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
Tailpiece
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.)
Nice summary, John- all valid points.
ReplyDeleteGetting 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.
Hi John,
ReplyDeleteA 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!
Cheers,
Don
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.
ReplyDeleteIn 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.