Electric Motor
I'm trying to replace my 2.5HP 2 stroke engine with an electric. Here's my search so far... damn is this stuff complicated.
First - from Minn Kota I get this in reply to a question:
The battery weight would depend on what battery you purchase and how
many. We generally recommend any deep cycle marine battery, group 27,
100-105 amp hours. Your running time would depend on what model you
purchase. Generally, we say to figure out what the boat weighs fully
loaded once you have that weight we recommend at a minimum 2 pounds of
thrust for every 100 pounds of weight. None of our motors will go
faster to 3-4 miles per hour.
To figure running time you will have to get an idea of which model you
are looking at. The formula to figure approx. running time at high
speed is as follows:
Battery amp hours X 85% divided by motor amp draw at high speed =
approx. running time at high speed
OK - so the answer to that is:
boat 350 lbs
max boat capacity 800lbs
total 1150 lbs
total thrust lbs needed 24 lbs
According to the Minn Kota chart a RT50/S/C with 50 lbs thrust more than qualifies. $300.
Now for the battery question. How much battery am I going to need to motor for around 5 hours?
I went Von Wentzels excellent site and used his incredible spreadsheet to try to figure this out.
According to his spread sheet I only need one battery (such as a 12 V 245 AH AGM for $329 at 158lbs). There are cheaper options but not much below $200.
Can anyone reality check this for me?
Then - if you're doing a multy day trip - how do you recharge this. I don't think there's enough room for solar panels on the W17, wind generator is out of the question. So, I was thinking a small Honda Generator.
Price $800
AC Output 120V 1000W max.(8.3A) 900W rated (7.5A)
DC Output 12V, 96W (8A)
So - if using that Honda Generator, can I use the ChargeTek 500 12/24V 5A charger ($99) to recharge the battery? And, if so, how long would it take to recharge?
Summary of costs:
Battery: $329
Charger: $99
Motor: $300
Total: $728 - which is competitive
with a Honda 4 stroke
Optional camp/sail recharging system
Honda Generator: $800
Update 03/19/2005 - Thoughts on charging and battery capacity
So - reading Battery Town there are definitely some adjustments to be made to this - particularly - the number of batterys needed.
Using their formulas:
1 Battery @245AH / 42A draw from Minn Kota Rt50 = 5.8 hours.
But - you're not supposed to totally discharge a battery. Battery Town suggests 3 times the number of batteries that the math says you need. So - this would mean 3 245AH batteries to motor for around 6 hours. That's 474lbs of battery.
If you were using a 45A charger like the Samlex 12V/45A charger ($319) it would take AHs per day / charger amps * 1.2 (fudge factor) so:
245AH / 45A = 5.44 hours * 1.2 = 6.5 hours.
This means, on top of the now absurd weight and cost (and the batteries only last a few years), you'd be running a generator all night to recharge. This sucks and isn't do-able in reality (I mean, it is, but it's stupid)
So - if you're taking my recommendation (and there's no reason you should) this is my thinking...
Get an electric engine and one battery for typical day sail motoring (leaving dock, getting boat back on trailor, possibly an hour of motoring if the wind dies). Do no more than 50% discharges - that gives you around 3 hours of motoring. And plan on plugging it in when you get home.
Still makes a honda 4 stroke a virtual necessity for longer trips - it's the only thing that makes sense. This is why green hasn't taken off yet. :)
Solar Panel Research
Also - if you're interested, I did a bit of research on just running directly off solar panels. Which, of course, is not really possible.
Minn Kota Rt50:
Max Amp Draw: 42
Volts: 12
Watts: 12 x 42 = 504
Largest flexible solar panel comes in at 32 watts and is 56"x17" or 4.6'x1.4' and is $245. You'd need about 15 of these to run the Rt50 straight out. That's 96 square feet of solar panels at a very reasonable cost of $3,645
The largest non-flexible solar panel comes in at 64 watts, is 54"x28" or 4.5'x2.3' and is $394. You'd need about 8 of these for a total surface area of 82 square feet at a cost of $3,152.
Again - this is why the planet isn't green yet. :) We're getting there - but we have a ways to go.
Updated 03/29/2005
Hydrogen and Fuel Cells
Ok - this is becoming obsessive. But, if anyone is interested. Here's what I found out researching Fuel Cells as an alternative to batteries.
For a cost comparison of Fuel Cell vs Gas see this excellent chart. Gas costs per kWh is around $0.14 while fuel cells are around $4.00.
That's outrageous! I thought fuel cell costs might be double or something, but not 29 times more expensive. Man, the green movement has a lot of progress to make.
If this doesn't phase you and you still want to go fuel cell the best option I found was the AirGen
I emailed the manufacturer and they said it's average lifespan is about 5,000 hours. At $6.5K that's not a very long life. This thing will run flat out for about 10-15 hours on a K bottle of hydrogen (which costs around $30 for the gas plus whatever you're paying to rent/buy a K bottle).
Even if you were cool with all these costs and serious size issues (K bottles are huge, and the generator isn't all that compact either) you'd also have serious weight concerns. The generator is around 100 lbs and a K bottle is around 133 lbs.
In conclusion - I guess I was really suprised by how immature the green market is at this time. It's a got a long way to go before a capitalist system would adopt green energy over gas. It fails on all levels: size, cost, weight, practicle use, etc.
posted by ben @ 8:26 AM
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