Page 7The Essential Needs List

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Having an engine that runs perfectly on the workshop bench is very far from having a
good Stirling powered boat.

And there is a great variety of types of boats.
Each type is different because of the different circumstances/purpose for the boat.

So, there are two lists. One for the engine to be compatible with the boat – and the other for
the hull to accommodate the engine

The Boat

1. The hull shape must be long and slim. Stirling engines are low power. The boat hull
must move through the water with the least resistance possible. Look at the racing rowing eight boats. Incredibly long and narrow.

2. The boat must have a counter or canoe stern. Absolutely no trace of a transom.
Canoe or counter sterns let the water flow cleanly past the end of the boat. Drag is reduced.
A square, flat back end for the boat which reaches down into the water is hopeless.
Transoms create huge turbulence and just waste engine power.

3. A Kort nozzle. Naval architects have long, long been focussed on how to get the best
performance out of a boat hull and its engine. We have all seen the bulbous “nose” on
the front of the supertankers. The racing sailing yachts rise out of the water on a wing.
The kort nozzle is a ring, a very short tube, around the propeller. It raises boat speeds by
about 10%. It is only works for speeds
under 12 mph. And, being underwater, no-one sees them or notices them. When you are short of power, it’s a “must.

4. The propeller must be as big as possible and rotate as slowly as possible.
Large slow propellers are the most efficient.

The Engine

1. It must be a pressurised Stirling engine. For many years, there have been many
attempts to power a boat with atmospheric Stirlings.
There is not enough power from an atmospheric engine.
The power of the Stirling engine comes from how many hot molecules of gas are
pushing at the piston. To get more hot molecules pushing, you can make the engine
bigger. But then it gets too big for the boat. Have a look at
Brian Lockwood’s 5 hp
Stirling engine
video. Good horsepower, but too large for a small boat.

To get enough power out of a Stirling, don’t make it bigger.
Put more molecules of gas in it by pressurising it.
The minimum need is for 2,500 cc of gas at atmospheric.
Or 1,250 cc at 2 atmospheres pressure (i.e. 1 bar = 1 atmosphere above atmospheric)
Or 625 cc at 4 atmospheres (3 bar)
Or 312.5 cc at 8 atmospheres (7 bar)
Regen” runs at 4 bar.

2. The engine must have gearing to the propeller shaft. Toothed drive belt is the best.
It is essential that, at cruise speed, the engine is running at peak power.
With petrol/diesel engines – more revs = more power.
Not so with the Stirling.
The Stirling develops most power at a particular rpm rate. (revolutions per minute)
Below that speed heat is flowing well into the gas. But it could go faster.
Above that speed, the engine is running too fast for enough heat to get into the gas.
The faster it runs, the less time there is to heat the gas. And the power gets
The “peak power” revs have to be measured on the workshop bench.
Then the gearing to the prop can be set so that the engine runs at those revs.

3. There must be a reverse gear. Reverse gear is the boat’s braking system.
The epicyclic (sun and planet) gear box is the only? choice.

4. There must be a speed control – The kill tap (more later) is the best?