help with porpoising

Discussion in 'Powerboats' started by wacreator, Feb 6, 2015.

  1. johneck
    Joined: Nov 2011
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    johneck Senior Member

    I think that the rocker on the additions is the culprit. If they were flat they probably wouldn't have had the same effect, though they do change the trim which might be an issue. Perhaps interceptors are the answer. They could be trimmed up when you want to wakeboard and down when you want to go fast.
  2. Pericles
    Joined: Sep 2006
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    Pericles Senior Member

    Planing boats have straight flat runs to the transom, which should be sharp at the lower edge so that the water leaves it cleanly, without inducing porpoising. The side pod extensions are rounded upwards, in effect you have grafted on part of a displacement boat stern to your boat, so that the bow is thrust down by the pods & the bow wave bounces it up again, because you now have a boat bottom with rocker at the stern.. Look at the sterns of some deep V boats. They run straight & level, because there is no rocker in the underwater shape.

    You could try straightening the aft run of the pods by screwing & fairing some ply in place to see if the handling is improved. Good luck.
  3. wacreator
    Joined: Feb 2015
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    wacreator Junior Member

    What are these interceptors you speak of do you have links to them so I can get some ideas.

    Thanks for the replies guys they are hugely appreciated.
  4. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    The additions suck (yes water sucks) the transom down, making a higher stern wake for surfing on. That, in combination with the existing CG location, creates a longitudinal imbalance and the boat oscillates from one unbalanced state to the other, which is what you see as porpoising.

    Making another hull addition to correct porpoising may work but may be getting into a vicious circle. Interceptors may work if they are positioned in just the right place and are of the correct height to break the water away from your neatly built rockered extensions. Enough interceptor height to avoid too much suction and not so much that they drive the bow down. That means that they have to be adjustable (at least on or off, if not variable) to satisfy both wake boarding and normal running. Interceptors would need to be placed in the curved portion of the hull extensions or they would drive the bow down..

    They could be manually attached when needed to avoid a difficult and complicated remotely operated system.
  5. Kailani
    Joined: Apr 2013
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    Kailani Senior Member

    In the photo in the post above. As Tom said interceptors are small plates that move vertically down from the transom and are a newer alternative to trim tabs. The water force on the blade creates pressure that lifts the stern in the normal install.
    Here is how interceptors work in general:
    I imagined a big plate on a hefty trim tab to give a flat bottom (eliminating your curved rocker bottom for high speed runs) or trim up to give the rocker you wanted for wake if that would work. I have no idea how interceptors would work combined with your curved rocker - I don't know. If you try it post back how your experiment goes :)
  6. wacreator
    Joined: Feb 2015
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    wacreator Junior Member

    Looking at a combination of two ideas including the interceptors about to start the mock up shortly. Will be full carbon.

    I'll post up once I've made the alterations.
  7. Moneymakerspy
    Joined: Feb 2015
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    Moneymakerspy Junior Member

    please delete this duplicte
    Last edited: Feb 14, 2015
  8. Moneymakerspy
    Joined: Feb 2015
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    Moneymakerspy Junior Member

    Hey wait a minute, your work is fine and you can fix this issue!

    Wow tough room here and I am surprised at the lack of out of the box thinking/advice offered to correct your problem. I am new here but I do have a lot of experience in getting ski-type hulls/runabouts/flat bottom/deep v-hulls to perform well. I have adapted many types of boats into "perfect" ski-wake machines and never once have I comprimised cruising performance/comfort. The thing is when you modify a hull to do something it does not normally do, all of the normal engineering dynamics are thrown off. Typical solutions will fail because you are now operating in unknown territory. That said I think your hull-mod is bad ***, took guts to attempt, and looks damn good for someone who has probably never modified the hull/extended the transom of an existing boat before. I think the paint drips near the chines attracted more criticism than it deserves. I am a believer of if you don't have anything of value to offer, you should just not offer anything at all and that goes for any snub-nosed comments about your work. I did not see any offer of help, or pictures of his mod looking any better so screw'em. It also hits me that you probably care a lot more about a perfect wake than you do about a perfect riding boat. This is something most people will not understand unless they are very passionate about being dragged behind a speeding boat. I totally get it and would cut my checkmate in half if I thought it would allow me to perform wonders when being dragged behind it. So, here are some suggestions that I have used to correct the porpoising problem after modifying a hull to make a better wake. First raise the motor (even a smidge) then/or try a dol-fin on the lower unit. yes I know it is not designed to eliminate porposing but it changes the ride of the boat and is sometimes enough to stabilize the ride. Second choice would be to change the angle of the mount itself with wedges (your desired deg. of angle change is TBD), more times than one I have solved this issue by adding a jack plate (maual or hydro). another thing to try would be to play around weight ballast and motor height/angle together. My favorite fix was always to try some further hull mods such as side-strips/under/strips (strakes?), stainless bolt-on rudders, or even scoops etc. I once had a porpoising problem on a hydroplane that was severely overpowered. I took some stainless vent plates from another boat and fastened them to the rear underside of each sponson. they acted as closed/dead end water scoops creating enough drag to change the dynamic of the ride and eliminating the porpoising problem. you might try some drag inducing attachments on the forward portion of each side of those "duckboards". Your idea of a stationary trimtab is right inline with the type of ingenuity needed to solve this while keeping your current mod. Trick is to keep trying and have fun while coming up with new modifications/solutions. We are not all trying to accomplish the same result in our boat building efforts and unconventional methods should never be frowned upon. Good luck.
  9. wacreator
    Joined: Feb 2015
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    wacreator Junior Member

    Hey guys

    Been a while but thought i would update everyone with the outcome of my mods.

    I essentially built a carbon panel which carried on at the same plane as the hull below my extensions.

    This completely fixed my issue.

    Ill chuck a pic up later on. The air gap i created was 20 mm over 300mm.

    Couldnt be happier.

  10. JSL
    Joined: Nov 2012
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    JSL Senior Member

    Fill in the 'rocker' with a wedge so the hull is prismatic. Try a temporary one first to see if works.
    There are 2 other 'fixes' that might also work.
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