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Old 02-06-2009, 14:24   #1
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Location: Central Coast NSW
Boat: motorsailer 17.7oa The Road
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Cruising Prop Size?

I was wondering if anyone could offer an opinion on my situation
I bought my boat a 52 ft motorsailer 45ft lwl/ 15 ft beam/ long keel/ 28 ton or 63000 pounds
I am chasing a vibration / rumble in the end of the drive train so as part of that search I wanted to ensure the prop was straight
I got the prop off on Friday and too it to Austral props in Sydney for testing on Mon
the prop was 30 inch dia with 20 inch pitch 3 blade
They say that the prop is a bit out but not too much - just like the three bears - one blade a little low one a little high one blade just right
The plot thickens - when I bought the boat I was told that the volvo (100 hrs old) had max revs of 2200, and that she should be cruised at max 2000 rpm
my research shows that the volvo (TAMD41P-A) is 200 hp and has max revs of 3800 / the transmission is a PRM402 with 3 -1 reduction
Based on the Volvo max rpm and 3/1 and boat spec - Austral did the calculations and are saying the the prop is too big
They are saying that a 25 dia with15 pitch will allow full horse power to be developed.
I know that they are correct - however I am wondering if I want full power as then I will be using a lot more fuel to cruise at the higher rpm.
Currently in still water the boat will make 6.5 knots at 1500 rpm and 8 knots at around 2000 rpm
I think the hull speed would only be about 10.5 so what I am asking is "am I doing anything to harm the Volvo in running it under these load conditions?" as it seems that the drive has been set up by the previous owner in its current configuration and it is fairly economical.
The boat is intended to be used as a family cruiser usually on week long local cruises in summer and once yearly 1000nm passage to great barrier reef for winter
Thanks for any advice

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Old 02-06-2009, 14:31   #2
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hmmm. I believe most will say that you want to be within 10% of max rpm. Loading at too low rpm can carbon up the engine. Also, reportedly, for displacement hulls, a larger prop dia is more efficient, so getting to your proper rpm might be best done with a larger prop with less pitch. On the other hand, that might be economically non feasible....

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Old 02-06-2009, 15:01   #3
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Sydney Harbour drag races...

I had 2" of pitch taken off my prop (should have been more but that's as far as can be practicably achieved) and got a (about) 400 rpm engine speed increase, a slight increase in boat speed and (from informal observation) a slight decrease in fuel consumption.

There might be an improvement in engine life expectancy with the correct prop.

At 10 knots you would do Sydney - Port Stephens in daylight. At 8 it's a struggle.

Given the drag race up and down Sydney Harbour why would you want to limit your boat speed?
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Old 02-06-2009, 16:35   #4
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You are overloading the engine so have to reduce prop load. The developed horsepower isn't higher with higher RPM's. Only the maximum horsepower is higher at higher RPM's. So the resistance of the hull through the water determines the actual horsepower (and fuel consumption) needed.

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