Hydrodynamics: Part 2 – The Science and Art of Gearcase Hydrodynamics

In our recent Intro to Hydrodynamics post we discussed the impact of drag on escalating boat speed, and the importance of using the science of hydrodynamics to reduce that drag. The gearcase is the one component of a high-performance sterndrive or outboard that is always in the water, and thus subject to drag. Mercury Marine and Mercury Racing have a long history of developing gearcases for racing and high-performance applications, with the first Quicksilver racing gearcase and shortened midsection introduced in 1950 for the Kiekhaefer Mercury KG-7 Super 10 Hurricane outboard, which promised to increase top speed by 20 to 30 percent on the racing runabouts of the era.

Drive development has a long history at Mercury Racing

Today, Mercury Racing engineers face some of the same challenges that complicated gearcase design 70 years ago. One is the trade-off between robust internal design – to handle ever-increasing power – and the desire to create a slim shape that’s as hydrodynamic as possible. All of the design work starts in the computer.

“We begin with some basic parameters – pinion and driven gear diameters, bearing sizes, and max prop diameter – this sets the propshaft-to-anti-ventilation plate dimension,” explains Jeff Broman, Mercury Racing Director of Engineering. “Then we have some basic configuration decisions around the shifting mechanism, which could be in the gearcase or further up the driveline, whether the exhaust will be through-prop or above-water and water inlets will be in the gearcase or on the transom, and the choice of single- or dual-pinion drive shaft.”

Sport Master Gearcase Cutaway

Once those mechanical parameters have been defined, engineers can start wrapping a hydrodynamic housing around the internal parts. The initial shape starts with established rules-of-thumb based on years of design experience. These relate to making a housing that has low drag, good handling characteristics, and resists cavitation, but also is able to be consistently and precisely manufactured, taking into account casting process capabilities and reasonable machining tolerances.

The anticipated boat speed then sets the shape of the torpedo – higher speeds require more length for a given diameter. Operating speed also defines the shape of the skeg, which is especially critical on surfacing drives. When a surfacing drive is running at speed, only the skeg is in the water, so the design of the skeg is critical to maintaining stability. The shape of the torpedo is also extremely important, particularly during re-entry and when accelerating onto plane. The torpedo also controls how the water flows into the prop. It is important to have a smooth flow of water up and around the gearcase, controlling the amount of lift generated. Outboard gearcases present the additional challenge of designing water pickups, which must be located and shaped based on the operating speed, minimizing sensitivity to trim and debris fouling and always providing enough cooling water to the engine.

Example of M6 / M8 Drive Overlay

Once an initial housing designed, it goes into CFD (computational fluid dynamics) simulation, which is the numerical analysis of fluid flow accomplished in powerful computers. Mercury continues to develop its proprietary methods and capabilities in this field. Current cutting-edge CFD simulations can capture air mixed with water – critical on surfacing drives like the Sport Master, M6, and M8 – and incorporates a moving propeller. The simulation offers guidance in refining the design.

1965 Speedmaster Assembly Drawing

The next step is the creation of a prototype, which may be a modification of a current gearcase or a new shape completely machined from billet. Then it’s time to head for the water. Depending on the application, in-water evaluation may start with a boat in the Mercury fleet. If an appropriate boat is not available, Mercury Racing will partner with a boat builder to gain access to a boat. Testing of a new gearcase housing is guided by Mercury Racing hydrodynamics specialist, Mike Griffiths. Griffiths has years of experience in all types of high-speed boats and has the expertise to safely evaluate any aspect of boat setup. He will put the new gearcase through a series of tests to evaluate how it will work in a real application. The output of these tests is a mix of objective data – top speed, acceleration, steering loads, and water pressure – and his subjective “feel” of the boat. Based on those tests, engineers will continue to fine tune the design until it meets Mercury Racing standards.

“While speed is always a goal, it’s actually more important that the drive behaves in a safe and predictable manner,” said Broman. “Once we meet all of the design objectives, we release the new design for production.”

The result is the kind of Wide Open performance boat owners have come to expect from Mercury Racing – the ability to enjoy the absolute thrill of speed on the water with confidence and reliability.

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One thought on “Hydrodynamics: Part 2 – The Science and Art of Gearcase Hydrodynamics”

  1. Thank you for this information. I have a long interest in propeller design, theory, and racing applications. There is not a lot of information on this subject available to the public.

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