600HP 42kts speed boat problem

Discussion in 'Boat Design' started by Hunterr, Oct 21, 2016.

  1. Hunterr
    Joined: Jun 2015
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    Hunterr Junior Member

    my boat has a problem
    LOA 12meter
    Beam 2.6meter
    Depth 1.25meter
    V hull and mercury300hp outboard twin.
    Maxspeed : 42kts


    until boating 30kts, boat motion is Okay.
    but when boating morethan 30kts, the boat has a list to the starboard side.. abt 5~6 degree.

    how can I fix this problem?
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    You have to work out what is causing it first. Doesn't sound like propellor torque, unless you have two left-turning propshafts, unlikely.
     
  3. CDK
    Joined: Aug 2007
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    CDK retired engineer

    With increasing speed, less hull surface is in contact with the water, so any error in propulsion and/or weight distribution becomes manifest.

    It probably can be corrected with trimming the starboard engine in, or adjusting the tie bar between the engines so there is some "toe-in".
     
  4. ChrisN67
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    ChrisN67 Senior Member

    If the imbalance is not noticeable at rest, my 10 cents is betting on a hull or propulsion geometry issue. Does the boar have a step? Is the engine mounting plumb?

    CDK do you prefer toe in or out for drives? I am running toe out by half a degree to prevent rudder port wear and yaw stability; but is there a difference?
     
  5. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Trim tabs???
     
  6. ChrisN67
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    ChrisN67 Senior Member

    I would not use trim tabs in a normal configuration.
    The tabs are helpful in balancing the boat when unevenly loaded.

    But tabs create lift, drag and yaw. They are a last resort.

    When I was first running my boat, going 50 knts I tried to use the one side tab to level the boat. Buttab movement on my boat is very fast and the boats' reaction was highly erratic when the 24" x 12" tab came to 1/2 travel. The boat heeled to Port and turned to starboard - not a good feeling.

    I have experienced the same effect (albeit slower) on a 36' pesca with 36" x 12" tabs.

    I actually had the tabs customized with negative angle to ensure they are completely out of the water while running.
     
  7. CDK
    Joined: Aug 2007
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    CDK retired engineer

    I've always used toe in with twin Mercruisers. Very critical is the exact same trim angle; a small difference becomes noticeable at high speed, even when the boat is level at say 20 knots. Trim indicators from Merc. cannot be trusted.
     
  8. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    Not enough information. When did the problem start,it can be anything. Tis rod between the engines, the propellers, defect in the hull engines or times trim not working properly.
     
  9. ChrisN67
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    ChrisN67 Senior Member

    The most reliable trim indicator is the propeller wash and eyeballs.
    We put a man on the moon over 50 years ago and we still can't get it right.....
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, 47 years, but who's counting. :D I can't see it is a differential engine trim issue, unless the gauges are in fact inaccurate, and it is being adjusted guided by them, rather than by eye or feel for how the boat is trimming.
     
  11. Barry
    Joined: Mar 2002
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    Barry Senior Member


    Your original post stated 42knots max, but before it ran 50k?

    If your boat sits absolutely level in the water tied to a dock, how do you ensure that when you have people on board that the boat is balanced?

    On our last boat, 30,000 pounds, 24 degree V, at 26 knots, on those rare calm days, I would set the leg trim and trim tabs up for side to side level, ie I could take my hands off the wheel for maybe a minute and the boat would track true.
    Then a passenger would decide to move from one side of the boat to the other, rough weight guess 175 pounds, from port then walk over to stbd, so the balance shift was 350 pounds and I would have to correct the trim, or steering, or the boat would begin a slow turn. And this is on a 30,000 pound wet boat.

    At rest, several hundred pounds of weight imbalance from batteries or fibreglass layup will probably not be noticeable as the boat is deep in the water and a half a degree off level will compensate for the imbalance.

    As the wetted surface of the hull gets much smaller at 42 or 50 knots, any imbalance becomes much more noticeable.
     
  12. ChrisN67
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    ChrisN67 Senior Member

    You said my original post said "42 knts"?

    Anyway, the movement of people on the boat is not very noticeable unless more than 2 people go to the gunwhale.

    I get a max of 52-54 depending on conditions; but I have relegated the use of the tabs under 40.
     
  13. Barry
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    Barry Senior Member

    Sorry Chris, thought that you were the original poster,
     
  14. JSL
    Joined: Nov 2012
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    JSL Senior Member

    It could be progressive heeling (chine running or chine walking) which can be influenced by hull form, weight (immersion), and running trim. Athwartship weight shift can often trigger it. Does the boat always list to stbd, or will it also go to port.
    If it is any consolation, I have been on a number of boats (and even a catamaran) that would 'chine run' when the bow is forced into a trimmed down attitude.
     

  15. ChrisN67
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    ChrisN67 Senior Member

    No problems Barry, you had me wondering...lol

    As I am running surface drive, when I redesigned the boat I moved the CG back 2 feet to prevent bow steer.
     
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