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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.

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    Thanks for posting I am curious to see what people post in reply. If it were me I would not try to make estimates I would do research—has anyone tried this and how did it work out? In practice what motor is typical minimum for the performance you require and what is the power? Do people with experience recommend excess power for emergency maneuvering or areas of increased water speed? Or the opposite can you use eddies to make headway with a very small engine? But it will be interesting to see what people say. – mmcc Jan 27 at 23:24
  • Without having a drag factor for that vessel it will not be possible to do this accurately. I think mmcc's comment is spot on regarding theoretical vs practical – Rory Alsop Jan 28 at 1:27
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    @JamesJenkins not because want saving weight, but because both things are already on-board. So, when I already have battery packs on the board (and i will have the batteries regardless of the engine) it is contra-productive to carry petrol boat-motor too. Especially, when (probably) don't need such power and the trolling should be enough. Also the battery packs are lithium batteries and their capacity is way above the needed level. 200+Ah (36v). – jm666 Feb 4 at 18:47
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    @jm666 hahaha yeah my friend crossed some big deserts in N. Africa in his frontwd Toyota stopping once in a while to clean the brakes with dental floss while people in massive 4x4’s stared...I felt cool and minimalist on my KLR 650 in Mozambique until I met the Italian kid on his 180cc scrambler I don’t even think he was taking any pictures just doing it...yeah the difference between software and wilderness adventure is of course penalty for a failed experiment so this list skews conservative with advice but I’ve read some great things here...Bon voyage... – mmcc Feb 9 at 1:06
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    @jm666 I’ve built some trikes that used arm power only, it is amazing how little power is needed at low speeds pbase.com/mccambridge/trikefrogs – mmcc Feb 9 at 1:22
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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.

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.

  • +1 :) Thanks. The informations are good, but (IMHO) against each other, because isn't possible directly compare gasoline outboard motors with trolling motors. Converting thrust to HP isn't possible also, because it depends on more factors (for example prop pitch). Another rule of thumb for lakes says that It is needed 2lb thrust for 100lb weight. But I need the estimates for the rivers. – jm666 Feb 1 at 7:11
  • @jm666 I have edited my answer, I am sure that it is not what you want to hear. Safety aside, you are just not going to be able to the performance you will need with an electric motor. If you are lucky you will only be frustrated buy the issues. – James Jenkins Feb 4 at 19:21
  • I know I can paddle my canoe faster then most electric trolling motors and I have been seriously challenged to paddle DOWN stream when a light head wind came up on the river I was traveling. – James Jenkins Feb 4 at 19:26
  • Thank you very much for your opinion. Good to see that experienced boater answered my question. Anyway, here are other opinions on the net too (similar or different) e.g.: sailnet.com/forums/learning-sail/… or michigan-sportsman.com/forum/threads/… or videos youtube.com/watch?v=QV2MRD6Ta8U or youtube.com/watch?v=vAHWl0nj1sE and many others. So, because the opinions and experiences are different - asked here for the "slow current river". Will continue to search. :) – jm666 Feb 4 at 21:33
  • @jm666 After you complete your research, be sure and come back to post your own answer :) – James Jenkins Feb 5 at 11:07
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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.

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