Arctic Tern Electric Power

[mcdenny]

Electric power makes a lot of sense for a sailboat auxiliary as generally both speed and range under the 'iron (lead?) genny' needs are modest. I did a lot of experiments with a trolling motor, a car battery and a canoe last fall to see if I could make a compact, low cost unit that would push my planned Arctic Tern about 4 mph for at least 10 miles.

I wanted to be able to mount the motor in a well and store it out of the water but inside the well when sailing; an aux drive that is silent and out of sight all the time.

The canoe results looked promising so here's what I did with the AT:

First the "bottom line" - a speed vs. range curve documented with a "Watts Up" meter and a Garmin Oregon 400 GPS. The 4 mph data point is averaged over 20 minutes each way and the others over 90 seconds. Measurements taken in a little inland lake in a dead calm. Mainmast up, CB up, rudder fully down, just me (170#) in the boat.



My canoe experiments showed an APC 10 x 6 prop was a good bit more efficient than the standard prop than came with the MK Endura 36. (it is also fragile and not weedless but only costs $3 so spares are cheap). An NACA 0025 fairing reduced the drag of the round motor shaft considerably and made for a stronger mount of the motor head to the well plug.





The standard (in low cost motors) resistor speed control wastes a lot of power so I used a PWM controller meant for an RC model car. (You need a car unit ot get reverse). It's the little green thing taped to the grey box.



The "throttle" is a servo tester also from the hobby industry. The pic shows a 12v outlet, grey box with motor on-off switch and throttle knob, and GPS.



The motor has plenty of power, 4 mph takes 19 amps, and WOT is 4.7 mph at about 35 amps. It easily drove us into the wind (but not very fast) on the 15-20 day. I did no experiments with motor sailing but expect spectacular upwind performance in lightish air and 5 - 10 amps of 'boost'.

The battery is an 80 ah AGM type weigning 53# and fits behind the foremast well and the front bulkhead. There is a built in 6 amp charger mounted next to the battery. You use an 110v extension cord in the front hatch to recharge.

[Hwyl]

Does the prop freewheel when you sail, did you consider having it retractable. Looks like a great solution.

Edit, am I seeing wrong. is it retractable?

[mcdenny]

Gareth, The motor is attached to a plug that drops in the well and is held down with a spring that kinks into place like the ones that hold up hatches. There is a plain plug for sailing. The wires are long enough to lay the motor on deck while putting in the sailing plug, then the motor fits back into the well for stroage

Sailing plug installed


Sailing plug


Motor installed


fish eye view of motor installed


Motor stowed


It's pretty quick to switch the motor for the sailing plug; certainly faster than unstowing a regular trolling motor than clamping it on. There are no threaded fasteners involved. It would be even faster if I had not made the well such a tight fit to the motor.

[Hwyl]

Thanks, it was the tight fit that fooled me. it looked impossibly tight. Do you get water in the well when you change under way.

[StevenBauer]

That is fantastic Denny. How long does it take to charge? Overnight? Will you be making plans available? With part numbers? How about total costs?



Steven

[Sailor]

TRES TRES cool!!!

[Robert W. Long]

I too am eager to know if your well fills with water when sailing withour the motor. My Caledonia Yawl well fill with the motor removed and the plug installed.

[mcdenny]

The well has a few inches of water in it all the time as the plugs are not water tight. They are tight enough, however, that the motion of the boat through the water does not force more water into the well.

The 6 amp charger could recharge a completely dead battery in about 15 hours. Recharging from a 3 or 4 mile 4 mph run would take about 4 hours. A nice feature of having the propulsion battery is it provides virtually unlimited run time for the GPS even in the "screen always on" mode. It also provides power for a bilge pump.

[mcdenny]

Steve:

From cabellas
MK Endura 36 $120
GP 24 AGM battery plus Prosport 6 charger $240

From Tower Hobby
Castle Controls Mamba Max $136 (less expensive Electronic Speed Controls would work fine)
Astro Flight servo tester $25
APC 10x6 pusher prop $4
Goldberg 3" spinner $7

From Jamestown Dist
50a circuit breaker $17
Plus wire, terminals, misc $50

Total: $600

Another option would be to use a trolling motor with a PWM controller like a Motorguide Varimax 40 ($160) or a MK Maxim 50 ($250 reconditioned). These come with an efficient speed control so you would not need the ESC or servo tester, but would make a somewhat bulkier package.

I already had the MK Endura 36 and the Castle ESC and servo tester left over from another project. If I were really starting from scratch I would probably have bought the MG $160 motor and used its built in controller and throttle. That would knock $120 off the package price and be simpler to wire.

[Duncan Gibbs]

Extremely impressed Denny! A gorgeous vessel as well! Well done and thanks for sharing this with us!

[Soundman67]

already an awesome setup but can it be made to charge the battery if you sail with the prop down?

[mcdenny]

Regeneration under sail is an elusive goal, definitely not possible with my set-up.

Electric drive manufacturers often promote regeneration because it is theoretically possible and seems just common sense. Actual people on actual boats with actual ammeters, not so much. People from the Yahoo electric boat forum with 25'-35' sailboats report only very small amounts of charging when sailing. The prop and drive ratio for propulsion is not what you would want to spin a generator and a small sailboat just goes too slow..

[htom]

This needs to be worked up into an article.

[kenjamin]

Thanks Denny, great information. Good to see that you've finished your daughter's Artic Tern and enjoying the design of its electric drive.

[BBSebens]

Very slick. I really like the interchangeable plugs... very handy. I like that you made it what it needed to be for you. nicely done.

[Phil Y]

Most excellent mate I'm thinking maybe a solar panel would be a nice addition-recharge while sailing, anchored, or just sitting on the trailer during the week. And of course you could add an interchangeable prop-big turbine blades, and a mount on deck, upside downt, so that when the wind is up for sailing you can put the motor to use as a wind turbine. Now I think we have an article for Popular mechanics;0

[vincentc]

Denny,
Thanks for a very interesting post. I am in the process of incorporating a 45# trolling motor into the rudder for an Oday Daysailer hull.

How do you mount the model airplane prop to the trolling motor?
Thanks

[mcdenny]

The MK motor I used had a 3/8 shaft but the props are either 5/16 or a close metric dimension. I just bored out the prop with a 3/8 drill and then used a dremel tool with the little ball rasp bit to machine a slot for the MK's shear pin. Loads are light and speeds are slow so precision machining is not critical.

Forward on the motor will produce reverse thrust with a normal "puller" prop. You can run the motor in reverse or use a "pusher" prop. Pusher is better as the blade foil will be pointing the right way but not so many sizes are available. I found a 10x6 pusher to work the best.

Buy extra props as they are somewhat fragile and they only cost about $3 each. Get a couple of extra prop nuts (3/8-20 nyloc IIRC, maybe 3/8-16) too as they will leap into the water when you are not looking.

[Thorne]

How does it work when you run aground unexpectedly or hit a snag? I take it that the boat's DB/CB is enough to protect the motor unit until you can pull it up? Looks like getting the prop oriented for removal might take a bit of extra time...

Looks like you've a lot of great research on motors, batteries and control setups - thanks for sharing it!

The slickest setup for thin water I've seen is a swing-up rudder with the motor mounted in the aft edge. The boat can be sailed with the rudder kicked mostly up and motor out of the water, or with the rudder fully down and prop spinning -




The other swing-down option I've only seen photo of -

[mcdenny]

Thorne, yes the rudder and CB do hit bottom a long time before the prop would but you could conceiveably hit something or run it aground with the foils up when beaching. I haven't used the boat that much but so far no problem. I did wrap the prop up in enough weeds to stall the motor (doesn't take much) so rowed out.

I do have to orient the prop to get it up through the narrow well but that's easy as there is room to reach your hand in to turn it. I thought this was an advantage as the well can be quite narrow, only about 4" or 5" wide.

An important goal of mine was to have the motor hidden whether stowed on in use. The drop down unit in your picture would work great on a transom sterned boat if you didn't care if it showed.

The motor built into the actual rudder blade is a good option too, and would give more steering authority when turning at slow speed. I chose not to do that to avoid the extra drag when sailing.

[openboater]

Denny, the shaft on your motor appears to be cut off with very little sticking above the plug. If water does enter the well, the top of the shaft does not look tall enough to be kept dry. I have read that minnkota's should keep the shaft hole open so what tiny bits of moisture do get in the motor casing can evaporate and have a way to escape. If you sealed the shaft the moisture has no way out.

Have you addressed the moisture issue, or have you found it not to be an issue ?

[mcdenny]

I filled the top of the shaft with epoxy. So far (about 9 months) no problems.

[yzer]

Excellent post and an innovative project. I loved the adaptation of RC equipment.

[Ann Noble's Dad]

Denny, I really like your idea and believe a motor like that would really help me fish from my Sooty Tern. Would you do anything different now that you have had it a while? Thanks!

[mcdenny]

Denny, I really like your idea and believe a motor like that would really help me fish from my Sooty Tern. Would you do anything different now that you have had it a while? Thanks!

Hi Anns's Dad, Everything still works fine. I gave the boat to my daughter who lives is Alabama so I don't use it but her husband says it works great.

That said I'd make the well a tad bigger to make it easier to get the motor in and out. Its easy enough in the shop but a little fussier with cold water and waves added to the equation.

If you are starting from scratch I'd get a PWM speed control motor and skip the RC parts I used. I already had an old trolling motor and the RC parts in my "junk drawer" so thats what I used.

[Ann Noble's Dad]

Denny, Thanks for the update. I am starting from scratch and will do as you say.

[JimD]

bump

[PeterSibley]

So about 2 hours to a 20% remaining charge ? Am I reading the graph correctly ? Between 19 and 35 amps ... that last little bit is expensive ! A very neat set up.

[QUOTE=mcdenny;2661587]Electric power makes a lot of sense for a sailboat auxiliary as generally both speed and range under the 'iron (lead?) genny' needs are modest. I did a lot of experiments with a trolling motor, a car battery and a canoe last fall to see if I could make a compact, low cost unit that would push my planned Arctic Tern about 4 mph for at least 10 miles.

I wanted to be able to mount the motor in a well and store it out of the water but inside the well when sailing; an aux drive that is silent and out of sight all the time.

The canoe results looked promising so here's what I did with the AT:

First the "bottom line" - a speed vs. range curve documented with a "Watts Up" meter and a Garmin Oregon 400 GPS. The 4 mph data point is averaged over 20 minutes each way and the others over 90 seconds. Measurements taken in a little inland lake in a dead calm. Mainmast up, CB up, rudder fully down, just me (170#) in the boat.

[bluedog225]

Great project. Thanks for sharing.

[mcdenny]

So about 2 hours to a 20% remaining charge ? Am I reading the graph correctly ? Between 19 and 35 amps ... that last little bit is expensive ! A very neat set up.

Hi Peter, You are reading the graph right. The range, measured on either distance or time, varies a lot with speed. My target was 10 miles at 4 mph (2.5 hours). Slow down to 3 mph and you can go 18 miles in 6 hours.

I used the boat for one summer then gave it to my daughter. She and her husband use it occasionally on a small lake in S Alabama. As far as I know they have never motored more than a few minutes to get away from the launch ramp and raise sail. I never used it for more than perhaps ten minutes either.

Because batteries store so little energy range is the holy grail for an electric setup but for a little day sailor a half hour of range would cover most peoples needs 99% of the time. Having a decent size battery aboard brings other advantages like running the gps all the time and an electric bilge pump.

[kbowen]

Does anyone know if the "speed coils" on Minnkota motors are actually resistors or are they additional field coils to suppress the rpm? My experience is that the amp load doesn't change much between slow and fast and my intuition tells me it should. But then, Einstein said that "intuition is a varnish of prejudice......" Or something similar....

[willmarsh3]

The speed coils are resisters. Its a very cheap but wasteful solution to controlling speed.

BTW Danny I'm impressed with your setup.

[mcdenny]

Yes the lower speeds put a resistor in parallel with the motor. As the speed drops the load on the prop lessens so the current doesn't go up.

If I recall speed 5 puts all 12 v into the motor and speed 4 cuts that to 6 volts.

[mcdenny]

Since this thread just popped up after a couple of years I thought I'd add: I took the boat out last week so now the electronics are four years old. The motor still worked fine. The battery seemed dead but after two days on the charger it came up to 12.6v. I used the motor for 10-15 minutes, no issues.

[suzyj]

The other swing-down option I've only seen photo of -

Me like!