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Old 05-04-2012, 22:08   #1
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Correct pitch

I have a damaged 3 bladed 24inch dia Hung Shen Prop on my 1979 Marine Trader trawler with Twin 120 Lehmans, This was an original prop and I can't find the pitch stamped on the hub. Anyone know what the correct pitch would be? The hull speed is 8.5 knots at 1800 rpms
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Old 05-04-2012, 22:12   #2
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Re: Correct pitch

A prop shop can measure it for you. The complicated technique is described in texts about propellors.
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Old 05-04-2012, 22:38   #3
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Re: Correct pitch

It's nice to know the pitch and diameter but you don't absolutely need to know that because the bottom line is whether or not your engines can reach their maximum governed speed. This is more important to know.

Can they?

If not, your hull is fouled, your engines or an engine is unhealthy, your props are over pitched or there is a combination of what I described.
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Old 05-04-2012, 22:50   #4
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Re: Correct pitch

boatdiesel.com has a great prop calculator that will determine the appropriate pitch based on your boat's hull type, waterline length, weight, cruising speed, gear ratio, transmission type, and RPM at cruising speed. In order to use it, you would need to become a paid member. I believe the fee is $25 per year, but the tool is invaluable. I have used their prop calculator to determine the appropriate propeller for my Diesel repower project and it has proven to be extremely useful and accurate. In my opinion, it is well worth the cost.
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Old 05-04-2012, 23:31   #5
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Re: Correct pitch

If I'm not mistaken, most of those parameters are used to help develop a figure for slip. While a large container ship might only see 5% at the 113 rpm shaft speed, we in sailboats if spinning the screw in a direct configuration would expect to see 8%~12% slip, and all other calculations are straight forward if I remember my old classes. prop pitch X RPM = inches per minute of speed / 12 = feet per minute X 60 = feet per hour / 6082 feet = nm/hr. So assume you are direct drive spinning a 6" pitch prop at 2000 rpm. 6" X 2000 rpm = 12000" per minute / 12 = 1000' per minute X 60 = 60000' per hour / 6082 = 9.8 kt X .88 slip = 8.7 kt That seems fast for the rpm, maybe there is more slip than my range of 8%~12%.

I hope I still remembered how to do this, if not I know David M knows.

I think the last container ship I was on had a 21' pitch and at full ahead sea speed was turning at 113 rpm. 21' X 113 = 2373 X 60 = 142380 / 6082 = 23.4 kt X .96 = 22.4 kt, it was a fast ship.
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Old 06-04-2012, 09:49   #6
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Re: Correct pitch

The OP stated that his boat is a twin screw vessel , so he would need to get the prop repaired to its original pitch to match the other prop. No theory is going to be able to exactly match the other prop. I would expect that the only way to be sure would be to take both props to a good shop and let them measure the undamaged one to get the pitch. The undamaged one would be the opposite rotation, but should be the correct pitch. I hope it works out well for him._____Grant.
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Old 06-04-2012, 09:54   #7
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Re: Correct pitch

You can do all the calculations you want but if the engines cannot reach their maximum RPM then the pitch is incorrect...or the hull is fouled or you have an unhealthy engine(s).
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Old 06-04-2012, 10:14   #8
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Re: Correct pitch

Yeah, I 've have prop shops do the calcs more than once and get it wrong more than once.... it's not a finite science because it doesnt take into account many variables like wetted area, skeg/strut turbulance etc....
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