Overvolting a minn kota trolling motor

[Monstarr]

I recentely bought a Minn Kota Endura C2 34lbs trolling motor for my boat. It runs of a 12V battery. I was wondering how it would react if i boost the voltage to maybe 20-24V. Any googling did not really put up results. I am not planning on actually doing it but i am curious.
Anyone of you have an idea?

 

[dnmun]

can you open the controller? take pictures and we will look. i assume it is a DC brushed motor so it will have a pwm IC that drives some mosfets in the outptut to allow current to go from the battery to the motor.

these need to be high enuff that they don't break down when the full 24V is applied. also the voltage to the regulator may need adjusting by changing the input power resistor, and the output caps may need to be increased in voltage, or buy a 24V brushed controller from Ecrazyman and use it.

 

[Monstarr]

I'm not opening it lol! What if i brake it

I was just wondering if anyone had tried it before.

 

[dutchlincoln]

I know from first hand that you fry your motor

Don't ask...

 

[Monstarr]

Pics?

 

[dnmun]

someone did go volt up on a trolling motor in a thread several years ago. since the motor is immersed in water, you would think it could handle more heat. you should be able to get the controller open, but i bet it is gooped in place with the tar they pore on some of these small controllers. that's why i thought buying a 24V brushed controller from Ecrazyman would be the ticket.

 

[dutchlincoln]

...since the motor is immersed in water, you would think it could handle more heat.....

That was my erronous thought as well...
It was on my holiday in Croatia. Had a 24 volt trolling motor, 2 sets of 24v, and everyone speeding around me.
I tried it on 36 volts, and it did go much faster indeed.
About 45 minutes later (and 2 miles from shore), it suddenly stopped...
No paddles with us (stupid, i know), we had to wade our hands in the water, and sort of swim to shore.
At the Tent, i disassembled the motor, and the complete (plastic) brush holders were fully melted away.
I threw away the complete motor, and had 14 days left without a motor..... And without paddles...


Now i can laugh about it. Last summer i couldn't...

 

[dnmun]

yes, that is how i have heard the Heinzman motors die when they get hot from over volting. i find that amazing since all the motors i have seen with brushes all had copper brush holders with a slot down the side for the stranded lead.

 

[dutchlincoln]

Mine had those as well....
As you know, copper conducts heat almost best of all metals, thus melting the plastic holders away...

 

[dontsendbubbamail]

My understanding is that the controller is in the foot with the motor. I opened the head of my 17lb thrust motor and all it had in there was a rotary switch.

Bubba

 

[Jeremy Harris]

My understanding is that the controller is in the foot with the motor. I opened the head of my 17lb thrust motor and all it had in there was a rotary switch.

Bubba

Depends what you define as a "controller". The cheaper models with the rotary switch have a few big resistors in the lower unit (called "power coils" in Minn Kota-speak!) that are switched in or out of circuit with the motor to control speed. They just dump power as heat into the water, rather than pass that power to the motor, so they are pretty inefficient. On full power the motor is fed directly, though, so you can add a cheap brushed motor controller to the full power wire and get speed control with minimal loss of efficiency. On it's own this will usually give a useful range improvement.

If you want to improve trolling motor performance, then the mods shown in this thread seem to make a big difference: http://www.boatdesign.net/forums/boat-design/efficient-electric-boat-27996-17.html#post305142

 

[cal3thousand]

I'm not opening it lol! What if i brake it

I was just wondering if anyone had tried it before.

I'd be more concerned with "breaking" it.




Sorry... couldn't help myself (car forum habits)

 

[DeaninMilwaukee]

The calculation is easy. 2x voltage means 2x amperage, so if you had say 200 watts at 12v you now have 800 watts at 24v.

Assume the original 12v setup was correct with the motor loaded down to 75% of no load rpm, a likely 70% efficiency, and 50 watts of waste heat. The 24v version will try to spin twice as fast, but since the prop load goes up with the cube of rpm increase, ( 8x the power needed to go twice as fast) what will happen is the motor will probably spin 20 or 25% faster, putting the motor into the sub 50% range of no load rpm, reducing motor efficiency to somewhere in the 30 to 40% range. This means you now have around 500 watts waste heat. to go along with your approximate doubling of thrust.

The motor simply will not live long while trying to shed 10x the heat it was designed for.

Dean in Milwaukee

 

[Jeremy Harris]

The calculation is easy. 2x voltage means 2x amperage, so if you had say 200 watts at 12v you now have 800 watts at 24v.

I'm sorry, but that's wildly wrong. Voltage determines motor rpm (because of the motor velocity constant, Kv). Power absorbed by the prop is proportional to the cube of prop rpm, so doubling the voltage doubles the rpm but the motor uses eight times the power. This means that motor current will increase by a factor of 4 if you double the voltage.

It's this cube law relationship between prop rpm and power that really causes problems when people try to increase the voltage to trolling motors like this.

 

[amberwolf]

Only slightly related: Does sufficient overvolting of such motors ever cause problems with cavitation causing destruction of the prop over time? Or do the motors A) not spin that fast and/or B) not last that long?

 

[Jeremy Harris]

Only slightly related: Does sufficient overvolting of such motors ever cause problems with cavitation causing destruction of the prop over time? Or do the motors A) not spin that fast and/or B) not last that long?

It seems to be B) 99% of the time, as these brushed motors don't tolerate higher than design current well. They don't generally have air entrainment prevention plates (often incorrectly called anti-cavitation plates) though, so the chances are that if run a lot faster than normal they would suck air down from the surface, unless mounted deeper. Cavitation might also be possible, but even at three or four times the normal power the blade loading would still be quite modest, I think, as they generally have a fair bit of blade area for their power.

 

[Honk]

Power absorbed by the prop is proportional to the cube of prop rpm, so doubling the voltage doubles the rpm but the motor uses eight times the power.
This means that motor current will increase by a factor of 4 if you double the voltage.

I'm not so sure this is true for all situations or electric outboard designs.
Myself I have built a submerged electric outboard from the Magmotor S28-400 with a 2-blade plastic Motor Guide prop.
The electronic controller was designed to handle both 12V and 24V input, but usually I just go for 12V as it provides enough power.
The Magmotor S28-400 is a brushed motor with a peak efficiency at 84%.

By curiosity I wanted to see how much current it consumed during full blast at 12V vs 24V with freshly charged batteries.
At 12V WOT the outboard consumed 20Amps = 240W from the battery.
At 24V WOT the outboard consumed 50Amps = 1200W from the battery.

My findings is that doubling the voltage only increase motor current by 2.5 meaning 5 times higher power consumption.
This is just a litte bit more than standard resistive power increase by a factor of 4. Far from the 8 times power increase as mentioned.
Perhaps there is some other rules if using a 3-blade prop, possibly made from heavier metal instead of lightweight plastic?

I'm soon about to start building a new outboard with a Slotless Brushless motor at high efficiency (>96%) using a 3-blade Metal Machete prop.
The sinusoidal controller will handle inputs from 9V to 50V so the current consumption at various voltages will be easy to test.
But don't hold your breath, the new build will take a few years to finish due to nice family concerns as small children taking a lot of time

 

[Jeremy Harris]

Power absorbed by the prop is proportional to the cube of prop rpm, so doubling the voltage doubles the rpm but the motor uses eight times the power.
This means that motor current will increase by a factor of 4 if you double the voltage.

I'm not so sure this is true for all situations or electric outboard designs.
Myself I have built a submerged electric outboard from the Magmotor S28-400 with a 2-blade plastic Motor Guide prop.
The electronic controller was designed to handle both 12V and 24V input, but usually I just go for 12V as it provides enough power.
The Magmotor S28-400 is a brushed motor with a peak efficiency at 84%.

By curiosity I wanted to see how much current it consumed during full blast at 12V vs 24V with freshly charged batteries.
At 12V WOT the outboard consumed 20Amps = 240W from the battery.
At 24V WOT the outboard consumed 50Amps = 1200W from the battery.

My findings is that doubling the voltage only increase motor current by 2.5 meaning 5 times higher power consumption.
This is just a litte bit more than standard resistive power increase by a factor of 4. Far from the 8 times power increase as mentioned.
Perhaps there is some other rules if using a 3-blade prop, possibly made from heavier metal instead of lightweight plastic?

I'm soon about to start building a new outboard with a Slotless Brushless motor at high efficiency (>96%) using a 3-blade Metal Machete prop.
The sinusoidal controller will handle inputs from 9V to 50V so the current consumption at various voltages will be easy to test.
But don't hold your breath, the new build will take a few years to finish due to nice family concerns as small children taking a lot of time

The physics I quoted are correct, doubling rpm cubes power. What I suspect happens in practice is that motor resistance comes into play and stops the motor from doubling in rpm when the voltage doubles; if the motor runs slower the prop absorbs less power. In that case, all that's happening is that efficiency is significantly falling off as current increases, as it does with all motors anyway.

All props behave the same way basically, whether made of plastic, wood, metal, carbon fibre or whatever. Even a bit of carved wood stuck on the shaft will follow the cube law power vs rpm characteristic. If the prop has overly flexible blades it can de-pitch with increasing blade loading, but this is the opposite of what's needed for good propulsive efficiency, so although it would reduce the motor torque (and hence current) with increasing rpm, it would very significantly reduce thrust gain with increasing rpm. I don't believe that the typical trolling motor props are flexible enough to do this, as they tend to be pretty stiff. The Kipawa after market trolling motor props might de-pitch slightly, as they use a fairly long swept blade that is cantilevered out a fair way from the hub, and that seems to be a bit more flexible than the standard Minnkota type prop.

The more blades a prop has the less efficient it will be. The downside is that often you need to add blades in order to keep the diameter down and still absorb the power, so lots of motors make a trade between efficiency and practicality. For example, my river boat was running a 13" two blade stainless prop, and at 380 rpm and about 85 watts that would push the 17ft 6", 500 lbs boat along at about 4 mph, our inland waterway speed limit. I've cruised with a friend with a smaller lighter boat, and his Minnkota was using around 200 to 250 watts to do the same speed. The main difference between the two of us was that my prop is around 85% efficient, the standard Minnkota prop is at best about 45% efficient (I suspect it's worse than that, TBH). Most off-the-shelf boat props are pretty inefficient, few are better than around 65%. Few people bother to match the prop to the boat hull resistance curve, either, which adds yet more inefficiency.

The very best thing you can do is pay attention to getting a good match between the prop characteristics and the boat hull resistance characteristic, then ensure you have the right reduction ratio so that you drive the prop at the optimum speed for its diameter and pitch. There is a very good free bit of software for optimising propeller design, I've used it a lot and it gives results that are always within a few percent of being spot on. Its called Javaprop, and needs fiddling with to work with water props, but it is worth getting to grips with, as the pay off is much less power drawn from a battery for a given performance.

 

[Honk]

Hi.

You do sound like you know what you are talking about, thats for sure

Can I ask you if it's possible to deisgn a boat propeller that can be efficient at higher crusing speeds, e.g 15 mph.
I looking to build my new electric outboard for fast crusing speeds to help me get to the sweets fishing spoots in a resonably time frame!
From what I read regarding efficency (there is not much to read actually) the prop should be low pitch to grip the water well, not exeeding 12000ft/min tip speed to avoid cavitating.
The idea is to have both the great handling of a low pitch propeller and still enough RPM to cruise fast when needed.

Do you know if such a propeller can be designed for good efficiency and yet deliver high crusing speed?

 

[Jeremy Harris]

You can design a prop to do pretty much whatever you want, but as with a lot of things there are some compromises that have to be made. In my case I started off (and in the case of the race boat am sticking with) the idea that I wanted the very best efficiency I could get from the whole system, boat hull, prop, motor, controller, battery pack etc. I spent a heck of a long time getting rid of wasted power, in things like bearings, seals, belt reduction drives, motors, controllers and eventually the prop itself. The downsides are that a really efficient prop will be big, have narrow blades (that tend to catch weed) and run at a low rpm (so need a low Kv motor and a reduction drive). You can up the motor rpm to use a smaller motor and get more power, but then you waste more power in the reduction drive. You can reduce the propeller diameter to increase the rpm and so reduce the need for as big a reduction drive but then you lose propeller efficiency. The whole thing is a series of trade-offs, and you can (as I found out) spend a long time wading through it all if you really want to get good performance and efficiency.

Your best plan is to start with the speed and endurance you want, see what the hull resistance at that speed will be for the boat you want to use (there are a couple of free hull modelling programmes, Freeship! and Michlet that will give hull resistance versus speed data, but they are time consuming to use), then work out how big a prop you think you could live with. I can say from personal experience that a big prop is a PITA at times, as it means the outboard leg has to go down a long way in the water, which makes launching in shallow water, or off a gently sloping slip, awkward - you can end up paddling a long way out just to be able to drop the leg and motor away. Once you have an idea of the biggest prop you feel happy with, then go into Javaprop, set it for water in the "options" tab (by using the values for the viscosity and density of water, rather than air) and then start playing with foil section options, required thrust (from the hull resistance data) and speed to see what sort of pitch and rpm you need for best efficiency. It's iterative, in that changing one thing means changing other things to get back to a better result, but after a while you should be able to get a feel for how to change things to get towards a prop that is reasonably efficient and will run at an rpm that's practical.

Then you can start looking at motors that will deliver the power needed (Javaprop will tell you the power the prop will absorb) and start trading with motor size, Kv and reduction ratios. I've found that under-running big motors with a low Kv seems to give better overall efficiency, and eases the reduction drive losses. Running at high voltages with large reduction ratios isn't always the best thing to do, but in you case you may find you have to in order to get the power that you may need. I've found that making toothed belt reduction drives is pretty easy, plus they use off the shelf parts, are fairly quiet and seem very reliable. I'm just putting together another drive so that I can do some better power measurement on my race boat; I reckon it will take me maybe two hours in the workshop to machine up a simple belt drive power head.

 

[Honk]

Thank you for your great help and input.

The thing is!!!
I'm designing the whole motor (8hp) myself from scratch and want to fit its RPM for direct prop drive without reductions gears.
The problem is the propeller, either to use a standard low pitch prop of shelf, or build one of my own.
I can go for higher motor RPM and lesser prop efficiency, or lower motor RPM and better prop efficiency.
What I found was a sweet spot at 125KV where the 3-blade Metal Machete would fit just nicely.
The advantage of a stock low pitch propeller is easy access, great trolling handling, good water grip and controllability.
The disadvantage is perhaps less efficiency. (I have not found any tests of low pitch prop efficiencies, only statements)

Battery power is not a problem as I intend to use 12pcs of 160Ah LiFeYPO4 cells in series for 38.4V.

I don't know much about propeller design and it would probably be difficult to get hold of all the right data for JavaProp.
One way to go is the Torqeedo prop but from my calculations it would consume to much power at high cruising speeds.
Iv'e read tests where the 3-blade Torqeedo 4.0 (5.3hp) is tested against 5hp gasolin motors and the gasolin motors always win in top speed.
The Torqeedo prop provides massive static thrust but the problem is the limited peak RPM. It's rated for 4KW at 1250 RPM and when you
take in account its lower pitch it can easily be calculated that it cannot reach high speeds at 1250 RPM. But it can easily run a heavy boat.
The gasolin prop has smaller dia, less efficiency and thrust but it can screw itself faster through the water due to its high RPM.

I wonder if the Torqeedo prop looses some of its efficient properties if reved to fast? Like 2500rpm instead of 1250rpm, of course at higher powers.
The 1 million dollar question: Will a good low pitch prop, (a prop that grips water with high efficiency), keep its good behavior when spun fast?

 

[Jeremy Harris]

You're pretty unlikely to get anything to work at all well with direct drive, unless you use a pretty large and very low Kv motor, which will be heavy. The Torqeedo motors use outrunners coupled to a high reduction planetary gearbox to get the prop speed down to something reasonably efficient, but even they would work a bit better with a bigger prop and a higher reduction ratio. A slower prop speed and a bigger prop diameter will pretty much always improve efficiency. Reducing the pitch really just makes the prop a low boat speed one, it doesn't really let you run at a higher rpm efficiently. The Minnkotas and other trolling motors run their props way too fast for good efficiency, because they are using direct drive and the lowest Kv motor for the power they need that they can fit into a reasonable size pod. They are forced to use a tiny little prop with a fine pitch to get enough low speed thrust, and consequently they are pretty hopeless at anything over a few miles an hour. I can do some rough numbers in Javaprop to give you a feel for power and efficiency if you let me know the thrust you want, the boat speed, the prop rpm and the prop diameter.

 

[Honk]

Hi.
Sorry for the late reply, I went fishing yesterady. Had to take advantage of the calm sunny day....

Yes, you are right, the motor will be big.
My own motor design with 125KV and 6.93mohm phase resistance weighs 8.5kg (18.7lb)
The entire outboard unit with the controller mounted onto the stering rod comes to 12.7kg (28lb)

I really want to avoid gearing as it introduces a lot of tiresome noise.
The Torqeedo has a high pitch whining sound. My present rig has a gearbox and it can be heard as a buzzing.

But I do have a couple of good quality planetary gears at hand that has 5:1 reduction ratio. Heavy though!
If gears is a must I could fit one of those to my motor to get down to efficient prop rpm's.

Here come some data using the planetary gears i've got, if you feel like helping me
The boat: Linder Sportsman 400 - Aluminum 125kg, planning hull, 4 meters long, width 164cm. Link below
http://www.linder.se/index.cfm?lg=2&x=cShopPub.viewItm&ID=6&bi=shop&pg=1&pr=0&activeIctId=3
Speeds: At least 15mph planning, perhaps more speed if possible.
Motor Power: Access to 6000W = 8hp to run the propeller
Prop RPM after gearbox: approx 800-1200 RPM
Prop Diameter: 35-50cm, you decide

 

[Jeremy Harris]

I think you will need to use a reduction drive of around 3 or 4:1 to get your prop speed down, as a 125 Kv motor and 38V supply is going to give you something like 4500 rpm on load, a bit too high for directly driving a prop efficiently at that speed. The easy way to make a reduction drive is to just use toothed belts, they are pretty quiet, don't cost a fortune and are easy to build a drive with, using pretty much off-the-shelf parts.

Comparing prop efficiency with rpm reveals the benefit of reducing the rpm. For example, if you wanted to directly drive a 350mm diameter prop (which is pretty big - probably bigger than you might want to go practically) then at 15 mph (6.7 m/S) and assuming an input power of 6 kW at 4500 rpm, then for a two blade prop with a 50 mm diameter hub you'd need a pitch of 141 mm, the efficiency would be about 69% and the thrust would be around 622 N (~ 140 lbsf). The blade loading would be OK, in the mid range, tip speed would be fairly high, over 82 m/S, and the chances are you might run into air entrainment problems unless you can immerse the prop to a good depth and fit a big "anti-cavitation" plate.

Dropping the rpm to 1500 at the same power and boat speed and with the same prop diameter and hub diameter gives you a pitch of 355 mm (nearly "square", so close to optimum) and efficiency improves to 78%, with a thrust of 699 N (~157 lbsf), a worthwhile improvement. You'd get about 13% better performance from the lower rpm prop, plus, as the tip speed will have come down to just over 25 m/S the prop tips may be less likely to draw air down from the surface, and prop tip vortices (so the prop wake) will be a lot less energetic.

Yesterday I knocked up a belt reduction drive so I can do some testing with an outrunner driving the drive leg that I used at the weekend for the cordless boat race. I want to try and get some accurate power measurements, something that's hard to do with the cordless drill driving the thing. It took me maybe two hours to make this low power (maybe 1kW maximum) unit with a 2:1 reduction (the motor is 122Kv, running on 18.5 V):

 

[Honk]

The 350mm prop at 1500 RPM you designed in javaprop sounds interesting.

When trying to input the data myself in javaprop I got nowwere near the results you got.
I changed the Kinematic Viscosity to 0.0000001004 m2/s and Density to 1000kg/m3
Please see the attached picture. What am I doing wrong? Perhaps the water data?
Is there a way to see the actual design or is Java Prop just crunching numbers?

A 1500 rpm prop by 5:1 gear reduction would fit my motor just nicely. That is 7500 motor rpm.
I just have to build the motor shorter to increase KV from 125 to 195. I also gain some motor
efficiency as the phase resistance drops from 6.93mohms to 4.87mohms.

How good efficiency is possible by a 3-blade design at 250mm dia, accordingly to Java Prop?

Thanks in advance / Honk