How big of a difference in speed does 50 horsepower make?

Power, power, power.

A glance at the different marine engine manufacturers’ website shows a constant focus on horsepower. However, how do you separate the marketing spin from reality?

What difference does horsepower make?

Shouldn’t torque be more of a concern?

Yes it should. As should an examination of both the engine’s power curve and torque curve. What matters is how much torque, its’ distribution along the operating rpm range of the engine, and how much of that torque is transfered to the shaft.

Let’s take a look at an example. We will show an 11m boat (RHIB), equipped with twin VGT 450 and twin VGT 500, showing you the difference 50 horsepower makes in vessel performance.

Vessel LOA: 11m

Beam: 3m

Displacement (full): 6.5t

2 engines

surface drives

Equipped with 2 X VGT 450, top speed 50 knots.

Equipped with 2 X VGT 500, top speed 52 knots.

This is a differential of only 2 knots in speed. Actually, since there are two engines, the differential on the vessel is 100 hp total.

Is the engine price differential worth the extra increase in speed? Only the customer can answer that question. Sometimes yes, and sometimes no. It really depends on the vessel’s mission and how it will be used.

Now, compare the same vessel, equipped with a Cummins QSB 6.7 engine, rated at 450 hp (The one we replaced). That vessel achieved a top speed of only 49 knots. Why the difference?

Easy.

The torque curves are different. Though the Cummins engine produces a slightly higher maximum torque than the VGT, that extra torque is concentrated in the middle of the rpm range. So, as the vessel continues to move faster after planning, the acceleration is lower, since the torque produced rapidly drops. This is fine when equipping a truck. However, boats encounter far more resistance than trucks, even after planning.

The VGT 450, on the other hand, produces a flat torque curve, much more evenly distributed along the operating rpm range of the engine. This allows a greater increase in speed that other engines.

VGT450 curves 100714

So, when powering a vessel, it is not simply a matter of horsepower. Torque and its’ distribution is far more important.