Re: [electricboats] trolling motor lower unit

Hi Luke,  I'm Dave.

I pulled the inboard from my Alberg 30' last year, plugged the stern tube, and substituted some MinnKotas.  We did a couple long runs over the summer to test battery life, speed, and range. I got a few random _used_ 50lb 12V trolling motors for cheap and hung a makeshift bracket off the transom for mounting.  They look like they'll stay submerged like that even in ten footers, even the shorter shaft ones, but the bracket probably needs rethought for the most extreme seas, and for aesthetics. I had another idea not entirely different from what you're describing. I'm very interested in hearing more of your progress.

Here's what I found on the Alberg:

Each 12V 50lb motor actually drew/draws about 50A or so. (~0.6kW or 0.8hp) As advertised by MinnKota.  I ran them full throttle, which they said would be most efficient.

Two 50lb motors pushed that ~10,000lb boat at a couple knots into a modest headwind.  This is my default setup. When the wind and waves both kicked up significantly against us, we did need more motors to get anywhere. However, slow burn appears to be most efficient.  Speed will cost exponentially more.  Surpassing, or even approaching hull speed is, it would seem, not a practical goal, unless you have plenty of reserve.

Four, or preferably even six 50lb motors would have been better (read as safer) for heavy seas and/or high winds IMO. Or if you're in a hurry.  Or in emergencies, etc. I'm trying to think of a way to mount a couple motors up front and maybe be able to rotate them to act like 'bow thrusters' as well.  That seems like it would be handy too.

Group 31 PbSO4 batteries (@60-70lbs ea) give somewhere around 100Ah depending on the draw so, two 50lb motors emptied one in an hour or so.  The first excursion we fed two motors one battery at a time, and this was the average.  Used AGM batteries maybe did marginally better than the _new_ flooded cells on amp-hours.  I expect battery longevity will be yet another variable.  We'll have to see how things age.  I found a guy that sells used AGMs for slightly less than the new flooded units.

As a practical matter, regardless of type, it starts being rather a lot of batteries if you need any distance.  One solution for this is of course to carry a small genset.  Honda makes a little (27lb) 1000W model for about $1k new.  A 2.2kW version is about 50lbs.  They're remarkably quiet.

I ran 18 large deep cycles with a dozen 25W solar panels (or shore power, depending) for charging.  Solar takes a _while_, but has the advantage of it's every day, all day, plus it's still charging on the water, which is nice. Lead charges best slow, BTW.  Quick charge should be avoided if you can. as should deep discharges.  25W panels seem to charge at an amp or two each @12V,  as expected.  The 12V systems are of course much more common, but you can do the 36V with three 12V charging systems if you're careful.  Lead acid is quite forgiving like that.  MinnKota said NOT MORE THAN 13.6V across the 12V motor.  I'd pay attention to their absolute max ratings if I were you.  Especially if you're planning on charging and running simultaneously.  Especially with a big genset.

The weight of all that lead is significant. As are the conductors required for the hundreds of amps I budgeted. I left the mast at the boatyard last year to be sure I carried enough battery. We burned through about half of the 18 cells in ~7.5hrs to get where we were going. Lithium worries me for safety, but it weighs roughly half. 

DO NOT use the motors' wiring as an example for conductor sizing.  As the wire gets long, you need a fire hose, not a straw.  Copper adds significant weight too, and probably not where you want it.  Just a heads up.  Higher voltage helps, naturally.  Twice the voltage means half the current.

The real advantage of having a pile of reserve batteries is of course the creature comforts. Coffee maker, heater, lights, stereo, etc.  That little coffee maker is 900W. I've got 1.2hp of coffee maker!  The stereo is even worse, but it sounds pretty darn good.  :-)  Again, with inverters and standard automotive accessories, the 12V rated bits were a LOT easier to come by, as a practical matter.

I hope this helps, and of course if I've missed something, I'll trust the group to correct me.  I'm an electrical engineer by trade, but by no means an expert on all this stuff.  It really is a lot of basics that a few simple calculations, some experiments and a meter can confirm or refute.  And beware of speculation too!  I had no shortage of nay-sayers going in, but the reality of making the 15 mile trek, literally on solar power (albeit stored solar power,) seemed to shush the peanut gallery nicely. 

There was no arguing the setup made it, and with plenty of reserve.  We could have probably blasted the stereo the whole way, but that would have been reckless before we had some solid numbers with those particular batteries, those motors, that boat, those winds, etc.  We're still not very far into the data collection phase of course, but that's how these things go.

Best of luck, and I hope to read more of your adventures this year!

Dave

Aquila

1965 Alberg A30 Hull #47

On Monday, January 3, 2022, 8:50:57 AM CST, Luke Johnson <jukelohnson@gmail.com> wrote:

Hi all,

I'm very happy to have found this group packed with such great people with so much valuable knowledge!

I am a fairly new owner to a '79 Catalina 30 with an old Atomic 4 and I'd like to go electric.
For context - I sail on Georgian Bay, Canada.  My goal is to explore the bay in a conservative, comfortable family cruising sort of way.

I have this crazy idea of mounting 4 or so 101 lb Minn Kota lower unit pods on the bottom of the boat.  Here's the link to the pod:
https://www.ebay.ca/itm/272728816739?epid=1953153684&hash=item3f7fe77c63:g:MpEAAOSwU~9hJQI5

Looking up the specs on the Minn Kota 101 lb model, it appears to draw 50 amps at 36 volts max, which is 1800 watts.
I'm thinking of using 4 of them for a 5.4 kw system and 400 lbs of thrust.  This seems to pass the two 'rule of thumbs' I've seen:
- Minn Kota site: 2 lbs thrust for every 100 lbs displacement
- - (10200 boat weight/100)x2 = 255 - - this system would have 404lbs (across 4 props)
- 'electric boat' rule of thumb of 1 kw / 2 tons

For mounting, I'm thinking I could drill a hole for the wires, and on the outside of the hull fibreglass in a threaded pipe that the pod would screw on to.  Probably mounted beside the keel to avoid haul in / haul out straps catching them - and to be close to the boat's pivot point to help with maneuvering.

The possible benefits I see are:
- no noise or space in the cabin
- more efficient than a single motor due to more prop contact with water - ie - more range vs a 1 prop solution?
- cooling would not be an issue
- would push the boat at least 5 knots 
- would be possible to make regen work
- would allow excellent maneuvering control in tight marina spaces
- excellent redundancy
- simple, cheap motor controls
- all components are max 50 amps
- easily available parts / replacements

Possible drawbacks:
- would need occasional brush maintenance - pods could be inspected when boat is pulled each winter
- lots of drag.  Not a bad thing if regen could be made to work.
- ??
 
I would truly appreciate any input anyone has about why this would be a terrible idea so that I can get it out of my head haha!

Thanks!