Now that's a real thought provoking question!

My initial and un-scientific guess it that you would get something less than
the paper calculations might show because you have the added physical drag
from the second motor that MIGHT be greater than that from a single motor.
I would suspect that the actual displacement of two motors would be greater
than that of a single motor I.E.; more stuff in the water equals more drag.
Secondly, if the single motor is rated at a higher efficiency than the two
motors are, you've lost ground in battery life. Conversely if the smaller
motors are more efficient, you're ahead of the hounds.

As I said... What a thought provoking question.

Here's bettin' that this stirs up a lot of opinions and will be fun to
follow!

Don Dando

I have considered this - my guess - like Don's - is that a single big motor
would be more efficient.

And if comparing two 15's to one 30, I would definitely opt for the 30.
But my boat (a 21' sharpie) probably needs 60-100 lbs. I got a 40 at
Walmart, cause it was cheap and available, and it is OK, but not quite
enough against a breeze. But above that size, the purchase price of a motor
increases dramatically - far more than in proportion to the thrust.

So I'm thinking two 40's. In addition to the purchase price economies, the
advantages of redundancy seem to be compelling. And 90% of the time, one
motor will be plenty.

Now - how to charge the d***ed batteries - maybe put them in the bed of my
pickup, wired in parallel with the main battery? Or keep the boat
near-shore on the lake, and run a long extension cord through the water....

Sal's Dad

-- My initial and un-scientific guess it that you would get something less
than
the paper calculations might show because you have the added physical drag
from the second motor that MIGHT be greater than that from a single motor.
I would suspect that the actual displacement of two motors would be
greater
than that of a single motor I.E.; more stuff in the water equals more
drag.
Secondly, if the single motor is rated at a higher efficiency than the two
motors are, you've lost ground in battery life. Conversely if the smaller
motors are more efficient, you're ahead of the hounds.

"Sal's Dad" wrote in message
...
I have considered this - my guess - like Don's - is that a single big motor
would be more efficient.

And if comparing two 15's to one 30, I would definitely opt for the 30.
But my boat (a 21' sharpie) probably needs 60-100 lbs. I got a 40 at
Walmart, cause it was cheap and available, and it is OK, but not quite
enough against a breeze. But above that size, the purchase price of a
motor increases dramatically - far more than in proportion to the thrust.

So I'm thinking two 40's. In addition to the purchase price economies,
the advantages of redundancy seem to be compelling. And 90% of the time,
one motor will be plenty.

Now - how to charge the d***ed batteries - maybe put them in the bed of my
pickup, wired in parallel with the main battery? Or keep the boat
near-shore on the lake, and run a long extension cord through the
water....

Sal's Dad

-- My initial and un-scientific guess it that you would get something
less than
the paper calculations might show because you have the added physical
drag
from the second motor that MIGHT be greater than that from a single
motor.
I would suspect that the actual displacement of two motors would be
greater
than that of a single motor I.E.; more stuff in the water equals more
drag.
Secondly, if the single motor is rated at a higher efficiency than the
two
motors are, you've lost ground in battery life. Conversely if the
smaller
motors are more efficient, you're ahead of the hounds.

If you're in the states, you're probably not doing too badly in terms of
price on these; over here in australia we're still paying well through the
nose for them. a small 13lb freshwater only motor is around AU$200 in the
shops, while something as exotic as a 55lb sal****er verision is going to
cost you around AU$1000. figure about half that price for a no-name made in
china version on ebay with the same thrist rating on the box, but who knows
what in terms of real thrust.

Im really looking for something to use on a small beack cat.... ive already
killed a couple of petrol outboards and was thinking i need something that
is really fully submersible, small, lightweight and instant to use. I had
thought of going with bilge pumps as propulsion, but that doesnt look too
likely now. the next best, though really pretty expensive still at this
stage, looks like single or twin small electric trolling motors, with all
the controls chopped off. Ill fit them to a custom hinged locking bracket,
with everything nicely sikaflexed up to prevent water from entering. I may
have to be a bit careful so it can all be re-assembled to look as it did
originally for if (when) it needs to be taken back under warranty. Sealed
gel cells placed low in the hulls look like the way to go for me. The
inside of my hulls stay almost completely dry.

For charging, Im probably just going to make sure i have plenty of excess
battery capacity and go with a 15-30 watt solar panel. Assuming that im
using the boat once or twice a week, and may or may not use the motor
depending on if the wind dies out on me, and if i do use the motor i may
draw, say 30A.... i think i'll do alright off about a 60-70Ahr battery with
this panel. Im guessing 5-10A/day charging and ill be ok. not having the
motor wont be life or death for me, its more of a luxury... but it still
does look expensive for what little power it provides! $300-$400 for a
motor, $150 for a panel, $150 for a battery, plus say another $150 for
cabling, whatever fuses/switches etc, brackets, sikaflex and maybe some sort
of lock to stop people pinching the solar panel and i figure i'll be luck to
get away with it for a total of AU$900.

a brand new yamaha 3.3 would cost about the same, and the real bummer comes
at re-sale time. I know that in 5 years, i could still sell that yamaha for
AU$400-$500. Id be pretty happy if the electric stuff would last half that
time, and i dont think id get much if anything for it. :-(

Shaun

Don Dando wrote:
Now that's a real thought provoking question!

My initial and un-scientific guess it that you would get something less than
the paper calculations might show because you have the added physical drag
from the second motor that MIGHT be greater than that from a single motor.
I would suspect that the actual displacement of two motors would be greater
than that of a single motor I.E.; more stuff in the water equals more drag.
Secondly, if the single motor is rated at a higher efficiency than the two
motors are, you've lost ground in battery life. Conversely if the smaller
motors are more efficient, you're ahead of the hounds.

As I said... What a thought provoking question.

Here's bettin' that this stirs up a lot of opinions and will be fun to
follow!

Don Dando






Try this site -- the fellow doing this seems to know his stuff far
better than most.

http://www.ecanoe.org/

BS