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From: (no name) (no email)
Date: Wed Apr 30 2008 - 13:16:01 EDT
This highly speculative think piece about alternate boat power sources was
prompted by a discussion with my son about the economics of hybrid power cars.
He has a 1.5 hour daily commute to work in the traffic chaos of Atlanta and was
wondering if the fuel saving would make it worthwhile for his next car to be
a hybrid. Eventually the discussion morphed into boat power, hence the
following:
Is stored electric power really practical for recreational displacement boats
in the >30 ft. range? A year or two ago the answer would have been a definite
NO. Now with $5/gal diesel fuel, I'm not so sure.
For example, moving my 30 ft., 16,000 LB Willard Horizon motorsailer at 7 KT
takes about 20 hp or about 15 kW. If I replaced my entire 4000 LB of ballast
with the best available lead/acid deep discharge storage batteries I could
store 54 kW hours of energy. This is just about enough to power the boat for 3.6
hours at hull speed or 5.2 hours at 6 knots. It is the equivalent of 4.3
gallons of diesel fuel. I would have to slow down to 4.3 KT to get 12 hr. of
cruising and I would have a range under power of about 51 nautical miles.
Good lead acid golf cart or industrial batteries last about 500 discharge
cycles, this is about 3 to 4 years of operation. Replacement costs of the
batteries would be about $4000. Electricity costs to recharge at 17.5 cents/kWh, my
local Con Edison price, would be $2500 over the life cycle assuming that the
batteries were recharged at the 50% depletion point. Total cost of expendables
is $6500. This works out to about $0.51 per mile at a 4.3 KT average speed. Of
this cost only $0.20 is for the electricity, the rest is for battery
depreciation.
Currently I consume .82 gal of diesel per hour at a speed of 6 KT. and .32
gal/hr. at 4.3 KT. Marine diesel is expensive in my locality, costing about $5
per gallon. At 6 KT my fuel cost per mile is $0.68, at 4.3 KT it is about $0.37
per mile. Now here is where it get tricky. Diesel engines depreciate too.
Moreover they require maintenance during the course of a season. I change oil and
filters several times a year. As needed I replace belts, engine zinc,
impellers, and hoses. Maintenance costs average $200 a year. My engine should last
about 20 years and will cost about $16,000 to replace. Over the engine's 20 year
lifetime, the total cost would be $20,000, averaging about $1000/year.
Now let's make a cost comparison between the two systems. Pick a comfortable,
economical speed. For a displacement boat that would be at a S/L ratio of
1.0, i.e., at a speed in knots equivalent to the sq. rt. of the waterline length
in feet. For my boat that's about 5.25 KT. To reach this speed in calm water
my boat requires 9 hp at the prop and consumes .5 gal/hr of diesel fuel. Given
that an average recreational boat cruises under power about 200 hours a year,
seasonal fuel consumption would be about 100 gallons at a pump cost of $500.
Add in the seasonal engine maintenance costs and the depreciation of the diesel
engine and the total cost comes to $1700.
Using stored electrical power, assuming that the storage and recovery of
power from the system is 80%, the electricity used should cost $284. Add in the
replacement costs of the battery based on a 4 year life and the seasonal cost is
$1284. If the batteries are replaced every 3 years, the cost would be $1617.
Given the uncertainties in the above speculations, the estimated costs are
almost equivalent. If diesel fuel rises much above $6/gallon, electric power may
have a cost advantage.
What about the utility of each power source. If the boat is used for day
trips, returning to a marina every night, electric power has a significant benefit
in convenience. Plug in overnight and you are ready to go the next day. No
problems with noise, vibration, exhaust. If lithium/ion batteries become
available as a byproduct of hybrid automobile production, four times the power would
be available in the same space and my trawler would have a range of 200 miles
between recharges.
On the other hand, diesel power has it all over electric power for extended
cruises. The energy density of diesel fuel permits even a small tank to provide
ranges of several hundred miles. Recharging the fuel supply with diesel takes
only a few minutes. Recharging a battery pack requires hours. Even if the
electric boat carries an onboard DC generator, recharging the power pack would
take an overnight run. You might as well use the motor for propulsion in the
first place. Or I could just hoist the sails and "fuggedaboutit."
We live in interesting times, indeed.
Larry Z
**************
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listings at AOL Autos.
(http://autos.aol.com/used?NCID=aolcmp00300000002851)
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