Riva classic speedboat - problem planing (porpoising)

Discussion in 'Boat Design' started by dckelly, Sep 13, 2010.

  1. daiquiri
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    daiquiri Engineering and Design

    Bear in mind that you have 80% more power than the original engine. I hope you have reinforced the engine bed accordingly.

    Since you say that sand bags up in front have helped, then you could try to move a step further in that direction by installing trim tabs, like Keysdisease mentioned in a previous post. Or, in alternative, the vertical interceptor plates at the transom. These two are commercial:
    http://www.great-water.com/pages/QL/QL_Trim_Sys.htm
    http://www.humphree.com/learn-more/interceptor-guide/
    but they can also be made manually. They are essentially vertical plates fixed at the transom (more or less all along it's width), protruding 50-75 mm (2"-3") below the transom edge. In alternative, you can think of L-shaped plates fixed below the hull, with the vertical part protruding downwards in corrispondance to the transom edge.
    They will give a similar result the sand bags do (trimming the bow down) but without large weight penalties. There will be some drag increase at low speeds - but you go low speed pretty seldom anyways, right? ;)

    But you never told us what speed do you reach with this engine?
     
  2. Alik
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    Alik Senior Member

    The reason of such behaviour is that bottom is not designed for that horsepower - it is too efficient to produce the lift.

    Can You post a photo of the bottom? Photos showing drive position, please.

    I would try to install running strakes to narrow down planning beam. I do not think tabs or interceptors will work, they will just make it worse.
     
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  3. HJS
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    HJS Member

    50-75 mm ????? What practical experience do you have with such solution? How did you come to that figure? According to my experience it should be perhaps 3-7 mm over the entire transom. The interceptor must work within the boundary layer, other ways it will be a brake.

    js
     
  4. daiquiri
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    daiquiri Engineering and Design

    Hey, please stay calm. :) You are correct and have been quicker than me in noticing the error. Those numbers (I believe I'd mentioned it at your thread about mid-ship interceptors) come from a tab we've applied on a 41 ft semi-displacement hull which, in fact, required much more protruding area below. We gave it 70-80 mm and it worked well, gaining 2 kts and levelling down the trim. It remained in my head but is clearly an overshoot here, and I admit the error.
    You're entitled to give your advice, I'm aware of the work you did with mid-ship interceptors, but please do it possibly with calm.
    However, 3 mm sounds too small to me for a boat this size and weight - but if you have made experiments with that then I have no arguments against, for this type of boat. I would make a variable-depth plate instead, fixed base plus bolted vertical plate and would experiment with height until the best solution is found. Just my opinion.
     
  5. daiquiri
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    daiquiri Engineering and Design

    Alik, that's an interesting thought. :idea: In fact, classic Riva hulls have a smooth bottom, so it could help indeed.
     
  6. Alik
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    Alik Senior Member

    Yes, I just studied Riva hull few weeks ago. The deadrise aft is very small and there are no strakes, so it will jump with such huge horsepower.

    So advise is: add one running strake per side at about 60-70% of bottom chord; extend them to the transom, and then see what happens.
     
  7. wdbeyer
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    wdbeyer Junior Member

  8. dckelly
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    dckelly Junior Member

    We don't yet have a way to measure it. Anyway, that's not our goal. We may look like extreme teenagers out to break speed records, but we're not, really :)

    We looked at some other engines before deciding on this one, but all needed too many interventions for proper installation.
     
  9. dckelly
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    dckelly Junior Member

    I'm going to post photos tommorrow.
    Thanks.
     
  10. daiquiri
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    daiquiri Engineering and Design

    You can take a common portable GPS road navigator with you. The map will show that you are drowning in the middle of the blue screen, but will also tell you the speed you are moving at. Every cheap GPS navigator has that feature. ;)
    It's not about speed goals, but about understanding at what speed the porpoising starts. Every info is useful.
    Cheers
     
  11. PAR
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    PAR Yacht Designer/Builder

    All of this discussion is fodder, as one afternoon on the water with this boat with any reasonable "prop tuner" or other skilled technician could find the balance needs of this boat.

    I still think the boat is "blasting out" with the extra power, reducing her planing patch, so the boat "falls down" then the process starts again. Strakes may help, as might shingles or a hook, but you have to determine if the CG is too far forward or aft first, which requires some test runs. You'll notice in turns, she carries her bow well and trim looks good, but as soon as she straightens out, she tries to drive up her shaft angle and runs out of planing patch.

    Then again, this is all speculation and relatively fruitless, without seat of the pants data input. In a 5 minute ride on this boat, I'd have a pretty good idea what she's trying to tell you. I know I'm not the only one with this skill set, as I've seen it many times from well seasoned skippers, tuners and repair craftsmen.
     
  12. baeckmo
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    baeckmo Hydrodynamics

    I can see two main issues here, one is the twisted bottom rising from a few degrees aft to some 45-50 degrees where the keel starts rising up front. Without additional forward strakes this bottom is unsuitable for the high speeds you attain with modern engines. The other is the propeller; the only thing we know here is that it was used for a previous (old?, original?) engine installation and that the porpoising has been there probably as long as the propeller.

    First to the bottom. As stated by others, the flat section aft is producing more lift "than required", forcing the nose down, while the propeller thrust line is producing a nose-up force at speed. When the nose comes down after the high-pitch sequence, there is very little dynamic lift (compared to the aft section) until it is deeply submerged with a substantial drag increase as a result.

    This forward section needs an additional "planing surface" in the shape of spray rails, to break the linear relation between trim and force and balance the lift of the flat aft sections. But they should be placed closer to the keel than Alik suggests. I would say something like 0.4 m from the keel and ending midships.

    In addition to the hull issues, I think PAR is putting his finger on the real culprit when he mentions the propeller. If the screw has been with the boat since the 80:ies (?) it most certainly has blade sections that are unsuitable to the ventilation that occurs when this boat is porpoising. The propeller thrust is suddenly reduced when the hull reaches its maximum trim position (in some frames you can actually catch a glimpse of the shaft cutting the surface), and then increasing when the bow is submerged. This is a strong driving force for a cyclic event. Before going into more detail on the prop, I would like to see some more facts on the boat (see Daiquiri's comments), and of course on the transmission and propeller itself.
     
  13. PAR
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    PAR Yacht Designer/Builder

    Like I said, one ride on this boat and it will tell me what it needs. I'll bet the engine/drive/prop cycle through a grab and release situation, she's climbing up her shaft and falling over, then repeats. When turning, there's enough chine bury drag to calm her down. Take it to a shop that specializes in prop tuning and have their shop old fart, take it for a ride. He'll quickly recognize what's going on.
     
  14. Alik
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    Alik Senior Member

    I worry if planning surface is too narrow at stern it can cause other problems as chine walk. I would suggest to start from values I specified just by affixing wooden strakes by glue; then moving them inside to the keel if this is appropriate.

    Anyway we should see photos first.
     

  15. dckelly
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    dckelly Junior Member

    Hi,

    Below are the photos (I hope).

    One thing I was now told was that there was a brass plate covering the hull right above the propeller. There is one picture which shows someone holding up a mock up of it. This fell off a long time ago, so this may well be the reason for the problem.

    My problem is that I don't have the specifications for that plate (the one in the picture is very approximate) and as I understand from the discussion, even a few millimeters makes a difference (?)

    How would I go about finding out how to make that plate?

    Thanks for all the help.

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