The Price of Fun

World events over the last two years have created a record high interest in recreational boating, but unfortunately they also generated record high fuel prices. To help you understand exactly how various boats burn fuel differently and how to run your boat at its most efficient, we’re turning to Steve Zimmerman, founder of Zimmerman Marine, a highly respected boat yard and boat builder with six locations in the Mid-Atlantic and Southeast.

Steve is knowledgeable in all aspects of boat maintenance and design that effect fuel consumption. I had the opportunity to ask Steve recently to dispel some commonly held misconceptions about fuel use in popular styles of recreational boats.

BOB:

When boaters talk about fuel consumption, they mostly speak in terms of gallons per hour, not miles per gallon. What’s the difference?

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STEVE:

Many boaters focus on gallons per hour (GPH); however, in determining how much fuel you use to cover a distance on your boat, we have to bring speed into the equation. For example, if someone asked which is more efficient, a boat burning 11 GPH or a boat burning 22 GPH? The answer is it’s impossible to say without calculating miles per gallon (MPG).

If the boat burning 11 GPH is traveling at 10 knots (nautical miles per hour), we divide 11 GPH by 10 knots to see it is getting 0.9 nautical miles per gallon (nMPG). If the boat burning 22 GPH is traveling at 22 knots, 22 divided by 22 equals 1.0 nMPG. So, in this example, we see that although the difference is minor, the boat burning double the gallons per hour achieves better mileage.

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BOB:

If people are considering a new boat, are some designs more fuel efficient than others?

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STEVE:

All boat hulls require a certain amount of energy to move through water. The more easily they move through the water, the less energy is required. The primary factors that influence how easily the hull can be moved include hull shape, length, total weight and drag. Hull shapes are sorted into three basic categories: full displacement, semi-displacement and planing. To determine which offers the best fuel economy, we introduce the most important variable of all: speed.

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BOB:

So, the faster a boat goes, the more fuel it burns?

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STEVE:

Usually that’s true, but not always. Different hull forms respond differently to the demand for speed.

As speed increases, boats move through the water in three basic ways. At slow speeds the boat sits fully in the water, riding between a wave at the bow and a wave at the stern. Full displacement boats live in this zone. As soon as speed increases, fuel burn rises sharply. Semi-displacement and planing hulls can apply more horsepower and begin to climb up onto the bow wave. In this phase the bow rides awkwardly high, and fuel economy plummets. By applying even more power, these hulls ride more on top of the water. The bow comes down, speed increases, and fuel burn levels off. All get better fuel economy at the slower speeds, but the penalty for higher speeds varies substantially between hull types.

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STEVE:

For all cruising powerboats, when it comes to fuel economy, speed trumps all other factors—but only at slow speeds. At full-displacement speeds going a knot or two slower can double or triple your fuel economy. Among the things that influence fuel economy on planing hulls are the condition and cleanliness of the props and rudders, alignment of shafts, health of bearings and a fouled bottom. Once you are on plane, increases in speed matter far less, but the importance of a clean underbody and running gear matters far more. Don’t be misled by GPH, taking the extra step to calculate MPG, which ultimately determines overall fuel use.

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HOW DO DIFFERENT HULL TYPES VARY IN FUEL USE?

The most common hull forms used in recreational boats are:

• FULL DISPLACEMENT
Let’s look at some actual numbers from a full-displacement trawler in the 40- to 50-foot range. At a speed of 7.5 knots, if it’s using 3 GPH, that equals 2.5 nMPG? If we push for a little more speed, the fuel burn changes, at 9 knots, burning 11 GPH, it’s down to 0.8 nMPG. Notice that by going just 1.5 knots slower, it’s using 300% less fuel.

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• SEMI-DISPLACEMENT
Now let’s look at a semi-displacement boat of similar size. If this boat is going 8.5 knots and using 3.4 GPH, it’s getting 2.2 nMPG. If we increase to 10.5 knots, using 14.2 GPH, we’re down to 0.74 nMPG. Once again, going just 2 knots slower increases fuel economy 300%. If we push this boat into higher speeds though, the fuel burn differs significantly. At 15 knots, fuel use goes up dramatically to 23.5 GPH, and our efficiency is down to 0.64 nMPG. At 20 knots, using 35.0 GPH, we’re down to 0.57 nMPG. When more of the boat’s hull is on top of the water, the penalty for increases in speed diminishes dramatically and economy levels off. As speed increases, fuel economy
will gradually decline in small increments.

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• PLANING
Finally, let’s look at a boat designed for speed, a lightweight planing hull. When going slowly at 7.5 knots, burning 2.6 GPH, that equals 2.9 nMPG. When we increase to 11.0 knots, burning 9.2 GPH, that lowers the fuel rate to 1.2 nMPG. At a top speed of 25.0 knots, burning 27.5 GPH, that gives only a small decrease in fuel burn to 0.9 nMPG.

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