calculating Minimum thrust needed against current

Hello all,
First time poster here. I was reading some previous threads where folks were performing calculations about thrust achieved by propellors and figured you might be the right people to help me with my question.

I am looking to put a small outboard motor on my Old Town Discovery Sport 13 square back canoe, in order to travel upstream against the current in rivers at certain flow rates, so I can fish my way back down and not have to deal with shuttling cars.
I am trying to figure out the minimum thrust/HP needed to make progress upstream against the current, because my boat is only rated for 3hp.

And even if I do have enough thrust to go upstream, if it's at a snail's pace it may not be worth it to buy the motors I'm looking at.

Attached is some information about my boat and hull from the original manufacturers documents. Basically I don't want to buy a new motor and then be stuck with it if it doesn't perform for my required use.
Obviously I could sell it, but I'd take a loss and it would be an expensive mistake.

The river I intend to primarily fish runs anywhere from 300 CFS to 3,500 CFS. I would likely only consider boating in it between 300-600 CFS.
Is there a way to do these calculations?

 

CFS, cubic feet per second, relates to a volume of flow per second. Not the speed of the water.
In a perfect world, ie a pipe with a known cross sectional area, you can easily calculate the speed but a river is not a perfect world
The speed of the water on a river depending on the curve of the river bed, the depth, gradient etc.


Beg, borrow or rent a 3hp motor ( while the canoe is rated for 3hp, check the weight, often the weight of a 3 is the same as a 4 etc, or a 4 the same as a 5.) Worried about weight, there are probably some 2 strokes around that will be a little less weight. Mind you, there is the hassle of mixing fuel.

I am not sure if the following info is current but

Yamaha 3hp 2 stroke 37 pounds
Yamaha 4hp 2 stroke 46 pounds
Merc 3.5 hp 4 stroke 41 pounds
Merc 4hp 4 stroker 57 pounds

 

Ok thank you for the advice and ideas. I called around to 4 marine shops in my area but didn't get anywhere with the renting/borrowing idea.

Can I confirm something with you ...are you saying that the HP ratings on boats are generally driven by the weight of the motor, not the force it exerts?

( I guess I thought those ratings were considering things like safety overall, like sure if theoretically there existed a 25hp motor someday that weighed what a 4hp motor weighs today, then the boat could hold the weight, but it still wouldn't be safe to put on the transom of the boat right)

 

I would imagine that the coast guard or ABYC has a formula to determine the horsepower that the manufacture applies to put a rating on your canoe.

BUT, with common sense if you can up the power by 1 horsepower and the weight is about the same, there should not be a problem.

Common sense being critical here.

 

The canoe appears to be a 13 footer. It is probably a displacement type hull. About the most speed you could normally expect with such a boat is somewhere between 4 and 5 MPH. You could overpower the canoe and displacement style boat would depress the stern and raise the nose up . It would then be wallowing in a hole of its own making and not going much faster.

The velocity of the water stream is a function of the volume (CFS) of the water and the space through which it moves. Say that your river is 50 feet wide and 3 feet deep. The flow volume is 600 cfs. You are stuffing 600 cubic feet of water into a pipe (the river) into an area of 150 square feet. The velocity of flow will be, again roughly, 600/150 = 4 feet per second. That translates to about 2.7 MPH. If the river is wider or deeper the velocity of the flowing water decreases......and visa versa. That SWAG estimate implies that when the flow is 2.7 MPH and your boat is good for 4.7 MPH then your velocity made good is the difference between the two factors is 2.0 MPH. You can walk faster than that. But if your river is 150 feet wide and 6 feet deep the water velocity will be so much lower that you can move along at a fair pace. All that is fun to tinker with but the configuration, depth and composition of the bottom affects the way the water flows. The flow is likely to be stratified and the flow rate at the surface will be different from the flow rate near the bottom.. So that is the reason for using the word "roughly. I should have used the words; very roughly.

Here in Florida the Ghanoe ^sp is often seen. It looks like a canoe but its bottom is designed as a planing boat. I have seen maniacs with 20 HP motors on their Ghanoe. They can speed along at 25 MPH and more. A six HP motor makes it go pretty fast if lightly loaded.

If you want to play with the numbers, one MPH is equivalent to 1.4666 feet per second.

 

Some more information regarding the 3 hp rating on your canoe

ABYC lists the maximum horsepower allowed on Canoes based on length

Less than 15 feet, the max hp is 3, which is why your decal should show 3 hp max.

Canoes have to meet floation standards and the manufacturer has to provide a quantity of floatation to keep the boat afloat in case of a flip , PLUS the weight of the motor.
ABYC supplies a maximum weight of a dry motor based on ranges of horsepower.
0-2,,, 2.1-3.9,,, 4.00-7 hp. The 2.1-3.9 hp allows a dry weight of 40 pounds (18.2 kg)

So as your decal shows 3 horsepower, you would be able to purchase an outboard that weighs 40 pounds or less to be legal.

Some issues"
Floation provided from the manufacturer regarding floation. I would doubt that any manufacturer provides information about how to deal with the extra equipment that might
go into a canoe on day or extended trips. Ie we used to tie in an aluminum wannagan with much of our heavy food stuff on long trips. In the event of a roll, the existing floation would more
than likely not been enough to keep the canoe afloat. We never considered the floation aspect with the extra weight tied to the canoe. On high rapid class water, we would take a tire tube,
push it under the mid thwarts and rope it in.

The round chines of a canoe do make them extremely tippy. If you have to much hp and turn the tiller hard to either of the directions and hit the throttle hard, the thrust will
want to START the canoe to tip over.