A little vocabulary:

The number that is important in looking at power required for a given
boat at a given speed is the Speed/Length Ratio (S/L). Take the speed
in knots and divide by the square root of the waterline length in
feet. Thus a boat with a 36 foot waterline driving at six knots has
an S/L of 1 (square root of 36 is 6). At seven knots, it's 1.16666,
and so forth.

At S/L less than one, the power needs are dominated by skin friction,
so wetted surface is the best measure of power need. The weight of
the boat (which we usually know) is a good stand in for wetted surface
(which manufacturers rarely give you).

At S/L above one, wave making becomes more important. S/L 1.34 is the
point at which the bow wave and the stern wave are making a big hole
for the boat in the middle. Some people call this "hull speed" and
say that you can't go faster. In fact, there is nothing in the math
that makes this particular speed special, and you can certainly go
faster if you have enough power. A good rule of thumb is that above
S/L of one, you must double the horsepower to go one knot faster.

Now, 120hp is a lot. A fast look at data on trawlers in the 40 foot
range, suggests that it takes around 40hp to do seven knots and that
120hp will get you above nine knots, burning three times the fuel per
hour. (see Voyaging Under Power, Robert Beebe, revised by James
Leishman, McGraw-Hill, 1994, which is the best collection of math,
data, and ideas for the tralwer world)

To give you an idea of the importance of waterline length, our new
boat, Fintry (see www.fintry.com), 79' and 350,000 pounds, will do
around nine knots on 120hp.

Now, as for single versus twins....
This is an old discussion, with strong feelings on both sides. I have
lived with both and like both (Fintry is a single with a 400 horse
main and a 60 horse bow thruster going in).

In favor of singles:
Much lower fuel consumption -- faster for a given power
Better protection for prop and shaft
Simpler, cheaper, to buy, install, and maintain
Most fishboats, cargo vessels, older tugs, many military are singles

In favor of twins:
Maybe a backup on single engine failure (but see below)
Easier to handle in very tight quarters

Comments:
While a single will go faster than a twin with the same total power,
it may be hard to get enough power in the water with one propeller if
you want to travel at S/L ratios above 1.5.

You can not simply shut down one engine on a twin and get the fuel
consumption of a single -- the trailing propeller and the rudder angle
needed to keep the boat straight produce significant drag. It may also
require special attention to gearbox lubrication.

Engine failure is much talked about. A well maintained diesel, once
started, almost never stops unless something outside it goes wrong --
fuel, air, lubrication, temperature. The most common problems are
dirty fuel, high water, and fire, all of which will stop both engines.
For coastal cruising I see this as a complete non-issue. For crossing
oceans, my choice is a single, but I respect those who disagree.

Manuevering a 40' trawler with a single screw, with or without bow
thruster, is a skill that must be learned. It will much easier than
your sailboat, as you have a much higher power to weight ratio and
more rudder in the propwash. You also have more windage, so it isn't
all gain. I would worry about this only if the only choice of marina
had only a very difficult inner slip available. Even then, patience
helps -- in a wind, put her on the gas dock and wait for the wind to
die down.

Jim Woodward
www.mvfintry.com



(Mark) wrote in message . com...
I am researchig trawlers for a future purchase that will take me away
from sailboats. I have loacted several trawlers in the 38-40 foot
range that I may be interested in. I have found most to have twin
engines but a couple come with a single 120-135 hp engine uaually with
a bow thruster. Singles do not bother me because I am not in a hurry,
don't care for the extra maintance and will only be boating in the
Great Lakes. What I would like to know is if these singles would
provide adequate power to cruise at least 7 kph. I plan on a larger
dinghy with a minimum 25 hp outboard to provide emergency get home
propulsion. I would be interested in some of your thought on the
single engines.

Thank You,
Mark