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Old 03 February 2012, 15:42   #1
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prop pitch to engine revs

Hi, I'm trying to learn a bit more about all forms of nautical construction and mechanics. Firstly does anyone know of any courses to learn about any and all forms of marine construction and mechanical works, would be more than happy to work as a free set of hands in workshop to learn more.

And finally am I correct in saying that by decreasing the pitch of the prop by 1deg the engine revs will increase by 300rpm? And vice versa?
Also lower revs (higher pitch) gives better grip and therefore pulling power and high revs (lower pitch) better acceleration and higher top end speed?

All and any help, thoughts or advice would be greatly appreciated.
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Old 03 February 2012, 17:36   #2
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In general 1" of pitch is 200 to 250 rpm (longer pitch = lower rpm - not a hard and fast rule, just a rule of thumb.)

1/4" of pitch is 100 to 200 rpm (smaller = more rpm)

Aluminum to SS, with an identical shaped blade loses about 200 to 500 rpm due to weight difference.

Best power and higher torque are generally both achieved towards the higher end of the power band, I think. The only reason I know of for propping towards the lower end is to reduce cruise rpm (fuel economy), but that comes at the expense of performance (hole shot, speed.) Grip is more a matter of blade design, prop depth, and a few other variables.

All my opinion; not necessarily fact.

jky
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Old 03 February 2012, 18:14   #3
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Quote:
Originally Posted by stew View Post
And finally am I correct in saying that by decreasing the pitch of the prop by 1deg the engine revs will increase by 300rpm? And vice versa?
Yes but prop pitch is measured in inches not degrees. As in inches of forward motion without slip per revolution.

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Originally Posted by stew View Post
Also lower revs (higher pitch) gives better grip and therefore pulling power and high revs (lower pitch) better acceleration and higher top end speed?
The other way around.

Higher pitches are like higher gears. You'll go faster if the boat is light enough to achieve WOT rpms. poor acceleration yet high top speed.

Low pitch props are used on tugs and work boats, like low gears in a car. The engine reaches full RPMs and full HP but without much speed. Good low speed acceleration, poor top speed.
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Old 06 February 2012, 05:10   #4
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Bigger blades also = more grip, but at the expense of speed.

Real wold example:

When I got my rib it had a 14" pitch by 10.<something small>" diameter prop. I had to throttle back before the engine threw a piston. It also "let go" and ventilated at the slightest provocation.

Swaped to a 14" pitch by 11.<whatever a standard K-series diameter is>" diameter and it topped out at an appropriate RPM. Only "let go" if I was being silly with the power.


Its one big balancing act with "equilibrium" being where you want or need it to be.
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Old 07 February 2012, 09:39   #5
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THE BLACK ART OF PROPELLERS
as a start all OBMs have a wide open throttle (W.O.T.)rev range, your boat should be propped so your W.O.T.goes into that rev range not above or below,if you rev over the W.O.T. range you are stressing the P.H. if you are at W.O.T. but under the W.O.T. range again you are stressing the P.H.it would be like starting the car in second or third gear, to prop a boat can be frustrating , to get the optimum performance its a question of do i raise /or lower the engine on the transom, do i use a stainless prop,an alluminum prop or if available a plastic prop. With a stainless prop you can mount the OBM higher,(cavitation plate above the bottom/keel of the boat),an alluminium prop level with the btm of the boat , at the end of the day the finall test is on the water, does it get to W.O.T. Does it chine dance? Does it feel good and safe to use?That in a nutshell is the black art of propellers good luck!
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