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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.