Outboard / Stern drive:

1. Q: What prop should I use on a 30' boat with 185 hp engine? 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. VicProp furnishes a propeller analysis form which our technicians can study to advise you.

2. Q: Can one propeller give top speed plus lots of power? Generally, no. To gain high speeds involves sizes or pitch ratios unsuited to load carrying or working conditions. Decide what is most important and select the prop accordingly, or change props with boating conditions.

3. Q: What's the best prop for my boat / engine? The best prop will allow the engine to reach it's maximum horsepower and RPM under normal operating conditions depending on the use of the boat. Many factors such as diameter, number of blades, and material can enhance various operating conditions. As an example, a 5-blade propeller could enhance water skiing and hole shot whereas a 3-blade high rake propeller could enhance top speed. Most propellers are a trade off, to gain top end speed you loose bottom end hole shot and vise versa. Another example could be a 4-blade propeller, which may enhance the hole shot without affecting or reducing the top speed. With some basic information about the boat and the engine size most propeller shops can recommend a propeller configuration that will work well for your intended use.

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

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

6. Q: If I change my prop for one with more pitch, will my boat go faster? The pitch of your propeller is measured in inches. It is the theoretical distance that the prop will travel forward in one revolution with out slippage. The higher the pitch, the farther it will travel and vice versa. It sounds easy, more pitch faster boat right? Not quite, as the pitch increases the load on the engine increases and so does the slippage. Basically, as the load on the engine increases the maximum rpm drops, so does the horsepower, so does the speed. Most engine manufacturers list a WOT ( wide-open throttle ) rpm specification on the tune up decal. This is the rpm range where the engine makes its rated horsepower, so it is important to select a propeller that will allow the engine to rev up to it's WOT rpm range. If you are using a prop with too much pitch it will be sluggish out of the hole and will tend to overheat. If you are using a prop with too low of a pitch it won't be using the full potential of the engine and the engine can be damaged by over revving.

7. Q: Won't a step-up gear increase my speed? 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 hp and rpm at the prop. With step-up gears, shaft rpm of 10,000 is not unusual in racing craft.

8. Q: My boat goes too fast to troll for fish, should I put a low pitch prop on to slow the boat down? This is not a recommended practice because the low pitch prop will allow the engine to over rev at full throttle possibly damaging the engine. You should use an auxiliary engine, or tow something behind the boat, or use a trolling flap behind the prop to slow the boat down.

9. Q: Will a reduction gear give me more power? Reduction gears in themselves do not provide power... they absorb power. They provide the means of using a greater propeller diameter, which may be beneficial. They are commonly used with high rpm engines in relatively large hulls, requiring greater blade area than can be used driving direct.

10. Q: You suggest a 13" diameter by 14" pitch for my boat. Wouldn't a smaller diameter with more pitch do the same? Pitch cannot be substituted for diameter. Diameter is directly related to horsepower and rpm, which dictate your requirements. If they call for a 13" diameter, a 12" cannot be substituted without adversely affecting performance by increasing slip or causing cavitation.

 11. Q: How much can you change prop pitch?  Depends on the individual propeller. Average standard bronze props can be changed up or down about 3-5", aluminum 2" and stainless steel 1". Remember that the metal near hubs cannot be changed. Excessive stress and strain can distort or kink blades, causing poor prop efficiency.

12. Q: What are true-screw pitch, variable pitch, controllable pitch? True-screw is the prop of constant pitch at any radius ... a section of a true helix. Variable pitch comes in many combinations and provides extra speed for fast bottomed boats. The practice of building true pitch is departed from in such prop construction -- either throughout the blade or at specific sections. Variable pitches reduce high speed cavitation. Many large boats use variable pitch to compensate for varying wake velocities hitting the blades as put into motion by underbody hull design. Controllable pitch is a mechanical means of pitch setting with the boat in motion.

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

15. Q: I hit something the other day, and there is only one little chunk broken off of one of my propeller blades. Will it be o.k. to keep using it like that? No, any damage to the propeller blades will cause performance loss and vibration. The vibration may be unnoticeable, but it will wear on the seals and bearings in your gear case causing more serious damage. The propeller should be checked and repaired after any impact.

17. Q: In building a boat: a) Where do I set the engine? b) Where do I locate the prop? c) What size rudder shall I use? d) What shaft angle is best? e) What size shaft do I need? f) What shaft materials are best? 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 percent of the load waterline length of the boat abaft the fore end of that waterline. b) Aft, just forward of the transom. c) For boats of 20 to 30 foot length, rudder blade should be about 5% of the figure found by multiplying the water- line length of the boat by the extreme draft. Boats of 40 to 50 feet need rudders of about 4%. Larger boats need an 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 degrees, 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 and 14 degrees 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.

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

19. Q: 1 have a right hand prop but need a left hand. Can it be changed? No. Propeller hand cannot be changed. Turning it around won't change the hand.

20. Q: Shall I get my propeller dynamically balanced? 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, etc., are 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.

21. Q: What is electrolysis, its cause, correction? 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.

22. Q: Do I use the same size wheel if I convert to a cupped-edge propeller? 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" dia.

23. Q: What benefits does a cupped-edge wheel offer? Cupped-edge props for fast boats and high rpm reduce slip, giving as much as 3 mph more speed on average, and eliminate cavitation.

24. Q: Shall I use heat to install or remove my prop? 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 wiII change the grain structure of the bronze setting up internal stresses resulting in hub splitting.

25. Q: Can I put an inboard engine in my outboard hull? The big question here is whether you will throw the hull out of trim with a heavy inboard mounted midships when the hull was designed to carry an outboard in back of the transom. Normally such conversions are never successful unless the hull was designed to carry the engine weight aft, or you have a V drive or a transom drive with the engine well aft.

26. 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)? For a 4-blade usually associated with pleasure boats, maintain the diameter and reduce pitch 1". For the 4-blade 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.

27. 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 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 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.

28. Q: What is propeller "slip"? 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%.

29. Q: Why do hulls of the same length, beam, weight and power vary so in performance? Here the effect of a real design shows up. The difference between an N.A. designed boat and a "built" boat can be many mph. Boat design is a science, requiring real engineers.

30. Q: Can't I get more than "X" miles per gallon of gas? 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 hp developed. For example, if you burn 15 gph your engine puts out 150 hp.

31. 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? 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.

 32. Q: How can I tell if I need a left or right hand prop? Stand in back of boat. If shaft turns clockwise, a right hand prop is needed. Most marine motors take right hand props.

33. Q: What causes squatting? 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.

34. Q: On a sailboat under sail, should prop be rotating or locked? There is less drag when prop rotates.

 35. Q: With twin screw, is opposite rotation required? 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.

36. Q: With twin screw, should rotation be inboard or outboard? Rotation is normally outboard looking at top of prop from stern of boat. Right-hand prop rotates to starboard, left-hand to port.

37. 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.