ESPcat Waterline Length and Hull Speed

The steamship Ernslaw of Queenstown, New Zealand has a pair of 250 hp engines.  She cruises at 13 knots and originally carried over 1000 passengers.

Ernslaw EnginesErnslaw

These days, there are trailer boats with twin 250 hp engines, although that is a bit on the large side.  How can a boat weighing over 14,000 kg (30,800 lbs) travel that fast with a pair of engines with a power rating smaller than the engine found in a family automobile?   The answer is waterline length.   Hull speed is 1.34 times the square root of the waterline length in feet.   At 50.6 m (100 ft), Ernslaw’s hull speed is 13.4 knots.  If the Ernslaw tries to go faster than 13.4 knots, her stern starts to fall into the trough of her own bow wave and she becomes very inefficient but as long as she doesn’t try to exceed 13.4 knots, her hull is very efficient.

Borderline is has a waterline length of 8.3 m (27.2 feet) so has a hull speed of 7 knots, yet according to her previous owners, she would make 9 knots on her 15hp auxiliary engine and had a maximum speed of 28 knots.   How is it possible for her to travel that fast when her hull speed is only 7 knots?  The answer is that if you make a hull narrow enough, the hull generates only a very tiny bow wave.  That bow wave will be so small that the effect of the bow wave is tiny compared to the drag of the water friction as the hull passes through the water.   The bow wave begins to get small at waterline length to beam rations of about 8 to 1.  With a waterline length to beam ratio of 13 to one, Borderline makes only a tiny bow wave.  Hull speed no longer applies when you are talking about very narrow hulls.

Borderline Hull Shape

So if there is no significant bow wave, that leaves only water friction to slow down the hull.  So what makes for low friction?  Just take a look at Borderline’s hull in the image above.  The hull is lovely and smooth allowing the water so smoothly travel past without having to make any sudden changes in direction.  Forcing water to make sudden changes in direction is what makes for inefficiency.    Ok but what about ocean waves?

I used to have a Hartly 18 foot trailer sailer.  She was a great little boat but she had one problem.  Every time she ran into a wave, she stopped.  That was because she had a fat bow rather than a fine entry like Borderline.

fat bow

ESPcat must have a hull that is efficient in every possible way if we are to reach our target speed of 7 knots with a pair of tiny electric motors.

Another consideration is weight.  The more weight you put on a hull, the further it sinks into the water and the larger the whetted surface will be.  More whetted surface means more friction so we must do everything we can to keep weight down.

Last but not least, an efficient hull must have a graceful stern.

graceful stern

See how Borderline’s stern gracefully turns up so the water doesn’t have to make any sudden moves as it passes.

At some point I am going to say something about antifouling paint and surface friction but that is for another day.


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