## €10 per hour for two extra knots

Everyone knows that water creates resistance to the movement of the boat hull in the water, some also know that this resistance consists of two components - the frictional resistance of the hull against the water and wave resistance. With the first, everything is clear - since water is to some extent a viscous medium, an object moving in it will be decelerated in a natural way.

The share of friction resistance in the total resistance will be greater at low speeds, but in absolute terms it is small, and directly depends on the area of the wetted surface and its smoothness. Wave resistance begins to impede the movement of the boat when it reaches a certain speed, and it increases sharply with increasing speed. Here, too, everything is clear - the larger the wave formed by the moving hull, the more energy is taken from the propeller of the ship (propeller or sails) to form this wave, and the less it remains forthe movement of the ship forward. Therefore, it becomes clear why the movement of even large hulls on smooth water at low speed, when it does not form waves, requires a very small amount of energy. But it increases dramatically when the boat reaches a speed at which its extremities are on top of two waves formed by the hull itself during its movement. In fact, this is the maximum speed of a displacement vessel, since without gliding it cannot “move” over the crest of its bow wave. This prevspeed is easy to find out. It (in knots) equals 1.34 times the square root of the waterline in feet.

All of these theoretical considerations start to make a lot of practical sense when we start to think about how fast we need to go under the engine in order to be reasonably fast and cost effective.

Nigel Calder experimented with his boat, and learned a lot of interesting things in this regard.

The hull of his sailboat had a waterline length of 38 feet, and the maximum speed calculated using the above formula was 8.28 knots. Each sailboat owner can understand when his boat reaches its maximum speed: this usually happens if the top of the stern wave is in the area of the rudder stock. When this speed is exceeded, the stern wave moves further aft, to the transom and further, and the boat “sits” the stern into the hollow, and a large “hump” of the wave appears behind the transom. The boat, as it were, pulls the entire mass of the wave behind it, spending a huge amount of money on this.your energy. But how much energy is required to reach 8.28 knots for this boat? Tests, depending on the screws used (and nine of them were tested), gave concrete results. From 44 to 55 kW per screw. The lower value was obtained with the most efficient propeller, and the larger one with the most unfortunate one.

What happens if we decrease the speed per knot, to 7.28 knots? A very impressive result! The required power will decrease to 19-23.5 kW. In other words, half! And if you reduce the stroke to 6.28 knots, then it will require only 13kW of motor power on the shaft, which is only a quarter of the power required for a full stroke. In other words, to add only two extra knots of speed (over 6.3 knots) we need three times more power.

The question immediately arises: how does this affect fuel consumption? At a speed of 8.26 knots, fuel consumption ranged from 14 to 19 liters per hour. At 7.28 knots, between 6.5 and 8 liters, and at 6.28 knots, only 4-5 liters of fuel per hour were used. That is, by reducing the speed from the limit by 1 knot, we save half the fuel, and by reducing it by two knots, we burn it four times less than at full speed! Converting this into money, we have an impressive result: two extra knots of speed cost us 10-12 additional liters of fuel, or about 10 euros per hour,whether 5 euros per mile!

Next, the screws were tested, some of which were polished, while others (of the same models) had blades and a hub overgrown with small shells. It was interesting how much more efficient a polished screw is overgrown and what savings it can bring. It turned out that at any speed, fuel consumption on overgrown propellers was on average 50% higher. If fuel consumption was limited to 3 liters per hour, then the speed dropped from 6.5 knots for a polished propeller to 5.5 knots for an overgrown propeller.

For those who are interested in fuel economy on their boat, there are two obvious and guiding conclusions from the tests carried out:

- keep screws clean
- slow down by a knot or two

And don't forget that much more savings can be obtained if we are a little more patient and learn to calmly sail towards our goal, and jerk the engine every time the speed drops below 5 knots.

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Thanks for sharing Artem!

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