Just Slow Down

Many readers may be familiar with the hull-resistance curves associated with displacement boats, which include most sailing monohulls. These curves show the amount of energy needed (in kilowatts or horsepower) to move a boat at a given speed through smooth water in no wind.At slow speeds, the principal resistance to motion is friction between the wetted surface area of the hull and the
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Many readers may be familiar with the hull-resistance curves associated with displacement boats, which include most sailing monohulls. These curves show the amount of energy needed (in kilowatts or horsepower) to move a boat at a given speed through smooth water in no wind.

At slow speeds, the principal resistance to motion is friction between the wetted surface area of the hull and the water. So long as the boat’s bottom is clean, this resistance is usually low and it therefore takes little energy to move a displacement boat in calm water. As the boat moves through the water, it makes waves. At slow speeds these waves are small and close together. As speed increases, the waves increase in size and lengthen until we reach a point where there is a wavecrest at both the bow and stern with a clearly defined trough between them. Now the length of the wave the boat is making is the same as the waterline length of the boat.



It takes energy to make these waves; not much initially, but the amount of wave-making energy expended rises rapidly as the wave length approaches the waterline length. Thereafter it takes enormous amounts of energy (whether produced by sails or an engine) to gain ever smaller increases in speed unless the hullform is capable of planing or is very light and narrow. For years, we’ve all been taught to think in terms of a maximum “hull speed” for most displacement boats (I am not addressing racing boats here) as the point at which the wavelength more or less equals the waterline length. This is defined by the classic hull-speed formula: 1.34 x ?(waterline length in feet). Most displacement boats must be surfing to significantly exceed this speed.

Our current boat, a Mal 46, has a waterline length of 38.25 feet. The square root is 6.18 feet, so this yields a nominal hull speed of 1.34 x 6.18 = 8.28 knots. We can tell when we reach this speed, because the stern wave we generate is then under the stern counter. If we go any faster, this wavecrest starts to move aft of the boat until we are trying to climb up our bow wave. This only happens in stiff winds or with a push from the waves.

How much energy does it take to achieve 8.28 knots in calm water? As part of our ongoing European Union-funded hybrid-propulsion research, we have installed test equipment on our boat that enables us to quantify this. Depending on the propeller used (we tested eight this summer, some with two or more different pitch settings) it takes between 55 and 74 hp at the propeller shaft. The lower number was achieved using the most efficient propeller; the higher number came from the least efficient.

What happens if we slow down to 7.28 knots? It’s quite dramatic. The power demand drops to between 26 and 32 hp, less than half as much. When we cut our speed to 6.28 knots, the power demand drops to between 15 and 17 hp, or approximately 25 percent of the demand at hull speed. In other words, the last two knots of extra speed requires three times as much power as the first 6.28 knots.



How does this translate into fuel consumption? At 8.28 knots our fuel consumption varied from just under 3.5 to 4.75 gallons per hour. At 7.28 knots it was between 1.6 and 2 gallons. At 6.28 knots it was between 1 and 1.25 gallons. In other words, as you would expect, fuel consumption pretty much mirrors the energy input into the propeller shaft. It takes half as much fuel to do 7.28 knots instead of 8.28 knots, and one quarter as much to do 6.28 knots instead of 8.28 knots. To put this in perspective, at $3 a gallon that last 2 knots costs around $9 an hour, or $4.50 a mile. At 8.28 knots we are paying around $1.50 per mile traveled, while at 6.28 knots we are paying only 54 cents a mile. Reducing our speed by less than 25 percent reduced fuel costs per mile to one third of what they were at hull speed.

We’ve also run some tests comparing a mildly fouled propeller to the same propeller when clean and polished, to see what effect a few barnacles might have. It really was just a few barnacles, but it was equally as shocking. At any given speed, fuel consumption was approximately 50 percent higher with the fouled propeller compared to the clean one, while for any given level of fuel consumption our speed fell anywhere from half a knot (at higher levels of fuel consumption) to a full knot (at slower speeds and lower levels of fuel consumption). For example, at 7 knots, fuel consumption went from around 1.06 gallons per hour without barnacles to around 1.6 gph with barnacles. If we locked in our fuel consumption at a steady 0.8 gph, our speed fell from around 6.5 knots without barnacles to around 5.5 knots with barnacles.


The European Union’s primary objective in funding our project is to reduce the carbon footprint of recreational boating. We are trying hard to design more efficient onboard-energy systems to service both propulsion and “house” loads. We already know we can often substantially improve upon conventional systems, but it will be a while before the marketplace reflects this.

Meanwhile, for those interested in achieving better fuel efficiency and/or reducing their own carbon footprint, I have two clear messages to share: keep your propeller clean and slow down a knot or two.

Currently, the quickest way to maximize fuel conservation on a boat is to install a miles per gallon (or gallons per mile) meter at the helm station. Most owners would be truly shocked at how fuel consumption doubles or even quadruples when they try to squeeze out an extra knot or two of speed. If the increase in consumption was immediately apparent, they would immediately ease up on the throttle.

Of course, even greater efficiencies can be achieved if we all learn to be more patient and sail to our destinations instead of starting up the engine whenever speed falls below 5 knots…



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