Its the prop.
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.
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.
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.
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.
--- 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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