Explain trolling motor lb thrust to me, more doesn't mean faster?

Josh

Dead in the water
Staff member
Admin
I had always assumed that trolling motor lb thrust was like gas engine horsepower. But now I understand that to be incorrect? In reality, more lb thrust won't push you faster, it just lets you push bigger/heavier boats? Is that right or am I missing something?

So if you have a, say, 12ft jon boat there's really no reason to get a 55-70lb thrust motor over a 40? I mean, aside from future-proofing your motor in the case of getting a bigger boat (or gaining a lot of weight).
 

Haggis57

Steelhead
Forum Supporter
In essence, the maximum speed a trolling motor can attain is based on the maximum speed of the electric motor and the propeller diameter. See below from the Minn Kota Website. This is a link to their calculation of 4.95 MPH top speed from their trolling motors. More thrust will get you up to the maximum speed quicker, but the motor speed and propeller will determine the maximum possible speed.

If you've got a really inefficient hull shape or a very heavy boat, you may need quite a bit more thrust to actually attain that maximum speed than someone with a very streamlined efficient hull. For instance, I use my trolling motor to "push" my Fish Cat Scout open end first so I can see where I'm going. However the front of the seat is low and pushes water so I am relatively inefficient and certainly slower than my friend with a similar motor thrust but with a conventional framed pontoon with two parallel streamlined pontoons. Pushing my boat in the direction I do, I will never be able to get to the 4.95 MPH speed with my 30lb thrust motor.

Ken

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Josh

Dead in the water
Staff member
Admin
In essence, the maximum speed a trolling motor can attain is based on the maximum speed of the electric motor and the propeller diameter. See below from the Minn Kota Website. This is a link to their calculation of 4.95 MPH top speed from their trolling motors. More thrust will get you up to the maximum speed quicker, but the motor speed and propeller will determine the maximum possible speed.

If you've got a really inefficient hull shape or a very heavy boat, you may need quite a bit more thrust to actually attain that maximum speed than someone with a very streamlined efficient hull. For instance, I use my trolling motor to "push" my Fish Cat Scout open end first so I can see where I'm going. However the front of the seat is low and pushes water so I am relatively inefficient and certainly slower than my friend with a similar motor thrust but with a conventional framed pontoon with two parallel streamlined pontoons. Pushing my boat in the direction I do, I will never be able to get to the 4.95 MPH speed with my 30lb thrust motor.

Ken
Excellent reply! Thank you sir.
 

krusty

We're on the Road to Nowhere...
Forum Supporter
Another factor is that maximum speed for any displacement hull (I think it's a safe assumption your kayak isn't designed to plane!) is heavily influenced by hull length at waterline. At some point no matter how much thrust is input the maximum speed is achieved and you simply end up with a bigger bow wakes.
 

Old Man

Just a useless Old Man.
Forum Legend
I don't believe that a 40lb thrust will put you upon a plane. I once had a 14' zodiac It was pushed around with a 40 lb. thrust motor. I never got anywhere to fast, but I got there without having to row.
 

DoesItFloat

Life of the Party
Forum Supporter
Someone explained it to me simply - the more solid volume of boat is underwater, the more LB thrust is required to push it at X speed. It's about volume not mass.
 

krusty

We're on the Road to Nowhere...
Forum Supporter
I don't believe that a 40lb thrust will put you upon a plane. I once had a 14' zodiac It was pushed around with a 40 lb. thrust motor. I never got anywhere to fast, but I got there without having to row.
And if the hull is not shaped to facilitate planing (flattened at the stern) the vessel will not plane). You're just 'plowing' water. Kayaks are just not designed to plane.
 

Rob Allen

Life of the Party
I had always assumed that trolling motor lb thrust was like gas engine horsepower. But now I understand that to be incorrect? In reality, more lb thrust won't push you faster, it just lets you push bigger/heavier boats? Is that right or am I missing something?

So if you have a, say, 12ft jon boat there's really no reason to get a 55-70lb thrust motor over a 40? I mean, aside from future-proofing your motor in the case of getting a bigger boat (or gaining a lot of weight).
More means faster, better trucking through weeds, wind and current.

My theory for outboards is max out your hull.

With trolleys get the biggest one reasonable for your boat
12 foot John boat I'd get the smallest 24 volt. Probably 70lb. Not because you need that much power but it'll last all day on one charge.
 
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