Over Prop/Under Prop? Confusion

Andydent2000

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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.
 

CLB

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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.
 
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CLB

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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.
 

PCUK

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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.
 

Andydent2000

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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)
 

CLB

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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.
 

sharpness

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Andydent2000 , I'm assuming your boat is the Powles 46 ? If so, she'll weigh a lot more than 11 tonne. A quick google shows old sales adverts of Powles 46 stating displacement 17,500 kg.
I remember enjoying reading your posts of fitting the Tamd61 engines. Did they used to achieve full revs of 2800 when they were fitted or have they always only achieved 2600?
 
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