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  1. #1
    Join Date
    Jan 2011
    Location
    Birmingham
    Posts
    127

    Default Over Prop/Under Prop? Confusion

    I am hoping you experts here can help.

    I had my pride and joy re-engined a few years back and over the past few seasons we have been working on a number of niggles - not the least being an overheat problem.

    My main problem is that the boat has never achieved the 21 knots it used to be able to do despite the engines being more powerful (305 rather than 270). We seem to top out at 17 knots.

    Or full RPM - we top out at 2600 rather than 2800.

    There is no smoke to speak of and now the engines sit nicely on temperature all day long.

    On the boat there are two plaques one saying the weight is 17 tonnes and the other 19 tonnes. When ordering the new props we went with 19 tonnes in the calculation as she is an old boat and there is lots of 'stuff' on board.

    I should also mention that I have had the hull peeled and copper coated.

    So - here is the thing. The last time I had her lifted I asked them to take a note of the weight. Now I know unless it is calibrated there will be some discrepancy but the guys are lifting boats every day and do know their stuff. They are telling me that she is weighing only 11 tonnes. Even allowing for inaccuracy there is still a 6 to 8 tonne difference meaning my boat is considerably lighter than originally thought.

    Would this cause the symptoms I am experiencing? More importantly - is this causing undue wear and tear?

    Answers gratefully received.

  2. #2
    Join Date
    Jun 2013
    Posts
    3,094

    Default Re: Over Prop/Under Prop? Confusion

    The first problem is that the plaques are not showing the weight, but the Tonnage, which is an archaic measurement of the amount of cargo the boat can carry. Your boat is 11 tons fully loaded as per the crane measurement.
    Last edited by CLB; 27-03-19 at 10:25.
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  3. #3
    Join Date
    Jun 2013
    Posts
    3,094

    Default Re: Over Prop/Under Prop? Confusion

    What engine did you have and what do you have now

    What size prop did you have and what size prop do you have now

    Was the gearbox changed at the same time and what were/are the gearbox ratios

    These questions may help narrow down a possible problem.
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  4. #4
    Join Date
    Jun 2005
    Location
    Mid Devon
    Posts
    6,327

    Default Re: Over Prop/Under Prop? Confusion

    If the boat is significantly lighter then it would over-rev so this is not your problem. Suggest you have a proper prop' calc' using the normal criteria. WL length, Displacement, Engine HP, engine max revs, gearbox reduction.
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  5. #5
    Join Date
    Jan 2011
    Location
    Birmingham
    Posts
    127

    Default Re: Over Prop/Under Prop? Confusion

    What size engine: then: TAMD-70-Ds - now TAMD-61-As
    What size prop - hmm - big ones - sorry. I need to look up the spec for the new ones. I will get back to you
    gearbox ratio.jpg
    We re-used the gearboxes. Attached is an image from the manual showing the ratios (I think it shows the original prop sizes as well)

  6. #6
    Join Date
    Jun 2013
    Posts
    3,094

    Default Re: Over Prop/Under Prop? Confusion

    So you used the same gearboxes but different props?

    As both engines rev to around the same, (2700 for the 70 and 2800 for the 61) I am not sure I see a need to change props. This may be the issue. You really do need to be able to find out the size of the new props that you had fitted. The answer may well be there.
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  7. #7
    Join Date
    Oct 2003
    Location
    west yorkshire
    Posts
    3,504

    Default Re: Over Prop/Under Prop? Confusion

    can you get max rpm on each engines when out of gear?

  8. #8
    Join Date
    Jan 2011
    Location
    Birmingham
    Posts
    127

    Default Re: Over Prop/Under Prop? Confusion

    yes

  9. #9
    Join Date
    Jan 2011
    Location
    Birmingham
    Posts
    127

    Default Re: Over Prop/Under Prop? Confusion

    Quote Originally Posted by kashurst View Post
    can you get max rpm on each engines when out of gear?
    yes I can

  10. #10
    Join Date
    Jan 2011
    Location
    Birmingham
    Posts
    127

    Default Re: Over Prop/Under Prop? Confusion

    I am checking with the engineering firm who made the new props to confirm the settings.

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