Riva classic speedboat - problem planing (porpoising)

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

  1. dckelly
    Joined: Sep 2010
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    dckelly Junior Member

    Hi,

    We have a Riva Super Florida classic speedboat.

    Since we bought it (in the 80'ies), its had the problem that when going fast, it won't plan like modern boats do. It may go half way there, or more, but at high speeds and never fully.

    Also sometimes it will start to "gallop" (I don't know what the right word is - its bow will start going up and down in a repetitive motion, splashing in the water). Then you have to go slower to break the rhythm, and increase speed again.

    We have tried to correct this by placing weight in the front (a couple of bags of sand), but although it helps a lot, it doesn't fix the problem.

    Does anyone know anything about this and how we could correct it?
     
    Last edited by a moderator: Sep 14, 2010
  2. Landlubber
    Joined: Jun 2007
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    Landlubber Senior Member

    ....try the search engine for "porpoising"...there have been may articles on the subject.
     
  3. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Twe two symptoms, difficulty in acheiving the full speed and porpoising (that's the name for what you call "galloping") sound like related to me.
    The porpoising has been discussed many times in this forum, for example here: http://www.boatdesign.net/forums/boat-design/understanding-porpoising-9509.html (in particular, Tspeer's post #11)

    More info would be necessary in order to assess what's going on. For example:
    • What is the running trim of the boat at maximum speed and when it starts to porpoise (degrees, possibly)?
    • Was the boat modified in it's lifetime, in such a way that it's hullforms or engine (or other components) placement has been changed?
    • Were the hull lines modified (during, for example, a restoration due to damaged hull or whatever)?
    • Was the shaft inclination changed?
    • Do you have trim tabs or other similar devices installed?
    • Does the basic boats' data (dimensions, engine type, fuel capacity etc.) coincide with these numbers: http://www.rivarevival.com/riva_spec.htm ?
    Sand bags can help diminish the trim angle but they will degrade the planing capabilities of your boat, and in certain situations can lead to other dynamic instabilities (like bow steering, for example). That's not a true solution to the problem.

    Cheers
     
  4. dckelly
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    dckelly Junior Member

    Thanks for the reply. I also think the problems are related.

    I have posted a video showing how she handles and the porpoising:

    http://web.matinaki.com/temp/Riva/riva-porpoise.htm

    I could not get much out of the thread you pointed me to.. I don't know enough to know what they're talking about.

    The boat has not been modified in any significant way since we bought it. I don't know if they had tampered with it before, but I doubt it.

    Thank you for any help.
     
  5. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    It appears to run too trimmed-up, or stern-heavy. The moments before porpoising starts, the keel appears to be wet only from about 40+% from the bow, which is not how a classic Riva hull should ride. They usually have the keel touching the water approximately at a point some 25-30% from the bow, at full speed.

    Can you tell what is the hull number of the boat and what engine does it mount? Do you have idea of the actual weight of the boat? What speeds do you reach before porpoising starts?
     
  6. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    The video shows that the boat starts to porpoise almost as soon as it starts out. This indicates to me that the CG is so far aft that the stern immediately squats drastically such that no equilibrium trim ever develops. Thrust drives the bow up beyond a sustainable trim angle and it falls back in the water, slowing the boat and initiating a new cycle as long as high power is applied. I guess that is kind of a definition of porpoising. Looks very uncomfortable unless you are a teenager and don't have a PWC.
     
  7. keysdisease
    Joined: Mar 2006
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    keysdisease Senior Member

    Trim Tabs

    Regardless of why, a very good "fix" would be to install trim tabs. Deploying them just a little will stop porpoising immediately.

    They are relatively inexpensive, usually easy to install on a vessel like yours and will also provide other benefits such as trimming the boat to an even keel when loaded unevenly, allowing slower planing speeds, getting on plane easier when heavily loaded, smoother ride in a chop, etc.

    Install trim tabs = end of problem.

    Steve
     
  8. dckelly
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    dckelly Junior Member

    The boat number is 867.

    It used to have the original engine which I gather is 220HP (although I could not find the label on it). We have now changed this to a Mercury Scorpion 300HP engine. This has the same block (normal GM v8 block) and approximately the same weight.

    I'm sure its not an engine problem though because it did the same thing before, with the original one...

    It may be something wrong or missing on the hull - I'll try to get some pictures and upload them.
     
  9. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Are you talking about a Mercruiser 5.7 MPI engine being actually installed on board?
    This one: http://www.mercurymarine.com/engines/inboards/inboards/5-7mpi_specs.php ?

    The hull number 867 (built in 1964) had a Chris Craft 283 185 HP @ 4000 RPM engine. The engine weighed about 712 lbs (322 kg). So the 220 HP was not your boat's original engine.
    Only after 1966 Super Florida models have been equipped with 220 HP engines, but only after some experimenting with hull variations - with the final adoption of a higher deadrise angle (the Vee at the hull bottom). That's probably the highest power which this (modified) boat model was designed for.

    On the other hand, the Merc weighs 860 lbs (390 kg) and gives 300 hp @ 5000 RPM. How much more weight for the reduction gear?
    So what you have now is a combination of an old (low deadrise) hull version, and a 62% more powerful engine than the original hull was intended for. It could easily be the reason for the behaviour of your boat. It is overpowered, compared to what it was designed for.

    The easiest solution, imho - depower the engine and change the prop accordingly. Or simply don't push it to the maximum throttle.
    But try to hear other opinions too - from PAR and Baeckmo, for example.
     
  10. dskira

    dskira Previous Member

    Who said the Riva's was good design?
    Daniel
     
  11. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Well, it depends, I guess... It's a design for protected waters. And I personally find it aesthetically very appealing. It is also a ruggedly built boat, with details cured to maniacal levels. But yes, it is not a good design for a sea-going craft.
     
  12. dskira

    dskira Previous Member

    You are right, I should have been more precise.
    Daniel
     
  13. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You can try moving some weight forward, but I suspect this has been done, probably more then once, so I'm recommending the opposite, move weight aft. Most are assuming the CG is too far aft and this is the reason for her porpoising, but I think the CG is too far forward, which also can cause porpoising.

    I also think that any reasonable person could solve this issue in an afternoon of testing. The first few base line runs with a skipper and a healthy crew member only. Next move the 200+ pound crew aft for the exact same route the base lines ran. Check the video, any better? How about the 200+ live ballast forward for the same run? Next would be shingles, starting with 1/4 taper across 12" shingles. Feel any different? Does the video look better?

    I honestly can't understand how this could be a 20+ year old issue that hasn't been resolved.
     
  14. dckelly
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    dckelly Junior Member

    Almost - this is the link: http://www.mercurymarine.com/engines/inboards/towsports/scorpion_350_specs.php

    Its 330HP (300 at propeller) and weighs 343kg together with the gearbox.

    I know its overpowered, but it was the best alternative for the existing engine because its the same GM block and has almost the same size and bases. It was very easy to install. I don't think however that overpowering is an issue because the propeller is the same size and as you said if we work it at the same RPM as before (which we do since we never go full throttle), the thrust is the same. Also the weight difference, if any, is not more than a thin person.
     

  15. dckelly
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    dckelly Junior Member

    We had resorted to placing a couple of sandbags up front and that had mostly solved the issue. Also we didn't use it that much.

    Recently though, we refitted it, and are looking at the problem again to see if there is something more we can do.
     
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