Frequently asked Questions about Inboards

A. Much more information on the boat, engine, gears, type of service is required to make a reliable prop selection. Past performance information is helpful too unless yours is a stock outfit with known performance characteristics.

Your marine dealer or a major propeller manufacturer can furnish a propeller analysis form which their engineers can study to advise you.

Q. Can one propeller give top speed plus lots of power?

A. Generally no. To gain high speeds involves sizes or pitch rations unsuited to load carrying or working conditions. Decide what is most important and select the prop accordingly, or change props with boating conditions.

Q. You suggested a 13" diameter x 14" pitch for my boat. Wouldn't a smaller diameter with more pitch do the same?

A. Pitch cannot be substituted for diameter. Diameter is directly related to horsepower, rpm, and speed, which indicate your requirements. If the operating conditions call for a 13" diameter, a 12" cannot be substituted without adversely affecting performance by decreasing efficiency.

Q. Will a prop change correct a disturbingly large wake?

A. Probably not, unless the change causes a non-planing boat to plane. Wake is directly proportional to hull displacement. Fast boats in planing position throw less wake than slow displacement boats even though the planing boat goes faster than the slow, wave-throwing hull.

A. a) Engine location is closely tied up with trim calculation so it's impossible to answer without complete scientific characteristics of the boat. Engine weight and center of gravity must be calculated in connection with all the other weights and centers. If it's impossible to obtain these characteristics, use a loose rule that the best location for engine weight is 55% of the load waterline length of the boat abaft the fore end of that waterline.

b) Aft, just forward or the transom.

c) For boats of 20 to 30 foot length, rudder blade should be about 5% of the figure found by multiplying the waterline length of the boat by the extreme draft. Boats of 40 to 50 feet need rudders of about 4%. Larger boats need and area of about 3%. Blade should extend downward, even with lower prop tip and aft far enough to get area from the above formula.

d) Generally, as low as possible, never more than 15^o, allowing proper clearance between blade tips and boat bottom. A modestly steep shaft angle does not have a noticeable effect on performance. Normally the difference between, say, 8^oand 14^o is negligible.

e) This is related to horsepower, rpm, and type of shaft material used.

f) Monel, stainless steel and bronze, in this order. All resist corrosion. Monel is strongest.

Q. I run on mile-high western lakes. Should I adjust prop size accordingly?

A. Yes, gasoline engines lose power when operated above 3000' altitudes. A smaller pitch prop will compensate some by bringing rpm to correct operating point. Boats operated at high and low altitudes need a prop for each condition.

Q. I have a right hand prop but I need a left hand. Can it be changed?

A. No. Propeller hand cannot be changed. Turning it around won't change the hand.

Q. How much can you change prop pitch?

A. Depends on individual propeller. Average standard bronze props can be changed about 2" up or down. Remember that the metal near hubs cannot be changed. Excessive stress and strain can distort or kink blades, causing poor prop efficiency.

Q. What are true pitch, variable pitch and controllable pitch propellers?

A. A true pitch propeller measures constant pitch at any radius … a section of a true helical surface. A propeller whose pitch varies radially, that is, is not constant but increases from hub to the tip, is a variable pitch propeller. Variable pitch propellers are used on most high speed craft and reduce cavitation which might result at such speeds. Many larger boats use variable pitch propellers to compensate for wake or varying inflow velocities which result from the depth of the immersed hull. With a controllable pitch propeller, the pitch angle of the blades can be mechanically adjusted when the boat is underway.

Q. Shall I get my propeller dynamically balanced?

A. Most premium grade propellers are dynamically balanced at manufacture. All other propellers can be dynamically balanced but only on special order and at additional price. Such a balance is inherent proof of manufacturing perfection insofar as blade thickness, width, spacing, ect. are not concerned. This inherent accuracy is not destroyed by blade damage, making it unnecessary to dynamic balance during repair. A good static balance at the repair station will suffice as the inherent accuracy of the propeller is still there.

Q. What is electrolysis, its cause, correction?

A. Electrolysis is the flow of electricity caused by the difference in potential of two dissimilar metals immersed in electrolyte. It acts as a plating process and one metal is eaten up and deposited on the other. Prevent harm to underwater parts by attaching sacrificial anodes (usually zinc). Replace from time to time as they deteriorate.

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 benefits does a cupped-edge wheel offer?

A. Cupped-edge props for fast boats and high rpm reduce slip, giving as much as 3 mph more speed on the average, and eliminate cavitation.

Q. How close to the bottom of the boat can the 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. Shall I use 2-, 3-, 4-, or 5-blade propellers?

A. Excessive horsepower, direct drive and high advanced speeds indicate that propellers of high pitch-diameter ratio be employed. Generally 2-blade propellers are used. At somewhat lower advanced speeds and shaft speeds, 3-blade propellers with pitch-diameter ratios of approximately 1.0 are required for best propeller efficiency. It is in this category that most pleasure boas lie. At even lower advanced speeds and shaft speeds, 4-blade propellers with pitch-diameter ratios of .8 or less are required for optimum propeller efficiency. On large cruisers and commercial applications such as trawlers, shrimpers, tugs and work boats, 4 blades are generally used. If excessive vibration is a problem, 5-blade propellers are used.

Q. What are normal slip percentages for various craft?

A. With propeller 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 and work boats 40 to 80.

Q. The salesman said my boat would go "X" mph. I go only 2/3 as fast, can that be right?

A. "Claimed" boat speeds are often overestimated. Were his speed trials and yours accurate? Were test conditions similar? It's hard to answer with so many factors in the picture.

Q. Shall I use heat to install or remove my prop?

A. Heat should never be used to install a propeller, and, therefore, should seldom be required for removal. If a wheel puller and hammering with soft hammer on propeller and /or shaft end does not work, gentle warming with a blow torch may help. Do not use a welding torch as the quick, harsh heat will change the grain structure of the bronze, setting up internal stresses resulting in hub splitting.

Q. Can I put an inboard engine in my outboard hull?

A. The big question here is whether you will throw the hull out of trim with a heavy inboard mounted mid-ships when the hull was designed to carry an outboard in back of the transom. Normally such conversions are never successful unless the hull as designed to carry the engine weight aft, or you have a V drive or a transom drive with the engine well aft.

Q. My outfit doesn't have the speed I expect, though I've tried several types of prop. Why?

A. Some boats do not have the power to "get over the hump" or attain planing speed. Other boats that achieve planing speeds may not have the power required to reach speeds desired. In such cases repowering is the only answer. Power requirements are proportional to the speed cubed.

Q. Will a reduction gear give me more power?

A. Reduction gears in themselves do not provide power … they absorb power. They provide the means of using a greater propeller diameter, which may be more efficient. They are commonly used with high rpm engines in relatively large hulls, requiring greater blade area than can be used driving direct.

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. For the 4-blade Dyna-Quad, usually associated with pleasure boats, maintain the diameter and reduce pitch 1".

For the 4-blade Work-Horse style, usually associated with commercial craft, reduce diameter 5% to 6% to the nearest even-inch diameter.

For the narrower 4-blade Trawler style, maintain same diameter and pitch.

Q. My motor is rated at 3200 rpm. Should I use a wheel 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, prop should be of a size that will turn 3000 at full throttle and be the best size for cruising at 2800 rpm.

Q. What is the 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 1065. 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 1065 equals 9.6 which divided by 29.6 equals 32.5%. Slip is not to be confused with propeller efficiency.

Q. Why do hulls of the same length, beam, weight and power vary so in performance?

A. Here the effect of an engineered design is realized. The difference between a boat designed by a qualified naval architect and one built by backyard techniques may be many miles per hour. Boat design is both a science and a skill which ahas evolved over many years. The designer must be cognizant of the effect of weight, weight distribution, deadrise and other hull characteristics which can affect overall performance. Otherwise, a poor performing boat results.

Q. Can't I get more than "X" miles per gallon of gas?

A. Don't think of miles per gallon … think of gallons per hour. Fuel consumption is related to the overall efficiency. Engines all use almost exactly one gallon of gas per hour for every 10 h.p. developed. For example, if you burn 15 ghp, your engine puts out 150 h.p.

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. Won't a step-up gear increase my speed?

A. With an extremely fast boat (step bottom hydro or drag boat), depending on your engine, a step-up gear may let you use a prop size better suited to your needs. Gears for "class" boats are usually worked out by trial and error, so stick to what has been done successfully for similar boats. Step-up gears are the opposite of reduction gears, but we still figure h.p. and rpm at the prop. With step-up gears, shaft rpm of 10,000 is not unusual in racing craft.

Q. How can I tell if I need a left or right hand prop?

A. Stand in back of boat. If shaft turns clockwise, a right hand prop is needed. Most marine motors take right hand props.

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 of ten 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.

Q. On a sailboat under sail, should prop be rotating or locked?

A. There is less drag when prop rotates.

Q. With twin screw, is opposite rotation required?

A. Theoretically it is best. It definitely aids maneuvering, by advancing or retarding one engine as required. For steering, non-opposite rotating motors cause no steering problems.

Q. With twin screw, should rotation be inboard or outboard?

A. Rotation is normally outboard looking at the top of prop from stern of boat. Right-hand prop rotates to starboard, left-hand to port.

Q. What do the stampings on my prop mean?

Most props are stamped on forward end with diameter, pitch and shaft size. Diameter is always first, pitch second. Size is likely repeated on the side of the hub with the manufacturer's name, type prop, hand, and date of manufacture.