Propellers:
Here is what I have found with respect to propellers being powered by an electric motor:
1) Choose the largest diameter propeller while maintaining 15 - 20% propeller clearance. Otherwise you will have prop walk in reverse.
2) Choose the highest pitch propeller that will fit on your boat. On a full keel boat, the aperture may not be large enough for a high pitch propeller.
3) Large diameter, high pitch propellers create more thrust at lower rpms. This reduces vibration and sound created in the drive train. The old atomic 4 propeller turning at 1800 rpm has less thrust than our new propeller turning at 600 rpm. Gear noise was very noticeable with the old propeller, but barely noticeable at all with the new propeller.
4) Choose a propeller that was designed recently. The propeller that came with your boat is probably as old as your boat. Propeller design has changed a great deal in the last 20 years. Your existing propeller is probably the least efficient component in your electric drive system. Increasing the efficiency of a propeller from 50% to 80% will increase your range by almost double.
5) Propellers work best when the angle to the hull is as minimal as possible.
6) Propellers work best in clear water. Keeping the propeller as far away from the hull increases efficiency. But beware of pushing your propeller out - the shaft will whip around if the propeller is too far away from the strut with cutlass bearing. Also a propeller close to the rudder makes the rudder harder to handle.
7) Each propeller requires a different gear ratio. Once you install an electric motor it is important to measure the current going to the motor. If the motor is an AC motor, make sure that your meter will read correctly for varying frequencies. It is better to obtain the AC current from the controller than from a meter. Motors are rated by current at the motor, not current consumed at the batteries.
8) If you increase the size of the propeller, you increase the load on the motor and increase the current consumed by the motor. Make sure the motor can come up to rpm. Air cooled motors that are not running at high enough rpm will overheat. Motors running at too low an rpm will not run efficiently. Motors need rpm to create horsepower. If your motor does not come up to its rated rpm, it will not create rated horsepower.
9) I think of the old style of propellers as flapper wheels. New propellers create lift from their profile, similar to how an airplane creates lift with its wing shape. An airplane's wing is longer on top than on the bottom, creating a low pressure above the wing which lifts the plane skyward. The same is true of a good propeller design. The leading (foreward) side is longer than the trailing (aft) side and the lift is low pressure on the forward side of the propeller.
10) Take a look at a new propeller, it is quite different from what you are used to. Our propeller's lobe tip is 2 inches behind the center of the hub, and really looks more like a screw than a flapper wheel. Changing the propeller and gearbox on our motor resulted in doubling the overall efficiency of the system. Propellers matter! If you are thinking of converting your boat to electric, be sure to change your propeller at the same time.
for more information you can look at this link:
James
James Lambden
Propulsion Marine
Santa Barbara, CA, USA
805 455 8444
On Mar 4, 2010, at 4:38 PM, gramplarry wrote:
Hi all!!
my yahoo name is gramplarry. the name came about because of another yahoo group to which i belong that had a number of larrys. My grand daughter being a member of the same group suggested i become gramplarry. Here I should be known as Larry.
I will be up front with this group. i do have a commercial interest in the development of electric motors for sailboat propulsion. I am a consultant in plastic propeller design and sales specializing in sailboat propellers. My main client is performance propellers a division oF CDI. They can be found at http://www.performancepropellers.com/
The reason for joining the group is simple. I have had many requests to select or design a propeller for an electric motor application. to the best of my knowledge there is not a good reliable formula to explain the interaction of the prop,(diameter and pitch) boat, amps,battery (size and life), heating and regeneration issues, nearly all simply say the motor is about like a xx hp diesel and give me a prop for that. Even the motor suppliers (performance propellers is an oem to one maker)will say send three or four props and we will try them out. there must be a better way! The existing formulas for prop selection have no consideration for constant torque and electrical power consumption rates. I am joining this group to learn about the requirements and practical applications. I hope to get some ideas about how to select a propeller for an electric application. In return I will answer as many prop questions as i can.
There was a discussion in January that had a few questions that i can shed some light upon.
Prop clearance to the hull? Should be 15 to 20% of the prop diameter. This is a hydraulic clearance to allow for propeller wash. too close generally results in extra noise and vibration resulting from the blade passing close to the hull. the frequency of the noise will vary with number of blades. This will probably not be as important with electrics which generally operate at lower speeds.
Moving the prop back by extending the shaft. This is not a good idea with a bronze propeller. this creates more overhung load. general design practice is to only have 2 or 3 shaft diameters clearance between the bearing and the back of the prop, just enough to allow a zinc or line cutter but not both. Overhung load is a second or third power of distance from the support. E.G. a 2 inch dimension may effect the overhang by a factor of 4 but a 3 inch might effect by a factor of 9 and a 4 inch by a factor of 16 etc. If a light weight plastic prop is used then the factor is not as important. a 4 lb prop at 2 inches may result in a load of 16 (4lb x2x2)but a 1 lb plastic prop could be mounted at 4 inches (1lb x4x4)might have the same load.
There is available plastic prop with an extended hub that moves the prop back 6 inches. Google extendo prop
I am in the process of rebuilding an Irwin 32 ketch for electric drive to be launched in may. One of the objectives is to gather information to be used in formulating a method for propeller selection. we will be trying different diameters and pitches on the same set up. we will be gathering volts, amps, temps, speed in two directions. Are there any other parameters that might be helpfull in characterizing performance?
larry
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