Odd one out - complicated porpoising

Discussion in 'Boat Design' started by DaanDM, May 23, 2021.

  1. baeckmo
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    baeckmo Hydrodynamics

    I think there is something wrong with your numbers, you would not get the speed you report with the mentioned weight; the resulting drag/lift ratio is too low for a deep V hull. Before we speculate about porpoising reasons, the numbers have to be checked. Propeller data could give a first indication, so please provide transmission gear ratio and propellers' pitch and dia. Are you actually reading "knots" or something else? And the crane weight, does it include the tare (lifting beam et c.)?

    Then the trim anomaly: the lifting point seems odd. According to the dirt line, the CoM is further aft. One reason for the shift might be a long, low tank with too small anti-sloshing walls, letting the fluid move longitudinally (have seen that problem in similar vessels). Please check the actual shape, position and internals of the tank.
     
  2. DaanDM
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    DaanDM Junior Member

    Hi Baeckmo! (we emailed about the VM/Mercruiser diesels did you receive my last mail?)

    Apart from being stainless quicksilver mirage 3 blade props, I don't currently have any info on them dia/pitch. I also don't have the gearing info of the B1's, just the serial numbers, if that is of any use?
    The boat really goes 70+km/h or 40 knots, measured with Simrad MFD GPS data and our mobile phones.

    The brand new truck mounted Palfinger crane (without weight readout) that you see on the picture in my first post, was beeping for overload warning all the time, and stalled out a few times, when the boat was brought to the marina for the first time in years. Don't know what the rating of that crane is. That was with an empty tank so that rules out this influence... but indeed its a low and long tank, but it has at least 3 baffled compartments with relatively small balance holes. I dont suspect the fuel gushing forward/aft to be the reason of porpoising.

    As said, I don't trust the mobile crane info. The weight figure of 5200kg was something that was shouted from the operators' cabin and he had to add counterweights on the back due to the span and unexpected high weight. I don't know if he tared the crane before lifting on haul-out, nor if things were calibrated or if he did or have to do any math due to the span. The diesel tank was holding about 280 liters of diesel in it's 340 liter tank at that time. However, the boat for sure has bigger bones than you'd expect.
     
  3. baeckmo
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    baeckmo Hydrodynamics

    Ok, the standard gear ratio for this engine/leg is 1.5:1, so if you can find some id for the props (something like 48-13703A41...), I can find the numbers needed. Mirage id is stamped on the hub.
     
  4. DaanDM
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    DaanDM Junior Member

    I've texted the marina guy to ask if he can make a picture of the prop P/N, I'll update when he did.
    Anyways, with the boat weight being a given (can't change it), the top speed a verified fact and the fuel not sloshing back and forward because of the baffles, what can you say regarding the porpoising?
     
  5. fastwave
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    fastwave Senior Member

    Just a comment because of something you said : the trim tabs are not supposed to lift the bow up, it’s not like the outboards you are used to.
    From your comment I conclude that you are running the tabs too far up. They should not be higher than being flush with the hull bottom. If you run them higher then you might get a separation which sometimes it’s attached and sometimes not, leading to porpoising. check that not what you are doing wrong.
     
  6. DaanDM
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    DaanDM Junior Member

    Appreciate your reply. I think I must have written something incorrectly because I do understand the mechanics of the tabs. Besides from memory they can't even go up beyond flush with the 'streamline' of the hull.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    I was a bit like fastwave looking at the tabs, it may be an optical illusion, but they did appear higher than normal. I don't think the weight calc in necessarily that far wrong, diesels =weight x 2. and look at that waterline mark, it is pretty indicative. I think you would be hard pressed to find many boats that length (and more significantly, beam) running twin diesels, it just doesn't compute that well, the engines become way too big a % of the weight of the boat, and where are they, right back aft, making the balance problematical, as if the hefty weight on its own wouldn't be enough to deal with.
     
  8. SolGato
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    SolGato Senior Member

    It might be worth it to try to film what’s going on with the outdrives underwater using a GoPro when the porpoising occurs.

    Is it so bad that perhaps the props start to cavitate and suck air due to a less than ideal overall weight or weight imbalance, and then the boat gets into a rhythm of biting and cavitating?

    I had a single hopped up small block with an OMC Cobra outdrive with small Bennett tabs once on a Cigarette boat that would suck air a bit when it had weight forward. Installing an anti-cavitation plate that bolts onto the built in plate/small fins of the lower leg but covers the whole top of the prop solved the issue for me and also allowed the boat to get on plane faster.

    The reason I mention this is because the porpoising reminds me a bit of the way sailboats behave when under motor power and the prop get close to the surface. The boat slows and rocks forward, then back and the prop bites, then forward, and so on.

    Just a thought. Those plates are an inexpensive and easy install. Looking at the photo of the boat it seems it would be hard to tell what the props are doing from above which is why I suggested filming them.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    He would notice the difference in engine noise in that case, I think.
     
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  10. SolGato
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    SolGato Senior Member

    Good point, although sometimes the brain tunes that kind of stuff out when it is syncopated with something like motion.

    But yes, paying close attention to RPM’s and sound of motors would be an important troubleshooting step.

    One of the nice things about using a camera is you can review the footage removed from experiencing the problem in the moment.

    And I always say it’s not one thing, but usually a combination of things that lead to a problem.

    This learned from restoring a lot of British Motorcycles, well I guess that’s not really fair to say, but rather vintage machines in general.
     
  11. DaanDM
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    DaanDM Junior Member

    I'm working on an underwater camera, forward and rear facing, using an obsolete paddle wheel through hull fitting that would otherwise need plugging anyways.
    With a bit of luck we'll get really cool views of the legs/props. (and fish, anchors, rocks)
    However, being a bit of an engine whisperer myself, I'm pretty sure that the engines did not experience varying loads due to ventilation.
     
  12. DaanDM
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    DaanDM Junior Member

    To remove any doubts about the trim tab max-up position going beyond the "streamline":

    [​IMG]
     
  13. baeckmo
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    baeckmo Hydrodynamics

    The main influence for the porpoising instability comes from the balance between trim angle, bottom rise and lift coefficient. Since the lift coefficient (Cl=m*g/(1/2*rho*V^2*B^2) depends on the weight (m), speed (V) and chine beam (B), I think you should focus on getting relevant info on these. It boils down to a maximum applied trim angle versus Cl for a given bottom rise. I'll be back with a diagram later today, but the most important test to run is trim versus speed. So although underwater footage may be inspiring in many ways, I recommend that you get hold of a decent inclinometer (there is one in your phone btw, download a physics app, like "phyphox" to use it) and fix it to read zero parallell to the keel, then run the test with say, five knot intervals and report back.
     
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  14. DaanDM
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    DaanDM Junior Member

    That's a good idea. I installed a precision 9 compass for the autopilot and should be able to see roll, pitch and yaw on the MFD. Before launching the boat I will ensure Pitch angle is properly calibrated parallel with the keel.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    On a boat like that, you would think a couple of 5 litre-ish V8 petrol sterndrives would be the limit of what could be accommodated, weight-wise. I don't know what these diesels weigh, but if significantly more, I think there is a hill to climb. Thanks for the photo of the tabs, does it seem possible to get bigger ones in there ?
     
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