Q. Should a two, three, four or five blade propeller be used?
A. Two blade propellers are mainly used on yachts. A narrow two blade propeller has minimum drag for sailing. Also some two blade propellers are used on high speed racing boats.
Three blade propellers are the most commonly used on all outboard, sterndrive and inboard engines. They normally give the best all round performance on boats to about 40 feet long.
Four blade propellers are normally used on large boats where load carrying is a requirement; particularly commercial fishing boats. They are smoother than 2 or 3 blade propellers and as the diameter used is smaller, they can be useful where the aperture is restricted.
Five blade propellers offer maximum thrust for minimum diameter. They overcome excessive propeller loading and are ideal for restricted diameter aperture. The are also very smooth, and reduce cavitation and slippage.
Q. To change from a standard 3-blade, what diameter or pitch modifications must be made for a 4-blade propeller (to retain comparable engine operation relative to rpm)?
A. Four blade propellers are normally 6 to 8% smaller in diameter
to give similar RPM to three blade models. As an example if a 28 x 22
three blade had been used it could be replaced with a 26 x 22 four blade,
to give similar RPM
Q. My motor is rated at 3200 rpm. Should I use a propeller small enough to turn this rpm at full throttle, though I want to cruise at 2800?
A. A propeller is most efficient at the maximum rpm your engine will turn it; the farther back from maximum your rpm is set by throttle, the greater the loss in efficiency. It is well to run somewhat less, say 200 rpm, than maximum. In this instance, the propeller should be of a size that will turn 3000 at full throttle and be the best size for cruising at 2800 rpm.
Q. How close to the bottom of the boat can blade tips run and what Is the required aperture clearance?
A. The clearance between the propeller tips and the hull should be
at least 1/6 to 1/7 the propeller diameter.
Q. A prop larger than I can swing is indicated for my boat, engine, and gear. Should I use as large diameter as I can and add pitch to hold motor rpm down?
A. Change the gear, increasing shaft rpm, reducing prop diameter requirement, or, change shaft angle or prop aperture to accommodate correct size. If impossible, use a wide 4-blade prop for diameters 18" and up. Under 18", a 3-blade, larger in diameter than required, can be clipped to maximum usable diameter.
Q. What Is a "Cupped" propeller?
A. This is a conventional propeller with the trailing edges scooped or cupped. It actually increases the effective pitch as the water passes across the last part of the blades. This reduces slip and cavitation and, as a result, can increase speed significantly. A propeller of less pitch than normally used is necessary, to give the same motor rpm.
Q. Do I use the same size wheel if I convert to a cupped-edge propeller?
A. No. Diameter remains the same, but additional load placed on the engine by the "cupped edge" requires that pitch be reduced 1", or 2" in propellers above 14" diameter.
Q. What Is propeller "slip"?
A. Slip refers to apparent slip and is a non-dimensional figure
expressed in percentage. It is the difference between theoretical mph and
actual mph divided by theoretical mph. Theoretical mph is calculated by
multiplying propeller pitch and propeller rpm and dividing by 1056. As an
example, a boat that goes 20 mph measured speed, is driven by a 12" pitch
propeller turning 2600 rpm. Theoretical mph is 12 x 2600 divided by 1056
equals 29.6 total mph. Subtracting 20 from 29.6 equals 9.6 which divided
by 29.6 equals 32.5%.
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:
Q. What causes squatting?
A. Many things. A slipping or cavitating prop causes a vacuum and
digs a hole which the stern settles into.
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