Electric mud motor - pic added

Rocky S

Rocky S

Member
I made an electric mud motor by welding up a tube frame and adding a electric trolling motor on the end. The motor has 44 foot pounds of torque and I estimate that the motor is turning about 250rpms at "full throttle", so that is equivalent to about 2hp. I mounted it to my Kara and I expected to get 5 - 10 mph, but was disappointed when I tested it and it struggled to make 4 mph.

I seem to remember someone posting a formula that would tell you how much motor you need to push your boat. Anybody have that?

PA210578.jpg

 
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From the web (Rickshaw site)

"Every boat hull will have a maximum speed before planing. Here is the formula for estimating the maximum speed for a given hull. Multiply the square root of the length of the hull at the waterline by 1.3. This will give the theoretical maximum speed in knots. Most small boats (10 to 14 feet in length) will max out at 5 to 6 miles per hour. You can push harder, but you will just make more waves, not more speed. If you push hard enough to make a boat rise up, plane, and skim across the surface of the water, you escape the limits of the hull speed formula. However, planing a small layout boat would not be safe."

Not exactly what you are looking for but it will get you close. Maybe your boats maximum speed is only 4 mph. I also think your 44 lbs of torques is not equal to 2 hp. Only a gut statement as I don't recall the formula to calculate hp from the numbers you provide.

Mark W
 
The horsepower formula is: (Torque x RPM)/5250 = hp. so 44 ft-lbs x 250 rpm / 5250 = about 2hp
Click to expand...

Me thinks you're using the electric motor THRUST of 44 Lbf which is not equivilent to a torque of 44 ft-Lbf.

Might this work: Thrust x Velocity = hp. Which I get to be 0.47hp for your case. Then there is the drag loss which I didn't guess at, but I still think your motor would be less then 0.75hp total after acccounting for drag loss.

Keir
 
That's about what you can expect from a Kara with a trolling motor. Even with a regular trolling motor, all I got was 4.5 mph. With a gas powered Mudbuddy, 6.9 mph. The Kara isn't a boat built for speed. You could put an Optimax on the back of that thing and it wouldn't go much over 7 mph.

Ed.

Check out my Kara website.
 
Thanks Ed!

I think I would like to see the video of you trying the Optimax!! You could put it on your website!

.
 
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