RPM to Speed

Discussion in 'Propulsion' started by Andy58, Oct 16, 2022.

  1. Andy58
    Joined: Oct 2022
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    Location: Yorkshire, England

    Andy58 Junior Member

    Can anyone help. Am I right in thinking the relationship between engine rpm and boat speed so long as the gearbox is good should remain fairly constant. My boat used to do 27 knots at 2600 rpm but will now only do 20 knots. I have checked for slippage in both gearbox’s and that’s ok. The bottom is clean and the props are not damaged what else should I look for.
    Thanks for any help.
     
  2. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    No, I'm afraid that you're wrong there. If the resistance increases for some reason, the forward speed will be reduced while the propeller rpm's stay fairly constant (to a limit). This happens because the propeller thrust increases with a lower speed, until there is a new balance point found, where available thrust exactly meets the new requirement.

    So, provided there is no damage to the propeller, what else is there that would increase hull resistance? How about weight; every time one leaves the boat, some stuff or other is left behind onboard. And most building materials, except metals, are hygroscopic.
     
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  3. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Welcome to the Forum Andy.

    I will second Baeckmo's suggestion above re weight - planing boats are especially sensitive to weight gain, and it is very easy to subconsciously add weight, and difficult to get rid of it (many people can relate to this personally as well :) )

    What is the time frame involved between when the boat could do 27 knots, but now can only manage 20? Is it years or months?

    And what type of boat do you have, and what type / size are her engines?
     
  4. Andy58
    Joined: Oct 2022
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    Location: Yorkshire, England

    Andy58 Junior Member

     
  5. Andy58
    Joined: Oct 2022
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    Location: Yorkshire, England

    Andy58 Junior Member

    Thanks for the welcome and thank you both for your input.
    The boat is a Sealine 450S with twin Volvo Penta 71 380 hp each.
    I take the point about weight and hull resistance and can look more into this.
    Am I right that if the engines had a problem this would result in the revs dropping as well as the speed.
    as to time scale this is only months.
    Thanks
     
  6. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Here is a sister ship that was for sale with Boatshed - some useful background info on the 450S here.
    Sealine 450 Statesman For Sale, 13.70m, 1991 https://portsolent.boatshed.com/sealine_450_statesman-boat-236535.html

    Re how you are only getting 20 knots now, is this at 2,600 rpm, or have your maximum revs also now dropped as well?

    Andy, you could also try asking your questions on the YBW Motor boat forum in Britain - there are quite a few Sealine owners on there.
    Motor Boat Forum https://forums.ybw.com/index.php?forums/motor-boat-forum.14/
     
  7. Andy58
    Joined: Oct 2022
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    Location: Yorkshire, England

    Andy58 Junior Member

    No the revs haven’t dropped at all it will still rev to 2600 and it gets there quite quickly and easily in
     
  8. yOyOeK1
    Joined: Jul 2022
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    Location: Poland

    yOyOeK1 Junior Member

    I wood recommend clean bottom and clean but really clean a prop. :rolleyes:
    Getting rid of weight will not any thing worse :p
     
  9. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    How are you measuring speed?
    Could your knot meter be fouled?
    Could an electronic connection be loose?
     

  10. Andy58
    Joined: Oct 2022
    Posts: 5
    Likes: 0, Points: 1
    Location: Yorkshire, England

    Andy58 Junior Member

    Thanks for your comments. I use the normal log to give me STW and gps to give SOG both are very close it’s not a very strong tide we’re I am.
     
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