Equating a trolling motor to oars?

Discussion in 'Electric Propulsion' started by MushCreek, Dec 20, 2020.

  1. MushCreek
    Joined: Dec 2020
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    Location: South Carolina

    MushCreek Junior Member

    Anybody have an idea how much thrust from an electric trolling motor would equate to using oars? In my case, I'm designing a 5M flat-bottom rowboat, designed primarily for oars. But I'd like to kick back and cruise with an electric trolling motor from time to time. The question is- What size? Is a 50 lb. motor going to be similar to oars? Underkill? Overkill? I'm going to buy a used motor and cannibalize it so I can build it into a wooden rudder. Above the water line, it will just look like a lovely varnished rudder, except for the necessary cord coming out of it somewhere. I'm just wondering what size motor to shop for. We're talking fair-weather cruising on semi-sheltered lakes, on a light (400 lbs loaded) easily driven hull. Thanks!
     
  2. hoytedow
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    hoytedow I'm not a cat.

    I like your idea. I would keep the original shaft though and add the wooden embellishments including disguising the original tiller handle.
    Or your spouse can row while you relax.
     
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  3. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Generally speaking, an adult can pump about 100 watts sustained (hours) into the water for locomotion.
    One HP is about 750 watts.

    How you equate that to pounds of thrust is beyond me but I think 15 would be generous.

    You could tow your boat (loaded) and measure the pounds of drag (='s thrust) needed at speed.
    A fish scale in the tow line would do it.
    Tow it from a side pole so it's in clean water.
     
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  4. ondarvr
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    ondarvr Senior Member

    Anything larger than 40 will work, but bigger is better. 12V goes up to 55.

    I have a 30, 55 and 80. The 30 is OK, it worked for years. I switched to a more efficient and powerful 55 lb model for the extended run time and more control in wind a waves.
     
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  5. MushCreek
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    MushCreek Junior Member

    What size/weight boat are you driving?
     
  6. ondarvr
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    ondarvr Senior Member

    Several, but I'm referring to using it on your size boat.
     
    Last edited: Dec 20, 2020
  7. MushCreek
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    MushCreek Junior Member

    It's kind of hard to do a direct comparison, I guess. Humans have so much more torque. I've seen 50 lb motors move pretty heavy fishing boats around pretty well, though.
     
  8. kapnD
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    kapnD Senior Member

    I’d err on the side of bigger!
    I recently spent an afternoon with both trolling motor and oars working hard to make headway against wind and current, and would have given a lot to have more push from the motor.
    Remember that Manufacturers thrust# specifications are derived under very ideal conditions, and the number deteriorates as the battery weakens.
     
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  9. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    Agree
    It's better to have a larger motor at 50% throttle than trying to throttle up a too small motor to 300%.
     
  10. MushCreek
    Joined: Dec 2020
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    MushCreek Junior Member

    We'll be boating on small mountain lakes, so current isn't an issue unless the dam breaks. Usually it's pretty calm, too, although like anywhere else, the wind can howl on occasion. I'm prudent about checking the weather forecast, and being retired, we can pick our days. I'm going to buy a used motor; probably under a $100 investment. I'll stick with 12 volts, as loading the boat down with heavy batteries kind of defeats the purpose of building a light rowboat. I'll test the motor clamped on before I hack it up to mount it in a rudder. I can even try battling the thrust of the motor with the oars to give it a real-world test. I think a strong man with a large set of oars has so much more torque, and such a large purchase on the water compared to a little plastic prop spinning madly that it's hard to compare. Ondarvr has some real-world experience.

    Budget is always an issue, too. The boat is going to be a simple, lumberyard plywood skiff, not a $10,000 Whitehall. I'll have a few hundred bucks in the hull, so auxiliary power needs to be inexpensive as well. Big trolling motors are expensive, as is a bank of deep-cycle batteries.
     
  11. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    You could keep that bellow the waterline. You might even be able to tap the gudgeons and pintle as an electric conduit. Just coat it in insulating epoxy. Maybe :rolleyes:

    -Will (Dragonfly)
     
  12. MushCreek
    Joined: Dec 2020
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    MushCreek Junior Member

    I'm going to make the rudder out of two pieces, laminated together. I'll machine the groove for the trolling motor shaft, then glue it together with the shaft already in there. I believe that the wires coming up from the motor are inside the shaft. I'll probably run them out from between the cheek plates, under the tiller. The controls will be mounted in a pretty varnished box instead of on top of the trolling motor. I'm going to have to look over the prospective motor to try to determine how easily the various components can be mounted in a box. I'd like to mount the controls near or on the battery box so I can keep the weight amidships. Otherwise, when I'm out fishing by myself, the back of the boat would be under water with me sitting back there.

    I was going to make a complete rudder, with a large opening to contain the trolling motor, but I could simply terminate the rudder below the waterline. The shaft of the motor could then be slipped in from the bottom and secured with a couple screws. That way, if the trolling motor dies, I can remove it and get another one.

    Electrifying the gudgeons and pintles is an interesting idea, but if I ever take the boat out in salt water, it would probably raise major havoc. However I set it up, I want to be able to completely remove the motor, controls, and battery easily for when I just want to go rowing, which is the purpose of the boat. I just have this romantic notion of putting a cute canopy on the boat, and sliding along effortlessly and noiselessly on a hot summer day. Our closest lake is 3 miles long, 1/2 mile wide, and horsepower-restricted, so a slow-moving boat is more appropriate.
     
  13. hoytedow
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    hoytedow I'm not a cat.

    If you plan on salt water get one designed for salt water.
     
  14. MushCreek
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    Location: South Carolina

    MushCreek Junior Member

    I also have a vintage 3 hp gas outboard I may use, especially if I go down to the coast and tackle bigger water.
     

  15. ondarvr
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    ondarvr Senior Member

    It doesn't take much thrust to move a floating object, achieving any speed and controlling it is where the extra thrust comes into play.

    The standard cheap trolling motors work OK, the PWM equipped motors extend battery life significantly longer.
     
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