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("Paul E. Bennett") wrote in message ...
In article
"Tony Kenny" writes:

Morning all,

I'm getting on good with this new hobby but we find that our old
mercury 20 is damn noisy! We can spend 4-5 hours on the lake with a
single tank of fuel so we're happy with that aspect.

I've been looking at electric motors and note that to equal a 20hp
petrol I will need some 400lbs of electric thrust. I think 6hp
(120lbs) will do for our leisure time on the lake as there is a speed
limit anyway. However, I am concerned about how long we will get out
of a battery.

Does anybody have experience of these things and how long a decent
leisure battery will last with constant use?

Is there anything else I could use? e.g. would an electric wheelchair
motor give enough torque? How much torque would I need?
I've considered looking around for a scrap seagul motor or similar and
experimenting with different motors from the scrap yard.

thanks in advance

The answer to this depends on how much work you wish to put into the
various options.

Off the shelf.
Ready made Electric Outboards are available from some of the Trade
Members of the Electric Boat Association (see web-link in sig). I
would suggest you try Lakeland Electric Boat Centre (Penrith) first
as they are agents for a USA imported outboard range that are quite
good as trolling motors or low power outboards.

I also found a Swiss company that do Electric Outboards up to about
4kW I think but I would have to get their web-site address from the
machine at work. I could do that tomorrow if I remember.

DIY Solutions.
If you have an outboard leg or could make a suitable bevel geared
outboard leg then adding one of Cedric Lynch's smaller motors and
a suitable controller to it would probably get you quite a good
unit. The Lynch Electric Motor Company (L.E.M.CO) are in Honiton
and Cedric is a very knowledgable person about highly efficient
outboard design.

Battery Life.
You need to know the kW rating of the motor and the Amp-hour
capacity of the battery your using. Deep cycle leisure batteries
are best for this application so you economise at the detriment to
battery life.

Battery Volts * Amp-hours = Watt-hours.

(Watt-hours * 0.8) / Motor Rating = Hours Duration (at full power)

As you will not usually want to use full power all the time the
duration on battery will be improved the slower you are prepared
to travel. The Power/Speed curve is a square law so you can see
that halving the speed will reduce power consumption to a quarter
of maximum and hence increase cruising duration by a factor of four
times.

Email me your boat dimensions (LOA, BWL and Draft) and I will provide
you with some motor power requirements for a range of speeds.

Thanks for that!

So, a 12v 140 Amp-hour battery on a 4kW motor will last 0.336 hours?
Or have I made a tits-up of the calculations there? At half speed
that would give 1.344hours? Seems like I'll need an extension lead in
the water with me

My bout is deep vee hull, 13' 6" long, 5' 6" beam I don't know what
LOA, BWL and Draft are, sorry, I'm new to the whole boat thing, I
usually stick with canoes and computers.

I think my outboard is worth about £200 in its current state so it
would be nice to be able to do all this within that budget or a little
more, is electric going to prove v.expensive to get the same times on
the lake that i do now? Ideally I'd like to get 10mph for 5 hours...
mission impossible maybe?

thanks again

Tony Kenny
Hoping to reduce headaches whilst enjoying boating.