This is exactly the problem I had when using available prop calculators and information on the web. The basic rule appears to be is that you use the largest diameter prop that will swing in the available aperture of your hull. There are also minimum clearance distances to be adhered from the hull, keel and propeller. In my case the existing prop setup met these guidelines. For my project I had the existing three blade gunmetal prop scanned, measured and cleaned by a marine engineering shop. It had no markings on it to identify pitch. The good news came back with a detailed report was that the propeller was in good condition and suitable for my electric repower. But the marine engineer was not happy with my original plan to use a 5kw motor and recommended that I go to at least 8kw. He advised that optimum maximum shaft RPM would be around 900 for my boat specifications. The propeller is right handed with a diameter of 13.5 inches and an average pitch of 12.9 inches. I allowed for propeller slip of around 35% with the 13.5 x 12.9 propeller and the propeller shaft rotation will be around 1000 RPM maximum to achieve the desired hull speed. This calculation is very rubbery and by no means predictable due to large number of parameters to be taken into account. It is at least some where to start to determine the reduction from the Motoenergy ME1115 motor speed of 3200 to around 900 RPM at the propeller shaft speed. I have toothed timing gears and and belt for the reduction 8M-HTD 22 tooth at the motor and 72 tooth on the prop shaft. This is a speed ration of 0.31 which is around 992 rpm at the prop shaft allowing for slip I hope to be in the ball park! Final ratio is still to be determined. It may be a case of sea trials to get the ratio right but it will be easy to go up or down a few teeth by changing pulleys and belt. From my understanding their is a sweet spot with electric boats where amp draw is minimal and speed acceptable.
Great tool available on the website below to workout gear sizes and reduction..
I'm studying a possible conversion to electric of my 35-year old full-keel sailboat. But the variable on which I'm having the most trouble finding experience-generated data is prop selection.
I'm aware of prop calculators out there, such as VicProp, but can anyone in this group report on the success of lack thereof, in the actual experience of using a calculator to make a selection?
Did you use VicProp or some other calculator to choose your electric-boat prop, and how did it work out?
If you are using an existing or stock propeller with your electric boat system, what are its specifications (# of blades, diameter, pitch, & mfg.); what power output is your motor (Kw); Max RPM/Reduction ratio (propeller RPMs in other words); and performance?
I'm aware of the data sheet available on this group, but the prop information is spotty. However, mainly, I'd be interested in hearing of anyone's experience with a prop calculator, and which one.
Any help out there?
[-tv]
Tom VanderMeulen
"Grace O'Malley"
Cape Dory 27
Posted by: bob jennings <sabre281@yahoo.com>
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