Three Vs. Four
Q: I own a '96 Centurion and have seen the ads and articles about and three- versus four-blade props. What are the advantages of the different props and are any of them significantly better for my boat? I currently run a factory-installed three-blade.
H. Douglas Ramsey
A: Your engine is designed to reach a specified wide-open throttle rpm. If it is a standard 350 or 351, that rpm level should be 4,200-4,500. Some changes can be made with propellers (such as adding more cup), but you must achieve the recommended rpm window for the engine to run most efficiently, whether that prop is a three- or four-blade.
Since WaterSki's three-blade test article (October 1995), four-blade props have made their way into the market, and I've switched out my reliable three-blade for the new wheel. With it, I've found improved hole-shot and midrange, with a slight decrease in top-end speed. The most noticeable difference is the four-blade's significant noise reduction in the boat compartment.
In either blade scenario, prop sizing for inboards usually starts with a square number (i.e., 13 x 13). The first number is diameter, the second is pitch. Theoretically, a 13-inch pitched prop should move 13 inches forward for every 360-degree rotation. When pitch is decreased, hole-shot is enhanced while top end is sacrificed. When pitch is increased, the opposite occurs. If diameter is reduced, the pitch should be increased; the opposite if diameter is increased. This combination of diameter and pitch is chosen for all-around performance, but must still meet the parameters for engine operation at wide-open throttle.
Get in Gear
Q: What exactly is gear reduction? What effect does it have on the boat?
A: There is a myth about gear reductions. Some skiers believe reductions will increase horsepower and torque, when in fact just the opposite occurs.
A gear reduction actually uses horsepower to drive the extra gears in the reduction unit. What gear reduction, like MasterCraft's PowerSlot or PCM's Power Plus, does is allow you to use a larger propeller because the engine rpm is greater than the drive shaft rpm. In doing so, off-idle to midrange power should be enhanced. For example, look at a tugboat. Even though the reduction ratios are many times more than what we see in the ski boat industry, the same theory applies.
Three Vs. Four