Be careful of boats' claimed top speeds

A while ago, I saw an ad for a small sport boat with the tagline: "The speed, up to 60 mph*, is simply incredible!"

Then, later, in the fine print: "*Achieved with a professional driver on a closed course using a specially tuned engine and custom propeller. Do not attempt."

Advertised speed capabilities are useful in understanding how a boat might behave, but they don't mean what you probably think they mean- even if the vendor isn't fudging them.

Very few powerboats are actually run at top speed. It's noisy, the ride can get rough, and it's hard on the engines. Typical well-designed planing hulls have a nice sweet cruise somewhere between one-half and three-quarters throttle, at which they'll run efficiently and comfortably. Displacement hulls hit a wall around the so-called "hull speed", beyond which adding more power simply makes the wake bigger and the fuel bill larger.

And yet speed sells. Being able to brag that your boat "does sixty" is a status symbol, particularly when comparing to a buddy whose boat tops out at fifty-five.

There are a few good reasons why, if you're in the market for a planing hull powerboat and don't care about the status symbol aspect of speed, you might look for something "slower".

To achieve high top speeds, a powerboat should have certain characteristics, including:

  • Centre of gravity is relatively far aft
  • Running surface angle of attack is relatively flat
  • Propeller is optimized for high speed (tends to result in smaller diameter, higher pitch)

If you spend most of your time at lower cruising speeds, though, the opposite characteristics are preferable:

  • Moving the centre of gravity forward will drop the bow, make it easier to get on plane, and make the ride more comfortable in choppy conditions.
  • A running surface shaped to provide more low-speed lift in the stern (at the expense of more high-speed drag) will keep the boat closer to level through the transition to planing speeds, and prevents the bow from pointing skyward at low planing speeds.
  • A larger diameter, lower pitch propeller provides more low- to mid-speed thrust (at the expense of a reduced top speed).

I frequently see small runabouts and bowriders that are advertised as being able to do 50+ mph (the small sportboat industry is inexplicably stuck on American statute miles). To achieve this speed, they place their weight so far aft and use such steep, high-RPM props that they are almost undriveable through the transition to planing speeds at 10 to 20 mph. Steeper reduction gearing, larger and shallower props, and moving the centre of gravity forward would cut the top speed of such a boat to perhaps the low 40s, but would greatly improve its time-to-plane and its handling characteristics at low planing speeds.

As another, more extreme example, a fleet of poker run boats crash Kingston's downtown once per summer. Although many of these boats can cruise at 80 to 100 mph, they are all but incapable of running below 30. With their long bows trimmed up by five to eight degrees at barely-on-plane speeds, the driver's visibility is badly obstructed and the wake they create is simply enormous. The result is that, when entering a busy harbour, these boats must either run so fast as to be dangerous, or run with the bow high and the wake huge- also not a good thing for smaller vessels that might be nearby. That's the price you pay for speed: a boat that, while simply amazing at full throttle, is a rather touchy and menacing beast in tight quarters.

The moral of the story? Yes, you should look at claimed top speed figures, but not because "higher is better".

An unusually high top speed tells you that the boat is tuned for running fast, often at the cost of relatively poor running characteristics at the lower planing speeds that many boaters prefer to cruise at. It may have a hard time slowing down to more comfortable speeds when the chop picks up and your passengers start complaining.

And if you have a fast boat that performs like a lethargic tortoise when climing to plane, consider sacrificing some top speed (by fitting a larger, shallower prop and moving weight forward) to boost its performance in the speed ranges where you'll really feel the difference.



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