Multi trolling motors on same boat - Disappointing performance

[Valley Boater]

I have a 15' Gheenoe that I use in mostly electric only bodies of water with 1-2 mile one way trip to the "crappie hole" not uncommon in my favorite electric only impoundment. I installed a 55 lb. bow mount Minn Kota Auto Pilot (from a previous boat I retired) and recently installed Co-Pilot (love it!!!). With a new, large deep cycle battery I can clock a top speed of 3 mph with the 55 lb. bow mount (tracking with my phone GPS application). I recently purchased an older 24v Minn Kota stern mount (claims to be 4 hp!!) and put it on the stern of the Gheenoe and also hit a speed of 4 mph with 2 new batteries. I then dropped the bow mount and ran both at full power (separate battery for the bow mount) and the speed dipped between 3 & 4 mph, indicating no increase and possibly a decrease in speed. I then borrowed a friend's 55 lb thrust 12v Motor Guide stern mount and tried the same trial. The Motor Guide hit 4 mph solo and when the bow mount was lowered and engaged the speed dropped down to 3 mph with both motors on full power. All trials were made on flat water with little to no wind!

I would have bet that running 2 motors would show some increase in speed but didn't see any increase in mph running both in my trials. I plan to still use both a stern mount for taking me to the fishing holes and then dropping the bow mount for trolling and casting as the Co-Pilot is a pleasure to control with my fingers.

Question #1 - are these results of twin power top speed to be expected? I assume the bow motor adds significant drag to the vessel but I would have expected it to contribute some to the top end speed.

Question #2 - I was surprised that a new 12v 55 lb would out perform (top speed) an older 24v Minn Kota (4 hp??). Is this a example of new technology is better?

Question #3 - If I take 2 batteries for the long haul trips for the stern motor and top speed is equal (or close) between a modern 12v and the older 24v Minn Kota, am I better to use one battery at a time with the 12v motor (switching when the battery shows signs of voltage drop) or to use both batteries all day on the 24v motor?

Any opinion, experience or educated theory would be greatly appreciated. Hundreds of crappie are staking their lives on your response.

 

[glenn property of pam]

Re: Multi trolling motors on same boat - Disappointing performance

the results are what you might expect but few people have tested the theory so we have to thank you for that. when you add a second trolling motor you are adding drag so a loss of speed or at least no gain would be expected.

most trolling motors are restricted by the pitch of the prop and most props are designed for the 3-4mph you are seeing so if you add a second trolling motor to the water on a calm day then the prop will not bite the water as its already at full speed and may in fact start acting like a brake. in real bad wind where one trolling motor can not get you to 3mph then the second one would be able to get a bite on the water and would increase your speed.

its hard to change thrust to hp so some makers use watts as hp as thats a bit fluffy at best as it does not take into account any loss from the motor or drive system. true hp is thrust X speed(fps) / 550 = hp

new technology is all about the way high speed gates control the voltage at lower speeds so in theory when run at max speed they should be close. a great test would be to swap props between the motors and see if the speed changes

 

[Silvertip]

Re: Multi trolling motors on same boat - Disappointing performance

Trolling motors are designed to provide a "reasonable" amount of push with a "reasonable" amount of current draw. To balance the two objectives, propellers and motor designs automatically limit the boat speed and boat size is of lesser consideration. In other words a motor of 55# of thrust draws a specific amount of current by design. Therefore you could probably put 5 or 6 motors on the boat and still not go much faster if at all because the motors are not helping each other -- they are all just providing xx amount of push but since prop speed is the same on all of them, there is no gain. Looked at from another perspective, a prop that turns 2000 rpm and which has 6 inches of pitch moves a boat forward 100 feet in one minute. Adding another identical motor does not and cannot increase speed because it too is doing the same thing as the first motor. It is propeller speed alone that determines distance traveled and two motors do not double distance because both props are still only turning 2000 RPM - they do not ADD to each other. However, load the boat up with people and gear to the point where one motor can no longer push it at the same speed it did before, then and only then would the second motor be helpful but it will still not make the boat faster than when the lighter load was applied.

 

[pootnic]

Re: Multi trolling motors on same boat - Disappointing performance

What everyone said above is true,especially the prop design,there not built for speed.
I just wanted to add theres no way that older minnkota is any where near 4 hp.
Your probably better off just using one(the newer bow mount)in parrallel,you could keep the other on board incase of emergency's but you would have to switch the battery's to series to run it.

 

[kfa4303]

Re: Multi trolling motors on same boat - Disappointing performance

+1 too much drag combined with low speed prop. Using 2 outboards of the same hp doesn't necessarily double your speed either. Cool to see a Gheenoe outside of Florida too. Have you seen the ones rigged with poling and casting platforms, helms and +20 hp outboards? They're SWEEEEEET! Check out cutomgheenoe.com for all kind of cool pics and vids.

 

[GA_Boater]

Re: Multi trolling motors on same boat - Disappointing performance

If you want ti try another test - See how long it takes to reach top speed with the bow mount only, transom mount only and finally with both bow & transom. I'm thinking the boat will accelerate to top speed in less time using both TMs. Maybe.

 

[H20Rat]

Re: Multi trolling motors on same boat - Disappointing performance

Question #2 - I was surprised that a new 12v 55 lb would out perform (top speed) an older 24v Minn Kota (4 hp??). Is this a example of new technology is better?

Rule #1, ignore any manufacturer horsepower rating... 4 hp = 3000 watts of input, assuming 100% efficiency. Lets say its really good and manages 90%, so 3300 watts required. 3300/24 volts = 140 amps of constant power. I highly doubt the motor is drawing that, and if so, that must be some MASSIVE power cable running to it, like 1/0 gauge. (to put it in perspective, that is more than what most gas engines require to start, so essentially think of the draw of holding the key trying to crank a battery)


(I'm not much on current trolling motor tech, do they use brushless or brushed motors? If they are brushless, double that amp draw above to get 4hp, so roughly 300 amps. 90% is a VERY efficient brushless motor running in optimum load conditions, so its a decent best case scenario.)

*edit, looks like all minnkota trolling motors are brushed... The only brushless that I could find are the torqueedo's.*

 

[Valley Boater]

Re: Multi trolling motors on same boat - Disappointing performance

Thanks for all the replies. From your input and from some other searches I can summarize:

Top end speed for trolling motors is secondary to getting torque and acceleration. Speed will be limited by the prop. Both of my Minn Kota's (55 lb bow & "4 hp" 24v) have 2 blade props. The 55 lb Motor Guide that I tested had a 3 blade prop so slightly higher speed with the MG that I recorded may not be unexpected.

I have also learned that although Minn Kota has a 4 hp decal on the (1985 vintage?) motor, this was a marketing ploy and an exaggeration of the power of the motor. In my initial trial I didn't notice a significant (if any) increase in top end speed with the 24v, 2 blade prop motor over the 55 lb., 3 blade motor. However, I now expect that the 24v motor will maintain its speed with a more loaded vessel better than a 12v motor as it should have significantly higher torque to maintain prop rpm.

The "hull speed" is another factor that I was uneducated. Apparently all vessels have a maximum speed that they can obtain up to the point where they go on a plane and once this max hull speed is achieved further increase in power will not increase speed until you hit planing speed (impossible with conventional trolling motors!) I have calculated that my maximum hull speed is somewhere around 6 mph (depending upon the load in the boat).

In my quest for speed I am going to purchase a Kipawa propeller for the 24v Minn Kota to see if I can achieve a slight gain in top end speed at the price of losing some torque (with the light weight boat I feel I can lose some torque). My goal is to see if I can come up with a combo that will throw me back in the seat with a blazing 5 mph! I have seen mixed reviews on the Kipawa, most were positive but many posters didn't "notice" an increase in top end speed. However, I will be armed with my free smart phone application (called "GPS Speedometer") I will post the near scientific results here. Due to fast approaching winter (I can see snow on the Appalachian's from my office desk) I may not have the new speed trial results until Spring. Thank to all for looking and sharing your comments. This is a great forum.

 

[glenn property of pam]

Re: Multi trolling motors on same boat - Disappointing performance

if you have a old spare plastic prop laying you can heat the blades and change the pitch slightly just make sure both sides are the same pitch and it would be good for a test

 

[fishrdan]

Re: Multi trolling motors on same boat - Disappointing performance

You need a higher pitched prop for more speed. MotorGuide has several 3 blade props (plastic, filled plastic, aluminum) and they all have different pitches, low to high respectively. I used to run a 70# 24V MinnKota on my 14' jon boat and it ripped along pretty fast with the MotorGuide high pitch aluminum prop, much faster than the 55# MinnKota I have on there now.

MotorGuide 3 blade props need to be modified a bit to use them on a MinnKota TM (shave down the edge where the prop's barrel meets the motor housing), but they will work if the motor diameter fits the prop.

That said, just because the prop has more pitch, doesn't mean you'll go faster, (maybe yes, maybe no). You may need more power to turn the higher pitched prop, to see increased speed. IE: I wouldn't put the high pitch prop on a 30# TM and expect more speed. MotorGuides disclaimer: The 3-Blade Aluminum Machete should be used with 36 volt motors only. I'm using that prop on a 24V motor, but I'm also using it on a very light boat...

heat the blades and change the pitch slightly just make sure both sides are the same pitch

Bond-o tried that and said it was a huge failure, lots of vibration.