Here is a web page with a quick explanation of how to determine the maximum prop size for a boat given the 15 to 20% of diameter constraint.
http://mysite.verizon.net/resqp86c/performancepropellers/id41.html
there are many factors in setting clearance all are related to how the water flows, how fast is the prop, etc and will vary with boat construction e.g. how thick is the hull and what is the resonant frequency of the system. All very complicated and often can only be determined by expensive testing. The rule of thumb 15 to 20% is used because we cannot aford the testing required to be exact.
bigger is usually better but the limit is usually boat construction.
--- In electricboats@yahoogroups.com, "Owen Tyers" <otyers@...> wrote:
>
> Hi James,
>
>
>
> Thanks for sharing your insight. It's great to read these bits from those
> who have literally gone before.
>
>
>
> One thing I noted is that you are using a prop that is 12.5 x 14 (pitch =
> 1.12 x diameter). In your previous post though you wrote: "The most
> efficient propeller has a pitch to diameter ratio of 1.3 :1." This would
> suggest a 12.5 x 16 prop. Why did you choose the 14 pitch? Would a 16 be
> substantially better?
>
>
>
> Along the same lines, is it possible to go too big? You state that: "A big
> propeller turning slowly adds enormously to the efficiency of the system."
> In my case where there is currently a 13 x 8 2-blade, a 1.3 ratio would
> suggest either a 13 x 17 or a 14 x 18 3-blade. Are these workable sizes in
> your opinion?
>
>
>
> Finally, you write: "Tip clearance is necessary especially when using the
> brakes. A boat at full boat speed when shifted to full reverse will cavitate
> if there is not enough tip clearance. When choosing a propeller we try to
> fit as large a propeller as possible onto the boat. Tip clearance should be
> at least 15 percent, and preferably a minimum of 2 inches."
>
> If one is not likely to ever be in a situation where at full hull speed they
> then need to go into full reverse (I've never done this and imagine the
> diesel would have ejected itself out the companionway hatch if it was ever
> tried) then how critical is the 15% rule? Assuming that 15% refers to the
> full diameter then 13 = 2" clearance and 14 = 2.1". In my case, I think a
> 13" prop would give 2" clearance while a 14 would give 1.5". Do you have
> advice on which prop size to choose... is the half inch loss of clearance
> more or less important than the 1" gain in diameter and pitch?
>
>
>
> Thanks again for any advice.
>
>
>
> Regards,
>
>
>
> Owen
>
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