Please select your home edition
Edition
Selden

Electric dinghy engine update

by Barb Peck & Bjarne Hansen 20 Jul 19:51 UTC
Seven years with electric outboard © Barb Peck & Bjarne Hansen

It's been a few years since we reported on our new electric dinghy engine (December 2012 Currents). We have updates to share and some observations about a new battery technology, but first a recap in case you aren't able to immediately get your hands on that valuable back-issue.

We bought 'Stealth', our MinnKota 55/S electric outboard in May 2011, and used it with our inflatable tender while cruising around Vancouver Island and the Gulf Islands. It accompanied us to Mexico in 2015, where we use it for snorkeling, bird-watching, mangrove-exploring, and general transportation.

Stealth has held up fine over the past eight years. It still runs smoothly in forward and reverse, and the moving parts (tilt, steering, propeller depth, transom clamps, and handle extension) haven't broken and function well. There are no signs of corrosion, though the white paint on the lower drive unit has blistered and is flaking off the cast aluminum housing. However, this is just a cosmetic problem. On the cosmetic topic, two years ago we spray-painted portions of the engine with a yellow acrylic; this was done purely for identification and as a theft-deterrent measure. We don't know if that was necessary, but our engine hasn't gone missing yet.

We'll just make a short statement about maintenance costs for our electric engine since 2011: none. No tune-ups, no spark-plug changes, no oil and filter replacement... nothing. We merely rinse off the motor with fresh water a few times per year before storage, and we've spread a bit of Lanocote (lubricant and corrosion preventer) on the threads of the transom screws. We initially had doubts about the durability of the plastic propeller, but so far it has held up fine, despite running it into sand and even some gravel a few times. The edges are slightly dulled, but it still pushes us around with minimal vibration.

Powering the Engine

Two sealed lead-acid (SLA) batteries of approx 35 Amp-Hours that we bought in 2011 have worked hard: discharges below 20% capacity, overcharging (before we obtained a proper regulator) and 6+ months sitting untended during the off-seasons. Despite this, they have survived decently. One battery seems as though it still has about 2/3 of its as-new capacity; the second perhaps 1/3. In November 2018, we decided to replace the weaker of the two batteries, so after a lot of research we spent the extra bucks for a lithium battery. The model we chose is the Smart Battery SB35.

You can see from the photo that the old and new batteries have the same dimensions, black rugged plastic housing, and screw terminals on top. There used to be a label on Lilith (our name for the lithium battery), but we removed it to avoid drawing extra attention. The Smart Battery line of lithium batteries are designed to be drop-in replacements for lead-acid batteries. Inside, however, they are quite different from lead-acids. In addition to lithium cells, the SB35 contains a charge-monitoring circuit that prevents over-charging, over-discharging, or too high of a current. The most striking difference isn't seen in the photo: Lilith, at 10 lbs, is less than 1/2 the weight of the SLA, and effortless to heft back onboard from the dinghy!

The weight advantage is one reason we willingly paid about $600 (5-times the cost of a standard lead-acid) for Lilith. Other advantages for us are:

  • Can deliver higher current. This lets us run the motor at a faster throttle. Powered by our SLA batteries, Stealth will not speed up any further past 60% throttle setting; with Lilith, we notice an increase in speed up to about the 75% throttle setting.

  • Much longer life. After 2000 full discharge cycles, the SB35 is expected to retain 80% of its original capacity (i.e. approx 29 A-Hr). A typical SLA might yield only 200 full-discharge cycles, and to reach a 1000-cycle life one would have to limit discharges to less than 30%.

    One of the characteristics of lithium batteries is that their voltage doesn't drop as much as lead-acid when discharging. This leads to a problem: one doesn't necessarily notice the battery is approaching empty. With a lead-acid battery, the motor just runs slower and slower as the battery discharges. With Lilith, the motor runs at about the same speed right up until it is depleted. At that point, built-in circuitry cuts off the output instantly. Thus there is no warning provided by gradually slowing - it just stops. This means we have to monitor the distance we have traveled or running time more closely, in order to predict when a recharge is due.

    So how does Lilith perform in real life? We took our dinghy for a spin with Lilith fully-charged and ran around the anchorage at approximately 2.4 knots (corresponding to a throttle setting of about 50%) until the battery was depleted. There was a light wind of less than 5 knots and the seas were less than 0.2m (1/2 foot). As shown in the GPS track of our test run, we covered 4.99 nautical miles before the battery stopped. This is 39% farther than the SLA battery could carry us, so we are pleased having a much lighter battery that also performs better in a real world test.

    We have summarized the comparison between SLA batteries and Lilith in the following table.

    You likely have heard accounts of lithium batteries in cell phones or laptops overheating or even catching fire. This is a legitimate concern. Lithium metal is highly reactive, and can burn if a cell casing loses its integrity. Consequently, battery manufacturers have been developing designs and chemistries that are safer and are required to include built-in circuitry preventing the main causes of battery destruction: overheating, over-charging, and over-discharging. Our lithium battery uses a LiFePO4 (Lithium Iron Phosphate) chemistry, which doesn't exhibit thermal runaway, unlike the lithium-ion batteries usually found in phones and other small devices. Driven by the widespread use of lithium batteries in such applications as electric cars, these batteries now have very low failure rates. We much prefer having a lithium battery rather than volatile gasoline on board.

    We'll close this article with three comments related to practical aspects of dinghy motor batteries. First, we keep two batteries on hand, so that one can be charging aboard Hoku Pa'a (via solar panels), while the other is in use on the dinghy.

    Second, as you can guess, we frequently connect and disconnect the battery and the motor. For safety, we detach one of the battery cables while our dinghy is hauled onto the beach (to prevent folks from injuring themselves by playing with the throttle: the motor starts instantly). The batteries come with threaded terminals for a bolt, which can be a little annoying if one needs a wrench each time. Our solution has been to use wingnuts, which one can tighten adequately by hand. Lacking ready-made ones for our new lithium battery, we made a set using two bolts, two short sections of hose, and two dowels, plus some epoxy.

    Our last comment relates to charging our dinghy batteries. We installed a socket (using the Anderson Power-Pole connectors described in a previous issue of Currents) into which we plug a short pair of wires ending in alligator clips. A switch sends the output from one of our solar panels either to this charging socket, or to our main house batteries. We also added a meter (optional) that shows us the battery voltage and how much current is going in. Some 12V lithium batteries are designed to charge at the same voltages and currents as lead-acid batteries; we purchased this type so no alterations were needed from our existing solar charge controllers.

    After eight years, our attitude towards electric dinghy engines remains very positive. We love the lack of maintenance and easy handling of the lightweight engine and battery. It feels good to be pollution-free when motoring about, and we are fine with the associated trade-offs. We are happy to discuss electric outboards further. Just catch us as we glide quietly by...

    This article has been provided by the courtesy of Bluewater Cruising Association.

  • Related Articles

    Provisioning Tip: Do you know Nido?
    Reconstituted Nido is almost as good as UHT milk When travelling for extended periods or in remote areas, fresh dairy products become unobtainable. In Mexico and Latin America, UHT processed milk (long-life box-milk) is widely available Posted on 15 Sep
    Ready, Aye, Ready!
    Sailing Avant in Mexico and Central America: Do you feel safe? We have been sailing Avant in Mexico and Central America for six years now, and one of the most common questions we get is "Do you feel safe?" Posted on 1 Sep
    Singapore to Sri Lanka
    Continuing to follow the adventures of Argonauta I In the last article from Memories of a Circumnavigation, Hugh and Heather explored the Java Sea and had some amazing encounters with orangutans. Posted on 25 Aug
    The return of Sea Turtle!
    Celebrate the conclusion of Jordan and Judy Mills' circumnavigation aboard Sea Turtle On July 3, approximately 30 excited family and friends, including many BCA members, gathered at Ogden Point with air horns, whistles and other noisemakers to celebrate the conclusion of Jordan and Judy Mills' circumnavigation aboard Sea Turtle IV. Posted on 7 Aug
    Postcards from a Caribbean Winter part 9
    Rod arrives in the Canary Islands The land was stark. The only colours we could see were earth tones of sandy beige, with brown rocks and sandstone cliffs as Oh! approached the anchorage at the south end of Graciosa Island. Posted on 30 Jul
    First Aid Kits for Cruising
    We looked carefully at what we would pack for first aid preparedness We could easily imagine a wide range of scenarios where one or the other of us, or another cruiser or local, was injured (grievously), and needed first aid in an inconvenient, remote location. Posted on 26 Jul
    May long weekend Rendezvous at Poets Cove
    Being boatless and living in Canmore can make it a little tricky to orchestrate Being boatless and living in Canmore can make it a little tricky to orchestrate as much time on the water as I'd like. So, when my daughter extended an invitation to join her aboard Easy Rider Posted on 22 Jul
    Medical Insurance: To have or not to have?
    Mike and Jacquie Champion catalog their experiences From La Cruz, we traveled the 11 miles to Nuevo Vallarta, put the boat into Paradise Village for a month and headed back to Vancouver for a week to celebrate my Mom's 85th birthday. Posted on 21 Jul
    Cruising life in Tahiti
    French Polynesia is often on the itinerary of BCA members French Polynesia is often on the itinerary of BCA members when they head offshore. We are in French Polynesia now, and would like to share some information that was not readily available prior to our departure from Mexico that might help others Posted on 19 Jul
    Getting into a life raft
    Not as easy as you think, and certainly something to practise Bang! My PFD exploded into action and wrapped itself around my neck and supported my head within seconds of getting wet. My husband, David, had already deployed the four-man life raft and I was swimming for it. Posted on 12 Jul
    B&G 2019 Q3 Cashback 600x600North Sails 2019 - NSVictoryList - FooterZhik 2018 Dongfeng 728x90 BOTTOM