What do all high-performance powerboat owners have in common? They’re always looking for more. More top speed. Better mid-range acceleration. Faster time to plane, also called hole shot. More.
There are all kinds of ways to get “more” in the go-fast boat world—from engine replacement and upgrading to entire hull and outdrive blueprinting—and they, too, have one thing in common. They’re expensive, not as expensive as buying a new boat, but expensive nonetheless.
As it happens, the most effective way to immediately up the performance of your go-fast V-bottom or catamaran is also the most cost-effective way, and that means replacing your propeller, or propellers in twin-engine applications. But there’s a catch: There is no single propeller choice that is “the best” for top speed, midrange acceleration, and hole shot.
“I get asked for that kind of propeller three or four times a day,” said Jeff Johnston of Hering Propellers, one of the top names in the go-fast boat propeller manufacturing game, based in Marysville, Wash. “If you’re looking to wring out that last tick on the speedometer, and that’s what’s important to you, we can do that. But it won’t be best prop for hole shots or midrange acceleration. If you don’t care about the last tick or two on the speedo and you boat with a full load of passengers and fuel, then getting on plane easily might be more important to you. We can choose a prop spec for that, too.
“That’s usually my first question to people when they call me: ‘What kind of results or improvement in performance are you looking to achieve?’” Johnston continued. “Although everyone wants to have it all, when I talk to clients on the phone we usually get down to what their true priorities are. That way, I can make a recommendation that produces the specific results they want.”
Propping for Speed
Volumes have been written on propeller technology, but there are three important definitions to understand before getting into goal-specific propeller applications, such as topping for speed. The first is pitch, the theoretical number of inches a propeller moves a boat forward with each complete rotation. (Gear ratios are typically used to control pitch and manage power, but that’s another article.) The second is diameter, the distance in inches from the tip of one propeller blade on the prop’s hub to the tip of the blade directly opposite. Last, there is rake, the angle at which the propeller blade differs from perpendicular (zero degrees or flat).
Generally speaking, when you have big power and you want your boat to run as fast it can, you choose the biggest-pitch prop the engine can turn without compromising its upper end operating range (maximum rpm). If the prop pitch is too large, the engine could end up 500 rpm or more lower than its capability at wide-open throttle, in which case the boat would be deemed “over-propped.” Conversely, if the propeller pitch is too small to handle the engine’s power, the engine will up hitting its rev-limiter to keep from over-spinning and damaging itself. In the case, the boat would be called “under-propped.”
Very generally speaking—because again gear ratios play a big role—the bigger pitch propeller your engine can spin without over- or under-revving, the faster your boat will go.
“The higher pitch propeller your boat can pull, the faster your boat is going to go,” Johnston explained. “In pure speed applications, we generally spec bigger-pitch props with smaller diameters, because that means there’s less propeller surface to drag through the water.”
Propping for Hole Shot
The problem with small-diameter “speed” propellers when it comes to maximizing hole shot, however, is the same thing that makes them so great for top speed—less overall blade area in the water. Regardless of horsepower or gear ratio, a twin-engine 40-foot catamaran with 40-inch-pitch speed props typically struggles to get on plane and requires more than a little driver experience and expertise to make it happen.
“There’s nothing more embarrassing than having a full load of friends on board your boat at a poker run and not being able to get on plane,” said Johnston. “For most people, though not everyone, that’s a lot more important than the last few ticks on the speedo.”
If hole shot is a top priority for a client, Johnston generally recommends a propeller that could be one to three inches smaller in pitch than what he would suggest as a speed prop, and one with a larger diameter than that of the faster wheel. By its own design, that propeller will turn slower and work better with an engine’s low- to mid-speed operating range. So in addition to getting the boat on plane in less time, the lower-pitch, larger-diameter prop will deliver better acceleration—thanks to better bite in the water.
Rake comes into play as Johnson helps his clients fine-tune their propeller choices to their specific boats. All hulls have different running characteristics. Generally speaking, higher rake angle translates to greater bow lift, which can be advantageous for V-bottoms and catamarans that need it. But in high-speed applications such as cats pushing 180 to 190 mph, too much bow lift can create problems. So in that case, Johnston would recommend a propeller with a lower rake angle, which would produce less bow lift than a prop with a higher rake angle.
The Strength Game
Go-fast powerboat propellers are either cast—created as single piece in a casting or mold—or forged—created from several hand-built pieces (blades and hub) that are welded together. Of the two construction processes, forging generally produces the strongest and most expensive results -- as much as $25,000 for a set of forged Hering propellers.
While a like-sized set of cast propellers would cost half the money, said Johnston, they’re also approximately half as strong.
Hering offers both cast and forged wheels. “We generally recommend our forged propellers in boats with engines of 1,400 hp or more,” said Johnston. “Our cast propellers work well in applications of less than 1,400 hp.”
Worth noting is that Mercury Racing offers only cast propellers for its 1,100- and 1,350-hp quad overhead-cam engine, which has become the power of choice in upper-end high-performance powerboats. To handle the incredible loads and stresses produced by the engine, Mercury Racing’s propellers are constructed conservatively, meaning with greater blade thicknesses. That probably costs 1100 and 1350 engine owners a mile an hour or two off the top end, but failures of those propellers have been close to nonexistent to date.
That said, thinner is faster, which is why a few owners have reportedly used 1100-engine-rated props in 1350 engine applications, which of course is against the manufacturer’s recommendations. But thinner definitely comes with one big drawback: reduced strength.
“Thinner is definitely faster, simply because you’re pushing less mass through the water,” said Johnston. “But you’re also reducing structural integrity and propeller durability when you go with thinner propeller blades.”