Page 1 of 4 1234 LastLast
Results 1 to 10 of 32
  1. #1
    Join Date
    Jan 2011
    Posts
    5,270

    Default Engines, props, and boatspeed

    As some of you know, I'm re-powering the boat this year.

    Having almost settled on the engine, my thoughts turn to props.

    My current prop gives a certain amount of drive and boat speed at a specified prop speed (so let's say at a prop speed of 1000rpm, the boat goes at 7kts.).

    Assuming that the engine has enough power, my assumption is that 1,000 rpm on the prop would give 7kts with the same prop, whatever size of engine was driving it.

    Is my assumption correct, or is there more to it than this?

    Also, my understanding is that torque affects accelleration and stopping power, but has little bearing on the above.

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

    Default Re: Engines, props, and boatspeed

    Unless the drag changes, (e.g. motoring into headwind, weed on hull, whatever), turning the prop at 1000rpm will generate the same thrust and the boat will go at the same speed.

    The torque needed to turn the prop at that speed dictates the power.

    Your understanding of torque is utterly wrong.


    Torque is a twisting force.

    Think of a simpler linear force.
    If you lift a weight with a force of 1N, lift it one metre you've done 1 Joule of work.
    If you do 1J of work every second, that's 1watt.
    So Power = force times speed.

    Rotationally, power= torque times rotational speed
    (times a constant in the unit we normally use!)

    If the engine does not have more torque than equates to the power needed to turn the prop at any given speed, the engine cannot speed up.

  3. #3
    Join Date
    Jan 2011
    Posts
    5,270

    Default Re: Engines, props, and boatspeed

    Ok, thanks for that. Very helpful.

    The engine I'm looking to fit is a lower-revving engine with maximum torque at about 1800rpm, whereas the current engine is higher-revving and produces the same torque at 2200rpm. The current engine's cruising revs (at prop) is about 1200rpm, but the new one will be about 1,000rpm. Both engines produce a similar amount of power and torque at cruising revs.

    So I assume that to get the same boat speed at 1,000 shaft rpm with the new engine as I currently get at 1200 shaft rpm, I will need to maybe go up a bit in prop pitch. Is that right?

  4. #4
    Join Date
    Aug 2006
    Location
    NE Coast
    Posts
    1,553

    Default Re: Engines, props, and boatspeed

    Archivist for the Colvic Watson Owners Group

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

    Default Re: Engines, props, and boatspeed

    Quote Originally Posted by Caer Urfa View Post
    Thanks yes, looked at that.

    I'm just trying to get my head around the principles, because everything I read give different answers, as do the engine and prop manufacturers.

  6. #6
    Join Date
    Sep 2001
    Location
    Home - Sothampton, Boat - Gosport
    Posts
    9,424

    Default Re: Engines, props, and boatspeed

    Quote Originally Posted by Bobc View Post
    Ok, thanks for that. Very helpful.

    The engine I'm looking to fit is a lower-revving engine with maximum torque at about 1800rpm, whereas the current engine is higher-revving and produces the same torque at 2200rpm. The current engine's cruising revs (at prop) is about 1200rpm, but the new one will be about 1,000rpm. Both engines produce a similar amount of power and torque at cruising revs.

    So I assume that to get the same boat speed at 1,000 shaft rpm with the new engine as I currently get at 1200 shaft rpm, I will need to maybe go up a bit in prop pitch. Is that right?
    Yes, or increase the diameter, which may not be possible.

    Plug the numbers into a prop calculator like this one: https://www.vicprop.com/displacement_size.php to see how much. While the physics is straightforward, boats are complicated things so sizing a prop is as much art as science, so it might be worth trying a few more, there are loads out there and go for an average, but I doubt you'd notice if it was a little bit out. I've heard it said that being slightly overpropped has advantages as it gives more relaxed cruising at the expense of not being able to reach peak revs, so losing a bit of flat out speed.
    Steve
    Never believe anything until it has been officially denied.

  7. #7
    Join Date
    Jan 2011
    Posts
    5,270

    Default Re: Engines, props, and boatspeed

    Quote Originally Posted by lw395 View Post
    The torque needed to turn the prop at that speed dictates the power.

    If the engine does not have more torque than equates to the power needed to turn the prop at any given speed, the engine cannot speed up.
    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?

  8. #8
    Join Date
    Jan 2011
    Posts
    5,270

    Default Re: Engines, props, and boatspeed

    : https://www.vicprop.com/displacement_size.php

    That calculator says that with my current setup, the prop should be 16.5 x 9.6

    My current prop is 19 x 15 and it is under-size.

    What am I to make of that? This is my problem. I don't feel that I can trust any of this information.
    Last edited by Bobc; 02-02-19 at 11:58.

  9. #9
    Join Date
    Dec 2014
    Location
    N of Ardnamurchan, winter Loch Melfort
    Posts
    825

    Default Re: Engines, props, and boatspeed

    You need to consider gearbox ratio. See what gearboxes are available for your engine and what the possible ratios are, then plug the numbers through Propcalc.

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

    Default Re: Engines, props, and boatspeed

    Quote Originally Posted by Minchsailor View Post
    You need to consider gearbox ratio. See what gearboxes are available for your engine and what the possible ratios are, then plug the numbers through Propcalc.
    I'm specifically using propshaft rpm here so not to bring gearbox ratios into the equation.

Page 1 of 4 1234 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Latest YBW News

Find Boats For Sale

to
to