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

    It certainly is not going to be dry, strictly speaking , but really it may attach without tabs, and not attach with them, but I think if it was attaching it would put the outboard legs above the splash plate in the way of solid water, and create a fountain feature that would be quite obvious. But there is nothing to be lost by taking a video over the stern to see what happens as the boat speed increases.
     
  2. Angélique
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    Angélique aka Angel (only by name)

    My gut feeling is this boat is a whole bunch of issues, originating from the long slender deep V hull, which as it is now best looks to be improved by removing the current pod altogether, and replacing it by a hull extension following the hull lines, maybe approx 1½ ~ 2 × as long as the current pod, and carrying all the fuel and batteries plus all moveable weight in the aft end of the new hull extension, since the displacement of the new hull extension is able to carry that weight where it in plane is directly above the water contact, and ditching the idea of carrying ballast wherever.
     
    Last edited: Apr 5, 2019
  3. Angélique
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    Angélique aka Angel (only by name)

    Note for efficient plane you put the weight where it is directly above the water contact . . .

    [​IMG]
     
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  4. fallguy
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    fallguy Senior Member

    I am a little surprised the incline folks didn't push back and suggest changes to other things in the boat.

    Wouldn't one first suggest changing other things before such major amounts of ballast were applied?

    If I understand the ballast report; it also restricted passengers in the front seat.

    If it were my vessel, I would have modifed other things before accepting 2150 pounds.

    When he goes about modifying this vessel; moving the lifevests forward to mid; isn't he technically going to need a redo of the incline?
     
  5. Angélique
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    Angélique aka Angel (only by name)

    My feeling is the boat would improve already when discharging the pod, and mounting the outboards directly to the transom, and placing all movable weight incl. fuel as far aft as possible, without any ballast at all.
     
  6. Angélique
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    Angélique aka Angel (only by name)

    From the complains the boat as it is now looks like a seesaw to me, when the bow is pushed up then the plane area isn't able to carry the forward weight and the bow drops back in the water. Place as much weight as far aft as possible (removing it from the mid and front) but keeping it within the hull, and the bow will stay up I hope. Especially so when the seesaw effect of the leverage of the on the pod free above the water hanging outboards is reduced by removing the pod.

    [​IMG]
     
    Last edited: Apr 5, 2019
  7. Yellowjacket
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    Yellowjacket Senior Member

    If there is water flowing on the surfaces of the pod it's creating negative lift. Period. For it not to create negative lift it has to be dry. A bit of spray may not be an issue, but if the flow is reattaching to that surface it's going to create negative lift. There isn't a lot of in between here. Also look at the aft view, the legs have spray deflectors that where the cowls meet the legs. If there is any spray on the legs this will deflect it. At higher speeds it's dry for sure, but down around 20 kts is where you're more likely to have that problem. At lower speeds the trim angle is higher and the boat is heavy. In the side view of the first post it would appear that there is no way it could reattach, so if it is at all it's a bad thing.

    One thing that the OP could consider that would likely address the problem, is to add an after plane to the transom that effectively just extends the regular planning surface. This could be a simple plate that has ribs to support it and is mounted to the transom. Adding 12 inches to the hull by this way would effectively move the CG forward relative to the trailing edge of the planing surface and it would also increase the vertical distance from the new trailing edge of the transom to the pod, effectively increasing the step height. This could be done very easily and for not much expense.

    Another thing the OP should do is to get a Savitsky spread sheet and figure out if you're in the porpoise region based on that analysis. You will need to input the basic geometry and the cg and thrust line info, but if the calculation shows you're in a bad region of the map you'll need to move the aft edge of the planing surface aft to get it to work.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    The essential thing to do first is to verify it actually does/does not attach to the pod, then work from there. I'd be surprised if it is the problem, but life is full of surprises !
     
    Last edited: Apr 5, 2019
  9. Mr Efficiency
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    Mr Efficiency Senior Member

  10. gofastguy
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    gofastguy Junior Member

    Thanks for your thoughts. Considering the hull extension works so swell on my 2 other Zodiacs, it's hard to imagine why it wouldn't work well here.


    Unfortunately the incline test is conducted for the sole purpose of satisfying USCG requirements for passenger carrying vessels. The Coast Guard doesn't care a bit about if the boat handles well. They just don't want it to flip over. So as long as the weight distribution of the boat in the test satisfies this section of CFR, it passes: 46 CFR § 170.170 - Weather criteria. https://www.law.cornell.edu/cfr/text/46/170.170

    I didn't really have any choice in the matter of this, at least that I knew of. I just ordered the boat to have the maximum reasonable number of passengers possible, this is what they gave me. I agree the amount of ballast is ridiculous and defeating.

    And yes under the strictest Coast Guard standard, that enforced at the federal level, a new stability test would be conducted when any weight is moved around. However, local inspectors have the ability to supercede federal authority, in my case we were granted an exception.



    Noted, thank you. My friend who is a boat manufacturer, upon describing my problems, offered I believe a simliar remedy. Here is his sketch. Will see how we can get access to a Savitsky analysis as well. IMG_0528.PNG


    We shall see! Thank you.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    That sketch show a different angle of attack to the hull proper, which would be no good.
     
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  12. Angélique
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    Angélique aka Angel (only by name)

    The bottom of the hull extension on the end of the post #51 Zodiac follows the hull lines, but that extension is narrower than the hull, for the topic boat however I think you'll need all the plane area you can get, so for a new extension I would make it as wide as the hull, so the extension sides also following the rigid hull sides.
     
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  13. rwatson
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    rwatson Senior Member

    The full width plate may be a backward step unless is is adjustable.
    You may still need trim tabs for load and other condition adjustments, which will not be available with a fixed, full width hull extension.

    The fact the the bottom of this plate is not the same "line" as the hull staggers me. Why ? You are building in the same problems as the Pod is suspected of.
     
  14. Angélique
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    Angélique aka Angel (only by name)

    Yes, the in the sketch added plate needs to follow the line, and I think also the full width, of the hull bottom panels, and then it start looking like the post #41 ‘‘hobbles’’ already . . :)

    boat plate.jpg
     
    Last edited: Apr 6, 2019

  15. Angélique
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    Angélique aka Angel (only by name)

    Some more examples of ancient hobbles successfully added after an power increasement . . .

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
    note the bottom of this hobble is a bit below the top board and picks up the hull bottom line a bit before the transom.

    (on a small desktop or laptop screen click the pics to enlarge for details, and click again to shrink)
     
    Last edited: Apr 6, 2019
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