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  1. #11
    Join Date
    May 2001
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    UK East Coast
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    36,520

    Default Re: Is This Fouling The Likely Cause of Failing To Reach Max Revs

    Quote Originally Posted by anoccasionalyachtsman View Post
    My thinking is that the flow into the prop would not only be turbulent, but also much slower. If the flow into the disc is reduced, the thrust increases (and power demand goes up). Same as boundary layer efficiency really. If/when cavitation happens then the thrust will reduce again. That's the tricky bit, which wins? Certainly, you couldn't really improve on that bundle of mussels for slowing the stream.
    What if the OP tries it in reverse gear?

  2. #12
    Join Date
    May 2009
    Location
    Wirral
    Posts
    688

    Default Re: Is This Fouling The Likely Cause of Failing To Reach Max Revs

    When did you last check exhaust elbow? common on these Perkins based engines to partially block and cause loss of revs when in gear but not much difference when in neutral.
    Leisure Owners Association

  3. #13
    Join Date
    Sep 2001
    Posts
    1,509

    Default Re: Is This Fouling The Likely Cause of Failing To Reach Max Revs

    Had the same problem suddenly appear with MD17D. Suspected something around prop. When lifted prop, cutter and P bracket fouled as yours. Yard blasted it off. Engine revs back to normal. Suspect disturbed water flow decreases efficiency of the prop which then has to work harder thus reducing revs. That’s my entirely unscientific explanation.

  4. #14
    Join Date
    Jul 2009
    Location
    East Coast
    Posts
    1,727

    Default Re: Is This Fouling The Likely Cause of Failing To Reach Max Revs

    Every time the prop blades turn, your deadwood in front of the blade drastically reduces power/suction. I know, because I owned a boat with a small aperture and two bladed prop.

    Your problem is further compounded, because of the mussels disturbing the flow. I would guess that you are losing about 40% of the potential power of the prop.
    It is never too late to have a happy childhood. Buy a boat.

  5. #15
    Join Date
    Feb 2008
    Location
    Suffolk
    Posts
    3,701

    Default Re: Is This Fouling The Likely Cause of Failing To Reach Max Revs

    Quote Originally Posted by peteK View Post
    When did you last check exhaust elbow? common on these Perkins based engines to partially block and cause loss of revs when in gear but not much difference when in neutral.
    Agree - check the exhaust elbow, my previous boat with a Perkins Perama engine(before renaming as VP)had a blocked exhaust elbow, revved easily to 3600 in neutral but only up to 1500 under load - when checked the elbow only had a 10mm hole in the centre, tried cleaning the carbon out but my screwdriver went through the casting... Fitted a new elbow and it immediately went up to full revs under load. ps - I actually removed the elbow completely as it was not obvious it was blocked as access was difficult.

  6. #16
    Join Date
    Apr 2004
    Location
    Portchester, Solent
    Posts
    5,013

    Default Re: Is This Fouling The Likely Cause of Failing To Reach Max Revs

    Did you ever get to full revs when the hull and prop were clean? Apart from the fouling it could also be that the prop has a pitch which is limiting the speed of rotation.

  7. #17
    Join Date
    Jun 2008
    Location
    Brixham
    Posts
    942

    Default Re: Is This Fouling The Likely Cause of Failing To Reach Max Revs

    When you say under load, do you mean tied to the pontoon and in gear or underway?
    If voting changed anything, they wouldn't let you do it.

  8. #18
    Join Date
    Jun 2015
    Posts
    1,721

    Default Re: Is This Fouling The Likely Cause of Failing To Reach Max Revs

    Quote Originally Posted by pvb View Post
    What if the OP tries it in reverse gear?
    Good question, no real turbulent flow, blocking is much the same*. My guess (like yours I suspect) is that it might be a little better but how much of any improvement is due to a less efficient prop would need a load of instrumentation to establish!

    I'm sailing with someone who, unlike me, actually finished a nav arch degree tomorrow evening, and it'll give us something to chat about.

    * If you've played with an indoor model helicopter you'll be familiar with how ground effect makes it hop into the air easily, but the climb need more power. It's because the first layer of air is stuck to the ground, and successive layers are held back with drag from the lower ones, so the 'exhaust' air can't get out of the way so easily. What you may also have noticed is ceiling effect and wall effect. In exactly the same way as on the ground, as the rotor nears the ceiling, suddenly the air can't freely come from above and has to come through the gap. That's got more friction and so the air doesn't arrive as quickly. Result is that the blades, climbing at the same speed, will now pull the helicopter up so fast that it's almost impossible to chop the power in time. Same at a wall.

    What I'm trying to put across is that it doesn't really matter whether the blockage is in front or behind, or to the ship designer's dismay, above. Trying to get the flow to arrive at the same speed all over the propeller is the propulsion specialist's nightmare.

  9. #19
    Join Date
    May 2016
    Location
    Suffolk
    Posts
    7,968

    Default Re: Is This Fouling The Likely Cause of Failing To Reach Max Revs

    I suspect the fouling will easily cause the problem. Either way, it needs to come off, so it makes sense to me to get it cleaned and see what happens.

    By the way, i know Garys boat and it isn't the elbow or the fuel filters, these are all new. Although without that knowledge they were perfectly valid suggestions.
    Rainbow Marine.
    www.rainbowmarine.co.uk

  10. #20
    Join Date
    May 2014
    Location
    March
    Posts
    571

    Default Re: Is This Fouling The Likely Cause of Failing To Reach Max Revs

    Quote Originally Posted by peteK View Post
    When did you last check exhaust elbow?
    It was new approx 50 hours use ago, presuming still OK, but I have also fitted a new water trap/muffler and swan neck, so back pressure is on my list of unlikley possible causes if clearing the fouling does not work. Ta for the suggestion

    Quote Originally Posted by Kurrawong_Kid View Post
    When lifted prop, cutter and P bracket fouled as yours. Yard blasted it off. Engine revs back to normal.
    It sounds more and more likely that clearing the fouling could be the end of this problem.

    Quote Originally Posted by LONG_KEELER View Post
    Every time the prop blades turn, your deadwood in front of the blade drastically reduces power/suction. I know, because I owned a boat with a small aperture and two bladed prop.

    Your problem is further compounded, because of the mussels disturbing the flow. I would guess that you are losing about 40% of the potential power of the prop.
    So close to the 50% that I am currently losing

    Quote Originally Posted by alahol2 View Post
    Did you ever get to full revs when the hull and prop were clean? Apart from the fouling it could also be that the prop has a pitch which is limiting the speed of rotation.
    Good point. Yes I have had full revs with this prop so I have discounted that as a possible cause.

    Quote Originally Posted by Laysula View Post
    When you say under load, do you mean tied to the pontoon and in gear or underway?
    So far testing limited to yesterday, underway.

    Quote Originally Posted by PaulRainbow View Post
    I suspect the fouling will easily cause the problem. Either way, it needs to come off, so it makes sense to me to get it cleaned and see what happens.
    Thanks for the advice Paul.

    I'll book a lift out, clear the fouling, sort a few other jobs, and test again.

    Will report back in due course.

    Many thanks for all the help and advice, very much appreciated.
    1976 Westerly Centaur, GEMINI

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