Gear Ratio vs speed

Discussion in 'Inboards' started by cheddar, Jul 6, 2009.

  1. cheddar
    Joined: May 2009
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    Location: Gulf Coast

    cheddar Junior Member

    I just purchased a 41 foot fiberglass shrimp boat. The engine is 671N DD and the gear ratio is 4:1. The prop is 4 blade and very large in size. The vessel only does 9 knots and that is pushing her.

    I would like to get more speed out of it as I am not interested in shirmping just fishing and motoring around.

    What are the rules as far as changing the gear ratio to 2:1 or changing the prop? Do they have to match?

    What should I be looking to do in order to get some more speed, is it worth it?

    please give me your opinions!
  2. TeddyDiver
    Joined: Dec 2007
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    Location: Finland/Norway

    TeddyDiver Gollywobbler

    You got to get some more information.. like rpm now and recommended, prop diameter, pitch and blade area, boat weight etc. Shrimp boat is not enough..
    For starters you could clean the bottom when you get the prop measured :)
  3. cheddar
    Joined: May 2009
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    Location: Gulf Coast

    cheddar Junior Member

    can you make any determination as to the best direction without any specific information as you stated above.

    I know the engine runs very hard to do 9 knots as it is 4:1 gear. I know the wheel is huge about 3' around. I was told the boat would pull a house. Until I find out the specifics that is all i know.

    I guess I was just wondering about the gear box and prop and how they do or do not work together.
  4. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Running at 9 knots is the best you'll likely get from the old gal, if it's typical of most moderate size working craft. At this speed, assuming a 37' LWL you're pushing S/L 1.5 and running into a wall of bow wave. If you quadruple the HP of your current engine, you might get to 14 knots, put it would require huge amounts of fuel, not to mention 4 times the HP.

    In short, you have a displacement hull that is pushing past it's limit at 9 knots. You could half your fuel use by throttling back to 7 knots. At this speed, your engine shouldn't be working very hard, whereas at 9 knots it's trying to kill itself.

    It's a hull shape issue, not a prop/gear ratio/engine issue. Your speed is limited by waterline length, friction and the wave train the hull makes underway.

    Without pictures of your hull (out of the water), HP, weight and a few other dimensions, these are the best answers that can be provided.
  5. cheddar
    Joined: May 2009
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    Location: Gulf Coast

    cheddar Junior Member

    Sorry par I should of told you guys it is a lafite skiff hull. The prop sits partially in a tunnel. She barley makes a wake.

    with the 671 DD and an allison 4:1 gear would changing the prop to a three blade 2.71:1 I had on my other 40' (steel displacement hull) twin 671 DD engines do anything for me. it appears to be the same shaft size.

    the prior capt said he has the Detroit dialed in for 3/4 throttle. She has more but he said it is not necessary for his operation and was better for the engine.
  6. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Work boats are usually geared down and overpowered to account for towing or trawling. To get more speed you may also have to add trimtabs or other way of preventing squat. The power should be enough for a lower ratio if you are taking a load off the boat.

  7. mydauphin
    Joined: Apr 2007
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    Location: Florida

    mydauphin Senior Member

    Like someone said, make sure bottom is really clean. At what rpm is 7 knots, 8knots, 9knots, and max rpms. This may tell you if something about engine loading before WOT. Perhaps a higher pitch might get you 9 knots at less rpms and may be a one more knot. Also lighten boat as much as possible, remove fishing gears and masts. Then remeasure performance.
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