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

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

  1. rwatson
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    rwatson Senior Member

    Yes. Negative pressure IS lower than atmospheric pressure. Water will flow OUT when the hull is moving, and it does not require high speeds. The only requirement is to have the hole in the hull pointing backwards, so water is not scooped back into the hull.

    The picture below says it all, a self bailer that operates at reasonable hull speed, even in a small dinghy. Of course, the effect is magnified by the larger foot, but the principle still applies.
    Bailer.jpg
    And yes "The step must be ventilated to atmosphere to work" with stepped hull. Why ? Because it removes the negative pressure caused by hull velocity.
    If the pressure under the hull was not negative, why bother ventilating ?
     
  2. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    The average pressure is above atmospheric when the boat rises up. However the pressure locally can be atmospheric or lower.
     
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  3. gofastguy
    Joined: Mar 2019
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    Location: California

    gofastguy Junior Member

    I want to thank everyone who has taken an interest in my struggles with this boat who was kind enough to reply. I have a lot of great suggestions and thoughts to digest here. We will be undertaking some tinkering with the boat in the next few months to correct the issues before summertime, and I hope to report back here with results.

    Thank you so much rwatson for taking the time to make these visualizations. This is worth a shot. I will consult with the bracket manufacturer to see what they think.

    I have found my 733 Zodiac Hurricane, with its 85 gallon tank far forward, rides perfect, ESPECIALLY when the tank is full of fuel. I think RIBs like a forward fuel tank.

    Interesting thoughts here, worthy of consideration. Thank you.

    Thanks for this explanation. Can you clarify what you mean by "not having a lot of step"? I think I can visualize pretty well what you are saying. What you are saying about having "two boats" resonates for sure. And the pod attaching to the flow.


    They are 16 inch diameter, 4 blades, 17 pitch. PowerTech props LFS4 is the model

    Armstrong, the bracket maker, says 2 3/8 inches above the bottom of the hull is the reccommended mounting height. I think this one is mounted 3 inches above the bottom. It probably could come higher a bit. I wonder if there's still a sweet spot to be reached with motor mounting height. I will be trying that as its pretty easy.

    The hydrofoil is an option to consider, yes. Worried it could break the engine plate with the high stress though. We put over 2,000 hours a year on these engines. I never received "hydrostatics", I'm not sure what that refers to exactly. I keep telling the manufacturer it's in their long term best interest to work with me to resolve these issues so their futures customers of this hull aren't making the same complaints. I think they are timid themselves acknowledge there is an issue because they don't want to spend money helping me fix it. The owner of the RIBCRAFT company came and rode along with me on the boat one day after my complaitns. Of course it was on the calmest day in a month, with a very light load of passengers and fuel.... the boat didn't really porpoise much with those conditions. The porpoising is way worse going into a swell. When it's lake like conditions (such as the conditions the manufacture tested the boat in, on a sound in the NE United States), the porpoising doesn't manifest itself strongly. But under our typical exposed pacific coastline conditions we have a lot to induce the porpoising, as indicated in my drone video particularly. And finally, yes, the more weight put on the boat the worse the handling. Particularly when when have 4 people sitting on the aft most bench seat.

    Yes, we have a TON of weight in the bow and middle of the boat and it still porpoises.... so I am slowly eliminating the CG issue. 17 people sitting forward of the console, at 185lbs a piece= 3145 lbs. plus 2720 in a full fuel tank. I also didn't mention there is 2,000lbs of LEAD glassed in the forward compartments to comply with USCG stability requirements (to keep the boat from having a hard list). So we literally have close to 8,000lbs in the forward half of this 36' boat under a fully loaded condition outside of the basic fiberglass hull structure. People want to blame the heavy Yamaha f350s... they are nothing compared to the immense weight that is carried in the front half of this hull. The porpoising and high planing speed must have to do with how the propulsion is mounted at the stern.
     
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  4. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    This is news, about this 2000 lbs of lead. You may have mentioned something apposite to the situation. Where is it along the length of the boat ? If it is well forward, you may have a pitching motion being helped by heavy weights at the ends of the boat.
     
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  5. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Oh, most definitely, you are pitching or shall we call it seesawing?

    This is a dynamic problem; exacerbated by static weights on the ends of the boat. I am no naval architect, but think seesaw. I thought stern static weights, but you have both!

    I doubt the pod is attached very long, if at all after you tell us a ton of xtra forward weight.

    The foils will give you stability because they have a dampening effect. If indeed the pitching did not go away; then the foils would be moving up and down in a vertical plane and getting 'stressed'. But they will dampen the hobbyhorsing because they make the back of the boat seesaw less.

    Interesting you were required a ton forward. That means you have to slow down to stop hobbyhorsing. The only solution.

    I still also think you are tucked a bit much, though less with the one ton news. The angle of attack is a bit high and this, too, would make the seesaw worse as the stern rises a bit in its turn on the seesaw, if you will. If you trim the engines up a tiny bump, not much; it should also help if my guess is right. But it won't be enough to fix the trouble. Engine height won't fix it.

    A list. That there is gonna give me humor all day. Does your stability adjustment have to do with pax weight changes, too? Adding forwrd weight to correct an assumed higher pax weight further aft? Or is it all based on incline testing? Something here is a bugaboo... if you lower your stern weight, could you then reduce the forward weights? See the picture forming? Less weight on each end of the seesaw..

    This is all the perspective from a casual lover of boats.
     
  6. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    depends where this weight is, it can't be too far forward, but might be far enough to be having an effect.
     
  7. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    Ouch, that lead ballast may well lower the pitching frequency if placed towards the ends. With a lower pf of the boat, it will pitch in resonance with longer waves, having more energy; movements getting worse in the process.
     
    Last edited: Apr 2, 2019
  8. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    The 2000 pounds of forward lead ballast doesn't come up until page 6 of this thread?
    What else are you holding out on?
    You have a 733 and you go to this vessel...?
     
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  9. gofastguy
    Joined: Mar 2019
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    Location: California

    gofastguy Junior Member

    I'm, sorry I mis-spoke. 2,000lbs of lead total. 1,000lbs of it is forward, another 1,000 pounds of it is aft.

    OK and I figured out I can upload the entire report from the incline attached here. This may also be helpful!
     

    Attached Files:

  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Ok, that is good info. The two masses are 18 feet apart, as I read that. Not much you can do about moving it. It may be amplifying the pitching, about the only things you might do, is bigger tabs, and possibly raise the pod, but I'd try the former first.
     
  11. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Bigger tabs reduce the lift/drag ratio, and increase the risk of disturbing inflow to the props in this boat. A better approach is to create a non-linear lifting function up front; that is an effective way of "breaking" a resonance. I am talking about either a "planing pad" at keel level forward, or widening of the lowest spray rails from the stem.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    I did mention early on in the thread, to use wider tabs, but no further inboard than at present. Worth a try, it is a big task, and risk, to be making alterations to the hull.
     
  13. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Adding weights to a small craft for 'stability'; especially 2,150 pounds in a 5-6 ton craft is an abomination.

    It is a little perplexing, to me, you wonder why fuel economy is poor on this vessel, when you are hauling two cars plus worth of ballast, gear, and engines before your payload arrives.

    And this is technically, not a large boat by any means.

    Too bad you don't have any data or ride experience before the incline changes.
     
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  14. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    What I'm referring to is the height of the step from the keel to the bottom of the pod. The pod keel line is at an angle compared to the hull keel. The intent of this is to keep the pod out of the water since aft of the transom the separated water flow rises. If this rising water touches the bottom of the pod it pulls the transom down, which starts the porpoise action. The combination of height of step and angle of the pod is what keeps the pod out of the water. If the flow reattaches that creates suction that is pulling down the transom. With the ballast you have a lot of weight to deal with. The higher loading in the hull requires higher trim angle to create the lift. Higher trim angle means you need more step, less pod length or more pod reverse keel angle to keep it dry.

    You really need to look and see if the flow is reattaching to the pod at lower speeds like 20 to 25 kts. If the pod is wet at all and making its own wake then you have an issue. If it is dry then it isn't the problem. It's really that simple. If it's reattaching then that is what you need to fix first. If it isn't then the problem is something else. Before you go looking at other things you need to determine if the flow is reattaching to the pod and if so at what speed does it get clean. Until you make that determination then all else is just speculation.
     
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  15. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Almost 20 000 pounds all-up.
    I'm not sure there's even that much buoyancy in the pontoons!
    A 733 has 12 000 pounds of buoyancy I think I once calculated.

    I'm with YJ here, GoPro the water flow around the pod at all your speeds.
    I didn't see any attached flow in your videos, but a closer look with a GoPro
    wouldn't hurt.
     
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