Propeller Basics: Are you SURE you’re Spinning the Right Blades?

You want to dramatically change the way your boat performs? The easiest way to do so is to change the propeller. Your prop has an effect on time to plane, cruising speed, top-end speed, and efficiency. And chances are, there’s room for improvement. Why? Because most people simply accept the prop their boat came with, and since everything is working fine, assume it’s the right one. Bad move – here’s why.

PITCH, which is a measurement of the theoretical distance your prop will move without slipping, has dramatic influence over how many RPM your engine will turn. To figure out if your prop has the proper pitch, check the owner’s manual for your engine and look for the wide-open throttle RPM range. If your engine operates in it, all’s well. If, however, the engine doesn’t reach that range, going down an inch in pitch will get you around 200 more RPM. This can not only gain you top-end speed, but you’ll also see an increase in cruising speed.

DIAMETER is the diameter (tip to tip to tip) of the propeller, and (if all variables remain equal) diameter increases as horsepower increases. Of course, all variables never remain equal. So diameter needs to be considered hand-in-hand with pitch, as you aim for that magic recommended wide-open throttle range.

BLADE NUMBER usually comes into play when getting onto plane with excessive weight or minimal power is a problem, or when turbulent water causes the prop to slip (such as is often the case with single-engine powercats, or tunnel boats). In either case, a four-blade prop gains some serious traction and can resolve the issue. But the resolution comes at a cost; that fourth blade also increases drag, which will cut a MPH or three off top-end speeds. Cruising speed may also go down a hair.

PROP MATERIAL is another consideration, one which has a dramatic effect on performance. Aluminum props are less expensive than stainless-steel, but they also flex more. As a result, switching to stainless can gain you several MPH of cruising and top-end speeds. There’s a drawback here too, though. If you run aground or strike things often, you’ll want to stick with aluminum. Those flexible blades will give and bend more easily, before the lower unit becomes damaged.

In some applications, to gain performance advantages with certain boats, you may also need to consider blade rake, vented props, and cupping. The bottom line? Every engine is a bit different, and every hull is, too. The very best way to make sure you’re spinning the right propeller is to try a number of different ones, varying pitch, diameter, blade number, and even material, until you find the one that’s best for your needs.

For more info on props, check out Prop Talk: They Spin, You Go.