60% sounds about right for most reasonably good boat props, few exceed this unless carefully optimised and matched to the hull requirements.
You can improve on this with some work, and the acceptance of some compromises on practicality, though. Increasing the prop diameter, reducing the blade count and blade chord and choosing a section for the blade that maximises local L/D ratio with radius, will get efficiency figures up into the mid-80% range.
There are downsides to going down this route, though. Props with a high aspect ratio are less robust than those with wider blades, plus they are more liable to suffer from weed entanglement problems. Making the high aspect ratio blades from a tough material, like stainless steel, plus allowing them to fold back, reduces these problems, but then adds the problem of ineffectiveness when trying to go astern.
I'm currently in the process of changing my large diameter, folding, two blade prop to a smaller diameter ducted prop of around the same overall efficiency (about 86%), but this only really works for a low speed boat (mine cruises at just 4kts).
--- In electricboats@yahoogroups.com, redu <reino.urala@...> wrote:
>
> My friend
> measured el-drive propeller efficiency by measuring the thrust force at
> drive-to-hull-assembly, while the boat was moving normally on sea. The
> prop output power is of course P = v * F, where v = boat speed, and F =
> measured thrust. Motor output power is of course current * voltage *
> specified motor efficiency. He has a super propeller with wide aperture
> and narrow blades. The measured super prop efficiency was 60%.
>
> reino urala
>
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