Georgs asked for comments on diesel/electric power for small trawlers. Here
goes -

The "rule of thumb" for adequately powering displacement craft used by a
number of naval architects is quite simple. To move a relatively clean
displacement craft, either trawler or sailboat, at hull speed in normal
conditions requires one engine horsepower per 500 lbs. of displacement. This
is a real world figure assuming driveline losses of 5% and a propeller
efficiency of 50%. As an example, let's consider the Nordhavn 40. This 36,000
lb displacement, 35.3 LWL trawler should be equipped with an engine of 72 hp
continuous power rating to reach its hull speed of 8 kt. in calm water.

A clean Nordhavn 40 hull would actually require only 53 hp to hold an 8 kt
speed but inclement conditions require additional power. To hold position
against a 20 kt wind requires an additional 12 hp per hundred square feet of
projected area. Wave bucking at displacement speeds marginally adds strain on
the engine as well as straining the crew quite a bit. Prudent naval
architects usually specify from 25 to 50 percent extra power to deal with bad
conditions.

The more complicated question is the frequently posed suggestion that a
diesel/electric drive system be used for trawlers such as the Nordhaven 40.
Diesel/electric drives are used in large ships and locomotives for their
flexibility and control. In both cases, reduction and reverse gearboxes
and/or transmissions are likely to cost more than the engines themselves. It
is far easier to change direction and speed on a 5000 hp engine by flipping
an electrical switch than to shift mechanical gears. Weight, size, and fuel
efficiency are rarely excluding factors for larger ships and locomotives.

Admittedly the idea is attractive. One system for both primary propulsion
power and electrical power certainly sounds ideal - until we crunch the
numbers. Most diesel/electric proponents make a fundamental error in assuming
that an 85 hp/65kW engine coupled to an alternator actually produces 65 kW of
electrical power. In fact alternators in this range are rarely more than 80%
efficient. The 65kW of the engine becomes 52kW of electrical power. To get
more efficiency you must use a lot more copper and iron in the alternator and
both cost and weight skyrocket.

The same holds for the traction engine used to power the propellor. Electric
motors capable of outputting 100 hp at propellor shaft speeds (900 to 1800
rpm) weigh as much as a Volkswagen. Smaller motors require reduction gears
able to handle the same power as conventional mechanical gearboxes. If the
drive motor is 80% efficient, the overall efficiency of the electric
driveline drops to 64%. A power controller will absorb between 10 and 20% of
the applied power as well. The upshot is that half the power (and fuel) is
wasted before it gets to the propellor. This will cut the normal 1800 nm
range of the Nordhaven 40 by at least 40%. Small increases in  overall
efficiency can be bought by large increases in cost, however in no case will
the efficiency of the driveline approximate the 95% power transmission
efficiency of the conventional mechanical drive.

In addition the electric drive system will be far larger and weigh
considerably more than a conventional system. One estimate showed a weight
penalty of 100% and a cost of at least twice that of a mechanical system.

But what of the recent advances in electric traction motor technology as
exemplified by the new wave of hybrid automobiles? Certainly mass production
of electrical power components will lower the price of these expensive items,
and some might actually be marinized for trawler use. However it is the
difference in power utilization between boats and automobiles that makes the
hybrid auto practical and the diesel/electric trawler impractical.
Automobiles typically utilize only a fraction of their fitted power for
normal operation. A small car with a 100 hp gasoline engine may only need 15
to 20 hp to maintain a 60 mph speed on a level road. The remainder of the
power is used only occasionally for acceleration and hill climbing, typically
less than 15% of the driving time. Gasoline engines are most efficient when
fully loaded near their maximum power point. The hybrid automobile takes
advantage of this infrequent high power use by fitting an efficient moderate
power engine capable of handling normal driving conditions and supplimenting
the internal combustion engine power with an auxiliary electric motor drive
for the occasional times when high power is needed. The loss in efficiency of
generating electric power, storing it in batteries, then using it to drive a
motor is acceptable in hybrid autos because of the non-continuous use of
electric motor assist. Additional fuel efficiency is gained by using the
electric motor as a generator during braking, recovering some of the power
used in acceleration.

Engine operating conditions in a trawler are entirely different. The typical
trawler diesel is sized so that it operates at 70 to 85% of continuously
rated power at normal cruising speeds. In a displacement boat, power
requirements tend to increase as the cube of the speed increase. In the
Nordhavn 40 example, it takes only about 7 hp to move the boat at a leisurely
4 kts but nearly 53 hp to travel at 8 kts. Fitting a lesser powered engine
and an electric motor supplimentary system offers very little advantage.
Increasing the cruising speed from 7 to 8 kts requires 18 additional horse
power. Electric vehicle engineers estimate that it takes approximately 50 lbs
of lead/acid battery to supply one horsepower for an hour. Maintaining that
one knot increment in cruising speed for a 10 hour boating day would require
4.5 tons of batteries. Moreover, this ballast would have to be replaced after
500 to 750 charge/discharge cycles. Clearly a diesel/electric trawler must
offer substantial advantages in convenience to overcome the inefficiency of
its main propusion system.

But there is a light at the end of the tunnel. If fuel cell technology is
developed for automotive use, and given a widespread hydrogen fuel
distribution infrastructure, fuel cell/electric drives do become practical
for coastal cruising trawler applications. A hypothetical fuel cell trawler
would have a power generating fuel cell using either stored hydrogen or
hydrogen generated by a hydrocarbon reforming process powering a 50 to 100 hp
high speed electric motor using a non shifting reduction gear to drive the
prop. By the time fuel cells are available, mass production of hybrid cars
should make these motors relatively cheap. The diesel engine might be
relegated to use as a get-home engine in case the hydrogen supply runs out.

How about comments?

Larry Z

Great stuff Larry, thank you.
It's always interesting to look ahead...or look at what might be.
I guess I've always felt that "simplify, simplify" as  Mr Thoreau said, is
pretty
important in a small cruising motorboat. The simplicity of diesel, gearbox,
shaft
makes real sense (which is where your post led me). I guess the other factor
in
the diesel/electric example, is the notorious dislike electricity has for
water.
I know that my diesel will keep running and keep the boat moving with water
up to its air intake!
Thanks again for a really illuminating  post.
Cheers
Glenn Williams
"Stirling" Universal36
Port Phillip
Australia
----- Original Message -----
From: LRZeitlin@aol.com
To: trawler-world-list@lists.samurai.com
Sent: Thursday, May 16, 2002 4:22 AM
Subject: TWL: Diesel/electric power NOT

<<In the Nordhavn 40 example, it takes only about 7 hp
to move the boat at a leisurely 4 kts...>>

I've often toyed with the idea of adding a very small
electric drive to Pooh, to be run off the house bank.
The idea is to have silent if slow capability for
sneaking up on wildlife and enjoying the silence in
places like the Wacamaw River.  I think 1-2 kts would
do the trick, providing steerageway while drifting
with the current.

Most of the speed vs power data I've seen are for
higher speeds: Beebe admits his curves don't work at
very slow speeds (much below S/L of 1.0).  I guess I
need to try a trolling motor on the transom to get
some very low speed data.

Great job, Larry.

Mark Richter, Winnie the Pooh
presently in Ft. Myers Beach, FL and northbound

Do You Yahoo!?
LAUNCH - Your Yahoo! Music Experience
http://launch.yahoo.com