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Imagine that you prop is spinning into something solid. Call it a block of cheese. The pitch refers to the distance the prop will travel in 1 revolution. So in a solid your prop will move 21inches in one revolution. It doesn't matter if the prop is 100 feet in diameter. If the pitch is 21 then it will only give you that much forward travel.
9" is pretty low pitch, but if it won't pull any more than 5100 rpm, it is not too low for the job. Maybe the motor is tired, as in down on compression. Is that an accurate speed measurement (15-17 mph), it should be going faster at that rpm with that prop, if the gear ratio is the 1.64 to 1, I see quoted elsewhere on the net.
Was told 1" change in Dia. is roughly equal to 1" pitch change. (about 400 rpm) Too much Dia reduces slip at the expence of Thrust. Too little Dia will increase Slip (decrease efficiency) I would guess 11 1/4 -11 1/2 Dia.is probably the smallest you should try. I think I may be in the to much diameter relm.
With a pitch ratio of 0.8 in the tip region, the blade chord angle there is arctg (0.8/pi), ie ~14.3 degrees. To get the blade chord angle for this propeller at the 0.7 radius then: the reference diameter is 0.7, giving a local pitch ratio of 0.8/0.7 (~1.143) and a blade chord angle of arctg (0.8/0.7/pi), or ~20 degrees.
DAR is based on the area required to avoid cavitation problems. Sometimes due to loading, you will have to increase diameter to provide sufficient blade area. In this case, due to the low power, a DAR of 0.5 - 0.60 would be enough to work fine (assuming a reasonable inflow and low shaft inclination). johneck, Oct 4, 2012.
Generally, yes, a 4- blade will be less efficient than a 3-blade for the simple reason that the leading edge of the prop loses efficiency as it encounters the disturbed water left by the blade ‘in front’ of it. Typically a 3-bladed prop will have more space between the blades to reduce that effect. Deering, Jun 29, 2019.
The propeller is an OMC three-blade SST with 15-inch pitch. It's my favorite all-around propeller on the boat. To plot the curve, we have to find the constant C. We take the rated horsepower, 225, and the maximum RPM that can be reached with this propeller, 5900, and use them to determine C from the relationship HP = C X RPM2.7
High pitch props are used where rpm at the shaft is limited by the gear ratio, such that the engine reaches it's rated rpm, but the boat is not yet at top speed, for that power. In applications where performance matters more than tradition, the designer includes a multispeed gear box, and gears up at that point, rather than using a prop that is ...
That is requires the least power to the propeller to drive the boat at the desired speed. Diameter, ptich and shaft speed are the major factors. Frequently the propeller can't be the optimum diameter due to lack of space so a smaller diameter has to be used. To maintain reasonable efficiency the shaft speed increases as the diameter decreases.
Here we describe the steps in a simple propeller design calculation, using lifting line theory. A table with an example is given afterwards. This corresponds to chapter 5.1 in Minsaas “Propeller Theory”, but be aware that the example in chapter 5.1 have several numerical errors, and the different lines in the example table don’t ...