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Old 01-08-2008, 04:03 PM
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Default prop speed

is it better to turn a big prop slow or a small prop fast . is the there a reccamended speed for a surface piercing prop

cheers gerard
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Old 01-08-2008, 04:52 PM
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Very simply (and neglecting internal drivetrane considerations):

Power In = Power Out
and
Power Out = Thrust Power - power consumed by drag

The ratio of of Thrust power divided by applied power, is efficiency.

Your goal is to maximize thrust and minimize drag, thus maximizing efficiency.

To achieve increased thrust, you can either spin a given prop faster, or grow the size of the prop.

Now, with increases in a prop's angular velocity (rpms) and therefore the relative tangential velocity of a given radial element, you inherit torque drag in a directly proportional relationship to the 2nd power. Conversely, increasing blade area to achieve the same goal only increase torque drag linearly. Therefore, it would lend itself to reason that a larger prop turning slower would yield greater efficiency than a smaller prop turning faster - all with limits, of course.

As an additional note, here's a handy rule of thumb:
Doubling your blade area, doubles your drag.
Doubling your blade velocity, quadruples your drag.
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Old 01-08-2008, 05:35 PM
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cheers nick
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Old 01-08-2008, 06:15 PM
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You need to spin it so the engine is at its max recommended WOT range. As for spinning a smaller prop faster and changing gears, that depends on the boat and gear ratios. Some boats like it and some don't.
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Old 01-08-2008, 06:38 PM
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Quote:
Originally Posted by CigDaze View Post
Very simply (and neglecting internal drivetrane considerations):

Power In = Power Out
and
Power Out = Thrust Power - power consumed by drag

The ratio of of Thrust power divided by applied power, is efficiency.

Your goal is to maximize thrust and minimize drag, thus maximizing efficiency.

To achieve increased thrust, you can either spin a given prop faster, or grow the size of the prop.

Now, with increases in a prop's angular velocity (rpms) and therefore the relative tangential velocity of a given radial element, you inherit torque drag in a directly proportional relationship to the 2nd power. Conversely, increasing blade area to achieve the same goal only increase torque drag linearly. Therefore, it would lend itself to reason that a larger prop turning slower would yield greater efficiency than a smaller prop turning faster - all with limits, of course.

As an additional note, here's a handy rule of thumb:
Doubling your blade area, doubles your drag.
Doubling your blade velocity, quadruples your drag.
That's EXACTLY the way Maryland Mark explained it
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Old 01-08-2008, 06:49 PM
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why im asking is i have a 26ft boat 2.2 ton all in and fexed surface drive do i go direct drve or reduction
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Old 01-08-2008, 06:54 PM
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Old 01-08-2008, 07:01 PM
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1.5 to 1 is most common for sport type boats with V8 gas engines. Some ski inboards will run 1 to 1.
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Old 01-08-2008, 07:04 PM
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Quote:
Originally Posted by marylandmark View Post
I think CigDaze did a cut/paste that from some thing I posted a while back- can't find the thread right now...
Dude, you crack me up.
As I saw in another thread, "Some new guys are entertaining, some are azz hat's, and then there is Mark"
You're sense of humor is just as phucked as mine.

CigDaze just always floors me with his technical responses, and after I sit and analyze what the hell he said, it begins to make sense.
Jeez - It's like taking a Physics course all over again though.

Sorry for the sidetrack of the thread Gerard - my apologies.
Back to the tech stuff.
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Old 01-08-2008, 07:13 PM
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im running a yanmar 440 diesel max rpm 3400
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