My 2018 Ford F-150 has a 5.0-liter V-8 rated at 395 hp and 400 foot-pounds of torque. The differential — the big round thing in the middle of the rear axle — is pretty robust at 9.75 inches in diameter.
Compare that to the gearcase on the lower unit of an outboard or sterndrive. Mercury Racing’s 450R outboard makes more horsepower than my truck’s engine, and its gearcase is 4.55 inches in diameter. The Duoprop gearcase on the Seven Marine 527 outboard has a diameter of 5 inches. The truck has a 10-speed automatic transmission, and an outboard or sterndrive has a single-speed transmission and one or two propellers. And, as it’s often said, a boat engine is always running uphill.
“I call it the Rodney Dangerfield of outboard parts,” David Meeler, Yamaha’s new-product introduction manager, says of the lower unit. “It takes all that power and makes it turn 90 degrees so it’s parallel to the water. It gets hammered on constantly, but quietly does its job.”
A lot of consideration goes into lower-unit design, particularly the gears. Undersized gears sacrifice durability and reliability. As Meeler points out, engineers want “gears the size of cantaloupes.” However, an oversized housing for large gears can result in a torpedo that’s too big, which can cause unexpected lift.
Arriving at a workable compromise is the biggest challenge. Computer-assisted design and computational fluid dynamics are the methods of choice for designing lower units, as well as for positioning water intakes, anti- cavitation plates and other accessories. Even with all that technology, Mercury Racing, Yamaha, Suzuki and Volvo Penta/Seven Marine each had unique challenges to overcome with the lower units on their newest outboards.
Mercury Racing offers two lower units for its 450R. The Sport Master lower unit is intended for lighter hulls that run faster than 80 mph, such as high-performance vee-bottoms and catamarans. It has a longer skeg because half the torpedo surfaces when underway, and the additional skeg length helps to maintain handling characteristics and stability at high speeds. Because the gearcase is partially above the water surface, the Sport Master is most effective when running as parallel to the water as possible.
“They’re designed to allow the boat to take advantage of the speeds they’re capable of,” says Steve Miller, director of marketing, sales and service at Mercury Racing. “It’s designed for boats that generate their own lift naturally.”
Mercury Racing’s 5.44 HD lower unit is for heavier boats that run slower than 80 mph; it typically does its work deeper in the water. It’s designed to be trimmed more and used as a fulcrum to generate lift. Both lower units run a 1.6-to-1 gear ratio, and the exhaust exits just below the cavitation plate.
When Yamaha introduced the XTO Offshore in May 2018, there was much talk about the size of the 425-hp beast. The outboard tips the scales at around 1,000 pounds, but it’s designed to push big, offshore boats. That means it needs a lower unit that can take some serious stress.
Meeler says Yamaha set a durability standard for the XTO Offshore twice as stringent as that for its other engines, focusing on the gearcase’s hydrodynamic efficiency and the strength of the gears. “If you take every consideration about size, weight and drag, this one by far has the least amount of [hydrodynamic] resistance,” Meeler says. “It’s the most efficient lower unit of any outboard we make.”
Inside the housing are gears made from a proprietary alloy. The prop shaft is the largest Yamaha has ever manufactured. “The key was durability,” Meeler says. “We had to have that. When you’re talking today’s horsepower — 300, 350, 425 — that’s a lot of power to be asking a gearset to transfer. It takes a specially designed gear and materials to do that correctly.”
Yamaha first introduced dual water intakes with its F350 in 2007. They work independently, so if one set clogs, the other still provides cooling water. This system is used on the XTO’s lower unit. Exhaust exits through the prop hub.
Suzuki designed the DF350A and DF325A outboards with a two-stage gear ratio. The setup includes an offset gear at the top of the vertical driveshaft gear, plus a gear in the gearcase. The greater gear ratio this produces translates to more torque, which prompted company engineers to consider twin contra-rotating propellers.
Gus Blakely, vice president of marine sales for Suzuki Motor of America, recalls a trip to Japan to see early versions of the lower unit for the DF350A. He estimates it was 30 percent larger than what’s now on the production model. Engineers reduced its size by moving the shifting point out of the lower unit and up toward the water pump, a change that allowed for a smaller, more streamlined housing for the gears.
To spread the torque loads, Suzuki used two prop shafts, with one spinning inside the other. The shaft for the rear propeller spins inside the forward shaft. “The rear shaft is similar in size to traditional shafts,” Blakely says. “The front shaft is hollow and has the rear one running through it.” Exhaust exits through the prop hubs.
When Volvo Penta put its diesel Duoprop lower unit on a Seven Marine 527-hp outboard, the goal was to give boaters who were used to the torque of an inboard the same feeling with an outboard. William Gremminger, manager of transmission engineering at Volvo Penta’s U.S. headquarters in Chesapeake, Va., says the change required minimal modifications beyond adapting the water pickups because outboards ride higher on the boat than sterndrives. “The biggest difference is that the trim angles are a lot higher on an outboard,” he says.
To manage the torque of the second propeller, the gears have a spiral bevel design and are made from case-hardened proprietary steel. Also, there must be enough water to feed two props without aeration, which also effects gearcase design.
Gremminger adds that one of the biggest considerations is noise. “It’s the biggest challenge to getting a gearcase right,” he says.
Volvo Penta turned everything around, so to speak, with the Forward Drive, which is a lower unit with forward-facing props. It’s designed to produce the wakes that surfers want at slow speeds, but also to perform like a conventional sterndrive because it’s trimmable.
Regardless of the approach to gearcase design, Gremminger says, there’s only one route to the desired result. “You build one and test with it until you get it right,” he says. “When you think about the amount of horsepower going through that gearset in a lower housing, it’s pretty impressive.”
This article originally appeared in the December 2019 issue.