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Brian Whatcott

On 5 Sep 2006 10:52:58 -0700, wrote:

hi,

not sure if i'm in the right group (?) but have a novice question to
ask about what sort of electrical motor (specs, batteries needed) I
would need for a boat that weights 275kg (for 4 people). It is 4.6m by
1.7m long and has a max.weight of 900kg. It is steered via steering
wheel and is a custom construction.

I would like to use the boat for 3-6 hours a day (for weekend use), and
be able to travel at a moderately fast speed (no powerboating, just
travelling at a leisurely speed).

What specs should the engine have, what specs should the batteries
have, and what companies can anyone recommend? would be grateful for
any replies!

Here's a stab at some numbers.

Suppose a 3HP outboard would satisfy the propulsion need.
3 HP = 3 X 746 Watts

What AmpHour capacity would give you a 6 hour
endurance?
3 X 746 X 0.9 efficiency X 6 hours = 12085 VoltAmpHours
or in the usual units: 12 kWhrs

Using 12 volt batteries, that calls for 12085 / 12 AmpHours
from them that is - 1000 AmpHours.

But that would represent discharging to flat - not
calculated to make the batteries last many cycles at all.
If you discharged them to half capacity - to provide some life
expectancy, then you double the AmpHour requirement.

If a 100 AmpHour 12 Volt battery is chosen, then
you would need twenty of them - and a 12 Volt charger
running for 16 hours a day at about 70 Amps.
The utility bill would be about $1.50 per day.

Capital costs At $85 per battery, 20 X 85 = $1700
Cables - say $50
Motor say $400
Charger say $120
Total: $3170

Depreciation (3 year life):
$1700/3 = $570 per annum

Running costs:
4 days/week and 50 weeks/year
200 days X $1.50 = $300 p.a. electric utility

Weight: 12 V 100AHr Batteries weigh 85 lb each
20 X 85 lb = 1700 lb.

That's a little too close to your gross weight limit - so you might
choose to accept a slower speed or a shorter endurance.

You specified a max weight of 900 kg or 1980 lb
less a live weight of 275 kg leaving 1375 lb.

If this were ALL used for batteries, that would allow
16 batteries giving at most,
3HP for 4.8 hrs or 2.4 HP for 6 hours.

You're Welcome!

Brian Whatcott Altus OK