Richard J Kinch wrote:
My Group-27 deep-cycle trolling battery weighs 53 lbs and provides 115
Ah x 12 volts = 1.4 KWh. Divide by 746 watts/hp and multiply by 80
percent trolling motor efficiency, you get about 1.5 hp-hours at the
prop, from a full charge to full discharge.

How much gasoline is 1.5 hp-hours?

My 25 hp Tohatsu burns about 2 gal/hour. So 2 gals for 25 hp-hour, or
12.5 hp-hour/gal, or 0.08 gal/hp-hour, times 1.5 hp-hours, is 0.12 gal,
which is to say,

1 GROUP 27 TROLLING BATTERY = 1 PINT GASOLINE

Gasoline has over 50 TIMES the energy density of lead storage batteries.


I still think that's a large exaggeration. And your math is
skewed. Why not multiply your gas outboard's power output by
it's theoretical efficiency of about 40% or less? FWIW I
agree that gasoline is very much more bang for the buck than
any other type of stored energy, that's why it's so hard to
find "alternative fuels."

A better way to compare would be to measure how many
ton/miles each power plant could move at a given speed in a
given hull form. For moving a slow efficient hull, the
trolling motor would be better than you think. And for
moving a light load fast, it won't do anything.



A plain old lead-acid battery
can easily run thousands of charge-discharge cycles if it's
treated properly.


Huh? They're good for about 200 cycles

Baloney. Even the advanced metals batteries go for over
1,000 ycles if properly cared for, and old fashioned
lead-acid batteries are limited only by how they're treated.
Their life is theoretically infinite.

If you're only getting 200 cycles out of your deep-cycle
battery, then you're doing something wrong. Maybe you need a
proper 3-stage charger with a desulphating function? They're
not expensive these days.

I know of many people getting 8+ years of heavy (practically
continuous) use out of traction or golf-cart type batteries.


Unless you mean by "treated properly" that you don't fully discharge, in
which case, your realized energy density is even less favorable.

Sure. But if you discharge it 100% and get to use it less
than 200 times, vs discharging it 50% and using it over
2,000 times, what does that do to the energy density
averaged over the life span?

Fresh Breezes- Doug King