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  1. #11
    Join Date
    Jan 2011
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    5,270

    Default Re: Engines, props, and boatspeed

    Quote Originally Posted by Stemar View Post
    Yes, or increase the diameter, which may not be possible.
    How does increasing the prop diameter increase your boat speed for a set rpm?

    Surely it is the pitch which does this. If a 15" pitch means that the boat moves 15" forward per rotation, then to increase the speed (i.e. distance per rotation) surely you would increase the pitch.

    I assume that increasing the pitch will require more torque to overcome the load of the additional water it is displacing. What does increasing the diameter do? I am assuming that you increase diameter to get physically bigger blades to reduce slippage, yes?

  2. #12
    Join Date
    May 2007
    Posts
    34,426

    Default Re: Engines, props, and boatspeed

    Quote Originally Posted by Bobc View Post
    So, what you're saying is that torque is ultimately more important than hp because it is the torsional force than pushes the water past the prop blades? If this is the case, why do engine manufacturers rate their engines in hp rather than max torque?
    Because torque without RPM is no use.
    What you really need to know is the power vs RPM curve, or the torque vs RPM curve.
    If you know one, you can derive the other.

    The only engines which aren't quoted in power terms tend to be aircraft jet engines, which are quoted in pounds of thrust. Power = thrust times speed, but there's a fudge factor for turning knots x lbs into watts or HP....

  3. #13
    Join Date
    May 2007
    Posts
    34,426

    Default Re: Engines, props, and boatspeed

    Quote Originally Posted by Bobc View Post
    How does increasing the prop diameter increase your boat speed for a set rpm?

    Surely it is the pitch which does this. If a 15" pitch means that the boat moves 15" forward per rotation, then to increase the speed (i.e. distance per rotation) surely you would increase the pitch.

    I assume that increasing the pitch will require more torque to overcome the load of the additional water it is displacing. What does increasing the diameter do? I am assuming that you increase diameter to get physically bigger blades to reduce slippage, yes?
    All props slip.
    So a 15" pitch prop might be moving the water around it say 14" relative to the prop, but the boat could be tied to the dock and the prop is just moving water around, the boat might move 10" and the water goes 4" back or any combination.
    A bigger diameter prop will want to make the boat move forwards more and the water back less.
    A smaller diameter prop of the same pitch will move a smaller volume of water backwards faster.
    Getting the diameter right balances efficiency against flexibility.

    Can someone post a link to explaining that better please?

  4. #14
    Join Date
    Jan 2011
    Posts
    5,270

    Default Re: Engines, props, and boatspeed

    Quote Originally Posted by lw395 View Post
    All props slip.
    So a 15" pitch prop might be moving the water around it say 14" relative to the prop, but the boat could be tied to the dock and the prop is just moving water around, the boat might move 10" and the water goes 4" back or any combination.
    A bigger diameter prop will want to make the boat move forwards more and the water back less.
    A smaller diameter prop of the same pitch will move a smaller volume of water backwards faster.
    Getting the diameter right balances efficiency against flexibility.

    Can someone post a link to explaining that better please?
    Ok, so what you're saying is basically that size of prop moves volume of water and pitch of prop defines forward motion?

    This is all good. Keep it coming.

  5. #15
    Join Date
    Jan 2011
    Posts
    5,270

    Default Re: Engines, props, and boatspeed

    So thinking about this a bit more overnight, based on the above comments, I assume that a big heavy boat that won't go very fast (like a Thames Barge) would have a big diameter prop with a small pitch (driven by a lor-revving engine with lots of torque), so it can shift a lot of water in order to get the bulk moving, whereas a hull that is easily driven and that wants to go fast, will have a smaller diameter (because it takes less to get it moving, but will want more pitch so that it can reach and maintain the desired speed, and would be more suited to a higher-revving engine with more hp and less torque.

    Is this right?

  6. #16
    Join Date
    Aug 2011
    Location
    On the Celtic Fringe
    Posts
    13,336

    Default Re: Engines, props, and boatspeed

    Personally, I'd be contacting the prop manufacturers to get their view on the matter.
    Cynical Scottish almost retired engineer.

  7. #17
    Join Date
    May 2007
    Posts
    34,426

    Default Re: Engines, props, and boatspeed

    Vaguely, yes.
    A bigger prop more tightly couples engine revs to boat speed.
    A smaller prop allows the revs to build before the hull speeds up.

    A powerboat for instance, you open the throttle and the engine revs up quickly, the boat takes a while for the speed to build.

    A big prop will be efficient at cruising speed, but if you run aground, it may deliver less thrust to get you off.

    If I overload my tender with its little outboard, full throttle won't allow full revs, so it doesn't develop much power, despite needing more...

    There are whole books on props.
    Sections in few books on yacht design, e.g. Larrson and Elliason (sp?)

  8. #18
    Join Date
    Jan 2011
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    5,270

    Default Re: Engines, props, and boatspeed

    Quote Originally Posted by Sandy View Post
    Personally, I'd be contacting the prop manufacturers to get their view on the matter.
    I have, but:-

    1./ They have all given me different answers
    2./ I don't just want to accept what I'm told without understanding why, as buying the wrong size of prop is an expensive mistake to make.

  9. #19
    Join Date
    Aug 2001
    Location
    Scotland
    Posts
    1,733

    Default Re: Engines, props, and boatspeed

    If you want to keep present prop and run (the prop) at around 1000 rpm then you could fit a gearbox with a 1.5 to 1 ratio. There are a few out there, I have Hurth 1.5 : 1 and run engine at 1500 to get 1000 prop.

    You could also download this excel workbook if you Google

    Surbaud prop calculator

  10. #20
    Join Date
    Jan 2011
    Posts
    5,270

    Default Re: Engines, props, and boatspeed

    Quote Originally Posted by Jcorstorphine View Post
    If you want to keep present prop and run (the prop) at around 1000 rpm then you could fit a gearbox with a 1.5 to 1 ratio. There are a few out there, I have Hurth 1.5 : 1 and run engine at 1500 to get 1000 prop.

    You could also download this excel workbook if you Google

    Surbaud prop calculator
    No, don't want to keep the current prop because a)it is a fixed prop, b) is is too small, c) the new engine rotates the other way.

    Just found this article which is quite interesting. Similar size boat and engine and has 19 x 14 prop which is undersized (so very similar situation to mine). I am surprised to see that the props he is testing are about 22 x 17, which is a big jump up in size.

    http://yaffa-cdn.s3.amazonaws.com/ya...ldingprops.pdf

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