There is no “perfect” propeller. There are only good choices—and inherent compromises.

By Matt Trulio

Looking to improve your go-fast boat’s performance but don’t have the bank for something such as a supercharger kit, much less a new engine? The simplest and most cost-effective move you can make is buying a new set of propellers. And yet choosing props is anything but simple.

“We think we know a lot about props, but I don’t think we do,” says John Tomlinson of TNT Custom Marine in North Miami. “There’s still so much to learn, but there are only so many hours in a day and so much money. If we had something like a Formula One program with millions to spend, well, I think we’d know a lot more.”

And this is coming from an offshore racing world champion and renowned pleasure and race boat setup man—setup being the science/art of choosing the best propellers, drive gear ratios and drive heights. Tomlinson has set up more than 2,000 V-bottoms and catamarans in the more than 25 years he’s been in the high-performance marine world. He is, quite simply, the best in the business.

So where does he start when it comes to picking props? With the peak operating range of the engine(s) in the boat in question.

“We want to be able to achieve the proper full-throttle operating rpm that the engine manufacturer recommends,” Tomlinson says.

If a propeller is too small—meaning its pitch is too low for the power—the engines will run on their rev-limiters, which keep them from over-spinning and damaging themselves. If a propeller is too big—meaning the pitch is too high for the power—the engines can be 200 to 300 rpm (or more) short of their peak operating range.

“If you over-prop, you end up with a boat that is real doggy everywhere but the top-end, and even then it takes you a long time to get there and you’re leaving engine rpm on the table,” Tomlinson says. “If you’re under-propped, you might have good acceleration and good rough-water handling, but in flat water you might be losing 10 mph on the top-end.”

Like other setup experts including Randy Scism of MTI—Scism managed the UIM Class 1 Dubai race program for several years before founding MTI—and Bob Teague of Teague Custom Marine, Tomlinson says there is no “perfect” propeller set for any given boat. Propping with top-end as your main priority likely means choosing the biggest propeller your engines and drives can spin, and that also likely means standing-start and midrange acceleration will suffer. Propping for good all-around performance could mean making a lower-pitch, conservative prop choice that will cost you up top.

To help mitigate these compromises, Tomlinson’s setup strategy is to maximize torque at the propeller shaft by using lower gear ratios (remember that a 1.35:1 ratio is lower than a 1.29:1 gear ratio) and higher-pitch propellers.

“The boat will accelerate better, but will still be able to get to the top-end we expect it to,” he says. “But I’ll adjust that, too. Let’s say I have a cat that I know will run 150 mph. I’m going to start with 1.35:1 gears in the drives and 40"- to 41"-pitch propellers. And let’s say the engines are pulling those props pretty easily at that speed. Well, I know I can’t get bigger props, so I’ll go with a higher gear, say a 1.29:1. As long as I can still pull that pitch, I’ll go faster.”

Balancing the equation—or at least a balancing point within it—is drive height. In pleasure-boat applications, even with surfacing drives such as the dry-sump No. 6 from Mercury Racing, Tomlinson leans toward the conservative side. He knows that the 1 or 2 mph he might gain in raising the drives might not be worth the compromises in handling in stability. In setups for offshore racing—as his multiple world titles would indicate—he’s far less conservative. But the vast majority of boats TNT, Teague Custom Marine and other shops set up are pleasure boats.

“Skater, for example, will send out boats with their drives one-half-inch deeper than I would run them because overall handling is better,” Tomlinson says. “There’s always a little room to tweak, but sometimes to go a little faster you lose drivability, and it’s just not worth it.”

Next to pitch, diameter is the second-most critical element in Tomlinson’s propeller selection decision-making process. It is particularly crucial in catamarans, he says, where in all but the largest cat he opts for smaller-diameter propellers.

“In a 36 Skater or a 42 MTI at 160 mph, big-diameter propellers will lift the boat out of the water too much and it won’t handle as well,” Tomlinson says. “Smaller-diameter propellers usually make cats feel more settled down. But on a heavy V-bottom, I go for the biggest-diameter props I can find.”

Likewise, five- and six-blade propellers, which are becoming increasingly popular thanks to their remarkable “hook-up” efficiency and lift (and let’s not forget taller drive heights), are not always the best choice in high-speed catamarans.

“In our kilo boat, when we started going 160, 170 mph, we found the six-blade props gave us too much lift so we went back down to five-bladers,” Tomlinson says. “Running with Dave Scott in his 36-foot Skater, we went from five-bladers to four-bladers for the same. When John Cosker throttled Dave Callan’s cat to 209 mph, he was using four-bladers.”

A hot topic these days, propeller rake angle—the angle of the blade as it “leans back”—also plays a role. Lower rake-angle propellers tend to provide less bow lift, which translates to higher stern lift. Conversely, higher rake-angle props tend to produce less stern lift and greater bow lift.

“That’s the general rule,” Tomlinson says. “Another general rule is that higher rake-angle props tend to work better in cats, especially when it comes to acceleration. That’s why all of Randy Scism’s MTI pleasure boats come out with higher rake-angle propellers. On the other hand, V-bottoms tend to work better with lower rake-angle props, but then you’re really splitting hairs. It won’t make enough difference for the average guy to know, unless he spends a lot of time in his boat.”

Last, but far from least, there is propeller rotation. In outboard rotation in a twin-engine setup, the starboard propeller turns to starboard and the port propeller turns to port. Switch the direction of rotation for each propeller and you have inboard rotation. Outboard rotation tends to create bow lift, whereas inboard rotation tends to create stern lift.

“When we run propellers inward on a cat, we usually get better handling,” Tomlinson says. “But in dead-flat water, outboard rotation is usually faster. With V-bottoms, they generally run better with the props turning outboard if the engines are installed side by side, but better with the props turning inboard if the engines are staggered.

“One thing I do like about outboard rotation on a V-bottom pleasure boat is that it makes it easier to pivot around the docks with one engine in forward and the other in reverse because you have more leverage through the props,” he continues. “On a big cat, where the drives are so far apart anyway, that doesn’t matter so much.”

Setup experience definitely helps, especially when it comes to seeing the larger picture. When Tomlinson looks at a Cigarette 38 Top Gun with side-by-side Mercury Racing HP525EFI engines, he knows that the boat should run between 81 and 85 mph with four-blade propellers from 28" to 32" pitch. He knows that the stepped 38-footer will accelerate strongly with 28"-pitch props, but likely will top out at about 81 mph. He knows the boat will run 85 mph with lab-finished 32"-pitch four-blades, but will be sluggish out of the hole.

“Look at a 42 MTI with Mercury Racing 1075s,” Tomlinson adds. “It comes from the factory with a 1.29:1 gear ratio and 38"- or 39"-pitch props. With inboard rotation it will run 153 mph and handle great in the chop. I can probably use a 1.35:1 ratio and 40"-pitch props turning outboard and the boat will run 160 mph, but it will be all over the place. I could also run a more aggressive 1.24:1 ratio, but then I’m going to run 36"- or 37"-pitch props to keep it from being so doggy coming on plane. But then I’m going to lose a lot on top because I’ll be on the rev-limiters.

“If I own the boat?” he continues. “I’m going with 1.35:1 gears and 38"- or 39"-pitch props. I know it will be 5 mph slower on the top-end, but it will feel a little more sporty when I hit the throttles.”

So are there perfect propellers for your boat?

“Only on a race boat, because we pick the props for those conditions on that day, and if the conditions change during the race, the perfect props might not be so perfect,” Tomlinson says. “But in a pleasure boat, there is always compromise. You pick props for overall performance, but there’s always a compromise.”

—Matt Trulio is a freelance writer for Powerboat.