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  1. #21
    Join Date
    May 2007

    Default Re: Engines, props, and boatspeed

    Pretty coloured graphs in the link showing all the props come out quite similar in most ways.
    The dominant effect is that it takes a lot more power to do ever incremental half a knot once you get above 6 knots.
    The test boat seems to have ample power in the engine, so can drive firmly into the diminishing returns zone known as 'hull speed', regardless of minor differences in the prop.
    The gearbox ratio will influence the pitch required. Article says 2.74:1 'which is higher than usual'

    In the real world what you might want to know is how the thing will go into the teeth of a gale and Solent chop. If you worry more about that than optimum flat water performance, it's going to move you to a smaller prop. If you want something in hand for when your engine is old and down on power, smaller prop.
    Last edited by lw395; 03-02-19 at 18:47.

  2. #22
    Join Date
    Jan 2011

    Default Re: Engines, props, and boatspeed

    Yes, it's starting to appear obvious now that there is a balance in prop size, from what might produce the ideal in calm weather and what will allow you to punch into the chop or into a head-wind.

    So much seems to come down to engine torque, the rpm which that torque is achieved at, and the volume of water that the engine is able to shift.

    My current engine is a Perkins-Prima based unit with a turbo, which has close to F**k-all low-down power and needs to be revved in order to get any grunt out of it (looks to me like it is more suited to a small motorboat than a large yacht). The engine that I'm looking to replace with has a lower ultimate hp rating, but at cruising revs, the current engine is only producing 30% of the rated hp and not a lot of torque at all, whereas the engine that I am looking to put in has similar hp and more torque at its cruising revs, even though that lower than the cruising rpm of the current engine.

    There's more to specifying engines than simply "how much hp it has", that is clear now. The same appears to be the case with prop size.
    Last edited by Bobc; 03-02-19 at 20:42.

  3. #23
    Join Date
    Sep 2008
    Costa Brava

    Default Re: Engines, props, and boatspeed

    Quote Originally Posted by Bobc View Post
    c) the new engine rotates the other way.
    Some gearboxes are bidirectional. I.e. can be run indefinitely in either direction.

    So you just need to change the linkage to the lever in the cockpit to work the other way around if you can't get your head around moving the lever forwards to go backwards and vice versa.
    My boat is for sale - ask me if interested.

  4. #24
    Join Date
    Oct 2012
    Southminster, essex

    Default Re: Engines, props, and boatspeed

    Quote Originally Posted by Bobc View Post
    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.
    Has it occurred to you that different blade shapes may require different pitch/diameters?
    Bruntons alter the effective "working "pitch of their autoprop, just by grinding a small bevel on the outer end of the prop blade
    So working on that basis it may be that different blade designs need different pitches & diameters which is why the manufacturers are giving you different figures. They may also be applying different requirements into the calculations, such as fuel economy, or maximum thrust at different speeds. Have you specified what you want. Ie speed through chop ( an important factor in my view as my boat stops dead in a bit of chop) or max speed in flat water (great if you only go out in calm weather) or fuel economy ( if you are a tight wad) . I would rather have a boat that went 1/2 a knot slower at top speed but lost less in a chop than do 7knots in a canal & 3 knots in Fishermans Gat in F6.
    There are so many factors that you need to question each potential supplier. Or so I would have thought.
    Last edited by Daydream believer; Yesterday at 09:44.
    It is all down to the fact that my wife does not understand me !!

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