Using Trolling Motor as an outboard

This is an interesting thread. I have just completed a wooden boat from my own design. Since I will be doing maiden voyage on an electric only lake I bought a used 55# thrust Minn Kota bow mount trolling motor. I have my 11hp Briggs converted outboard on the stern and I will also have a smaller borrowed trolling motor on the stern for added power and to make it easier to unload and load on the trailer. I also purchased two new large deep cycle batteries I will determine their best location after I see how she sets in the water. After maiden voyage the boat will then be on the Mississippi river and I would not think of being there with out back up power with barge traffic and large boats. A friend of mines son died when their gas motor quit. A barge was coming and they had the boys abandon ship they had life jackets and knew how to swim. but the one boy got trapped under the barge and drowned his name was Ryan Norlan I think he was 9 years old. The barge missed the boat. If they'd had a trolling motor for backup power he would still be alive.

 

Folks wanting to do electric power must be sure to use only deep cycle batts , not car starting batts for the power.

Start batts do not live if drawn down to 1/2 full very often.

The label on start batts will show CCA , cold crank amps.

The label on deep cycle batts will show the "20 hour rate" .

"Everything" batts that claim to be both starts and deep cycle do poorly at both tasks..

There is now a trend to use old trolling motors to power a cruising vessels dink.

Many ladies can not conquer a small OB and feel trapped aboard.

Electric solves the problem well, and will even push a B Whaler a good distance.

FF

 

This is very helpful info gathered over some years. With my new Com-Pac Sunday Cat weighing 1500 lbs I want to avoid gasoline and use an electric trolling motor to get away from the launch ramp to more open water and back.

Some of the good wisdom I have heard is 2 lbs of thrust per 100 lbs of weight ---- seems very little, so I am thinking more like 40+ lbs. And I need to get a deep draft battery to go with it and a charger for when I bring it home.

Does one use a separate battery for the running lights or the same one.

And what good wisdom do you all have for me? With my thanks.....

 

Welcome to the forum Tedster.

For your boat, a 55 pound trolling motor (the biggest of the 12 VDC units) will do fine. You could get by with smaller, say a 40 pound. Range will be determined by how much battery capacity and how fast you push the motor. For your situation, use the "out and back" approach, which is a two battery setup. Use the first battery and when it's down to 70%, switch to the second battery and turn around, so you'll have the capacity to get back without dragging the batteries way down. On your boat, you'll get about an hour on each battery. You can stretch this to 1.5 hours on each, but you'll prematurely kill the batteries if you do this often. If you're willing to run at half speed, you can double your range (time), though of course at a limited speed. Considering the likely electrical elements you might have aboard this boat, you don't need a third battery, unless you do a lot of night sailing.

 

Having been thru this, get the biggest motor you can. Even a large trolling motor has minimal thrust compared to a gas engine.

I like Par's idea of an in and out battery, but would restrict usage to 50% not 70%. The batteries will last longer this way. And if you do run out of power, there is a large enough reserve to be meaningful.

Just remember, power draw goes up exponentially with speed, so unless we are talking really short trips, going slow is always better.

 

My thanks to the both of you.
A question: by what means do you measure that percentage of battery power remaining available? That is helpful to know, and I really would not want to guess.

And I am not expecting hard use, extended use, or speed. What I am setting out to do is to get away from the launch ramp or pier to where I can raise sail and be in more open water.

I used to do a lot of racing when I was a kid, and sailing up and into a busy pier was always an adventure!

 

My 70% recommendation is because of the current battery market. You want real deep cycle batteries, but unfortunately, many batteries marked deep cycle are actually "duel use" units, which are little more than heavy duty automotive pieces, not real deep cycles. These batteries don't like getting sucked down hard, so 70% is my recommendation. If you do pick up real deep cycles (the price will reflect what they really are) then 50% - 60% (or more) is desirable.

Most new trolling motors have a battery monitor built in, though I wouldn't trust one to be especially accurate.



SeaDog sells a reasonable unit, which will give you a good idea where your batteries are at.

 

Folks with much larger boats that have a dink might contemplate the use of one of these electric motors on the dink.

Many brides are not good outboard starters , and may feel "trapped" aboard while cruising.

Electric power solves this problem "If She ain't happy , You wont be happy".

Just a thought,

 

I came by to review these posts and thanks to Par and Stumble for your good wisdom.

And I went by my boat today (Com-Pac Sunday Cat) today to take measurements.
With the motor mount in the "down" position, the measurement to the base of the transom was 19 inches; in the "up" position it came to 24 inches. The photos don't look to have a cavitation plate, but the prop clearly needs to be below the hull.

I called Com-Pac, and they just said "get the longest one you can" -- which seems ill considered and makes the whole draft longer than I probably want near the launch ramp.

And the Minn Kota 55 lbs. models look good. It is amazing how much more money is involved with the Torqeedo models and the propane units. It is easy to make this a lot more complicated than what I just want to do about getting away from the launch ramp and pier to nearby open water and raise sail.

This boatdesign.net website is new to me -- and enjoying reading here a lot!

 

Do not launch your boat with the motor in the lowered position, you'll just break it. Lower only once floating.

 

A 40 lb thrust trolling motor isn't nearly enough for a 30', 3600lb boat in wind or current. I've been out on a windy day in a 12' aluminum boat with a 33 lb trolling motor and could barely make headway.

The biggest bass boats are in the 20-22 ft range and much lower profile than a sail boat, and they use 36v 100 lb thrust trolling motors or better.

If it doesn't need to work in the wind it'll move the boat OK. If you might need to go into the wind you'll need a lot more thrust.

 

The Com-Pac Sunday Cat is a 17', 1,500 pound sailboat Srimes, so my suggestion for a 3/4 HP trolling motor is quite reasonable, with his request to just get away from the ramp into his sailing area.

 

Ah, missed the switch from the olson 30.

 

Thank you, Par, and I do know that.
One of the nice things about a light weight engine like that is the ability to lift it away easily to eliminate that problem and also to reduce drag.