Damn, this is a crazy idea. Those trolling motors are really weak--much weaker than your rowing. And yea, sure, just toss two lead acid batteries on the boat. Naw, no chance they will get wet. But, it's worth trying the whole thing for $500.
This is the kind of thing tha sounds good conceptually, but the technology isn't as good as you want it to be.
whoa!!! easy there trigger. overdose on the angry koolaid today?
to the question
experience. as a kid, i went out on chatfield once in a 13' raft equipped with a troller. i think it was a pretty shitty one, the troller. we got out just fine in the morning when the lake was dead. once it got crowded, it sucked. big boats whizzing by, seemed like they couldnt even see us. scared the piss out of me a couple times. then the storm came. there we were in the middle of the lake and all we had was this piss ant troller. by the time we got back to the car everyone was gone.
opinion . id like to think that with improvments over the last 15 years that if you got a new good one and if all you were looking for is a little help, on the flats or for fishing around a lake it would be fine. i also like how quiet they are compare to gas and i can think of other benefits too if you could find one that was good enough. of course you would take all precautions as to not dump batteries or acid into the drink. those marine batteries are pretty good these days and contrary to some opinions, they can get wet.
now, if you are looking to make serious miles or actully run the boat through some shit with the motor, i'd steer clear of the trollers. at least the one we had. we may have gotten 3 or 4 miles an hour out of her with the wind at our back.
Take a look at what horsepower they are and that might give you an answer. An electric 12 volt motor is about 575 watts or .75 HP. That will also mean that you will kill your battery in just a few hours. My guess is that it will not work that well unless you have a lot of time on your hands.
You might try mounting a generator on the front of you raft to keep the battery charged. That would work.
I've seen a couple of trolling motors launch at Westwater with a single deep cycle battery and no solar panel. They were going to be used to push out the last 6-7 miles of relatively flat water. Never paid attention to how they were going to keep the battery dry in the whitewater.
When I've asked the folks using them, they said that they worked OK for that little amount of distance. But they conceded they were slow and pretty much of limited use when the wind was blowing very hard upstream. Of course when it's windy, every little bit helps.....
The solar panel thing is something I've got some personal experience with. You would have to have a huge panel, lots of continuous direct sun, and no shadows at all cast by clouds, passengers or gear to even begin to stay ahead of the current draw from a trolling motor.
But for relatively short flatwater trips where you needed a little assist, a trolling rig with a sealed battery would work as long as you were sure the battery terminals were adequately protected from shorting -- bad day when that happens.
Anecdote and data often get confused. My 75 watt solar panel, which is 2 feet x 4 feet in size, yields about 4 amps of power in full sun. That's 18-19 volts regulated down to 13-14 volts, to charge the battery. That's in full sun -- no clouds, no shadows, sun directly overhead. For every thin cloud, passing tree branch, and lower angle of the sun, less amperage.
All you really have to do is look at the motor you intend on using, and find out what amount of amperage it uses every hour. Then look at the amp-hour capacity of your battery, and divide by two (since if you drop the battery below 50% charge you'll kill it). That will tell you more or less how many hours you can run without a solar panel before having to re-charge or switch batteries.
Then look at the rating of the solar panel you think you want to use and add some fraction (say 75% due to various inefficiencies) of that back in for every hour you think you'll get sun on the panel.
I can give another example. I also have a 10 watt panel that I use for charging a small gel cell battery for special applications. It puts out -- at best -- 0.4 amps every hour, and is about 14" x 14" in size. That means that on a really good sunny day with 10 hours of honest charging, you could put about 4 amps back in a battery with a small panel.
I doubt that would run a trolling motor very long, because when I did a quick Google search for trolling motor amp-hour usage, I learned that at fast speeds they draw 30 amps an hour and at slow speeds they still draw 5 amps. Now if you had a true 100 amp-hour battery and could only draw it down 50%, you would only get abot an hour and a half at full speed.
I have done this. I have a 14' Cat and have used the rig twice at Westwater. It is faster than people rowing (burly guys rowing hard).
My Minn Kota trolling rig is 15 years old and also a cheaper model when i bought it. I purchaased a $50 marine battery from Walmart and place the battery inside a splashproof black Tuffbox (those balck with grey top walmart jobs) that i use as a seat in the rear of the cat. Works great for 2-3 hours at medium to high setting.
Well, my experience was different. I had a Minn Kota too. It was pretty much useless.
By the way, 746 Watts==1 Horsepower. Most small gas engines are in the 10 horsepower range. I think the top end Minn Kota is 500 watts. There's a reason these are marketed as trolling motors, not power motors.
And if your 2x4 foot solar panel produces 75 watts in ideal conditions, then to produce the 7500 Watts to equate with a 10 Horsepower motor, your solar panel would have to be 20 x 40 feet. In ideal conditions.
It kind of shows how much energy gas contains and how it is hard to replace gas.
10 HP Motor is like 80-100 lbs. way too heavy. Most Cats/rafts use a 2 hp gas motor. Who knows.....all i know is i'll use the electric motor again on westwater this year...no stroking on the shaft and laugh at my buddies sweating.
I think the point is that you can't run a motor off the solar panel itself -- it has to be off a battery. And whether you have enough battery power to get you where you want to go is something that you can calculate, if you know the amp draw of the motor and the available amp capacity of the battery or batteries available to you. Of course, whether it pushes you fast enough to suit your purposes is a whole other story.
This doesn't account for the impact of prop design, which motor boaters will tell you is critical to getting maximum performance from a given hull. When I worked commercial trips on the Grand Canyon a few years ago and we got new Hondas, the first thing we did was throw away the stock prop and put on one that was more suitable to 37' baloney boats. So it's possible one trolling motor prop might be better suited to pushing a raft than another and that might account for the performance differences mentioned here.
As to horsepower, I've used a five horse Honda (weighs 65 pounds) on lots of rivers and lakes with my 16' cat. It will get you there, but not very quickly -- 8-10 mph would be tops, depending on wind and wave conditions. That motor does, however, get about 25 mpg on an average trip with the cat, which is incredible fuel economy. It gets about 15 mpg when I put it on my 14' NRS SB -- a shorter boat with more frictional surface on the bottom and more frontal area for the wind to work on.
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