How Do I Calculate the RPM of a Windmilling Boat Propeller?

Propeller Calculator

http://www.rbbi.com/folders/prop/propcalc.htm

Now give me a GA and a vote please

Nice try:) I will not use a online propeller calculator, as i have no chance of knowing which calculation method it is using. But thanks anyway! i will however consider using it to see if it's numbers are anywhere close to my own.

Sorry, no GA from me.

The OP already understands that the propeller rpm is inversely related to propeller pitch. (That's what he was saying here: Theoretically the rpm should be equal to ships speed/pitch(v/p), as pitch indicate the distance a given propeller would "drive" forward for each full rotation. But then the resistance...) The online calculator does not help him come closer to the answer he is looking for. It simply takes the rpm times the pitch, adjusts for slip and outputs a speed (or calculates any variable, given the others). What the OP wants to know is the slip value to use. That slip value is different for a prop driving a boat vs a boat driving a freewheeling prop, because the angle of attack of the blade is higher for the powering case (in other words the blade is generating more lift, and therefore more drag).

Unfortunately, there is no simple answer, because it depends upon the drag on the prop shaft (e.g., how much stuff is turning with the prop), the influence of the hull shape on the prop, the propeller's profile shape (with that shape being effectively reversed during windmilling), etc.

My guess would be that 5% is a reasonable average value for slip with a windmilling prop, but the actual value for a particular prop in a particular installation at a particular speed could vary up or down from this. Testing with an rpm pickup on the prop shaft would be the best way to find out the value for a particular installation. CFD would cost more, and would not be as accurate. The slip value will change with speed, particulary at the low and high extremes, but there have been many propeller-base knotmeters that treat the slip as if it is a constant. (In a propeller-based knot meter, I'd guess slip is 1-2% at medium speeds.)

Thanks for the answer. I do not have a particular boat to measure on, so this is all theoretical. I'm looking for a general way of doing this, independent of any particular ship/boat. Therefor the propeller does not need to be one of a ship. Actually I imagine dragging a separably propeller, to generate power. I guess i could use the slip, speed, pith method, if i cant find anything else. i'm about to look into force components in flow theory. This theory is used when calculating on hydro turbines, mainly kaplan turbines.

Unfortunately as you have stated, you can calculate the theoretical RPM at a given speed , but the losses are specific to your own boats design with all its fittings and fixtures and can only be tested under actual sailing conditions. That is why testing tanks are used in hull design for all these sorts of characteristics.

Guess you are right. still, If you have all the data on a particular propeller, there must be a more precise theoretical way of calculating it.

there must be a more precise theoretical way of calculating it.

There is. You simply do the thousands of iterative calculations done by CFD. Given a year's time you'd get an answer. One large difficulty in hand calculation is predicting at what point along a hydrofoil the flow goes from laminar to turbulent, and at what point separation occurs. Even computers do not predict turbulence well.

The problem (for a simple quick calculation) is that the slip varies with the power transmitted by the propeller. Consider a 10 HP outboard on an 8' hydroplane: the boat does 30 mph. The same engine, using the same prop and same rpm will power a 24 foot sailboat to its hull speed of about 7 mph. So in the first case, the slip is very low. In the second case, the slip is very high. In the first case, the prop is (mainly) carving through the water, moving the boat along in about the same way that a wood screw advances through wood. In the second, the propeller is (mainly) creating a jet of fluid driven rearward at high speed while the boat moves at low speed.

If you are generating electricity with a towed propeller, the amount of slip varies with the amount of power produced. These devices are designed for a particular output at a particular boat speed, and slow down the boat to which they are attached. Obviously, they are only used with sailboats (because in a motor boat, the engine's alternator drive is more than twice as efficient as a propeller). When they are designed for a sailboat, a reasonable amount of slip at which the propeller efficiency is high (say 10% slip) is designed in. For a unit to generate more power from a given sailboat speed, the propeller area then must be increased to maintain an efficient slip angle.

It is possible to calculate, given a particular speed through the water, and the amount of power to be extracted (assuming the boat is large enough to generate that amount without excessively slowing the boat) the prop size and pitch to most efficiently transfer power from wind to alternator, using water as the coupling. The hydrofoil characteristics of the prop blade must be known, because the objective is to have the blades operating at the lift coefficient where lift/drag is best. (The calculations are like those done to select a pitch for a particular engine/motorboat combo. For example, in the hydroplane vs sailboat example above, in practice, a different prop is used for each application, with the sailboat prop having lower pitch, and, ideally, larger diameter.

So without knowing the power to be transfered, the characteristics of the blade section, etc. there is no way to calculate rpm from pitch. As a rule of thumb, propellers are about 40% efficient, so if you know that you want to extract 1kW of power from a sailboat's forward progress, the propeller would be about the size of that on a three hp outboard. It would be pitched to generate 10 - 15% slip at the designed towing speed. In actual use, the slip would vary, because the boat will often be at a speed other than the design speed, and the load on the alternator will vary.

I would take a different approach here:

You will need the boat speed; use a stethoscope touching the bearing for the shaft and use a stop watch. You will be able to hear each revolution. Time 100 revs and this should give you a good value.

I don't have a boat. I'm looking for a way of calculating the rpm, not measuring it. Thanks for you reply though:)

Get a boat first