An Intro to Hydrodynamics

There are two basic ways to increase the top speed of a powerboat: add power or reduce drag. Adding power is the obvious path and the easiest to understand, and nobody understands power like Mercury Racing, home of the Mercury Racing 450R outboard and the Mercury Racing Dual Cal 1550/1350 sterndrive engine. Finding a reduction in drag can have a surprisingly significant impact on speed, and basic physics tells us why.   

Drag is proportional to the square of speed – so going twice as fast requires that we overcome four times the drag. To make a simple math example, to increase boat speed by 10 percent, say from 100 mph to 110 mph, will require overcoming about 21 percent more drag (1.1 x 1.1 = 1.21). However, the power required to overcome that drag is proportional to the cube of speed. So to use the same example, increasing boat speed by 10 percent will require approximately 33 percent more power (1.1 x 1.1 x 1.1 = 1.331). To create a real-world example, if a boat powered by a pair of Mercury Racing 1100 QC4 engines (2,200 total horsepower) can reach a top speed of 125 mph, pushing that speed to 135 mph (an increase of 8 percent) will require approximately 26 percent more power, or 2,772 horsepower – a pair of Mercury Racing 1350 QC4 engines would do the trick. This assumes no increase in prop slip, losses to drivetrain friction, or significant change in air density, among other factors. 

This formula works backwards as well, so any reduction in drag translates into more speed if the available power remains static. A fast boat experiences both aerodynamic and hydrodynamic drag. At high speed, most performance boats have very little hull in the water, so much of the drag to be overcome is aerodynamic. The two key points of hydrodynamic drag that concern Mercury Racing are at the gearcase and the propeller. The engineering challenge lies in finding ways to reduce drag while retaining real-world practicality. For example a very slim gearcase might be hydrodynamically efficient but unable to contain gears with a practical ratio, or gears large and strong enough to offer acceptable durability. Improving gearcase hydrodynamics may have a negative impact on water flow to the propeller, decreasing prop efficiency. Chasing hydrodynamic improvement can be a complicated game of compromise, but is always a worthwhile effort because any reduction in drag is like gaining horsepower. And horsepower is expensive. 

Modern engineering tools like computational fluid dynamics (CFD) allow engineers to study in detail the flow of air, either through and engine intake or around the hull and deck of a speeding boat, and to identify areas of drag. But those tools are much less reliable in the study of hydrodynamics, making years of experience and on-the-water testing vitally important. In future columns we’ll look at ways hydrodynamics influences the design of Mercury Racing propellers and gearcases as we support your quest for a Wide Open life at speed on the water.  

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