- Currents Bluewater Cruising - https://currents.bluewatercruising.org -

Electric Dinghy Engine Update

It’s been a few years since we reported on our new electric dinghy engine (December 2012 Currents [1]). 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. [2]


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:

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.

Battery terminal detail.

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 [3]) 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…