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. • 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. • 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. Notice that at the slow displacement speeds, a slight increase in speed causes a large decrease in fuel economy. But once the boat is out of the water at planing speeds, a significant increase in speed had a smaller effect on fuel consumption. It should also be pointed out that weight matters, but it matters considerably less at displacement speeds. A full displacement trawler can pack on the cruising weight without much of a penalty. The other hull
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. BOB: Generally speaking, going slower saves fuel? 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 condi- tion 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. m l
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types won’t pay a penalty at lower speeds, but at higher speeds the additional weight will take its toll.
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SUMMER 2022
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