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© 2009 Mark Kennett

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Displacement motor boats versus planing motor boats

Conventional sailing boats, rowing boats, tugs, trawlers and ships all have
displacement hulls.

This means they push through the water instead of skimming over it. A portion of the water is turned down under the vessel and returns to the surface behind it. The advantages of this type of hull are many. They include:

  • They can be strongly built without undue concern over weight.
  • They will tolerate variations in loading and changes of trim
  • They can be very economical to power
  • They can be very seaworthy.

One of the limitations of this type of hull is that its absolute top speed (terminal hull speed) is restricted by a transverse system of waves formed by the hull’s passage through the water. All displacement hulls are subject to this condition, which is related to the vessel’s waterline length. Essentially this means that the longer a boat is, the faster she has the potential to travel. Naturally other design factors will determine how efficiently the hull achieves its potential speed, but short, displacement hulls are restricted to relatively low speeds (about 7.5 knots in boats with under 30 ft waterline length).

The other side of the displacement coin is that displacement boats can be very fuel efficient and economical. Well-designed displacement hulls of moderate weight require relatively low power and small engines to bring them to their terminal hull speeds. This factor, coupled with the displacement hull’s load-carrying ability, which allows for generous fuel tanks and cargo provision, means long range operation is a practical proposition.

Planing hulls are designed for high speed above all other considerations. They have to be built lightly to get up on the plane and skim over the water, and they require large powerful engines to do so. Maintenance costs for these large engines, needless to say, are high.

Some inexperienced boatmen have argued that simply running a high speed planing motor boat at low speeds would equal the economy of the displacement hull while retaining the advantages of high-speed potential. Unfortunately a high-speed hull is only efficient at high speeds and is often grossly inefficient at low speeds.

An example of this is a comparison I made with a friend of mine some time ago. At the time I had a 40 ft waterline length displacement boat which used about 1.5 imperial gallons per hour at 7.5 knots. It was powered by a single 80 hp engine and had a top speed of about 9 knots. My friend had an ex-pilot boat, 48 ft long and capable of 26 knots. This boat was powered by two 250 hp engines. At top speed the pilot boat used at least 20 imperial gallons an hour. When it came off the plane and slowed down to 7.5 knots however, its high speed hull created excessive drag and it still used 8 imperial gallons per hour, 5.3 times more fuel than my displacement boat at the same speed.

Now imagine these two boats on a 750 nautical mile voyage.

At 7.5 knots my displacement boat would use 150 imperial gallons and get there in 4 days.

If my friend were to travel at 26 knots he would use 577 imperial gallons and arrive at the destination in a day and a bit. That’s 3.85 times more fuel than my displacement boat used. Slowing down to 7.5 knots he would use 800 imperial gallons and arrive at the same time as me, having used 5 times the fuel! This shows that the planing hull is more efficient at high speeds, but it still uses much more fuel than the displacement boat at all speeds.

But in reality, my friend would not arrive at the destination at all, because his planing hull could not carry enough fuel to make 750 miles at any speed. In order to plane, planing hulls need to be light, so they can only carry a relatively small fuel load, and this coupled with their very high fuel consumption and large engines makes them impractical for long-range operations in both a physical and economic sense.

If you want to go far on the water, or go cheaply with all the practical advantages that a seaworthy motor vessel can give you, then displacement boats are the way to do it.