Best way to modify a hull to stop poropise?

Discussion in 'Boat Design' started by nickireson, Mar 31, 2019.

  1. nickireson
    Joined: Jan 2011
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    nickireson Junior Member

    I have a boat im running thats a modified version of an Aquasport 24. It has a 13.5 degree deadrise. 24 feet long. 40" tall transom, running a 26" Porta Bracket (as specified by Scott Porta) Yamaha 4 stroke 225. 19 pitch Inertia Eco Prop. Top speed 51mph. From what ive been told the chine has been made wider to help make the ride drier.
    A friend has the mold for this hull and I plan to build these boats and sell them. The current mold is in 6/10 condition as far as fine surface finish and some shrinkage/mild distortion. If things go well I will build a new mold for the hull eventually. So i could tweak the plug at that point if needed. I plan to build a new cap for the boat to modernize it. Before I do all that id like to make its ride less dependent on trim tabs. Right now i am running lenco 18 wide x 12 long trim tabs. They do an incredible job, nut smaller tabs dont cut it, and no tabs at all just isnt an option. I can slide the fuel tank (94 gal) forward on future boats, but in not sure if that alone will be enough. What else can I do to control the porpoising? Im willing to modify a hull out of the mold and make a new mold if necessary but im not sure where to start from a design aspect. The boat has no strakes at all, im not sure it that affects it or not. It only takes a little bit of tab down to fix the porpoise, but its still too essential. If a customer had a failed tab they would have to come home very slowly and thats not much of a selling point. Im including some pictures. Thanks
     

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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    I'd suggest if you have access to the boat on the trailer, put some blocks of wood under the frame of the trailer at the rear of it, and roll the boat back on the trailer till it starts to tip, and note where that point is. Roll it forward gain, them measure from the transom to where that tipping point is. That is useful information, it tells you where the centre of gravity is, and a good starting point in deciding where the problem originates. The photo of the boat in the water isn't that suggestive of being stern heavy, but doesn't mean COG is not an issue.
     
  3. nickireson
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    nickireson Junior Member

    Thanks for the response. Thats the kind of advice im looking for. Its not stern heavy im sure of that. I think it maybe a running surface issue? I had considered adding a "hook" to the transom a little
     
  4. fallguy
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    fallguy Senior Member

  5. Mr Efficiency
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    Mr Efficiency Senior Member

    There you go, one suspects this hull wasn't podded in its original conception, and maybe did not have the porpoising problem. Does engine in-trim work to lessen the porpoising ? It should, by the sounds of it. I don't think I'd be fooling around with the molds, adding a hook or whatever, at least with tabs and engine trim, you have the option to take them out of the equation, when needed, as in running downhill in big seas, I suspect this hull might be a touch broach-prone with that modest transom deadrise. There is a school of thought that a hull that needs tabs must have something wrong with it, but if it is OK for planes to have flaps, I don't see a need to apologise for tabs, which have other benefits, like keeping a boat on an even keel in strong cross-winds, allows a lower cruise speed in difficult conditions, and improves the ride by getting the forefoot more into the water. You have too much guesswork, and actual work, involved in modifying molds.
     
  6. Yellowjacket
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    Yellowjacket Senior Member

    First you need to make sure that the bottom is true. If there is any rocker in the bottom that could be what is causing the issue in the first place. You need to know where you're starting from before you think about any modifications. While you can move fuel forward what are you going to do when the tank is empty??? If the bottom is true then the biggest contributor is the setback of the motor. You've got the kind of setback that you see in a race boat. There is no reason to put the motor back that far unless you want maximum speed and are willing to give up everything else. If your hull was light enough to go 70+ mph it wouldn't porpoise at all. Get the cg in the right place and it won't be an issue. If you're building new boats and the bottom is true, then get the motor closer to the transom and put a well so that you can still tilt the motor and you should not have an issue.
     
  7. kapnD
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    kapnD Senior Member

    Molds that are not well cared for and/or not properly supported can change the bottom configuration substantially.
    I’d first check the beam of the hull compared to the original plug, if possible, and check the keel for rocker.
    Also get under the boat with a long straightedge and look for concave or convex surfaces where they should be flat.
    Layup in uncontrolled conditions and lacking specifications can also bend bottom surfaces into wierd configurations.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    I guess one assumes he has checked the hull for such defects.
     
  9. kapnD
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    kapnD Senior Member

    That “one” being you!
    I didn’t get that impression at all.
     

  10. Mr Efficiency
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    Mr Efficiency Senior Member

    He did mention "mild distortion", I see :eek:
     
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