600HP 42kts speed boat problem

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

  1. gtflash
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    gtflash Senior Member

    Tabs are very handy to compensate for windage. I couldn't live without them, on my single engine outboard boat at all speeds. If the boat has a lot of warp it will lean over very easily into wind and I don't know any other way to correct it?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    A lot of warp ? :confused:
     
  3. mydauphin
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    mydauphin Senior Member

    Trim tabs correct design issues with boats besides load balancing. But if your boat is loaded right you should not need them, they waste a little power. Sometimes cg, or engine placement is wrong, resist the temptation to just fix it with tabs.
     
  4. gtflash
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    gtflash Senior Member

    Apologies I mean sharp deadrise forward. Although every power boat I ever driven leans into the wind when it's on a side or quarter. Thus making a harder ride.
     
  5. gtflash
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    gtflash Senior Member

    I doubt any designer knowlingly builds in design issues. Yet he might compromise speed for stability at rest or ride quality over speed for example.

    That aside, I can't understand how a single engined offshore boat can not behave differently when driving from point a to point b, if point a was north and point b was south and there was a westerly wind over ride, when I turn round and run b to a, I can't change design or load balancing much (shift some people maybe) but the ride will be different. I'm not a designer though, just a boat driver. Trim tabs are essential to maintain speed and comfort offshore to me, although I am Aware thy add weight and drag.

    A twin engine is obviously different such as OP as trim can be controlled by engines individually.
     
  6. ChrisN67
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    ChrisN67 Senior Member

    A single engine boat will have a torque steer issue at most speeds, torque steer trim correction found on the Anode above the propeller is only a compensator and designed to compensate at cruise speeds, not less or WOT.

    Virtually all single engine boats (except for duoprops) will have this propensity.

    I have never seen winds having such a significant effect on a boat underway and at higher speeds as to require corrective trim.

    The statement of using tabs to "compensate for windage" is a peculiar.

    The vessel is far more effected by weight distribution, the single propeller torque, and sea state. I would not even think of compensating for "windage" unless you are in a typhoon or hurricane in which case it is better to be on land.

    I only use my tabs assisting in a faster plane; I have yet to use them for compensating for weight distribution; as a matter of fact over 40 knots I find that tabs (in my case) adversely effect boat handling. I use drive angle to compensate for trim issues. I rarely leave the dock with less than 6 passengers and sometimes as many as 12.

    Winds are only an issue during docking and changes in direction and attitude (sea spray); but have little effect on a vessel underway.

    Gtflash, what boats have you been in the heel into the wind? Your statement :
    " every power boat I ever driven leans into the wind when it's on a side or quarter. Thus making a harder ride."

    I do not doubt your observations; but is it possible that there were other factors at play? Are you 800 pounds and like to lean on the upwind gunwhale? :)
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    This is no great mystery. I comes about because the centre of lateral resistance of the immersed portion of the boat, is further aft than the centre of lateral resistance (air) of the rest of your boat. That produces a turning couple in a cross-wind, which you then compensate for by use of the helm. With an outboard motor, this puts the line of thrust off-centre, and introduces heel, much as a normal turn heels the boat. You can easily verify this by looking at where the engine is pointing, when the lean is evident, it will not be straight ahead. Trim tabs largely solves the problem, as you mentioned. Boats with a lot of deadrise, combined with high structure forward, are most prone to cross-wind lean.
     
  8. mydauphin
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    mydauphin Senior Member

    The problem with boat designs is that the boat changes over time. The boat starts out as 40 knot boat with 400hp, then the an idiot like me adds 200 more hp or worse the marketing tells the designer they need a faster boat so add the bigger engines. The hulll and boat are now close to the tipping point as far as controlabilty. Then you add a couple of overweight people in the back or front, and the boat is jsut not working as designed.

    This is all happen to a boat I owned, we move the batteries to the front. Another interesting the boat had different issues at low speed than high speed,. We eventually fixed the issues. One of the problems was the props were not counter rotating so with the extra horsepower it would walk to the side.

    Trim tabs are important but again if your boat has a problem try to find out what it is, don't just cover it up.

    On windage, never felt it on a small fast boat, but on a very large light slow boat like I have now, you can feel it push you I add a little more rpms to one engine if it is a liong trip.
     
  9. gtflash
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    gtflash Senior Member


    I agree :)
     
  10. gtflash
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    gtflash Senior Member

    A chequered boat past, (all planning hull) U.S. day boats like regal, and searay, before British built speed boats, Shakespeare, phantom, ring. British rib boats of numerous makes and twin stepped offshore racing hulls. Never driven anything bigger than 9m or faster than 65knots.

    One local port (and favourite pub) is south west in location from my home base by only a couple of mile. The prevailing winds are from west and the tide runs
    East and west. Leaving port to come home with substantial wind on my port front or rear quarter, the waves will kick up and break on boat and make us wet at slow speed. Up on the plane the boat will lean to port thus landing harder on the outer chine I presume. A little tab to lift the port side and the boat will land straighter, presenting the sharper part of the v bottom to the waves and keeping my passengers happy and allowing me to take a more direct route.

    My first boat a regal 2150 cuddy thing with not tabs would slam so hard I would have to run head into or away from waves, tacking across to avoid quartering sea. It didn't run great into a sea neither so was only comfortable with everything behind us.

    maybe the point is that it is the helm input to maintain course, what causes lean, not the wind??
     
    Last edited: Oct 27, 2016
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Maybe ChrisN67 has a boat with not much superstructure forward, the offshore race boats with the low foredecks and reverse sheer forward got rid of the issue to a substantial degree.
     

  12. mydauphin
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    mydauphin Senior Member

    Just remembered a flat bottom skiff with 5 hp, only way to get it to plane was running it into the wind at an angle, it would then plane one corner, and go a little sideways.
     
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