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.performa
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
No comments:
Post a Comment