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Assuming the same geometry (or at least shape) what would be the expected performance trade-off on the bigger wheel with less pitch? Will I lose top speed but gain mpg with the bigger wheel or can such a correlation be made?
-Gillie
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Good question Gillie - I have a 150 page book called the "Propeller Handbook" by naval architect Dave Gerr that says a rough general rule of thumb is that 1" of diameter is equivalent to 2-3" of pitch, and that 2" of pitch is worth about 450 rpm. That seemed to be about right on the MWC Vortex that I had repitched, which picked up about 500 rpm with a 2" pitch reduction. It also said that in general, larger diameter props were more efficient, unless you want to run over about 35-40 kts. It says 4 blade props theoretically lose a little efficiency due to the drag of the extra blade, but my experience is that they have other advantages that may offset that. All I know is that compared to Sandy's 3 blade Mirage 3+ that I tried, which is supposed to be a good prop, my 4B Apollo was about 3 mph faster, planned about 3 mph slower and was about .4 mpg better on optimum cruise fuel burn as measured by the I-Command gauges! It's still too much prop for my rig unless I'm running light. Would likely be a great prop for someone running a 175 or 200, maybe on a lighter boat. I've been trying to get Sandy to try it on his 20cc, but no luck so far! Denny
02-17-2009, 01:27 AM
Re: Seafari CG and stern-lifting props
Hybrid Mode
