Following some calculations I first saw here, I made a table of known shaft rpm to boat speed, converted rpm into revolutions per hour and knots into inches per hour, got inches per revolution from that, put it over the pitch and found slip to be right about 54 per cent. This was a 12x8x2 on a t27.
If you know how many rpm it takes to get hull speed, you should be able to calculate slip, for that speed at least.
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Ned,
While I have used a general prop sizing calculator that use 50% slip factors, I know that the propeller shop that sold me a new prop for my 5 ton, full keel, 30' ketch witht the prop in a aperture used 55% slip to find the appropriate pitch. I found the following information on th Michigan Wheel propeller website:
Q: What are normal slip percentages for various craft?
A: With propellers correctly selected for the operating conditions, the slip percentages would be as follows: racing hulls 10 to 15 percent, planing runabouts 15 to 25, planing cruisers 25 to 35, displacement cruisers 30 to 40, sailing auxiliaries 35 to 40 and work boats 40 to 80.
I also found this reference on the Castle Marine Propeller Repair Specialists website: "The pitch is the distance a propeller would in theory move forward in the water in one revolution if there was no slippage i.e. a 21" pitch propeller would in theory move forward 21" in one revolution. In practice most propellers do slip the amount being dependent on the boat and as a general guide slippage would be between 8% for a race boat with a planing hull to as much as 55% for a full displacement craft. A 10" pitch propeller on a displacement craft with a 50% slip ration would only move the boat forward 5" per revolution of the propeller. The pitch is the second number in the description i.e a 13 3/4 x 21 propeller is 13 3/4 diameter and 21 pitch."
So I would say that we may both be right, we're just talking about very different use cases.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Ned Farinholt <nedfarinholt@...> wrote:
>
> Eric,
> Your slip factor seems unnecessarily pessimistic, at least for driving forward. I have no experience with regen. Perhaps you are using slip factor for reverse which may simulate regen. See http://continuouswave.com/cgi-bin/propcalc.pl for estimates and formula. They suggest 6-10 percent which agrees with my experience with my Torqeedo 801 on an easily driven very light hull at nearly hull speed.
> Ned
>
> On Dec 1, 2010, at 3:03 PM, Eric wrote:
>
> > Myles, your numbers look good. I have noticed that most prop manufacturers use a 50-55% "slip" factor in determining the proper pitch for displacement auxilliary sailboats, so your 65-70% prop efficiency may be a bit optimistic for many of our conversions....
> >
> >
> > Fair winds,
> > Eric
> > Marina del Rey, CA
> >
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