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.

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 '19 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 '19 at 01:27
@RoryAlsop I think that jon boats with defined length has similar beam width aka also similar drag too... so for the rough estimates the needed constants could be found on the net? or no? — clt60, Jan 28 '19 at 01:33
it's pretty easy to measure: how fast does the current go, and how fast can your motor push your boat? — njzk2, Jan 28 '19 at 04:34
All I'm finding on this is that the boat should have a plate on it telling you the max hp on the engine for that hull and you want at least 70% of that figure, but there's no real guidelines beyond that. — Separatrix, Jan 28 '19 at 09:56
Can you rent or borrow a similar boat to see how that boat's set up performs? I have a 12 foot jon boat with an electric motor pushing at 54 pounds of thrust. WIth 3 adults and gear (close to 700 pounds) in the boat moves quickly on the lake, even against the wind. I would trust it to move the boat in a light current also. Try to test it on your waters. — B540Glenn, Jan 30 '19 at 18:52
@njzk2 :) yes. Good point. Unfortunately I haven't any boat, nor motor and event havent any river... :) So, therefore me asking for how to estimate. :) — clt60, Feb 01 '19 at 06:39
It looks like you want an electric motor to save weight, but you are planning to carry batteries and a gas powered generator. A small 4 cycle gas outboard weights 30 lbs while a single deep cycle battery weights 45 lbs — James Jenkins, Feb 04 '19 at 14:12
@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). — clt60, Feb 04 '19 at 18:47
@JamesJenkins but bookmarked the small-gas motor link. Will consider it. :) Thank you. :) — clt60, Feb 04 '19 at 18:53
Hey sounds like you’re making a highly custom setup—any chance to put in a sliding seat and oar locks? There is a massive difference between rowing with a fixed and sliding seat in terms of your peak and sustained power output. This rig, plus the skill and fitness you develop using it, will give you great speed in short bursts and enhanced reliability (if you are alive you can move). Also on long trips the workout passes the time... — mmcc, Feb 08 '19 at 03:24
The other thing we haven’t mentioned yet is a small sail and mast deployed only in occasional wind conditions—not a thing people normally bother with, but you are going for something unusual here — mmcc, Feb 08 '19 at 03:31
@mmcc You're (of course) right. Highly unusual. We (party of friends) are brainstorming on some amphibious hybrid (human-electric-solar) tricycle. Check this: https://imgur.com/gallery/hijEr8n . It looks like as an funny project, but here (in the stackexchange network) we must ask "no opinion based" questions. (I know, because I have 40k+ rep on stackoverflow). So I asked just about one exact thing - most important for me now - how to estimate the motor power. :) — clt60, Feb 08 '19 at 13:07
@mmcc also, we are little bit tired from answers like, "it's impossible". (Not here, but in average about the project). It's similar as: years ago, almost everyone has claimed that it is impossible to cross Africa safely in health, without kidnapping, without a 4x4 car. Until came few Czechs and did it on two Trabants (worse cars in kommunistic Europe), without any support vehicles. Impossible things dosn't exists. Here are just an fearful, poor-spirited peoples. :) :) :) — clt60, Feb 08 '19 at 13:08
@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 09 '19 at 01:06
@jm666 I’ve built some trikes that used arm power only, it is amazing how little power is needed at low speeds http://www.pbase.com/mccambridge/trikefrogs — mmcc, Feb 09 '19 at 01:22
@jm666 https://outdoors.stackexchange.com/a/20182/15978 didn’t mention this in the post but the bike and trailer fit in the kayak easily if I’m padding it solo so this rig is amphibious...way less room than a jon boat but I think the more efficient hull would perform w a trolling motor. this is a Wilderness Systems Pamlico 16. — mmcc, Feb 09 '19 at 01:32
@mmcc Wow. Impressive hand cycles. Let say, take this, https://pbase.com/mccambridge/image/64011694 add a bit more clearence, more length, change the bottom side to "boat" add electric motor instead of hand crank, trolling motor to the back, canopy and it is done.. ;) ;) — clt60, Feb 15 '19 at 09:55
@mmcc yes - the "penalty for the failed experiment" - this is for why it is funny. :) Having everything work at the 1st try - not funny. :) thanks :) — clt60, Feb 15 '19 at 09:59

score 4 · Answer 1 · edited Jun 18 '20 at 08:23

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. — clt60, Feb 01 '19 at 07:11
@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 04 '19 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 04 '19 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.: https://www.sailnet.com/forums/learning-sail/297073-trolling-motor-river.html or https://www.michigan-sportsman.com/forum/threads/electric-trolling-motor-on-river.281499/ or videos https://www.youtube.com/watch?v=QV2MRD6Ta8U or https://www.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. :) — clt60, Feb 04 '19 at 21:33
@jm666 After you complete your research, be sure and come back to post your own answer :) — James Jenkins, Feb 05 '19 at 11:07
My research? :) Mean watching videos and reading blog entries or (forum) answers like this? Of course, if I will find something interesting. ;) — clt60, Feb 06 '19 at 08:56
@jm666 I am assuming you are going to make a decision and to try it. Hopefully you will be able to return and post an answer based on your experience. — James Jenkins, Feb 06 '19 at 10:53

score 2 · Answer 2 · answered Feb 06 '19 at 17:03

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.

score 2 · Accepted Answer · answered Apr 20 '20 at 04:46

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.

Flat bottom boat trolling motor power

EDIT

3 Answers3

River Specific Rule of Thumb