There is no perfect propeller. Correct propping for your boat is going to involve a compromise between hole shot, midrange and top end performance. Depending on the specific boat/engine combination, some propellers will perform better than others at attaining all three objectives. The best hole shot is generally achieved with a propeller that allows an engine to gain rpm as rapidly as possible without detrimental ventilation or slippage. BLACK MAX, ALPHA and HIGH FIVE are excellent propeller families that would fit this application. Most engines develop maximum horsepower within the recommended wide open throttle range. Within the range, however, horsepower is usually increased with rpm. The propeller uses power to transfer a higher propeller pitch (inches traveled per revolution), and more rake (bow lift) into more speed with a result of lowering engine rpm at wide-open throttle operation. Mercury offers many excellent performance propellers including LAZER II, MIRAGE PLUS, and the BRAVO ONE Performance Series. Mid-range performance is a compromise of the above two applications. Choose the propeller with the larger diameter that allows the engine to operate in the upper rpm range.
Diameter is the distance across the circle made by the blade tips as the propeller rotates.
Pitch is the distance that a propeller would move in one revolution if it were moving through a soft solid, like a screw in wood. When a propeller is identified as 13 3/4 x 21, it has a 13 3/4? (35 cm) diameter with 21? (53 cm) of pitch. Theoretically, this propeller would move forward 21? in one revolution.
When the trailing edge of the blade is formed or cast with an edge curl it is said to have a cup. Cupped props will usually allow a faster top speed and more midrange efficiency by allowing more positive trim with less prop slip. Cupping benefits are so desirable that nearly all modern recreational, high-performance or racing propellers contain some degree of cup. Cupping will usually reduce full-throttle engine speed about 100 to 200 RPM below the same pitch propeller with no cup. A propeller repair shop can increase or decrease cup to alter engine rpm to meet specific operating requirements on most propellers.
Ventilation occurs when air from the water’s surface or exhaust gases from the exhaust outlet are drawn into the propeller blades. The normal water load is reduced and the propeller over-revs, losing much of its thrust. This action most often occurs in turns, particularly when trying to plane in a sharp turn or with an excessively trimmed-out engine or drive unit. Ventilation can also be caused by aerated water from step bottom hulls.
Many high-performance boaters are aware of a phenomenon that limits their top speed below what would otherwise be possible with the available horsepower. This phenomenon is commonly called “gearcase blowout,” “propeller blowout,” or just “blowout.”
As a shape passes through water at an increasing speed, the pressure that holds the water to the sides and back of the shape is lowered. Depending upon the water temperature, when the pressure reaches a sufficiently low level, boiling (i.e., the formation of water vapor) will begin. The collapsing action, or implosion, of the bubbles releases energy that chips away at the blades, causing a “cavitation burn” or erosion of the metal. The initial cause of the low pressure may be nicks in the leading edge, too much cup, sharp leading edge corners, improper polishing, or, sometimes, poor blade design. Massive cavitation by itself is rare, and it usually is caused by a propeller that is severely bent or has had its blade tips broken off resulting in a propeller that is far too small in diameter for the engine.
Generally, yes, a steel propeller will be faster. If the designs are the same, the steel propeller can be made with thinner blades that run faster and with greater strength. Also, most stainless steel propellers take advantage of performance enhancing designs to gain even more advantages over aluminum.
Best all-around performance is achieved when wide-open-throttle (WOT) engine operation occurs near the top of the wide-open-throttle rpm operating range designated by the manufacturer for that specific improperly propping an engine can not only reduce performance, but also, in fact, damage the engine. An engine that does not reach the rated rpm at wide-open-throttle is in an “over-propped” condition, resulting in “lugging.” This severe strain can lead to detonation, piston seizure, and engine damage. On the other hand, an engine that revs past the recommended rpm will have higher than normal wear and can also be damaged by fatigued parts breaking and passing through the engine. This is why it is so critical to be sure your engine is propped correctly for your boat/engine combination and the type of boating you want to do. To make this selection, propeller charts are published as guidelines for general applications of Quicksilver propellers. They are not intended, however, to be an absolute recommendation, as boats and operating conditions vary. Use the guidelines suggested here, but remember, the best propeller for your boating needs, can be determined only by experimentation. You really should have more than one propeller if you use your boat for more than one type of activity, such as cruising, fishing, and skiing. You may well need different propellers for the best performance in each type of activity. In any event, you should keep a spare propeller on board at all times, along with a wrench that will fit the propeller nut, pliers, a spare nut, and tab washer.
The most important concern to be aware of is the correct rpm range for your engine. The propeller pitch regulates the engine rpm. Lowering the propeller pitch (going from a 23 pitch to a 21 pitch) will increase the engine rpm, just like shifting from third gear into first gear. Increasing the pitch (23 pitch to a 25 pitch) will decrease engine rpm. Most applications will match up pitch to pitch and this is the best starting point. Changing from Black Max aluminum to a Lazer II SS will usually decrease maximum rpm between 50 and 100 rpm and increase top speed by 2 to 3 mph. If your aluminum was operating in the mid rpm range for your engine, the next lower pitch Lazer II propeller should be best for you. Changing from Black Max aluminum to a High Five SS the rpm should hardly change. In most cases when moving to a High Five from a 21? 3 blade aluminum at the top of the rpm range test have shown a 23? to work best. This would seem to contradict previous information. The 23? High Five has exhaust vents, which the 19?and 21? pitch High Five props do not have. These vents allow the 23?’ to slip at low rpm allowing the engine to make hp quicker and to accelerate as well as the 21? without vents. In most of these cases the 23? will use less fuel at cruise and have a faster top speed.
150 to 300 RPM higher at WOT. Blade design and diameter are two of the indicators that will tell how one propeller will perform vs. another.
No, we do not have an exchange program once a propeller is run on an engine, it becomes a used propeller. We have no provision to deal with used propellers.
Essential to good propeller maintenance is periodic inspection to detect even small dings, which can lead to blade failure if not dressed or repaired. A damaged propeller, even one that only appears slightly damaged by running through silt and sand, can significantly reduce performance efficiency and fuel economy, and can more severely damage itself through cavitation erosion emanating from the blades’ irregular leading edges. In one test with a damaged propeller, top speed fell more than 13%. Acceleration was off over 37%. Optimum cruise miles slowed 21%. Worse yet, damage usually is not done to each blade uniformly and, therefore, the damage can set up imbalance vibrations that can cause fatigue damage to other parts of the engine or drive. If you boat in shallow or rocky waters, you will want to check your propeller more frequently for possible damage.
Stainless Steel is just as its name implies — it “stains” less than normal carbon steel. Only the highest quality stainless steel available is used. Rust can occur under numerous circumstances, including polluted water and galvanic corrosion. This can be caused by the boat, environmental galvanic activity, (i.e. a marina’s electrical system), and oxygen depletion which can be caused by shutting off air to the propeller (shrink wrap or wrapping drive/prop in plastic). Since this is a post purchase cosmetic defect and has no bearing on performance, there is no warranty for rust. The rust can be removed with rubbing compounds. A Scotch brite pad is OK but do not use steel wool. The propeller can be polished and sealed with a chrome polish.
A stainless steel propeller that is turning white has been exposed to a high lime and or calcium (oxidation) condition. Readily available commercial products will remove this and keep deposits of calcium and lime away. A coating of chrome polish will help deter this formation.
The pitch on most propellers can be changed. On aluminum propellers a maximum change of one inch of pitch up or down is recommended. Stainless steel can be changed up to two inches of pitch, up or down. One inch of pitch equals approximately 125 rpm. Lower pitch to gain rpm, increase pitch to lose rpm. Modifications will void the propeller warranty. The work of individual prop shops cannot be controlled, especially when adding cup.
When observing from behind a boat, the propeller turns clockwise when underway with a normal right-hand propeller. As water resists the clockwise rotating propeller, it causes the boat to roll slightly in the opposite direction (counterclockwise) or down on the left (port) side and up on the right (starboard) side. To offset this slight imbalance, the driver’s seat is placed on the starboard (right) side. Boats differ significantly in the degree of their reaction to prop torque.
The standard propeller recommended by manufacturer is best for all-around performance under average use. When we select a standard propeller, it is the one with the best combination for top speed, cruising, skiing, etc. In other words, it is a “general use” propeller. No one propeller can give you the maximum performance for “out of the hole” versus “top-end” expectations; one comes at the expense of the other. The only way for you to achieve maximum performance in both categories would be to carry two props, each designed to maximize their respective feature. For example, if you have a 19ft boat with a 220 HP EFI V6 and a light load, the engine will hit the rev limiter. Replacing the standard 23? propeller with a 25? propeller would reduce the “general use” features of the standard prop and bias the performance to top-end only. You will lose “out of the hole” performance or the low-end torque as it is frequently referred to. The higher pitch 25? propeller will usually produce higher top speed, but is typically unsuitable for adequate planning or water sports usage, particularly as your load increases.