull are made, such as a new hull extension and/or a full width added trim plate, then be sure all extensions are level and in line and fair en even to the original hull panels, so no "hook" is introduced in the transition to the extension. A "hook" in this regard is an unfairness under the bottom that hooks up the water flow.

    For some "hook" info in this regard see the thread: Dory in aluminiumpost #33, by the highly valued forum member Messabout.
     
    Last edited: Apr 7, 2019
  8. Angélique
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    Angélique aka Angel (only by name)

    Edit: Important note about this post, there are parts I revoke: see post #114 & #120.
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    I've just reread the article which was linked by Pafurijaz on the thread Dory in aluminium in post #28, the above quote from Messabout's post #33 is also about this article.

    From the article and the complains about the topic boat my impression is that at a certain speed the water flow attaches to the pod, which Mr Efficiency calls on this thread for checking all the time already.

    Now if the water flow attaches to the pod then at speed I think the pod starts acting as a giant "hook" in the sense of the linked article and Messabout's above quoted post, then at more speed I suspect the attached upward flow from the bottom starts violently pushing upwards the pod and pushing down the bow, and so in my view is causing the porpoising.

    Below a quote of the article, I've enlarged the part which I think is about the topic boat's problem.
    [​IMG]

    small pod hull step.jpg
    * That looks like a small step to me, the important thing I think is to have no step at all and have an even to the hull extension like on your Zodiac, or have a large step, so the water flow can never attach to the pod, not even when the boat is considerably reclining backwards when getting to full speed, because I think at more speed the to the pod attached upward flow from the bottom will start violently pushing the pod upwards and thus the bow pushing downwards, and so in my view will cause porpoising when getting to full speed, and this I think will bring the boat into a self-repeating porpoising cycle.

    Now if this is causing the problem, then I think the easiest solution is removing the pod from the transom, and re-attaching it much higher, so the water flow can never ever again attach to the pod. If the outboards legs than can't reach deep enough anymore, then mount the outboards lower on the pod's bracket, it might be necessary to cut off a part of the bracket's mounting edge for this.

    But while the pod is off, I would also tackle another issue, which is the leverage the pod gives to the outboards weight on the hull, and so in rest is causing a light bow and unstable boat, hence the USCG calls already for ballast in the bow for the current situation.

    So I would cut off the front end of the pod and re-attach the pod with the outboards mounting bracket so close to the hull that the outboards can just move free of the transom. This has the additional advantage that the bottom of the pod will be higher due to its underside slope, so the top of the pod will require less raising, or none at all, to reach the goal of a larger hull bottom vs pod bottom step.

    The other improvement options are the full hull extension I've suggested in post #92 & #102, or mounting the outboards directly to the transom as I've suggested in post #95.

    Whatever you choose, best test it with and without the current bow ballast or wherever ballast, and with sandbags on the seats to simulate the passenger's weight.

    If it all works well without the current USCG required ballast, then inquire it they can lift the ballast requirements because of the weight of the outboards moved forward, and because of the elimination of the leverage of the pod's length on the outboards weight to the hull. Or the outboards weight is directly supported by flotation in case you choose for a new full hull extension.
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    Edit: Important note about this post, there are parts I revoke: see post #114 & #120.
     
    Last edited: Apr 8, 2019
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I'd say not, it actually creates a low pressure area, drawing water up, and the stern of the boat down. I think this boat has suffered by the need to install a lot of ballast to meet regulations, it would be adopting a more nose-up attitude as it rises on to plane, as a result, but whether there is solid water contact with the pod, remains doubtful.
     
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  10. Angélique
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    Angélique aka Angel (only by name)

    At least I think we agree a major improvement of the boat would be Gofastguy finding a regulatory approved adaptation to the boat to get rid of the ballast . . :)
     
  11. Angélique
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    Angélique aka Angel (only by name)

    I've reformulated the part from which you took the quote a bit, and edited the post accordingly, not sure if it's correct, but it's my current view on the possibilities with the information we have now, below a quote of that part from my edited post #113 . . .
     
    Last edited: Apr 7, 2019
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    It sounds as if it is in there for the duration (epoxied ?) There are other possible ways this boat could be improved, I think, to counter all that lead. One might be to increase the width of the bottom, as per this sketch. Not a huge task.
    PAINT.png be to
     
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  13. Angélique
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    Angélique aka Angel (only by name)

    The sketch looks like a good option . . :)
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    If you have a lot of weight to carry in a planing boat, slenderness is no help.
     

  15. Angélique
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    Angélique aka Angel (only by name)

    I've just looked again at the article and now found you're right, attaching flow to the pod is always causing suction below the pod, and while that's bad it can't be the major cause of the porpoising, so I revoke that part and equivalent parts of post #113, while some other parts still stand, it's up to the reader to filter out which parts these are from the info in your post #114 and from this post, I've left a note about it at post #113, thanks for correcting my errancy . . :)
     
    Last edited: Apr 8, 2019
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