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| Propellor question- overpropped? |
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Richard mac
| | Posted on Thursday, May 16, 2002 - 05:12 am: | 
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Hi again,
My 31 ft steel yacht is very heavy, at approx 8.5 tonnes. It has a 27 hp 3hm Yanmar diesel, which runs very well, except will not drive the boat at over 2200 rpm. It just bogs down and starts blowing more smoke. This is about 6 knts. I would imagine hull speed for the boat would not be much above this, so it would seem that the boat is overpropped, as the Yanmar is supposed to rev to 3200 rpm. However, it is the original yanmar prop, which comes with the Motor, gearbox, shaft, all matched to each other. So it seems strange that it could be massively overpropped, to the tune of losing 1000 rpm.
It also does not look over big compared to plenty of other props at our boat club. In fact it looks quite small.
Thoghts anyone? by the way the engine revs cleanly all the way to redline in neutral.
Thanks for your advice.
Richard. |
Kim Boehm
| | Posted on Thursday, May 16, 2002 - 08:02 pm: |
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Richard, sound like you are overpropped.You can do several things, but first I would contact a local marina or boat shop or even a prop shop. You can go down in pitch or diameter, but pitch is the best way. You may be able to repitch existing prop or swop for one of less pitch. exactly how much pitch you need to lose can be determined by existing RPM obtained at WOT. (Wide Open Throttle). Good luck and keep us posted . Kim |
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Visitor
| | Posted on Monday, March 06, 2006 - 07:27 pm: |
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Murray McGehan
New member
Username: murray_m
Post Number: 1
Registered: 08-2009
| | Posted on Friday, August 21, 2009 - 04:11 am: |
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Richard what reduction gear do you have and what size prop. If the the prop did come with the motor something is really out of wack to drop 1000 rpm assuming your boat is around 26ft on the waterline hull speed should be closer to seven knots, square root of waterline x 1.34 will give you hull speed. Changing diameter or repitching will work fine for decreasing engine load but probably won't gain you any more speed. What you gain in revs you will probably lose in reduced bite in the water. Your suggestion that it may be something mechanical seems quite feasible and here is a list of the things I would check
air & fuel filters
check exhaust from manifold to overboard discharge for any obstructions.
Get injectors and fuel pump in for a service
Get a compression test done on engine.
Back to the propeller if the engine is that badly over propped and assuming that your engine has a 2.2:1 reduction rather than cut the blades down or reducing pitch I would consider changing the reduction gear from 2.2:1 to 3.2:1 as in displacement hulls a larger slower turning propeller is far more efficient than a smaller one turning faster. To give you a brief run down on my own experiences I have done this twice once on a yacht and once on a very heavy displacement launch.
I had a 26 ft 1.8 tonne yacht which we repowered with a Yanmar 1GM this model was earlier than the 1GM10 and only 7.5hp experts suggested the prop size but they were way out and like you I was 1000rpm down and only just getting 6 knots. A little bit of a disappointment as the previous Yanmar YSE8 gave 6.5 knots. A friend had the same class of yacht and just repowered with the same engine and he also struck the same problem, he decided to alter prop dia and pitch from 13x8 inches to 12x6 inches although he gained 600 rpm his engine was still not reaching full revs and was still down 400rpm. Straight away I realised this was the wrong way to go as I was also swinging a 13x8 so I changed my reduction ratio to 3.2:1 once this was done the boat was now slightly underpropped so I had another two inches of pitch put on the blades.
Unfortunately I couldn't go any larger in diameter as it would reduce the tip clearance to the hull too much.I would have liked to try a 14 x 8 but tip clearance would have been below 10% of the propeller diameter
After this pitch adjustment the engine would rev to max (3600) and give a boat speed of 6.75
On the launch we did the results were very much the same with propeller slip being reduced from 33% to 24% |
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