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

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.

cheers nick

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.

That's EXACTLY the way Maryland Mark explained it:panda:

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

:evilb:

1.5 to 1 is most common for sport type boats with V8 gas engines. Some ski inboards will run 1 to 1.

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.

im running a yanmar 440 diesel max rpm 3400

Your question was size & speed?

We ran diesels nothing else, we tried to keep prop speed at 4,000 - 4,500 rpms, which is pretty fast. The big props breaking and then re-entering the water drag the diesel down it can't recover because you've lost some boost.

Size - whatever you could spin at that speed, we liked small dia and more pitch.

At 3,400 rpm even at 1:1 you won't see big speed numbers.

Without any fancy math or fromulas, use the example of a gas engine behind a stern drive. When do start to see real speed? In the last 1,000 to 1,500 rpm of engine speed.

5,500 rpms X 1.5 reduction = 3,666 prop rpm

prop needs to operate above 3,000 rpm

The problem is getting on plane, with small props, they cavitate to no end and you have no useable cruising speed. Big props take forever and you over load the engine trying to break those blades free.

Get yourself a 2 speed and run a slight overdrive you'll have cruise and top end

cheers mate thats the answer i was looking for but just when you mention that i have a 2 speed first gear is 1.30-1and the second is direct

thanks again

If you can ex-change for OD gearing you'll be happier?

funny enough with my set up the last 200rpm seems to give me about 6mph

the "good old days" were better because we were younger, single and didn't have 3 kids!!!

It's not that life today is bad but it is different!

As far as cut & paste - you didn't think I was smart enough to do something on my own - did you?