March 4, 2012

It pays to be single

IT ALWAYS GIVES ME a good feeling when a boat I'm on has two means of propulsion, sails and an auxiliary motor.  So I'm always rather glad, for safety reasons, that I'm not aboard a motor cruiser that has only one diesel engine and no way to make sail.

There are an awful lot of powerboats that have only one engine, including the majority of the world's fishing boats, and it used to puzzle me why this should be.  But after a little research I discovered the answer:  In general, two engines are not twice as good as one, whether they be inboards or outboards.

As a rule, twin-screw installations are comparatively wasteful of power. They also cost more, need larger fuel tanks, require more servicing, and weigh far more.  In addition, twin-screw installations are commonly rather cramped, and leave little room for access, thus almost guaranteeing that the engines will be poorly maintained.

The commonest reason for having twin engines is safety. But that can be misleading. Many twin-screw powerboats with planing hulls are almost unmanageable under one engine in bad sea conditions — the very conditions under which engine failure is most likely.

What is often overlooked is the fact that a boat with two 100-h.p. engines cannot make the same use of all the available power as a boat with one 200-h.p. engine. Added weight, added friction in drivetrains, and added underwater drag from extra struts and rudders are high prices to pay.

The rule of thumb is that a twin-screw installation wastes about 20 percent of the power available compared with a single engine of comparable horsepower. With fuel prices reaching record levels, that's a formidable price to pay.  Besides, modern diesel engines are extremely reliable if they're given the simple maintenance they require.

I still prefer to have an alternative means of propulsion up my sleeve, but now I can see why designers and builders turn out so many single-engined motor cruisers. They're trading fuel economy and efficiency for safety, certainly, but the odds are on their side.

Today's Thought
Power is so far from being desirable in itself that it sometimes ought to be refused, and sometimes to be resigned.
— Cicero, De Officiis

A sailor rowing a dinghy came across a man in the water fighting off a shark. Nearby, on a 45-foot ketch, the man's wife stood calmly by with a rifle in her hand.
"Why don't you shoot the beast?" the sailor asked.
"I will if I have to," said the woman, "but I'm waiting to see if the shark will save me the trouble."

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)


Matt Marsh said...

Nice summary, John- all valid points.

Getting real redundancy with twin engines requires that they are in individual fire-control compartments, have separate controls, have separate electrical systems, and draw from separate fuel tanks that are always filled at different gas docks. Nobody does all that.

The number-one killer of well-maintained engines is cruddy fuel. And if one of your engines dies from bad gas, you'll have barely exhausted your vocabulary of English curse words before the second engine dies from the same cause.

Don P. said...

Hi John,
A dingy with either oars or a small OB is a good alternative source of propulsion. The outboard on my 25 ft. boat failed once as I was entering the marina. I was surprised to find how easily I was able to tow her 5000 lbs to the berth...with the bow line held in my teeth!

Bill Ray said...

A common mitigation for the bad fuel issue on singles is a dual Racor setup with quick switchover. Rough seas are a common time for stirred up tank crud to block a filter and a bad time to have to change one. The second filter buys some time to seek shelter or sea room (we hope). The day tank used in larger vessels is another.

In theory a single prop is better protected than twins, so maybe it rides over a log without damage to prop, stuffing box or rudder.

There is a certain seamanship challenge, and pride when it works, to handling a single-screw powerboat without a keel -- though there are moments docking in a wind or current when I wish for twins or a bow thruster.