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Propeller FAQs

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
Generally the pitch would remain the same, however sometimes the diameter reduction can be less, combined with a corresponding pitch reduction.

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.
Clearance is equally important along the forward edges of the blade where they swing in line with the deadwood and should not be less than 1/12 of the 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%.
Slip is not to be confused with propeller efficiency.

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
work boats 40 to 80.

Q. What causes squatting?

A. Many things. A slipping or cavitating prop causes a vacuum and digs a hole which the stern settles into.
Correction of prop size often corrects this. Poor hull design, or a good hull put out of trim by misplaced engine or load is a leading factor. Heavy keel, unfaired strut, scoops, or automatic bailing fixtures or anything that disrupts smooth, free flow of water to prop, can
cause cavitation and subsequent squatting.

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Article: Propeller Sizing, Have you got the correct propeller?

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