Outboard Horsepower Ratings for Tiller-Steer Boats

Builders and Coast Guard spar over formulas

6th February 2011.
By Charles Plueddeman

When I tested the new Crestliner 1850 Pro Tiller last fall, Steve Magers of Crestliner commented that “this boat sure runs well with a 115-horsepower outboard.” So why were we testing with a 90? Because that’s all the motor the United States Coast Guard permits on the 18’8” boat. Funny thing is, according to Magers the same hull with a remote-steer outboard (that means it has a steering wheel) can take up to 175 hp. And if the Pro Tiller was just 16 inches longer –20 feet – it could have unlimited horsepower, even with a tiller motor. Here’s how the rules work.

90hp is max for this tiller-driven boat.

For any outboard-powered boat over 20 feet in length, the maximum horsepower rating is essentially at the discretion of the boat builder. If the boat is less than 20 feet long, the power rating is determined by a formula.

For remote-steering boats, the formula is: (stern width in decimal feet x length in decimal feet x 2) – 90 = max. horsepower.

For tiller-steer boats with a flat bottom, like a jon boat: (W x L x 0.5) – 15 = max. horsepower

For all other tiller-steer boats: (W x L X 0.8) – 25 = max. horsepower

Now, it seems logical to permit more power when the operator has the mechanical advantage of steering the boat with a wheel, versus a tiller. Or at least it did until the advent of power-assist steering systems for tiller outboards, like the Mercury Pro Tiller system that was on the Crestliner that I tested. If the motor was equipped with steering assist, couldn’t the tiller rating be raised a little bit?

“We have had many discussions with the Coast Guard on that subject,” says Chad Keskitalo, compliance specialist for Lund and Crestliner boats, “and the Coast Guard won’t budge.”

Po Chang, a civilian boating safety engineer for the Coast Guard, explains that the agency has three concerns when considering max power for tiller- versus remote-steered boats. The first is operator placement.

“With the operator sitting further aft at the tiller, the boat is not in ideal trim,” says Chang. “He also can’t see forward as well, and the boat may be more prone to porpoise.”

The second concern, says Chang, is operator security.

“A steering wheel provides a brace for both hands, and the seat offers additional stability, that the operator of the tiller boat does not have,” explains Chang. “The tiller operator is just hanging on the gunwale with his right hand.”

Finally, there’s the difference in reaction to helm input between the two systems.

“With a steering wheel, it takes a pretty deliberate motion to change course,” says Chang. “The tiller is connected directly to the motor, and it’s much easier to make an inadvertent course change.”

Adding power assist to the tiller does not really address any of these issues.

Chang also explained the history of the outboard power rating formulas, which date back to the early 1970s.

“The goal was to come up with a simple formula that would apply to a variety of boats, and would be easy for the boat builders to use,” says Chang. “It was an entirely subjective process. Engineers ran about 150 different boats with increasing amounts of power, executing a quick, 180-degree turn and a course-avoidance maneuver. Basically, when they no longer felt comfortable doing that, they’d reached the power limit for that boat. With that experience in hand, they devised a formula that would work with a typical boat.”

So why does the power-rating mandate end at 20 feet in length?

“The 20-foot limit is based on accident statistics,” says Chang. “Once a boat is 20 feet long, accident incidents go way down. There are fewer boats of that size on the water, and they are less likely to swamp or to be tossed out of control by a large wake, and people don’t fall overboard as easily.”

Makes sense to me. But I’d still like to run that Crestliner with a 115.

plueddeman-head-shotCharles Plueddeman is Boats.com’s outboard, trailer, and PWC expert. He is a former editor at Boating Magazine and contributor to many national publications since 1986.


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Charles Plueddeman is Boats.com's outboard, trailer, and PWC expert. He is a former editor at Boating Magazine and contributor to many national publications since 1986.

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