Perhaps you're intrigued by the notion of zooming across the water a couple of knots faster without spending tens of thousands of dollars on a high-performance gas engine. If so, there's a common sense solution you really ought to know about: Improve the efficiency of the inboard or stern drive already mounted on the stringers. To do so is relatively inexpensive and the projects we're about to tell you about can be accomplished without special tools by anyone with average skills.
Begin at the top of the heap. Scrap the stock flame arrestor. While the factory original does an adequate job flowing clean air and preventing backfire from igniting latent fuel vapors, it's not all it could be. Frankly, it restricts airflow by just enough to leave room for improvement. The aftermarket, USCG approved K & N air filter, does its good work by straightening and smoothing airflow into the combustion chamber. A greater volume of air equals higher horsepower. In this case the bump is worth a couple of percentage points. Priced from $99, models are available to fit small and large block, carbureted and EFI, Mercury Marine inboards and stern drives. Installation takes minutes.
Next, peruse the owner's manual, paying particular attention to the octane requirement. When running low octane (87) in a gas engine rated for 89 octane or better, the black box retards ignition timing to prevent cracked pistons. If the good news is modern engines head off self-destructive behavior, the bad news is failsafe systems dumb down horsepower. So when the correct octane is unavailable dockside, dope the cheap fuel with an octane booster. Read the label. Wear gloves. Follow the directions. The pay-off is like a tune-up in a can.
Synthetic motor oil is as slippery as glare ice. So it comes as no surprise to learn a crankcase full of the stuff significantly reduces friction generated by the reciprocating mass. With pistons, rods and crankshaft journals lubricated with synthetics, revs climb higher and acceleration comes on stronger. Even if just a little. Similar to burning the correct octane, swapping synthetic oil for conventional doesn't actually boost horsepower, but instead allows an engine to generate its full potential.
A propeller can make or break a boat's performance. Hands down the best way to go faster is to certify that the propeller package is optimal for the kind of running intended. That said, the perfect propeller for one man's boat, would be a dreadful mistake on an identical boat whose owner has different expectations. Propeller pitch, like gearing in a tow vehicle, determines acceleration, fuel economy and top speed.
To determine how well your engine and propeller are matched, make a test run at Wide-Open Throttle (WOT) and note rpm. Then refer back to the owner's manual for engine operating range. When top speed is most important, and with an operating range from 4500 to 5500 rpm, the engine should rev right up to the ragged edge of the envelope. Say 5300 or 5400 rpm. Conversely, if you're more interested in seat of the pants acceleration WOT ought to register in the middle or lower end of the operating range.
Also vitally important is the alloy. Aluminum wheels are less efficient than stainless steel for the simple reason more intricate and precise shapes can be cast into stainless. Stainless blades are also thinner and stronger. Naturally, in order to eke out the maximum efficiency a propeller ought not be dinged, and its blades cannot be bent out of whack. One final tip. In order to go fast, travel light with a minimum load of water and fuel.