More about slip versus efficiency.
3.) Propeller Slip
- We have shown that the propeller, in order to generate thrust must accelerate or move a mass of water astern. Now the propeller, being shaped like a screw might conceivably, as it rotates, slide through the water as a machine screw would into a nut, without displacing any water aft. If this happened the propeller (and the boat) would, in one revolution of the shaft, advance an amount equal to the propeller pitch. This would be called zero slip. But in order to produce a thrust,
we must accelerate or move some water aft, and therefore it is apparent that the propeller will not advance the full
amount of its pitch in each revolution, but will advance some lesser amount, depending upon how much water it accelerates astern in the process of producing enough thrust to offset the resistance of the boat to being driven ahead.
If the boat were tied to a dock, the propeller would not advance at all but would generate maximum thrust because full engine power would go into accelerating water astern. This would be called operation at 100% slip. - The term "apparent slip" is used to indicate the difference between the theoretical speed that the boat would obtain on the propeller pitch and the rpm of the propeller shaft, and the actual speed of the boat.
- Slip must not be confused with efficiency that is a measure of the percentage of engine power converted to thrust by the propeller. We have seen that we must have slip in order to generate thrust and the amount of slip will be proportional to the amount of thrust required by the boat.
(from "propeller 101", www.vicprop.com)
You can search this group for my previous posts on the observed propeller slip of my boat, both before and after my conversion.
Fair winds,
Eric
1964 Cheoy Lee Bemuda 30 ketch, 5.5kw Propulsion Marine drive, 8kwh LiFePO4 batteries
Marina Del Rey, CA
---In electricboats@yahoogroups.com, <ewdysar@...> wrote :
Jason,
Good post and explanation about why we gear our motors. But in the lower part, you are calculating propeller slip, not propeller efficiency. There is a difference, propeller slip is easily measured and calculated. Propeller efficiency calculations are more complicated. I don't think that anyone in this group has been able to find a reasonable way to objectively determine their conversion's prop efficiency with accuracy and precision.
Fair winds,
Eric
Marina del Rey, CA
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Posted by: ewdysar@yahoo.com
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