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

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

  1. Squidly-Diddly
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    Squidly-Diddly Senior Member

    sounds like the Thing To Do would be attach a big Vee "wing" under the existing pod, so as to form a more or less flush and full width hull extension.

    You'd have new Big Wing/Extension and some struts connecting it to the pod, and the Wing would have some re-enforcement especially where it is full width but the pod is not. Any appropriate (can-do, rough and ready) welding shop should be able to do that for under $1000 not including a few sheets of aluminum.

    Did this boat deal include blue prints for the pod and transom? That would help. Put that into a CAD and that will allow you to get dimensions for Wing and struts. PS-field verify any blueprint dimensions against "as built".

    Look on the bright side. :) Avon purpose built rescue RIBs had big holes in the back of the ridge hull that would flood and get them ballast and stability when stopped, but instantly drain to lose the weight under power. You'd have something like that. Maybe the struts/slabs could also be baffles/flume-tank to reduce rocking at rest. Its a feature, not a bug.
     
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  2. rwatson
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    rwatson Senior Member

    It does stand out. Any sudden water-plane change, sudden negative angle etc at the end of high speed waterflow can not be good, especially on a boat that makes its living on the plane.

    It would probably take an expensive analysis to uncover every single issue that it would create.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Many podded boats use that kind of pod. There is no continuity between the bottom and the pod, and the reverse slope has minimal effect underway.
    pod.jpg
     
  4. baeckmo
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    baeckmo Hydrodynamics

    It is all a question of propeller loading, what works in one setup is not automatically suitable for another!
     
  5. rwatson
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    rwatson Senior Member

    The comparison is not equivalent to my eyes. That white boat is much smaller, and lower powered, and the motor is powered in the centre at the deepest part of the pod. Comparing the hull supporting a 100HP, say 25 knot to one supporting two 350 HP motoring at 40 knots, offset from the pod V seems unbalanced. Have we got any photos of a bigger boat ?

    Looking at the black boat, that pod has a flat section with a sharp turn to the pod chines , it has two high powered motors, offset from the centre, in the disturbed wash first from the V on the main hull, then split by a flat section, then further redirected away from the props on either side by the Pod chine flats.

    The motors cavitation plates seem nowhere near "clear" water. Baekmos comment on the props performance may be relevant, but the OP apparently hasn't detected any sudden prop surges and the sound of the motors in the video seem ok, so the hull shape appears to be the biggest suspect.

    And that without analysing the possible hydrodynamics of the prism shape of the pod, two inches above the keel line of the main hull where the water is supposed to be chopped off suddenly while planing.

    If the side mounted trim tabs help the porpoising, then perhaps it indicates a downward pressure from the pod shape ? Or can the motors be trimmed a bit ?
    outboard-engine-trim-illustration.png

    Watching the unloaded porpoising video, makes me suspect that the heavy motors are contributing a bit of unbalance. That video really needs more organisation showing the effects of fully loaded with passengers, with and without the trim tabs. etc etc.

    Heres a similar, but larger boat, with four engines, no add on pod. The only porpoises are on the bow - I know, because I have travelled on it.

    What I really love, is the chance to analyse without having to wear the cost and trouble if actually fixing it :)

     
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  6. Mr Efficiency
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    Mr Efficiency Senior Member

    There isn't anything novel about the bracket for a twin engine installation, here is something similar (but full-width) that is moulded-in.
    edgewater260Transom550x336.jpeg

    It is the case that the wrong height of either the bracket, or the engines, can cause problems, too low and the unprotected (by any hull ahead of it) outboard leg can generate a great deal of drag, and too high will give too little water into the prop and engine racing and surging, as the prop disc leaves and re-enters the water, but stainless cupped props have quite a bit of tolerance before "letting go". It is interesting how this pod/bracket idea developed, initially it seemed to come originally from a desire to get extra speed by racers, to have a small gain by having the props working in water further behind the hull, but seems to have morphed in several directions, mainly to gain more internal space, and as a way to re-motor sterndrive boats. Bit it does pre-dispose a lot of boats to being more inclined to porpoise, especially if the end result is the same hull with the COG further aft. The pod that is a continuation of the hull bottom, less so, but that has no pretensions to the supposed benefits of engine set-back.
     
  7. Dejay
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    Dejay Senior Newbie

    Maybe you can borrow a camera that can film at 60 or 120fps (relatively cheap now) and put it on a selfie stick to record what exactly is happening.
     
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  8. gofastguy
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    gofastguy Junior Member

    Thanks all for the continued input. I appreciate the group discussion and the back and forth, it's very helpful. I took some pictures today at 30 knots of around the pod and engine area. Insert at the bottom of this post.

    I like the idea of the hull extension wing, probably the cheapest route and if it turns out to be detrimental we can remove it without much damage.

    RE: the heavy motors.. it's really not that much motor weight in the grand scheme of things since it's a 36' boat, and it doesn't appear at all to sit stern heavy in the water while at rest?

    I think our props are good. You mentioned antiventilation plate height... I wonder if I keep raising the engines the angle of thrust, even if directed an inch or two higher could make a difference? At any rate it would be one of the cheaper things to try before a new bracket.



    OK, today while underway I took some still pictures. Not sure what it will tell us. Here they are. This is with no trim tabs, in a very calm condition going downswell. We were able, considering the ocean conditions and the speed of the boat, to avoid a porpoising. You can see the pod surface is entirely free of the water, enough so that I was able to stick my iPhone down there and take a picture at 30 knots!

    IMG_0268.jpg IMG_0272.jpg IMG_0273.JPG
     
  9. fallguy
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    fallguy Senior Member

    Not to beat a dead horse, but the boat manufacturer ought to be able to tell you the design waterline for that boat; the 36' version before all the additional weight.

    Let's say, for semantics, it is 12" above the bottom at the centerline.

    Then, let's say all the customization and stern loading you did has the boat sitting at 9".

    It would mean you are 3" too deep in the water. When you call them and ask for the waterline on the transom; avoid telling them it is a custom boat and find out the hull design waterline. My guess is you are deep. This goes along closely with the other fellows concerns about the narrow hull.

    Then I'd ask armstrong for a review of the bracket. Did they design to the hull or to the weightier version.

    My opinion is the boat is overweight aft and sitting well below the design waterlines of the hull and might perform better with lighter engines and any other possible weight reductions aft.

    Then, despite the pushback, you need some prop work that goes beyond a forum post. The hole shot is an indictment of the setup.

    If it isn't too hard to do; remove all non essential aft weight inclusing the hardtop if you can and retest depth of the transom and all your numbers running.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Good pictures, seems OK, the motors were too low before you raised them, they are probably about right now. If anything can be done to improve matters, it might be bigger tabs, but no closer to the centre-line.
     
  11. fallguy
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    fallguy Senior Member

    Can't see the cav plate, can you?

    Can you get any venting in a hard turn?

    I'd say you are low.

    If you go up the hole you habe and the cav plate is still buried; then you probably need to jack or lose some weight aft.

    Taking the engines up isn't going to help enough with the boat buried at the stern.
     
  12. fallguy
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    fallguy Senior Member

    Another thing is to straight edge the hull to the lower unit. While this isn't any panacea, if you are in a good place there, you know you have a weight problem.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    If you look at the side-on view of the outboard in the opening post, you see the cav plate only develops some distance back from the front of the leg, the splash plate above that does not appear overly congested in the picture, I think I'd leave it at that height. If he has another hole left, not that hard to trial that though.
     
  14. Angélique
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    Angélique aka Angel (only by name)

    I see the bottom of the pod on the B&B Cape Lookout 28 also having a negative trim angle and step compared to the hull bottom panels, and also having trim tabs in the drawing.

    There's a thread about this boat on the The Hull Truth, and here the 2008 build pictures of hull #1.

    B&B - Cape Lookout 28 LOA hull + pod 30' 8" (9.35 m), 2 × 300 hp

    [​IMG]

    [​IMG]

    [​IMG]

    Same pod type on the B&B Ocracoke 256 and on the B&B Ocracoke 20B

    [​IMG]
     
    Last edited: Mar 28, 2019

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    I would be leery of installing a pod that was stepped up, but with the same angle of attack, to my mind that would be asking for porpoising problems, as the pod contacted the water at speed, or left it as the boat pitched, it being a potent source of lift, being a "leading edge", and with a positive angle of attack. These things don't have the reverse slope for no reason.
     
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