Trolling motor speed control aka PWM

[IslandSpirit]

For a small sailboat, sometimes electric power is used to maneuver from dock to a point where the sails can be hoisted and vice versa. To fill this need I purchased a Minn Kota Endura 30 for my 14' LOD Skipjack.

Unfortunately the Endura line doesn't include the "maximizer" circuitry from Minn Kota which is effectively a Pulse Width Modulation speed control (PWM). A PWM accomplishes two things: 1) maintains motor torque at any speed setting which results in propeller rotation 2) only delivers usable power to the motor without any waste power going to "speed coils" which are effectively resistors in the motor designed to carry a portion of the power and leave the remaining power go to the motor.

It is this portion of power carried by the speed coils which is dissipated as useless heat and of course battery drain.

What a PWM does is deliver a train of DC pulses to the motor and vary the on/off duty cycle of those pulses. Because motors pulsed in such a way have the full voltage delivered to them for a percentage of the overall cycle, but result in current built up to an average level based on the duty cycle, only usable power is delivered to the motor during the on cycle with residual current decaying on the motor as usable power until the next on cycle. No power is wasted in the form of heat (or for the purists, no SIGNIFICANT power is lost), because to use a PWM, you essentially set the motor speed handle at full on and control the motor speed with the PWM.

Minn Kota claims their maximizer circuitry extends battery life by up to 5x longer on a single charge. Couple this with solar panel recharging of your battery and you always have a motor in reserve albeit at a low cost.

I did a search on Google and found this little gem from Virtual Village in Shanghai, China: http://cgi.ebay.com/12V-30A-DC-Motor-Sp ... 4005r33189

This item is a 12V 30A PWM speed control complete for around $35 plus $20 shipping FedEx from Shanghai

It arrived in 3 business days! Schematic on the box for hookup is self explanatory, but the Yellow lead on output is the motor POSITIVE.

I built one of these years ago which worked quite well but has since been relegated to control the trains on a train set which is in storage.

Works very well to control trolling motor speed down to ZERO while maintaining driving torque and can run up to full speed in either forward or reverse. You use the full forward or full reverse setting on the handle of the motor, and use the dial on this control to vary the speed to what you need.

[upspirate]

Interesting....this may be a good post to put under the electric topic too

[razopp]

Island,

Very interesting post on electric auxillary power. Much of what you said went WAY over my head (I am an accountant), but I have thought of using a smaller trolling motor in lieu of an outboard. It sounds like you did a lot of testing of this motor. Is there a way to equate the torque and RPM to horsepower equivalent.

The question I have tried to answer is how large an electric motor I would need to push a Minuet (1100 lb displacement) in my local lake. I have no current to deal with, but want the motor to be able to push into the wind and waves for a couple of miles in case I get stuck out in a storm.

Any ideas and thanks for the post. Also, what is the total cost of this set-up? I have seen the Endura motors at walmart for $100-120 and it sounds like you have $55 in the controller and shipping (plus battery).

Robert

[IslandSpirit]

I don't have a correlation between thrust and horsepower, but I do know that Minn Kota has a motor chooser based on what kind of boat, displacement, use, etc.

I've used a Minn Kota 10 before for a canoe/kayak and sold it to my buddy a few years back. He now uses it to troll his 18 foot pontoon.

As far as cost, I've got $120 into the motor (Cabela's), $55 into the speed control, and I haven't bought the battery yet but it looks like an Optima blue top is about $160. However the battery will be running my nav lights, tiller pilot and GPS, lights for my compass, and I'll have a solar panel and charge controller to keep things charged while I'm out on the water.

[Cranky Badger]

If it delivers a pulsed voltage, it ought to work as a dimmer for LEDs as well, no ?

I'm thinking for cabin and courtesy lighting on a (future) Lord Nelson....
I was planning to use the BlueSea LED dimmers but they're pretty pricey. For comparison, a BlueSea 20A module is around 150 bones, but the 5A are about $90 (then add the DT momentary switch for another 20...). With the unit you've got (given the price and country of origin ;^) would you de-rate it a bit to provide better reliability or does it seem pretty robust ?

From the pic, it looks to me like it wouldn't be a problem to lengthen the pigtail on the pot...you figure there would be any issues with that ?

[IslandSpirit]

First answer - you can definitely use it to dim cabin lights, leds, etc (I have an electrical engineering background and these work great for that - you're applying the full voltage so the light has a wider range of dimming, you're just choosing how long that voltage is applied and thus controlling the current.

As far as robust - I know what goes into these units and IMHO there is no way that a 20A module should cost anywhere near what you indicated above. The most expensive part is the aluminum extrusion that constitutes the housing.

Mine is rated at 30A and I've tested it with the trolling motor running dry. Results as expected.

BTW, after I bought the first one I got a coupon for 20% off anything from the same merchant so I bought a second as a spare. Got it in 3 days again. I'll likely never need the spare or as you have indicated I can use it to control the power to something else. Definitely well built and works great.

[Cranky Badger]

Thanks for the info. Based on it, I think I'll order one and try it out. 30A is overkill, but in the marine world that's just called 'reliability'

I've got a pontoon boat (potential production prototype) on the go and I want to use 2 engine-mounted MinnKotas as the primary drive. Unfortunately, their modules don't allow you to combine a single user input to control both motors, which makes them difficult to rig with a conventional throttle control layout. I emailed their tech support for suggestions but I haven't heard back. These units look like I could rig a single actuator to operate both pots, and if I can adjust the pot's attachment, I can synchronize the two motors.

As for the price quote - I agree it's a bit nuts. I was looking at a BlueSea 7505 module and an 8234 switch. MSRP is 146 and 20 respectively. While my cost is considerably less, I still can't get anywhere near those modules. I can understand the switch price because their switch contacts never seem to wear out and the actuator can be replaced again and again and they never seem to get loose or wobbly, but I think the price for the module is out to lunch.

From what I can tell, the rotary switch on the Chinese unit is a pot so the module needs a varying voltage as a signal. The BlueSea unit uses a dual-throw momentary switch so its input is constant voltage, either 'increase' or 'decrease', not 'set the output to X' . Since it's manufactured as an IC, that difference still doesn't justify the cost IMO. That said, I have nowhere near your electronics background.

[IslandSpirit]

The rotary control is a pot. You could extend it back to the helm through a shielded cable to cut down on RFI noise that might make the motor control "jumpy".

As far as 30A I wouldn't call it overkill. I am pretty certain the motor load increases drastically as you start wrapping weeds around the prop or hit something that stops the rotation completely.

I haven't opened one of these up yet to see if it's dynamically current limited at 30A, fused or if you should put a 30A fuse inline with the load.

I may also rig a bypass switch so that I can bypass the output of the PWM drive and connect the motor directly to the battery through a fuse. The alternative would be for me to install both PWM drives and a toggle switch that connects the motor load to either one but that might more applicable in a dual battery setup. I don't plan on dual batteries because I will be installing a solar panel and charge controller to charge the main battery while underway, towing, etc. Sun energy is free and I've already solar powered a shed with the same setup - works great!

[Cranky Badger]

Hehe...I meant 'overkill' with respect to LED lighting - I'm figuring around 5A per zone so I'll guage the wire for double that and fuse it down to just above the actual draw. I tend to overbuild my stuff anyway cause you can't exactly pull a boat over to the side of the road.
I totally agree with you about a potential 30A load being a perfectly reasonable operating limit for a trolling motor. If I go with the MinnKotas on the pontoon boat I can have as much as 202 lbs of thrust per (double) unit. I'm guessing that if they're loaded they'll draw even more than that, though I haven't even put the ammeter on my current (38lb Duramax) motor yet.

Thanks for mentioning the shielded cable, I never thought of that.

I'm figuring on solar as well (on the pontoon's hard bimini), though in my case it'll just be a float charge because I'm going to have some pretty hefty amp hours. Are you buying a charge controller for your panels or are you going to make up your own ?

Last OT question (hehe): How do you like your SuperSkip ? I have the Vac plans and still poke around the byyb forum once in a while. Check out Jim Saunders' Vac...it's impressive. Enough about non-Glen-L's though !

Actually, while we're on the topic of charging, I have another thought I'd like to run by your electronic engineer's brain if you don't mind....
Are you familiar with the taffrail logs that were used to record distance before GPS ? It functions like a paddlewheel speed sensor using a body that's fixed in the water with a propeller aft.This 'fish' is towed behind the boat by a cable like a speedometer cable connected to a mechanical counter that keeps track of the miles sailed. What if one were to tow (for example) the head of a trolling motor in such a way that the prop spins but not the body ?
This would generate a voltage up the line, but I have no clue how to do the math on the drag caused vs the power generated to find out if it would be feasible. Obviously one would be looking for a purposely-designed unit to maximize efficiency for a given drag, but the lack of units already on the market says that either it's not feasible or I should be looking at getting a patent...quickly.

[IslandSpirit]

Every transducer is by virtue of what it is bi-directional - that being, a motor with a propeller with power applied generates forward (or reverse movement of the thing it is attached to). By the same token an electric motor dragged through the water with a propeller on it will generate voltage, though it's not optimized for that and will likely not produce enough voltage or current to put power back in the battery to a reasonable service level. You would have to couple the motor to the battery through a diode to prevent the battery from trying to power the motor while you're trying to charge the battery.

If however you are looking to keep track of distance, the motor will only generate an instantaneous voltage proportional to the instantaneous speed of the boat dragging it at any given point in time - it is therefore a tachometer measuring speed through water (not speed over ground, which is what we're concerned with for navigation).

To measure distance, a tachometer (i.e. speed transducer) will need to be integrated to measure distance. Electronic integrators are by virtue of their construction unstable - that is, they drift until they reach saturation at which point the distance measurement is over.

An inexpensive GPS unit like the first gen ones that came out after the first Gulf War make a simple job of tracking distance. For a couple hundred bucks more you can get one with map capability, track measurement and a whole lot more.

BTW, I am quite happy now with the progress of the Super Skipjack. Today I filled all the exterior screw head countersinks, put a first coat of epoxy on the exterior and glassed the chines, side/transom joints, and the hull-bottom/transom joint and installed the 3rd of 4 rubrails. After I get the last rubrail on tomorrow I'll proceed to fairing the keel, filleting the keel/hull bottom joint and glassing the same as well as the keel bottom to make it more durable during beaching, launching, etc.

UPS showed up today with my custom cast bronze Kranze iron (from Port Townsend Foundry) for the bowsprit - a perfect fit, and man is that polished bronze gorgeous! Between that, and the bowsprit shroud chainplates (salvaged off an older gaff schooner, also polished bronze, and the polished bronze bow line chocks, polished copper bowsprit heel socket and polished bronze Herreshoff-style cleats, she'll be a beauty.

As their ad says "let us give your girl some jewelry". Wow!

At that point, she's ready to turn over and work on the cockpit seating. Wife went up north this weekend with her folks and told me to work on the boat all weekend without the honey-do list!

Who can ask for more?

[Cranky Badger]

or reverse movement of the thing it is attached to...

gotcha, same reason helicopters have tail rotors and helm seats are to starboard.

though it's not optimized for that

That's my thought, yet most aircraft use a starter/generator that turns the engine over and once it matches engine RPM begins to provide usable current. I'm using a trolling motor head as an example, perhaps a sealed alternator would have been a better analogy, though I'm not sure how you'd vent it. The F4U Corsair had and offset vertical stabilizer to counteract the prop wash over the rudder, so I guess a canted skeg might work as well. Even though it would have to be set for a particular speed, this would be simple on a sailboat.

it is therefore a tachometer measuring speed through water

With a fixed pitch prop, a particular voltage would provide the necessary time component to calculate distance per revolution, i.e the time it take for the prop to travel through its pitch (1 revolution).
As it is, I'm thinking of basically a 'wet' wind turbine. Preventing the counter-rotation without increasing drag is the sticky wicket at the moment.

(not speed over ground, which is what we're concerned with for navigation)

From what I've read (never used one) they were compared with sextant sights to figure out SOG. Quite right though, current and leeward drift would throw it out to lunch, although (average) currents are pretty well documented.

until they reach saturation

S'cuse my dumb@ss, but what's saturation ?

An inexpensive GPS unit

For nav, definitely but I'm thinking of a redundant, passive unit that will trickle charge a start battery, in the event that everything else has gone sideways in the middle of the Pacific. Apparently they make pretty good lures in bluewater though...

As their ad says "let us give your girl some jewelry". Wow!

You know that means pics right ? I'm sure there's a few gaffers on here with sprits....

told me to work on the boat all weekend without the honey-do list!

Hehe...she's a keeper for sure ! I found one who actually LIKES sanding - or so she says. Either way....


Good to hear the SS is plugging along. Being a new design, I'll be interested to hear how she sails (I wasn't sure if your avatar was your own boat or not)

[Nova SS]

Man this is an informative thread. From what I remember from my days in the Navy, as a Marine Electrician, an electric motor and a generator are largely the same thing. It just depends on how you drive it that will determine if its a motor or a generator.

[IslandSpirit]

That's my thought, yet most aircraft use a starter/generator that turns the engine over and once it matches engine RPM begins to provide usable current. I'm using a trolling motor head as an example, perhaps a sealed alternator would have been a better analogy, though I'm not sure how you'd vent it. The F4U Corsair had and offset vertical stabilizer to counteract the prop wash over the rudder, so I guess a canted skeg might work as well. Even though it would have to be set for a particular speed, this would be simple on a sailboat.

It will work. Question is how much will you get back vs. how many knots will you give up in drag in exchange for that. All one has to do is take a computer fan with those led lights built in (removed from the computer of course), and hit it with compressed air in the right direction - those LEDs will light up as bright as if they had been powered by the motherboard. However keep in mind it only takes 20 milliamps to run an LED. You will get usable current to charge a battery, diode connected as i indicated. Question is how much - you need a fairly good amount of speed.

With a fixed pitch prop, a particular voltage would provide the necessary time component to calculate distance per revolution, i.e the time it take for the prop to travel through its pitch (1 revolution).
As it is, I'm thinking of basically a 'wet' wind turbine. Preventing the counter-rotation without increasing drag is the sticky wicket at the moment.

It will provide a voltage proportional to the rotational speed. This voltage needs to be integrated to get distance.

Remember first derivative of distance is speed (rate of change of distance with respect to time), second derivative is accelleration (rate of change of speed with respect to time), but I digress to Calculus 1 principles. To get speed from accelleration you need to integrate (first integral), to get distance from speed, integrate again (second integral).

From what I've read (never used one) they were compared with sextant sights to figure out SOG. Quite right though, current and leeward drift would throw it out to lunch, although (average) currents are pretty well documented.

You would need to take 3 sextant readings - the first two to get a running fix of the position of the second reading, and the next one to get a running fix of the position of the third reading from the second - this would give you two lat/long points and if you dragged one of these through water you would get a crude average reading of speed over those two lat/long coordinates.

However for a sextant reading keep in mind that the ground position (GP) that is the position directly under the sun for which no shadow is cast moves a mile every 4 seconds. To get an accurate reading from a sextant through either manual or calculated sight reduction you need something with time accuracy down to the second and a relatively stable time (i.e. low drift) over the course you plan to voyage. It is why when you are taking sextant readings you take your reading and then immediately observe the sweep second hand, and then write down the local time in hh:mm:ss in that order. If you don't then your sight reduction and your running fixes will suffer by potentially many nautical miles, which is worse if your running fix readings are too close together. I can't get into the details of why because it took me months to understand spherical trigonometry upon which sextant nav is based but basically if you are cutting a slice of pie and your two cuts are close to a right angle, the point at which they intersect is very easy to figure out and you can slightly change the position of the second cut without moving the apex of that triangle of any significance.

However as you narrow the angle of the two slices the certainty of the position of the apex can vary greatly (this is what happens when your running fixes are close together).

Bottom line, If I wanted to know my speed, sans electronic navigation, I would compute a running fix using my sailor's chronograph (i.e. high-accuracy quartz wristwatch ) and then when I got a running fix for the second position I would simply calculate my distance from the two coordinates, and the time it took me to get there which is an average.

If I'm that far away from land that I need to use a sextant as a backup to my GPS, then I'm really only concerned about 1) whether I am tracking to my destination and 2) my average daily speed which gives me an indication as to when I'll reach my destination.

until they reach saturation

S'cuse my dumb@ss, but what's saturation ?

It's when an electronic integrator (used to get distance from speed), no longer integrates because it's reached its maximum output. Your odometer on your car is a mechanical integrator. Another word for integrator is accumulator. They have a definite range before they're maxed out, rollover, etc. Once that is reached you are no longer integrating.

An inexpensive GPS unit

For nav, definitely but I'm thinking of a redundant, passive unit that will trickle charge a start battery, in the event that everything else has gone sideways in the middle of the Pacific. Apparently they make pretty good lures in bluewater though...

You could use it for that with the above caveats. I'm not sure I want something on the end of the line that is big enough to consider a trolling motor head "lunch"!!

As their ad says "let us give your girl some jewelry". Wow!

You know that means pics right ? I'm sure there's a few gaffers on here with sprits....

Pics will be forthcoming

Good to hear the SS is plugging along. Being a new design, I'll be interested to hear how she sails (I wasn't sure if your avatar was your own boat or not)

It's not my own boat but a representation from a very general sense of what she'll look like.

It will be replaced with the real girl when she's ready

[IslandSpirit]

Here is what you are talking about: http://www.swanseaheritage.net/article/ ... CLE_ID=350

The integrator (i.e. accumulator) in this picture is the log itself which is calibrated to measure distance. Calibration of the log is based on the pitch of the spinner. Since the log itself rolls over (one complete revolution), you would have to make your measurements over a short period of time.

Using your trolling motor analogy, the voltage generated by the prop spin would have to be used to drive a dial that rotates in a similar fashion (similar to the watt-hour meter on your house - also an integrator). The faster you go the faster the dial accumulates.

If you're really looking for a distance measurement device like this you can google for it using the term "buy taffrail log"

[marwan.abdelnaby]

Dear IslandSpirit,

I have just purchased a trolling motor (conventional ) without maximizer,
and I have built my own PWM control circuit, but when I opened the upper part
of the trolling motor ,I found 4 terminals (black, yellow, red and blue).

I don't know how to connect it to my 2 terminals PWM control circuit.

I am waiting for your response

Thanks in advance
Marwan Abdelnaby