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  1. #1
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    Default the relationship between prop size and thrust

    I am confused

    Not sure I really understand

    I can see that a small prop revving fast can push a small boat along at high speed

    but I am not sure I understand why the same size engine ith a slow revving big prop might be more efficient and push my yacht hull faster than a small high speed prop

    yours

    Thicky of Buckingham

  2. #2
    poter's Avatar
    poter is offline Registered User
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    Default Props

    The prop size is important & needs to be the correct size for your boat but more impotantly is the pitch of the blades & the cut.

    Check out with a good engine or prop manufacturer for details.

    Poter

  3. #3
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    Default

    Quote Originally Posted by dylanwinter View Post
    I am confused

    Not sure I really understand

    I can see that a small prop revving fast can push a small boat along at high speed

    but I am not sure I understand why the same size engine ith a slow revving big prop might be more efficient and push my yacht hull faster than a small high speed prop

    yours

    Thicky of Buckingham
    Drag
    I may be wrong, but I'm not confused.
    ⛵ Ex-SolentBoy

  4. #4
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    Default

    This may be total b*****cks...
    Think of the column of water that the engine can shift past the prop. The small prop, spinning fast, is shifting a narrow column at high speed. The big prop, spinning slowly, is shifting a wider column at lower speed. The idea is to match the speed of the column of water to the (cruising) speed of the boat. If the small prop is moving the narrow column faster than the boat can travel then it is wasting a lot of its energy. The wider column, from the bigger slower prop, is more likely to be moving at a speed commensurate with the speed of the vessel.
    Of course, to change from a small, fast prop to a large, slow prop also requires a different gearbox or you will be running the powerhead at the wrong rev range for maximum power.
    That's roughly my theory anyway

  5. #5
    William_H is offline Registered User
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    Default Prop size

    You have got as I see it two aspects.
    Firstly the pitch of the prop is just like the gear ratio of your car or bicycle. A small high speed hull (planing) needs a prop coupling the engine speed and torque to the water speed effectively going past the prop.
    So for low speed you need fine pitch for high speed you need course pitch.
    However you need to be actually pushing the water so you need more speed of water pushed than hull speed.
    The next question is of efficiency in coupling this thrust to the water. As said a small column of high speed water pushed out from the prop will push the boat along a bit like a jet. However a large column of water (or closely coupled prop to all the water) generates less slip and turbulence and is more akin to oars rowing a boat. This is what a large diameter prop does but this requires that the engine revs prop speed and pitch all be much more closely aligned to the hull speed. ie just a small amount of push (slip)

    So you have for tug boats a variable pitch large propeller which gives just the correct coupling of propeler to the water regardless of very low speed tugging a ship or relatively high cruising speed.
    In a similar way small light aircraft have a fixed pitch propeller. But with any sophistication the propeller is made variable pitch (AKA constant speed prop) for thrust at acceleration take off and thrust at high cruise speed. Similarly we find prop aircraft are more efficient than jets. (for other reasons jets are more efficient at high speed and high altitude)

    So for your boat you need an o/b with fine pitch prop for low speed (compared to the engine on a dinghy) and especially hard work against strong winds and waves. On the other hand you want normal course pitch for low engine revs at easy cruising. Most people go for the former but the latter are more standard props so more common.
    The diameter however is dictated by the engine dimensions. good luck olewill

  6. #6
    penfold is online now Registered User
    Location : On the Clyde with a good view of where Kip chimney used to be
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    Default

    Basically, the larger the prop and the fewer blades it has the more efficient it is; the most efficient prop on paper would be a single blade of infinite size spinning infinitely slowly. Obviously that isn't very practical; single blade props would be very difficult to balance, very large props cost a lot(bronze isn't cheap) and cause a lot of drag when not motoring. There are also practical limits from proximity of the hull(the gap between the blade tips and the hull needs to be at least a minimum size to avoid interaction), engine torque curve, size/weight/cost of gearbox, size/weight/cost of propshaft, etc. Outboards have the props they have because bigger props are more expensive and heavier and require bigger, heavier, more expensive gearboxes to gear down the output from the engine.

    On big ships there are other concerns like harmonic vibration; at certain shaft speeds there will be resonant frequencies which can have an adverse affect on things, like destroying bearings and couplings or causing the propellor to drop off. These are calculated by the naval architect who will specify propshaft size, bearing scantlings and propellor size and blade number so the resonance does not occur at a shaft speed likely to be useful.
    Grünkraft? - nein danke.

  7. #7
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    Default

    Specifying the correct prop is complex. To look at the subject in simple terms you need to seperate diameter from pitch. First, regardless of pitch the diameter is related to the hull resistance and speed you wish to achieve. A hull with high resistance or pulling a large load like a tug at slow speed will have a large diameter running relatively slowly. A fast light boat will not have such a large prop and will run faster. Its a matter of torque rather than power which is generated by the area of the blade. Too small a prop on a big tug would be like an egg whisk and would "slip" and cavitate. Too big a prop on a small light vessel would cause the torque to literally try and turn the vessel rather than drive the boat along (too much torque) It would also cause drag when not rotating on a sailboat. So there is an optimum diameter and speed of rotation somewhere and this is mostly decided by the naval architect or hydrodynamicist before you get the boat so there is little room to increase and no advantage in decreasing.

    Having got diameter out of the way there is a lot of choice regarding pitch. You need to decide on gearbox ratios and engine speed before choosing pitch. To get the optimum efficiency again you need to consider load and displacement. A heavy boat will be better with a course pitch and slow rotation speed where a lighter boat can probably get better efficiency by the opposite. The wrong choice will reduce efficiency and increase slip. Having got that right you then decide on the speed your hull can reasonably achieve with the power available and pitch the prop accordingly taxing the efficiency into account. Its a compromise therefore between rotation speed, diameter and pitch to give your boat the optimum speed with the least power consumption. All in all it's much more complicated than most owners realise but having a boat with a certain engine, gearbox and diameter of prop means usually the only choice is pitch.

  8. #8
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    Default

    Quote Originally Posted by dylanwinter View Post
    I am confused

    Not sure I really understand

    I can see that a small prop revving fast can push a small boat along at high speed

    but I am not sure I understand why the same size engine ith a slow revving big prop might be more efficient and push my yacht hull faster than a small high speed prop

    yours

    Thicky of Buckingham
    Slip.

  9. #9
    Join Date
    May 2003
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    Default

    Quote Originally Posted by Salty John View Post
    Slip.
    and displacement Vs a planing hull
    I may be wrong but not always

  10. #10
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    Default

    It is a black art not a science. I have just gone from a fixed three Claude 16 x 12 prop to a folding two blade 16 x 12 prop and the difference is amazing, much more bite and grip on the water and revs are well down now. But the pitch and diameter is the same.

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