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From: Faure, Marin (no email)
Date: Mon Dec 15 2003 - 22:26:20 EST
>I have heard that having one engine is more fuel efficient that having
twin engines, can anyone put a % on this? ... I am interested in twin
engines / twin props for reliability, maneuverability and having twin
keels that would let the boat sit on the bottom but this is not my prime
question.
I can't speak to the fast vs. slow prop issue except to say that high
rpm is generally the way to get maximum horsepower out of an engine,
assuming the engine is designed for it. So when taking off in a
constant or variable pitch propeller airplane, or running a racing
hydroplane, high propeller rpm is what you want. But it's not very
efficient in terms of fuel economy, and it's hard on the engine, which
is why with the airplane, you gain efficiency in cruise and extend
engine life by reducing engine rpm by increasing the prop pitch and
taking a bigger "bite" of air.
As to single engine vs. twin engine, you'll get as many opinions as you
do answers. For a trawler-type boat, I believe a single engine will be
more fuel efficient, although perhaps not by a huge amount, depending on
how you run the boat. If you are content to run at hull speed or below,
you don't really need much power to do it. But with a 36-foot boat, for
example, the hull speed is going to be pretty slow. So manufacturers
started putting in larger engines so their more impatient customers
could go faster if they wanted to.
I'm talking a bit over my head here, but I suspect you could probably
run a Grand Banks 36 at hull speed, which I believe is only about 6 or 7
knots, with an 85 hp Perkins for just a couple of gallons an hour.
However, the smallest engine (so far as I'm aware) that was put into a
GB36 was the ubiquitous Ford Lehman 120. Compared to today's marine
diesels, the 120 Lehman is a low-tech, low-power, low-speed antique,
albeit it a very good and reliable engine if properly operated and
maintained. But in its day, this represented more power than was
necessary to push a GB36 along at hull speed. The design of the GB hull
permits relatively efficient cruise speeds above hull speed, so the 120
Lehman was perfect for pushing the boat along at a knot or two above
hull speed but still burning a relatively small amount of fuel. There
were GB36s made with single Lehman 120s and GB36s made with twin Lehman
120s. At a conservative engine rpm of 1,600, I'm guessing that the
single-engine boats cruise not much slower than the twin-engine boats at
the some rpm, and maybe even the same speed since they weigh less.
We have two 120 Lehmans, and the total fuel consumption at 1,600 rpm or
so is between five and six gallons an hour. This gives us about 8.5
knots. I've run a much newer single-engine GB36 with a Cummins engine,
210 hp I think. The recommended cruise rpm was 2,000 rpm. I ran it
just a wee bit under that, and it gave us a cruise speed of.... 8.5
knots. This was a charter boat about six years ago, so we didn't use it
long enough to get a feel for the fuel consumption.
So far as I can see, the only advantages of two engines are redundancy
and maneuverability. If you have a fuel problem common to the whole
boat, the effect of two engines not running on a twin is pretty much the
same as having one engine not running on a single. However if you have
a cooling problem, exhaust problem, pump problem, etc. on one engine of
a twin, the other engine will probably get you home. We've not (yet)
had a fuel problem, but we have had a couple of cooling problems, so we
were very happy we had the two engines.
Redundancy of engines is a value that will vary widely with the kind of
boating you're going to be doing. Short runs in an area with lots of
boat traffic and good coverage by commercial towing operations reduces
the value of engine redundancy. Longer runs in more remote areas where
assistance may be a day or more away, or in areas where strong currents
and narrow passages don't give you much "drift" time may increase the
value of a second engine. While I have no experience at this, I would
actually think a single engine would be better for a long ocean passage.
Or put another way, I don't think a twin would give any real advantage
on this kind of cruising.
The disadvantages of having two engines are several. Like a four-engine
jetliner vs. a twin-engine jetliner, the chances of an engine problem
are doubled. You will probably use more fuel, although perhaps not a
lot more fuel. You will have two engines to maintain instead of one.
You get to pay for twice the number of pumps, hoses, clamps, impellers,
alternators, exhaust and transmission parts. You get to change oil and
filters twice. You have two shafts that can get out of alignment. Your
boat will be heavier, which may or may not be an advantage depending on
its design. The engine room will be more crowded. The shafts and props
of most twin designs are more exposed and therefore more at risk of
being damaged by underwater debris.
>From my observation, a single has only two disadvantages. You'll have
no redundancy unless your boat is equipped with a so-called "get home"
powerplant of some sort. And the boat will be harder to maneuver in
close quarters under some conditions, although the fact that a bazillion
commercial fisherman have successfully maneuvered their single engine
boats since the early 1900s would indicate that the single-engine boat
can be mastered. (On the other hand, a commercial fisherman doesn't
usually run around threatening to sue everybody when their boat whacks
into the dock and scratches the paint. At least it's AT the dock, which
is the important thing.) And a single can be made as maneuverable as a
twin, if not more so, by the addition of a bow thruster.
My own feeling, based on not much experience in this kind of boating, is
that if I was having a new trawler built, or was in the market for a
recently built one, I would prefer a single-engine boat IF... and it's a
big if... the type of engine had a proven record of reliability and IF,
on a used boat, the engine had relatively low hours and there was no
doubt it had been conservatively run and well maintained. But if I was
shopping for a much older boat, which is what we did, I would prefer a
twin unless I intended to repower the boat. Since you don't really know
what sort of problems you might be getting into with older engines that
have been run and maintained by Lord knows how many people, I'd prefer
to have a "spare" just in case, particularly if I was not mechanically
experienced with diesels.
I also prefer "old fashioned " engines, even if they were built
yesterday, which to me means a heavy-built, normally-aspirated,
inline-six with enough power so they don't have to work hard to move the
boat at a reasonable cruise speed. I know Lugger makes such an engine
today, and there may be others. This kind of a new or newish engine in
a single would, to me, put the boat's reliability right up there with a
twin.
As to twin keels to let the boat sit upright on the bottom, you've
described just about every small fish-boat in the UK (and a lot of
sailboats, as well). It works for them, so I would imagine it would
work for anyone.
______________________________
C. Marin Faure
GB36-403 "La Perouse"
Bellingham, Washington
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