Hi Jeremy,
When you're dealing with displacement auxiliary sailboats, even the good propellers run at slip rates above 35%. If you look at the propeller calculators from most of the propeller manufacturers, the typical figures for displacement sailboats are around 50-55% slip. Here's the figures that came from my boat, before and after:
"Doing the calcs, the old prop showed 39% slip at 3kts and 58% slip at 5.5kts. The new prop calcs to 27% slip at 3kts and 37% slip at 6kts. Better, but not 6 times better."
I know that for lightweight canal shells like yours, the numbers are radically different, but I'm a little fuzzy about what kind of regen scenarios that you would get while using your boat.
Variable pitch will make a more effective hydro-generator as proven by "the amazingly expensive french hydrogenerators (more than $7000 US each) that have been discussed on this board more than once this year."
http://www.velux5oceans.com/#/ocean-sprint-4-news/brad-hydro-power-sustainability/1902
The variable pitch props are computer controlled to maximize the charge while minimizing drag. I get the impression that they have an algorithym that optimizes the tradeoff for different speeds. They have the advantage of sailig at much higher speeds than most of our boats and they are only looking to supply house loads.
I agree that larger diameter props are more efficient in both directions (propulsion and regen), but we're all limited in the size of prop that we can pratically fit under our boats.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "Jeremy" <jeremy_harris_uk@...> wrote:
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> Its the prop.
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> The reason is this. A prop being used to power the boat has a slip factor, probably quite modest for a good prop, but likely to be around 10 to 20%. This means, in essence, that the prop is spinning 10 - 20% faster than it theoretically needs to to propel the boat.
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> A motor used as a generator needs to spin at enough rpm to generate a voltage that is high enough to charge the battery. The relationship between voltage and rpm is the same for generation as it is for propulsion.
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> Because of the slip factor, the prop being used as a turbine will spin more slowly at a given hull speed than it will when used for propulsion. To get the prop speed up to the same rpm needs a different pitch when the prop is used as a turbine.
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> If you have a variable pitch prop then it should be possible to alter the pitch when in regen mode to get better efficiency. Ideally you really need a larger diameter prop for use as a turbine, as diameter has a greater effect on efficiency than pitch.
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> --- In electricboats@yahoogroups.com, "Tim" <haywardt@> wrote:
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> > I am curious as to why it is so challenging to achieve good regen rates when towed impeller generators produce such high charge rates. I would think the placement and size of the prop would produce better results than a towed generator. Is it a problem with the controller? Have you tried sailing with the throttle slightly in reverse?
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