Quote:
Originally Posted by Limecc
I'm thinking of buying a 55lb thrust electric auxiliary as backup motor. My thinking is it would be stored away and only mounted when needed, I'd probably have to extend the cable but connect it directly to the battery.
Are these credible? I'm only after displacement speed but how long/how far is realistic presuming the 110ah battery is fully charged before the main engine fails? Would it be able to overcome a 4 or 5 knot current?
Anyone else done this in preference to a small fixed petrol outboard?
Bonus: I could make a transom bracket and also use it on the kayak.
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In a 4 or 5 knot current, you'll be going backwards. I don't remember who said 55pounds of thrust is equal to 2.5hp, but that is definitely false. Assuming 100% efficiency to keep it simple, 1hp=745.7 watts. If you're operating a 12v system, this means you're looking at roughly 62amps (745.7/12) to obtain a single horsepower. Minnkota tells us the amp draw on 12v motors of 55 pounds or less is roughly 1 amp per pound of thrust, so in this case 55 amps=55 pounds of thrust. A 55 amp draw couldn't quite make 1hp, even at 100% efficiency, and since 100% efficiency is a pipe dream, you're actually looking at less than that.
Moreover, ALL trolling motors are designed to push you at 5 knots in perfect conditions. Bigger will push more weight, and may even be more efficient at doing so, but it will not move you faster that 5kts with the OEM prop. An "electric outboard" is a different species. These are attempting to make the transition into primary power sources, but the efficiency is far from there, because a typical deep cell battery contains perhaps 10% of the power of the equivalent power of gasoline per pound, if you're lucky.
How long will it last is relatively straight forward. 110ah will get you just that, 110 amp hours. At "wot" with a 55 amp trolling motor, you'd get 2 hours. However, ah assumes a constant draw over 20 hours, and if you suck the energy out faster than that, you lose efficiency, so you're probably looking at closer to an hour and a half. You could combat this loss of energy with an appropriately large battery pack, but who's going to carry 700 pounds of batteries and cables on board to reach the 1,100ah battery storage that would be required to draw 55 amps for 20 hours?
And this is with a battery in new condition, that hasn't been abused in the precisely fashion described in the last paragraph. You'll find that 110ah battery won't be holding that much power for long. Also consider you'll knock 75% or more off the life of the battery itself, if you routinely draw it until dead.
Quite simply, you can get 3 times the power out of a 2.5hp gas motor, and likely get more range than even the 1,100ah example out of less pounds of gasoline than a single 110ah battery weighs. Throw in the weight of the motor, and you're still WAY ahead. I don't think my 1984 6hp Johnson 2S weighs more than a typical 110ah battery. And it would probably get you your 5kts well short of full throttle.
P.s. The only reason I know this stuff, is I desperately wanted to go electric on my 3M SIB, but there's just no reasonable way to cost-effectively get the range I wanted, even at a snail's pace. FWIW, I'm still going to pack a tiny trolling motor (16 or 18 pounds, can't remember which), and have ordered the components to build a 200ah battery pack that should come in under 40lbs (lithium)... but that's not so much for an emergency at sea, per se (I'm 5 miles from Long Beach/Los Angeles Harbor and do plan on hitting the seas), rather it's to creep around in near silence beyond some tunes under the stars without disturbing a soul, and to retain the ability to drop into waters that don't allow gas powered craft.
spy-sea-one
Sula
