Prop efficiency?

[MagicMike]

Hi folks,

may anybody has precise information about the prop efficiency factor of a standard 3 blade outboard propeller (or similar) of a stock outboard engine, or simply common efficiency factors of outboard-, sterndrive-, and surface drive-props.
Or described with other words: how many hp are brought into the water if there is a shaft hp of 60hp? What I heard is that the efficiency varies with speed, what seems to be logical because the prop is calculated for just one speed and weight which is usual the top speed. I have heard values from 20% up to 90%. What is a typical factor for a stock outboard in planing mode?

I don't mean the prop slip, I mean the efficiency factor!

Thanks for your help
Greetz MagicMike

[jehardiman]

A well designed, high speed, fully-submerged, non-cavitating prop, operating at its proper open water design point, should have an ehp/shp efficency ~65-70% . Surface piercing and supercavitating props will have lower efficencies due to the cavitation numbers that they operate at.

[MagicMike]

Thanks for your reply.
But the overall efficiency of a surface drive should be better than a stern or OB becoause of the less friction (no gearcase and skeg). Isn't ist right?

[RANCHI OTTO]

Here you have various ETA0 according to boat speed.

I've checked for 2 speeds various propulsion systems using the KR coeff=(BHP)^0.551/(displ.)^0.476 based on sea trial results.

Obviously the fixed displacement is not real because for each propulsion system you will have another one but....


Displ. = 8 t
Speed = 30 /50 knots

Outboard > 725 / 1335 bhp
Sterdrive > 580 / 1460
Duoprop > 437 / 1384
Shaft line > 380 / 1240
Piercing > 507 / 1000
Waterjet > 550 / 1120

[jehardiman]

Originally Posted by MagicMike

Thanks for your reply.
But the overall efficiency of a surface drive should be better than a stern or OB becoause of the less friction (no gearcase and skeg). Isn't ist right?

No, it will be less because they pay a higher torque penalty due to the higher face pressure needed to achieve the same thrust (Most gearboxes BTW are about 95+% efficient, you will get more loss in the stern tube/thrust bearing which all systems have). Additionally, the energy cost to form the cavitation bubble is also greater than the surface friction of a similiar sized blade.

As RANCHI OTTO's grapth shows, the only reason you move to cavitating wheels is if your required speed is too high for the size of wheel you can fit. Otherwise, for maximum efficency, you want the largest, thinest, highest aspect, slowest turning, blade you can get away with.

[naturewaterboy]

Jehardiman
So, the best efficiency you can expect out of a well designed prop is 65-70%? And that is at it's design point (one rpm, one boat weight)? What happens to efficiency at slower speeds - does it usually change dramatically?

[jehardiman]

Originally Posted by naturewaterboy

Jehardiman
So, the best efficiency you can expect out of a well designed prop is 65-70%? And that is at it's design point (one rpm, one boat weight)? What happens to efficiency at slower speeds - does it usually change dramatically?

First, weight has nothing to do with prop efficiency directly. Propeller efficency is the ability of the prop to transfer torque (Q) to thrust (T) at a given rpm (n = revolutions per second) at a given speed of advance (Va). Since thrust=drag at a given speed, vessel weight only indirectly effects thrust required based upon other hull factors not directly related to the propeller. Efficency is therefore the amount of thrust delivered at a given speed divided by the torque deleived in the same time: eta= (VaT)/(nQ). Note the efficency is 0 at zero speed, regardless of thrust.

The ratio of n to Va is the advance ratio (J = Va/nD where D is the diameter) Maximum efficency occurs at a particular J. Keep the rpm constant and vary the speed of advance you vary J which also varies T and Q. The same is true in reverse... keep speed constant and vary the rpm you change J. At a J lower that the optimum J, the higher the thrust and asorbed torque but the lower the efficency (think tugboat). At a J above the the optimum J the thrust is reduced and torque goes down and the prop overspins again reducing efficiency. If you continue to increase J, eventualy the thrust, and then the torque, go negative and you have a generator rather than a propulsor. See my discussion in this thread for a more full discussion of efficency and marine propellers:
http://boatdesign.net/forums/showthread.php?t=6890

The maximum efficency I have ever built a prop to is ~ 85% open water and about 110% behind the hull (i.e. I was recovering about 25% of the energy in the wake). This wheel was designed for a very specific application and is to fragile for general use. With a broad brush...most general use marine propellers are about 70% efficient and when well designed with the hull efficency about 85% overall. Because of the nature of most prime movers and vessel hull drag, most propellers generally operate at just higher than optimum J at speeds lower than the design speed. If the prop was done correctly, it will have it's maximum efficency at design speed. At speeds over the design speed the prime mover will load up in torque until it is unable to support the required RPM and the maximum achievable speed will be reached.

[tom28571]

Ranchi,

Thanks for the graph. It's the clearest that I've seen.