Boaters: Multiple vs single trolling motors for max thrust

benton0311 · Sep 25, 2019

I've searched the internet for an answer to this and posted on The Hull Truth but, so far, I have not found an answer.

Has anyone compared dual trolling motors to a single high-output trolling motor?

What provides more usable thrust/higher top speed: a single 110 lb thrust trolling motor or two 55 lb thrust trolling motors?

If it is the single motor, is the difference substantial?

I have a 40 HP outboard as well as 55 and 30 lb trolling motors. No issues with outboard power but I need more power for electric-only reservoirs and lakes where I have to run without the outboard. With the 55 lb thrust motor, the best I can manage is 3 mph on full power and looking to get about 5 mph, at least for short bursts.

Wondering if I should add another 55 or look at a single 110 instead.

High-end "electric outboards" like Torqeedo are out of my price range while used 55 lb trolling motors are relatively cheap.

pbharvey · Sep 25, 2019

I’d poke around on the tin boats forum

benton0311 · Sep 25, 2019

pbharvey said:
I’d poke around on the tin boats forum

Thanks, I'll give that a try.

edit: First time I've seen it but it looks like they've got a lot of discussions about trolling motors and electric motors. Looks like a great resource!

pbharvey · Sep 25, 2019

I’d like to know what you find out, I’ve wondered the same myself.

Speed3 · Sep 25, 2019

In my younger years I had a john boat that I built a platform out of wood on the front to mount a "pedal" style trolling motor. Some of the lakes (ex:black hills) were pretty large. So I bought a rear mounted trolling motor. With the 2 on at the same time, it would move pretty decent(guessing 5pmh +). I didn't have anyway to tell how fast I was going so I can't say how much the extra trolling motor gave me in MPH.

danb · Sep 25, 2019

Knowledge about trolling on this forum is vast and deep... but not the kind that involves motor thrust, I imagine.

benton0311 · Sep 25, 2019

Speed3 said:
In my younger years I had a john boat that I built a platform out of wood on the front to mount a "pedal" style trolling motor. Some of the lakes (ex:black hills) were pretty large. So I bought a rear mounted trolling motor. With the 2 on at the same time, it would move pretty decent(guessing 5pmh +). I didn't have anyway to tell how fast I was going so I can't say how much the extra trolling motor gave me in MPH.

My problem is weight at about 1000 lbs fully loaded with family, fishing gear, coolers, etc. I tried running my 55 and 30 side by side but the output difference meant the 30 was almost creating as much drag as it was putting out thrust so the speed gain was under .5 mph. Wasn't worth drawing 80 amps of juice for a tiny boost in speed. Another 55 would probably work pretty good having equal thrust but it might also be throwing good money after bad compared to getting the single if the single works a lot better. Or it may not, I just don't know yet.

danb said:
Knowledge about trolling on this forum is vast and deep... but not the kind that involves motor thrust, I imagine.

Yes, lots of experience with the other kind of trolling on here I imagine...

I'm hoping there are some waterfowl hunters on here who run trolling motors to stay quiet but run a full load with gear, blinds, and hunting buddies who may be knowledgeable about trolling motor power.

steves1911 · Sep 25, 2019

Not sure about top speed but a 24v TM will last longer on 2 batteries than 2 55lb TM on a single battery each. I have a 17ft jon boat I had a 55lb up front and a 45lb on the back at one time to help with the current on the upper potomac in spring while bass fishing. I now have a 80lb motor up front and it does better than the 2 combined.

randomuser · Sep 25, 2019

I'd head to cruisersforum.com or sailing anarchy.
lots of knowledge on those 2 places about boating and there are at this time enough people who have done electric conversions and crunched out all the numbers to give some really solid answers.

benton0311 · Sep 25, 2019

Ok, I did some reading over lunch and think I've finally got it figured out. If you're running additional trolling motors, and you have adequate thrust to push your hull through the water, the props would need to be changed out to higher velocity props to realize any speed gain.

5 mph is the general rule of thumb for trolling motor top speed although many of the props on the smaller motors are optimized for 3-4 mph. You can keep adding motors and add thrust which would manifest like having added torque in that you can push more weight at the same speed, you can resist current and winds better at the same speed, but cannot gain additional speed until the motors are propped to move through the water faster.

The higher output 24 volt+ trolling motors already have more aggressive props and/or spin them faster (24v). The prop that would be ideal for smaller 12v trolling motors running in tandem would be too aggressive for the single motor, but since the load on the motor is effectively halved when running two, the prop can be more aggressive to pass more water.

In my case, using twin 55s on OEM props I would probably hit 4 mph and not go any faster although I could increase my weight by 50% and not feel a difference. Re-prop those motors with a steeper pitch and I should be able to exceed that.

It's almost never done that way although Minn Kota actually lists the prop formula on their website so it's right there. The only report I found of someone re-propping their trolling motor to take advantage of availabe extra thrust was on the Hobie website were a guy went so far as to experiment with RC aircraft props and was able to hit 6.5mph.

So it's doable.

Speed3 · Sep 25, 2019

Awesome...hope it works out for you.

Ranchero50 · Sep 25, 2019

benton0311 said:
Ok, I did some reading over lunch and think I've finally got it figured out. If you're running additional trolling motors, and you have adequate thrust to push your hull through the water, the props would need to be changed out to higher velocity props to realize any speed gain.

So it's doable.

Props are made from FRP so you should be able to heat and contour the blades to a different pitch without much effort. I'm still a moderator on tinboats under the jet section but haven't been active in a while. Good site.

randomuser · Sep 25, 2019

benton0311 said:
Ok, I did some reading over lunch and think I've finally got it figured out. If you're running additional trolling motors, and you have adequate thrust to push your hull through the water, the props would need to be changed out to higher velocity props to realize any speed gain.

5 mph is the general rule of thumb for trolling motor top speed although many of the props on the smaller motors are optimized for 3-4 mph. You can keep adding motors and add thrust which would manifest like having added torque in that you can push more weight at the same speed, you can resist current and winds better at the same speed, but cannot gain additional speed until the motors are propped to move through the water faster.

The higher output 24 volt+ trolling motors already have more aggressive props and/or spin them faster (24v). The prop that would be ideal for smaller 12v trolling motors running in tandem would be too aggressive for the single motor, but since the load on the motor is effectively halved when running two, the prop can be more aggressive to pass more water.

In my case, using twin 55s on OEM props I would probably hit 4 mph and not go any faster although I could increase my weight by 50% and not feel a difference. Re-prop those motors with a steeper pitch and I should be able to exceed that.

It's almost never done that way although Minn Kota actually lists the prop formula on their website so it's right there. The only report I found of someone re-propping their trolling motor to take advantage of availabe extra thrust was on the Hobie website were a guy went so far as to experiment with RC aircraft props and was able to hit 6.5mph.

So it's doable.

I'm probably going to get this wrong in explanation, but the general idea will be fairly sound.

It sounds like when you are trolling you are going to be hitting hull speed.
Hull speed on a sailboat (displacement boat), once that is gained any extra force you use to push your self is very very very inefficient. Unless you were to be able to get up on a plane.

I used to tell people that a waterskier takes a ton of energy to get up, but very little to keep up once there. the engines you are talking about using are not enough to get the boat onto a plane so you will achieve hull speed and never plane.

https://www.boatus.com/magazine/2018/february/getting-on-plane.asp
As the boat starts to move forward through the water, speed creates hydrodynamic lift. As more power (and speed) is applied, lift increases, and the boat, in effect, rides over its bow wave, reducing wetted area and thus reducing drag. At this point, the boat is said to be "on a plane" or simply "planing."

Hull speed:
As a very general rule the maximum speed of any displacement hull--commonly called its hull speed--is governed by a simple formula: hull speed in knots equals 1.34 times the square root of the waterline length in feet (HS = 1.34 x √LWL).Mar 26, 2010
Crunching Numbers: Hull Speed & Boat Length - boats.com

https://www.boats.com › reviews › crunching-numbers-hull-speed-boat-length

Biggfoot44 · Sep 25, 2019

Ok , I'm game to read some about long dormant interests . What's the actual name of this tin boat website ?

Back in the day, Trolling Motors were for , you know , Trolling , and 20lb was a big one . What are the current expectations and norms ?

benton0311 · Sep 25, 2019

randomuser said:
I'm probably going to get this wrong in explanation, but the general idea will be fairly sound.

It sounds like when you are trolling you are going to be hitting hull speed.
Hull speed on a sailboat (displacement boat), once that is gained any extra force you use to push your self is very very very inefficient. Unless you were to be able to get up on a plane.

I used to tell people that a waterskier takes a ton of energy to get up, but very little to keep up once there. the engines you are talking about using are not enough to get the boat onto a plane so you will achieve hull speed and never plane.

https://www.boatus.com/magazine/2018/february/getting-on-plane.asp
As the boat starts to move forward through the water, speed creates hydrodynamic lift. As more power (and speed) is applied, lift increases, and the boat, in effect, rides over its bow wave, reducing wetted area and thus reducing drag. At this point, the boat is said to be "on a plane" or simply "planing."

Hull speed:
As a very general rule the maximum speed of any displacement hull--commonly called its hull speed--is governed by a simple formula: hull speed in knots equals 1.34 times the square root of the waterline length in feet (HS = 1.34 x √LWL).Mar 26, 2010
Crunching Numbers: Hull Speed & Boat Length - boats.com

https://www.boats.com › reviews › crunching-numbers-hull-speed-boat-length

I’m aware of hull speed and mine is about 5.5 mph (4.83 knots IIRC) which is what I’m aiming for.

The significance of the guy who used RC props and trolling motors to hit 6.5 mph in his Hobie is that he likely exceeded hull speed ever so slightly. I’ll have to dig that one up again to see if I can figure out hull length but I think it was a run of the mill one-person kayak at 13-14 ft. The hull length vs bow wave formula was exceeded but the weight was really light in a very hydrodynamic hull.

I don’t put a whole lot of credence into the bow wave formula as a hard and fast rule for max hull speed. It’s good for a rough estimate when using very limited power so it applies here as a general guideline but is not a hard and fast rule since it ignores hull design and width even if the hull is still in displacement mode.

benton0311 · Sep 25, 2019

Biggfoot44 said:
Ok , I'm game to read some about long dormant interests . What's the actual name of this tin boat website ?

Back in the day, Trolling Motors were for , you know , Trolling , and 20lb was a big one . What are the current expectations and norms ?

Unfortunately, trolling motors are still just for that - trolling - and the higher output ones are really just intended to maintain a trolling speed on bigger boats as opposed to pushing a smaller boat faster. However, they will to a small extent which is what we’re discussing.

The problem is, many lakes are electric only, so trolling motors are the only viable means of propulsion for the masses.

There are “electric outboards” of various designs such as Torqeedo and Elco but they cost thousands to tens of thousands for power output that pales in comparison to a run-of-the-mill gas outboard.

Trolling motors are up to 180 lbs thrust now. I have a 55 that works fine for trolling but sucks as a means of propulsion.

Biggfoot44 · Sep 25, 2019

So rowing is no longer a Thing ?

So a 55lb is the new normal ? Do people still commonly use A battery, or are dual ( or more) typical now ?

benton0311 · Sep 26, 2019

Biggfoot44 said:
So rowing is no longer a Thing ?

So a 55lb is the new normal ? Do people still commonly use A battery, or are dual ( or more) typical now ?

I don't mind rowing a little kayak or canoe one bit, I actually enjoy it. But 1000 lbs worth of boat and gear with a 6 ft beam is not pleasurable to row several miles to your destination if you're the only one rowing. It can certainly be done, but you just become a human motor at that point and you're not really taking in your surroundings and couldn't care less about fishing.

Seems like 55 is kind of the baseline and about the max for 12V power. The tiny 30 lb thrust motor I also have would probably be fine for a small 1-person Kayak where you want to stop rowing so you can troll and/or concentrate on fishing. It's all about sizing the motor for your particular application.

Anything over 55 lbs is generally going to be 24v or more so two batteries in series.

Boaters: Multiple vs single trolling motors for max thrust

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