Searching for the Sweet Spot

Searching for the Sweet Spot

Do you know the best cruising speed for your boat?
If not, here’s how to find it.

Boat throttle

They are questions all of us have probably asked ourselves at one time or another: What’s the best cruising speed for my boat? Am I running my motors too hard, to the point that something is going to break or they’ll wear out prematurely? Could I be going faster without incurring these negative effects? Should I just run my boat at the speed that “feels right”?

Assuming you do not subscribe to the “feels right” school of boat operation, the first step in finding your best cruising speed is to define what “best” means to you. Are you looking for maximum cruising speed? Maximum cruising range? Maximum engine longevity? Your priority in this regard will go a long way to establishing an answer.

Let’s assume you’re after the most speed you can get without damaging your engines. Sorry to obfuscate again, but before proceeding, you need to understand that “damage” as being distinct from “wear.” Damage means something breaks, and fortunately for all of us, most modern marine engines are able to run at or near full throttle without, to use the racer’s term, “hand-grenading” or otherwise incurring a mechanical catastrophe. But you will pay a price for doing so: The faster an engine turns, the faster it wears. This is because the major causative factor in engine wear is the amount of fuel the engine has burned, and I’m sure it will come as no surprise to you to learn that the harder you push an engine, the more fuel it burns per unit of distance traveled. 

The logical corollary of this is that if you want to get maximum life out of your engines, you should run them at the point of maximum efficiency. While intuitively true, such a conclusion is neither practical nor palatable for most boaters, since the point of maximum fuel efficiency is typically at or close to idle speed. Despite the greenest of intentions, not many of us could stand to creep along like that for very long—except perhaps for ex-sailors.

This explains why for most boaters the best cruising speed is the one that best balances three factors: velocity, decent fuel efficiency, and a relatively low wear rate. But again, a little hedging is required. Just how much more money you are willing to pay to get to where you’re going a little sooner is a personal decision that’s beyond the purview of this column. But there are some basic considerations.

Perhaps most elemental is the one that’s borne out of decades of testing here at Power & Motoryacht. With a few exceptions, planing boats do not have a magical threshold above displacement speed at which optimum running angle occurs, the hull uncovers, and fuel efficiency peaks. In 90 percent of the boats I’ve tested, the faster the boat went the more fuel it burned per unit of distance—end of story. Thanks to the wonders of electronic engines and the plethora of data they put at your fingertips, most of you can verify this yourself on your own boat. If you do, and discover that you are one of the lucky ones who owns a boat with that elusive sweet spot, your search for the best cruising speed is over and you can stop reading here. If not, read on.

At this point you could just pick your poison—choose a speed-fuel consumption figure based on your personal preference (i.e. what feels right) and be done. But before you do, consider the following: For reasons too complex to deal with here, diesels can run at higher rpm for an extended time without incurring excessive wear compared to gasoline engines. It used to be an accepted rule of thumb that a diesel could safely tolerate a cruising speed of about 200 rpm less than full throttle, but that was before the advent of new high-speed diesels that may max out as high as 3600 rpm. Today the sustained cruising-speed guideline I hear quoted most frequently is 10 percent off of full throttle.

Gasoline engines are another kettle of fish. If you run them at full throttle, the increase in wear and the fall-off in fuel efficiency are both much greater than they are for diesels. If wear and efficiency are important to you, consider the speed at which you cruise. The full-throttle limit for modern marine gasoline motors ranges from 4800 to 6000 rpm. (By comparison, your car’s engine probably turns less than 2500 rpm at freeway speeds.) Most mechanics and engineers suggest the best cruising speeds are achieved at 3400 and 3800 rpm respectively. 

This article originally appeared in the July 2014 issue of Power & Motoryacht magazine.