Propeller Efficiency

[fredrosse]

I have some data on a small displacement boat:

15 ft LWL
0.75 tons
4.5 Knots
1.0 SHP
Prop 500 RPM 14 inch dia x 14 inch pitch with 22% apparent slip.

I am interested to determine the relationship of this propeller propulsion/horsepower compared to a test with a long tow rope. This could give an indication of the efficiency of the propeller drive system.

In this case, 1 horsepower towing power would correspond to 72 pounds towing force ( Force x Velocity = Power, 72 Pounds Force x 7.6 Ft/sec = 550 Ft-Pounds/Sec = 1 Horsepower).

Would I expect the tow test to have a lower force value at 4.5 knots (7.6 ft/sec), since the propeller drive system could not be as efficient as pure towing. Any quantification would be helpful.

What is a reasonable range for propeller efficiency, and what factors can be used to improve the propulsion efficiency?

Thanks in advance.

[jehardiman]

Pardon me if this is long and been said else where better.

First, propellers are usualy describe by three critical factors. The advance coefficient (J) which is a function of the propeller advance (Va), diameter (D) and the RPS (n): J=Va/Dn. The Torque coefficient curve (Kq), a function of torque (Q) at a given J and water density (rho): Kq=Q/(rho*n^2*D^5). And the Thrust coefficient curve (Kt), a function of thrust (T) at a given J and water density: Kt=T/(rho*n^2*D^4). the "open water" efficency (eta O) of a propeller is a measure of the ability of the prop to turn torque into thrust when operating by itself free from any surrounding influnces: eta O = (J*Kt)/(2*pi*Kq). For most propellers eta O is about 70% at some discreet J.

Now the propeller absorbs hp as torque. Torque * rpm / appropriate constant = shaft hp (SHP). Now hp is absorbed by the boat as drag. Drag * speed (V) / appropriate constant = effective hp (EHP). The quasi-propulsitive efficency (eta D) is a measure of the ability of the wheel to change SHP to EHP eta D= EHP/SHP.

As the propeller operates in the wake of the hull, the speed of advance of the prop may not be the same as the speed of advance of hull. This is important as one of the critical factors for propeller/rpm selectionis the advance coefficient (J). The Taylor wake fraction corrects vessel speed (V) to propeller advance (Va) by the wake fraction (w) which is the precentage of ship speed the water is moving at the propeller (+ being in the direction of travel): Va=V(1-w). Additionally, the wake may be uneven, which will cause the prop to absorb more or less torque. This is called the relatative rotative efficiency (eta R) and is equal to torque behind/torque open water.

Finally, there is the matter of slip and its effect on the wake and drag. In order to generate thrust, the prop must suck water in and push it out faster than the propeller is moving forward. As we have shown above, the prop operates in water of a different speed than the vessel. Therefor there are two slip ratios the real slip ratio: 1-Va/(pitch*n); and the apparent slip ratio: 1-V/(pitch*n). The real slip ratio is the only one we need to worry about but it is very difficult to determine without rigrious testing and is usually estimated from the wake fraction. What this slip does is to speed up the water around the stern of the hull, causing additional drag. Rather than think of it as additional drag, as it is a function of the propeller, it is normally called a thrust deduction (t) so that required thrust = (1-t) delivered thrust.


Now to make the boat go at a given speed, drag must equal thrust, so you can see that SHP and EHP are linked by the ratio of input torque to output thrust at a given speed and rpm for a specific propeller operating behind a specific hull. To write it in efficiency terms:

eta D = (1-t)/(1-w) * eta R *eta O

That said, you can see that the eta D is directly related to the RPS (n) of the propeller by the advance coefficient (J), which is related to the speed of the vessel by wake fraction (w). What this means is that (for a given prop)having too much rpm is as bad as having too little rpm as both can cause the prop to come of its operating point.

Now in your case, if you give more information about the hull, prop, and prolpusion arrangement; we me be able to estimate t,w, eta R, and eta O which will not only enable you to size the motor, but give you the proper RPM for most efficient operation also.

If you want to change a prop and make it more efficient (at a given speed and thrust) there are only two things to you can do; make it bigger and make it slower. And those both have other considerations whit things like arrangements, gearing, structures, material effects,..etc.

[ABoatGuy]

Well said in a couple of paragraphs!

Off subject: Went to elementry/jr high in Port Orchard a long while back. How is the old place?

[jehardiman]

Originally Posted by ABoatGuy

Off subject: Went to elementry/jr high in Port Orchard a long while back. How is the old place?

Getting bigger . We've been here about 10 years and the place has really grown in that time (4K to about 8K). We like it, better than Silverdale and Poulsbo which have become CA north. It still has a more rural feel.