Most so-called "pleasure boaters" probably leave their boats as is. After all, we're not looking for jaw-dropping top speeds, or a race-winning holeshot. We want a reliable, all-around performer. There is one modification, however, that I think is worthy of serious consideration — a new propeller. Why? To understand, let's take a crash course in Propeller 101.
Propellers are designated by their diameter and pitch, expressed as D x P. A prop designated as 14 1/2 x 21 means that it is 14 1/2 inches in diameter (the distance across the circle made by its rotating blade tips) and features a 21-inch pitch (the distance the prop would move forward in one full revolution through a soft solid, i.e. like a screw turning through wood). Stock props are often made from aluminum; alternatives are commonly constructed of stainless steel.
Got it? Now here's why you might want to change your stock setup.
For starters, aluminum is soft. Hit that inevitable sandbar (or worse, a rock) and it's going to get dinged, maybe even seriously chipped. The good news is that aluminum will take the brunt of the impact and save your driveline from a serious jolt. The bad news, however, is that your prop may need to visit the nearest service department more than you'd like. Leave it damaged and you're likely to cause a vibration that will ultimately damage your driveline, not to mention seriously hinder your boat's performance. Aluminum is also more subject to flex, which can affect your boat's acceleration.
Stainless steel, on the other hand, is tough, roughly five times stronger than aluminum. Hit something and you likely won't incur much, if any, real damage. This stronger material can also be cast thinner, making the prop more efficient. If you live in an area prone to shallow waters or shifting bottom surface, stainless steel can prevent a lot of headaches. I hit a submerged piece of concrete with an aluminum prop within weeks of taking ownership of the boat, and lost a healthy amount of metal. In the eight years since, my stainless replacement has nary a scratch. And yes, I've hit other stuff along the way.
Obviously durability is a reason to upgrade. Another, however, is to fine-tune your boat's performance to the way you use it.
That average three-blade prop is a compromise; it balances vibration, size convenience, efficiency, and of course, cost. Additional blades, however, can improve acceleration by putting more blade area on the water, as well as further limit vibration. You'll likely lose a little top speed with additional blades, but the tradeoff is often worthwhile. On my own boat, I swapped the stock unit for a Mercury High Five, a five-bladed stainless steel prop. The reason? We do a lot of skiing and wakeboarding, and I was more than happy to give up a little on the top end for the increase in pulling power. You might be in the same figurative boat.
If you're thinking about changing your prop, do your homework. First, check a manufacturer's prop selection guide to see what choices are recommended for your particular boat and engine. As a rule of thumb, a higher pitch will generally provide better acceleration, while a lower pitch will improve acceleration. More blades should improve acceleration, but may come at the expense of top speed.
Then, find your engine's ideal operating range in RPM. An engine's best all-around performance is typically considered to be when wide-open-throttle occurs within 200 rpm of the engine manufacturers designated operating range. Check your owner's manual or the manufacturer's website to find the range for your engine. Then do a test run with a light weight load and compare your RPM to that ideal.
Using this combination of info, select a prop. In general, each 1" change in pitch will alter the engine's RPM by 200. If RPM is too low, decreasing the pitch of the prop will typically raise it up. If RPM is too high, increasing the pitch of the prop will lower it back down.
Find the difference between your engine's actual peak RPM and its manufacturer's suggested peak operating range, then divide by 200. The resulting number will be a starting point to how great a change in pitch is necessary to get the best results from your engine. (Example? Say the peak RPM range is 5200; your boat hits 4800. Divide the difference — 400 - by 200. The next size prop to try would ideally be two inches less in pitch.) When in doubt, consult your dealer or the prop manufacturer. They likely will have suggestions based on your goals, whether they be more speed or more pulling power down low.
Keep in mind a prop that causes your engine to rev above its intended range will likely also cause premature wear and even engine failure. If it prevents the engine from reaching that ideal range, excess strain will be placed on the pistons, bearings, even crankshaft. Stay within that ideal range, and select a pitch that provides the results you want, and you'll be a more satisfied boater.