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Join Date: Nov 2014
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Prop Drag on a Sailboat Motor

11/24/2014 4:39 PM

I have a battery powered trolling motor assisting a small sailboat. Its prop remains in the water at all times. Let's say with no friction (i.e., in air) it runs at 1500 rpm. In the water (now hard at work), it runs let's say at 750 rpm??...and moves the boat at 3 knots. Now the wind begins to blow gently at first. The combination of prop and wind moves the boat at a somewhat faster speed. Presumably the prop will also be spinning somewhat faster because it is being assisted by the wind. First of all, is there a formula for what that combined boat speed might be....roughly? Secondly, at some point the wind induced speed outruns the above combined speed (when the prop reaches 1500 rpm??) and we now have drag. I understand (or not) the arguments as to which ...a free spinning prop or a fixed prop.....produces the most drag. That is not my question. My question is whether a POWERED prop (as opposed to a free spinning prop) even though it is being outrun by the above mentioned combined speed of motor plus wind is producing less drag than were it stopped?? And finally, with a 12 sq. in. prop surface area and a 4" pitch, what IS that drag at various speeds from when the drag phenomenon begins on up to perhaps 12-15 knots (it is a catamaran) vs the drag with the prop stopped??

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#1

Re: PROP DRAG ON A SAILBOAT MOTOR

11/24/2014 4:58 PM

A powered prop will always produce less drag, even at hull speed.

There may be some way to calculate the drag but I don't know how.

I have several apps on my phart smone that can give my speed, and heading and location and directions to the nearest bar, marina, boat ramp, etc.

I'd go.with an app and not over think it.

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#8
In reply to #1

Re: PROP DRAG ON A SAILBOAT MOTOR

11/25/2014 8:01 AM

Had to give you a Chuckle Vote over the term "Phart Smone". Haven't heard that one before!

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#11
In reply to #8

Re: PROP DRAG ON A SAILBOAT MOTOR

11/25/2014 1:32 PM

I've developed a healthy contempt for my Android phone, and it for me.

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#2

Re: PROP DRAG ON A SAILBOAT MOTOR

11/24/2014 5:10 PM

First, you must be familiar with the computational methods for fluid dynamics. (Phooey the link only shows the TOC. This link has all 333 pages of a similar textbook.) As you should notice chapter nine deals with boundary layer equations which will probably be the most relevant section for your condition you have with a water screw in the stream. Once you master this, your probable brushed DC series motor will be a piece of cake to explain.

Happy sailing.

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#3

Re: Prop Drag on a Sailboat Motor

11/24/2014 10:03 PM

This is a fairly complicated problem. Here is my take on it...

The thrust (or drag) from your prop is the mass of water (i.e. volume x density) flowing through the prop per unit time times the change in velocity of the water (velocity of boat - exit velocity from prop). F=rate of change of (mass x velocity). The force is either thrust or drag, depending on whether the boat is moving faster or slower than the exit velocity from the prop.

The volume of water per unit time can be estimated by the area of the prop times the exit velocity. The exit velocity from the prop can be estimated as the RPM x the pitch of the prop.

If the boat is moving at the speed that water exits the prop at 1500 RPM, the prop is neither providing thrust or drag. Call it the "no drag" speed.

If the boat is moving at a speed faster than the "no drag" speed (i.e. excess speed), if the prop is powered and the water is slowed to the "no drag" speed , the force is proportional to the excess speed times the boat speed.

If the prop is not powered, the drag would be proportional to the square of the boat speed (assuming zero exit speed for stopped prop, the excess speed is the boat speed).

So the ratio of the drag forces (powered/unpowered) would be (excess speed) / (boat speed).

The assumption is that the prop does not speed up appreciably due to the water flow (for example if it is connected to a gear train).

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#4

Re: Prop Drag on a Sailboat Motor

11/24/2014 10:45 PM

My take on this would be that the powered prop will produce more drag than a free spinning one if the hull speed is such that the water speed over said prop is greater than what the prop would produce at its current RPM. The prop would be unable to auto rotate due to gearing and engine compression, and so must be restricting water flow over it.Obviously, the removal of the prop from the water would be the most efficient option with a folding prop being a close second.

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#5

Re: Prop Drag on a Sailboat Motor

11/24/2014 11:24 PM

Eliminate the problem, your question and all the calculations. Change the prop to a "sailboat folding prop". When the forward speed of the boat from the wind exceeds the speed generated by the motor the prop will fold and present a small profile with much less drag from the prop. Although most are made for inboards some are made for outboards and electric motors. You just have to search around for them.

The following will give you a good explanation and pictures of what they look like both extended and folded.

Don't try to solve the problem and the question, eliminate them!

Good Luck, Old Salt A sail boater for many years!

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#7
In reply to #5

Re: Prop Drag on a Sailboat Motor

11/25/2014 2:00 AM

The OP's term about the driven prop being "outrun'' by the speed of the boat can be misleading. Typically, the water leaving the driven prop will be moving much faster than the forward speed of the boat when under power, so the boat would have to be travelling quite a lot faster than the full ahead speed for the driven prop to create drag.

But I too always used a folding prop on my sailboat. As said earlier, just try it and observe the speed.

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#9
In reply to #7

Re: Prop Drag on a Sailboat Motor

11/25/2014 8:27 AM

Yes, if the prop is not properly matched with the sail boat and motor. When purchased most small outboards and electric trolling motors are supplied with a high pitch 2 blade prop such as used for a much faster boat (there are more of these than sail boats) such as a 12 ft to 16 ft utility V-hulled or flat bottom utility. This would be the type used for a protected water fishing, crabbing or general recreation power boat

Unless there is an unique situation, a 3 blade prop with a lower pitch should be used on a sailboat gasoline outboard or electric motor. The main reason is to reduce the torque on the prop by increasing the rpm and allowing the motor to rotate at a higher speed. This also reduces the disturbance of the water by the prop and therefore reducing the "outrun" speed of the boat. The prop matches the boat more and the "matching process" takes this into consideration. Gasoline outboard engines do not run well at reduced speeds nor with a high torque load at low speeds. Matching the prop with the motor and boat allows a more efficient utilization of everything. Just as a boat with a cruising speed higher than a sailboat needs a matched engine and prop so does the sailboat.

Some outboard motor dealers that have a high percentage of sailboat customers usually stock motors with either 2 or 3 blade props and short or long lower legs (shafts).

Diesel engines use a higher pitch prop since their rpm speed is lower and they are higher torque at slower speeds. A diesel will seldom exceed 2,000 rpm while a gasoline will operate in speeds in excess of 3,600 rpm without straining since its torque is delivered at a higher rotational speed.

Sailboat folding props are available in both 2 and 3 blades.

Prudent procedures when sailing with either an outboard motor or electric motor dictate tilting the motor and its shaft up out of the water and not leaving it in the water. To do otherwise is an invitation to inefficiency. If sailing, why not do it right and get the speed (if wanted) available.

With my sailboats I never use(d) anything but a 3 blade prop, folding or fixed.

Good Luck, Old Salt

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#6

Re: Prop Drag on a Sailboat Motor

11/24/2014 11:35 PM

Finished for the day. Just wanted to say thanks very much for the responses.

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#10

Re: Prop Drag on a Sailboat Motor

11/25/2014 10:00 AM

Measure the current draw and polarity at the prop motor. If it is a permanent magnet motor, then the current draw will decrease and then reverse when the prop is driven by the wind, and the prop becomes a generator.

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