Flat bottom boat trolling motor power

Question

I am wondering how to estimate the needed motor power for boating upstream on big rivers like, for example, Danube. (mean the bottom to middle part, i.e. where the Danube is wide and "slow"?).

To be more exact, let say having an 12' (3.6m) long jon boat, fully loaded with lets say 800lb - how much thrust is needed for going upstream (even with slow speed - but upstream) i.e. having enough power to "beat" the river's current).

The practical merit of the question is: will it be enough to use some electric (12V) trolling motor e.g. with 30lb or 55lb thrust?

Do not need some rocket-science based calculations, enough some rule-of-thumb based estimates.

EDIT

Because of the comments trying be more precise:

• I haven't any motor
• and I haven't any boat (yet)

So, I can't try it. (What, of course, would be the obvious solution if I had any boat.) :)

Moreover, I haven't defined any exact river. This question is in the idea-gathering phase - when trying to do first and rough estimates of the needed gears and or possible solutions.

Everything what I currently know (more or less precise) is:

• I need to move against the current of some rivers (more different rivers, not only specific one). Mostly only for straight river-crossing, but not limited.
• I will never use the boat on high-velocity rivers - only on slow & calm waters. e.g. let say, the max. velocity of the current where I will use the boat is 4 km/hour (aka approx: 1.11m/s or 2.16 knots)
• also, if the wind against will be too strong - will not use the boat. (I'm not sure about this, how to define it more precisely as calm wind).
• My (custom made, riveted) boat dimensions will be: max. 3.0 up to 3.6m long and the beam will be approx. max. 0.8 to 1m wide (it is not defined exactly yet).
• It will be (sure) flat bottom & straight nose boat - aka like an common jon boat
• The total weight of the boat+motor+batteries+persons+other will be approx. 250 (max.300) kg only. The above 800lb was only for safety. :)
• For some reasons (smallest as possible, lightweight motor) I must to use electric motor. (trolling motor)
• I will have enough batteries (and also 2Kw electric generator) already onboard...

So, is possible to have a jon boat with electric motor under the above conditions? How to estimate the needed motor size (type, power, thrust, etc..) :D.

Answer 1

I wouldn't recommend going with the 30lb thrust motor. Most jon boat trolling motors come in 5 speeds and if you load 800 pounds into your boat you'll be unable to use the first three speeds. The minimum I would recommend for even slow moving rivers (like the Danube) is 40-45lbs of thrust so you can actually make use of the lower speeds.

If you do opt for 30lb thrust then expect to move very slowly even at the fastest thrust possible. You can always compare trolling motors and opt for the faster one and then stick to the lower speeds.

Answer 2

I found a couple of references.

This one has a chart that says the minimum thrust for a 12 foot boat is 30lb, it says 50lb for 17 foot boat, and 101lb for a 22 foot boat.

This speedboat artcle says, you should have at least 20 Horse Power for your 800 pound boat.

The rule of thumb is based on weight alone, and says you should have between 40 and 25 pounds of weight for each horsepower.

From experience I can tell you that I can paddle both a Canoes and Row Boat faster then most trolling motors can move a similar boat.

I would never "rely" on a trolling motor for movement, particularly on a river where either/both current and wind can easily overcome small motors. Even with a gas motor, you should have oars or a paddle as backup and not go out in conditions where you are not able to safely return by manual power (row/paddle).

An additional consideration, is judging remaining power. It is relatively easy to gage liquid fuel, batter power less so.

River Specific Rule of Thumb

There is not one, and if you find something don't rely on it.

You clarified that you will have a generator and batteries present, even if you don't have an electric motor. So the electric motor seems like an optimal plan.

On a lake the only variable to account for is wind. On a river you have a current, and you either have a dam or you don't have a dam. The lake is broad open body of water where the wind is relatively predictable. A river is a narrow winding body of water in a valley where the wind changes drastically in moments.

The current can change without warning. If there is a dam upstream, the operator can open the spillway at anytime increasing the current significantly. If there is no dam, then a flash flood anyplace upstream can alter increase the current where you are. Combined with wind considerations, make for a risky venture.

Additionally If there are any bridges across the river, you are unlikely to be able to motor under them with a trolling motor. Terrain changes can cause similar issues.

See related, attempts to overcome river challenges.

• What is the most effective means of paddling a canoe, against the current, under a bridge?
• How to manage the sail of a sunfish when paddling?

Note that I looked for an outboard motor with an integrated generator and there are a couple of DIY projects, but no off the shelf solution.

Answer 3

As a reference, see Patterson's "The Dangerous River, Adventures on the Nahanni" book. He would prospect for gold, and had a camp a good ways up the river. His boat was in effect a 30 foot long 5 foot wide flat boat. As I recall he had two motors, a 3 hp and a 1 hp motor. The 3 hp was for ascending the river. The 1 was used for putting about using a smaller boat from his base camp. He would carry all his supplies for the year which included both food, gear, mining stuff. There was a reason for having a 30 foot boat.

Caution: I read this book some 30 years ago.

Sanity check on canoe forums suggests that 2-3 hp kickers will get a canoe up to the speed that it planes.