I don't know about the first item, but the second item seems right on target. Talk to any prop vendor and they will tell you that typical prop slippage for an inboard driveline in a displacement sailboat is around 50%, put the prop in an aperture and the slippage increases.
And if you think about it, it makes sense. Let's asy that you've got a 10 inch pitch on your prop. This means that it would travel 10" forward for each revolution if cutting through a solid medium. Spin it at 750 RPM and it should theoretically travel 7,500 inches per minute or 625 feet. Do it for an hour, and the theoretical distance is 37,500 feet or 7.1 miles. We all know that a propellor with 10" pitch spinning at 750 rpm (1500 engine rpm with a 2:1 transmission) won't get anywhere near 7.1 mph on our boats. Speed up and the effficiency drops a bit more. You can check this out for yourself, just do the same calcs using the pitch, engine/prop speed and observed boat speed from your boat. I'll guess that you'll be right around that 50-55% mark.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, redu <reino.urala@...> wrote:
>
> What a ...
> Instant check of Daniel Michaels .XLS:
> 1. Prop diameter:
> sqrt(MaxBeam * MaxHullDepth) * 4.07
> Recommended prop dia becomes 30" for example 27 feet hull? Prop dia from
> hull beam and depth?
> 2. Prop slip:
> In sheet example slip is 55%. What? Should better be 5.5% or something?
>
> Maybe something was misunderstood?
> redu
>
>
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