Propellor selection electric drive

Some years back, I towed a WWII troop carrier to the ship breakers. she had dozens of 50ft long, 9ft beam, galvanized lifeboats in davits.
These boats were powered by muscle.
Siding bars with upright levers socketed in them, turned bicycle type cranks at the stern.
The prop was huge. Had 7 scimitar shaped, fat blades. Appeared to be stamped out of sheet metal and galvanized.
Obviously, the designer believed this prop design most appropriate for such a low RPM and minimum HP system. Maybe 20 men could push and pull on those levers simultaneously.

A healthy man can produce about 75 watts during an hour.
That's a tenth of a HP. so 20 dudes pushing n pulling, 2 HP.

I can't find any reference to this type of lifeboat propulsion on the web. I didn't take pictures. So I drew an approximation of what I remember the prop looked like.
A properly designed prop of this style might be appropriate for low power electric system.

 

So in actuallity you DON"T want an efficient prop.

Since you don't seem to know much about props and how they work, you have made the typical mistake of equating high efficency to high thrust at high speed, which is not the case for a marine propeller. In your case, for maximum efficiency, the wheel would be ~ 50" in diameter with blades ~2" wide and 1/8" thick. What you are really asking for is a prop for an auxalliary sail which allows for maneuvering. In this case, just use the folding prop that came with the saildrive on the boat. If you are specing a new prop, and don't want a folding prop for whatever reason, just use a Wageningen B2-30 or B2-38 so you will have some bite when maneuvering.

 

My apologies for not being clear enough. The CW Hood example was for clarifying there might be other reasons to go for electric i.s.o. choosing the most efficient drive. By the way, for ultimate efficiency we hoist the sails and get free propulsion.

Indeed I don't know much about props, if I did, what would be the point to ask?

I am asking if anyone knows or has advice on choosing an efficient propellor which takes advantage of the typical properties of an electric engine when used on lets say a displacement speed launch, not for a lecture to stay away from them. Nor am I asking on propulsion advise on our sailboat, it works fine.

The best advice until know comes from yobarnacle, at least he answers the question, thanks for that!

As for your statement prop selection/efficiency has nothing to do with prime mover selection, I am quite sure a lot of people and experts will disagree. Mr. Gerr from Westlawn has written a nice book on propellers. The first sentence states that hull design, propeller and engine selection directly influence each other.

Probably your 50" diameter is not a bad idea, thanks for mentioning.

 

the mistake is to compare HP and not torque (at an rpm which is HP yes)
Your pencil could drive 1000hp if you could turn it fast enough

 

Yay for TORQUE.

 

I'm reluctant to mention this. Just a reminder of a design aspect well known.
Design isn't about hardware components.
It's about systems. Systems where the components are subsystems.
And making sure they are all compatible.

The best hardware isn't the best, if it's incompatible in the system.

The best SYSTEM for moving a boat from point A to point B, is a truck, trailer, driver, and roads.
That's why I have two trailer sailors. They move 60 mph behind my truck.
A 25 ft boat is too small for a couple to live aboard for an extended period.
But two identical 25 ft boats are twice the space, and only need one trailer.
As a system, my truck and trailer (and roads) are twice as efficient with two boats than with one, because with no additional outlay for equipment, only for fuel and time, TWO boats get moved.

This system, as a subsystem is incompatible with a design for world cruising only because the ROADS are missing from the equation.

The same logic is applied to all the choices in choosing and equipping a boat.

Diesel electric as a propulsion subsystem has advantages over conventional ICE.
While slow, batteries can be recharged with solar, wind, and current power.
They can be recharged from the grid, if it's available.
If fuel is available, the diesel can charge the batteries.

In conventional ICE setup, you are entirely reliant on fuel availability.
Hardware is subservient to system requirements.
I apologize for preaching.

 

Mr jehardiman makes some good points on the prop design.

He suggests blades only 2 inches wide and a wheel diameter of 50 inches.

The width of the blade is important as is the diameter.

Narrow blades push less water and have less surface friction resistance. That allows greater RPM for a given torque. In fact REQUIRES greater RPM, to move the same size column of water as wider blades.

If high RPM isn't a desired factor, as in man powered or smaller electric drives, then greater blade area gives more thrust at a lower RPM because it moves more water mass.

In your situation, a trade off on blade width, juggling thrust and speed, to meet your requirements.
Wider equals more thrust and narrow more RPM, but not necessarily more boat velocity.

With narrow blades you can have more pitch, ergo potentially more speed (velocity).
More pitch, more speed; less pitch, more thrust.
Less pitch, more blade area, lower RPM, gives more thrust with same power.
More blades = more blade area. Most efficient prop is 2 blades. at sacrifice of area.

Likewise more diameter equals more thrust and smaller diameter allows more RPMs and speed.

So, do you want to move a large diameter column of water slowly? or a small column rapidly?
Boat speed is relevant to water column speed and thrust to mass of water moved.

And do you want to do this with low RPM or high RPM?

There are numerous articles on low speed flying, that are helpful to understanding precepts. Of course planes and boats are different, but both operate in fluid.

Good luck.

One other factor. Drag. A large propeller has a large drag resistance. However, if you can tune the motor to provide JUST enough torque to spin the prop a few RPM faster than your speed under sail, the prop drag disappears. This may require only a few amps of electricity and may be obtained from solar. The best solution regarding drag, is remove the prop from the water while sailing.

One final comment. Yes electric motor use more current when maintaining high torque.
ICEs use more fuel maintaining RPM.
An ICE lugs down, loses RPM when the fuel system can't keep up with the power demand.
Same thing in my book, electricity is the fuel for electric motors.

In reference to "As you can see torque increases only with slowing down (i.e. loading) the prop (i.e. motor speed), but power out is relatively flat while power in is always increasing (and so is current which leads to other issues...large gage copper cables are not light)." comment by jehardiman

 

you have to work out the optimal blade loading, thats the secret of the correct blade size.

 

Thanks for the useful comments! Will keep you posted on future selections.