Thrust, is this a fair guide ?

[Canada Bob]

I have a 12ft solo {just me and a small dog} canoe, we just go fishing and discovering little coves etc, total weight is around 200lbs. I'm looking at buying a trolling motor for those lazy days and wondered about how much thrust might be needed. An old fella told me that as a rough guide, if you divide the total weight by 10 that will give you enough lbs of thrust to propel the boat at 1 knot.

For my loaded boat I'd need {in clam waters this is} 20 lbs thrust for 1 knot, 40 lbs would get me 2 knots and so on until other factors start to kick in.

I know there's hull shape, etc etc etc, but is the above a fair guide ?

Bob.

 

[Alan Gage]

I think hull shape will be much more important than weight and I don't think that's an accurate guide. At least not for canoes.

The software I use to design canoes gives an estimate of how many foot pounds of effort it takes to move the boat at a given speed. For normal cruising speeds (2-4 knots) you're looking at well under 10 pounds of force. As speed increases beyond a certain point the force required goes up exponentially.

When I was younger I used to fish a lot in 17' aluminum fishing boats. Not the heaviest thing out there but not the lightest either. I usually used 55lb thrust trolling motors and they had no trouble moving the boat around at a good clip. For a canoe I'd be looking for something in the 20 lb. thrust range but I have no personal experience in that regard. I'm sure a Google search would find lots of info on it.

I also wonder if trolling motors used in canoes wouldn't benefit from a deeper pitched prop.

Alan

 

[Guest]

Interesting Alan. has anyone ever analyzed and published popular canoes by this method? Cruising effort, not top speed is what is important to me.

 

[Canada Bob]

Thanks Alan for the interesting and informative comments, appreciated, I'll be looking into all aspects. What I'm looking for is as Turtle says, cruising rather than getting to a particular place fast...

 

[frozentripper]

One way to test might be to tow the loaded canoe behind a motorboat at different speeds (speed indicated by GPS or some kind of speedometer, or time over a known distance), and measure the drag of the canoe with spring scales attached to a rope. To maintain constant speed, thrust must equal drag. IIRC drag increases as the square of speed, so doubling the speed will need four times the thrust.

Your call on how fast you want to go... it should be interesting to see if the theory that squaring thrust to increase speed twice is real. The canoe will begin to plane beyond some critical speed so the relation will break down, but... this is science and this is how we get to understand reality. Having a few beers will also help. Don't forget to note wind speed, water temperature, air pressure, turbulence, and tow rope location and factor those into the hydrodynamics. I'm sure the sales guy at the outboard dealership will be extremely interested.

 

[Muskrat]

Trolling motors are high acceleration low top speed. A 20 lb thrust or a 30 lb thrust will get your canoe to the same top speed. They are limited by the prop angle. I've seen people put model airplane props on trolling motors to increase top speed. They're cheap and it shouldn't be hard to swap. I can't recall any specifics but a Google search should yield more details on prop angles and mods.

Turtle: guillemot kayaks publishes hull resistance charts for their canoes and kayaks. I don't know of anywhere else that does. John Winters certainly has that information for his designs since he developed some of the calculations.

 

[frozentripper]

A quick read through a reference confirms that drag, or resistance, increases as the square of velocity (speed) as an object moves through water. So the simplified formula (ignoring the form coefficient for different hull shapes since only one hull is being considered here and ignoring water density which is greatest at 4C, decreasing with higher temps)... is:

Resistance = frontal area x velocity squared

So the hull moving through water at two km/hr will need four times the thrust needed to move the canoe at one km/hr... aaaah, more or less, in theory, generally speaking, without putting too fine a point on it.

IIRC Rolf Kraiker (when he used to post on message boards) measured resistance by fastening a canoe to a bridge and then used spring scales to get a resistance number. This was done to compare different hull shapes and loadings (? fuzzy memory) but let's say that for this one canoe only, measuring resistance in a river could yield resistance at a certain value at that current speed. If the speed needed from a motor was twice that, the resistance should be four times the above according to the formula and so the thrust needed from the motor should be equal to the resistance, four times the resistance measured in the river.

<if you think this is bad, you should see what happens with maps>

 

[Canada Bob]

THANKS to all for the info, very interesting. Algonquin Park hey, 20 years since I was up there {from Thorold at the time}, are they still losing hikers / canoeists to the bears ? used to be about 5 a year if memory serves me well...