My experience of an electric outboard

The previous owner of my Cruz had an electric outboard which he sold me with the boat. I’d never had an outboard before and an electric one was even more mysterious. It didn’t come with instructions so I was unable to read them. The inscription on it said it produced 28 lbs of thrust, but I couldn’t easily relate that to the outboards at the sailing club (4hp — OK, 4Ohp — quick). The motor required a large (90 ampere hour) 12 volt leisure battery but the calculation for converting that amount of power into movement through the water didn’t seem obvious either. A bit of physics and electrical engineering converted 90 ampere hours at 12v into less than 2 horsepower for an hour, with very high currents in the cabling. That didn’t seem very impressive and as batteries represented an investment, I chose to go without.

For the first months of sailing my Cruz I relied on sails and oars. I borrowed Brian McClellan’s Seagull on Coniston for the petrol experience (thanks Brian). No one else seemed to know very much about electric outboards either. I was beginning to feel auxiliary power would be an added safety feature but couldn’t find anyone to convince me I should experiment with a battery rather than splashing out on a petrol outboard. By the 1998 Falmouth rally I’d decided I needed an outboard, left myself too little time to find a petrol motor, but knew where I could buy a battery. I bought a 100 ampere hour leisure battery and fitted it to the boat. I really didn’t fancy having a heavy battery moving round in rough wind and water, and fixed some straps in the rear compartment of the Cruz. It’s not supposed to be a good idea to put the weight at the ends of the boat, but it would be shut in and wouldn’t get in the way.

I had no idea how the outboard would perform or how long the battery would last. I first used it to push me out of St Just Creek into a force 4 to 5. I made it, but not very quickly and with nothing in reserve. I used it a few times during the week, mainly to surmount the fickle winds in King Harry reach. Jim at Pascoe’s boatyard recharged the battery mid week for substantially less than a can of petrol. While I wasn’t confident that electricity would do the job I wanted, on a number of raffles the motor got me out of difficulties where sailing space was limited or tide and wind adverse.

1999 started badly for the motor. Moving to a mooring off the beach at Waldringfield at four in the morning against the ebb tide, in fog with the tent rigged, the motor died just as I was about to pick up the mooring I needed. This did not endear the motor to me, but my folding grapnel, hastily assembled, miraculously held and disaster was averted. Either the connectors failed or the battery was not fully charged when I set out. The 1999 Falmouth rally saw the motor tow a West Wight Potter (not DCA I hasten to add) off the shore after a mooring mishap, more pushing into the wind, and additional passages past the chain ferry. The motor seemed to have adequate power, was nearly silent in operation, and reasonably easy to use. The rear compartment smelled of hot insulation, but that was a less intrusive and pervasive smell than petrol.

So how long did the battery last? I charged it up on return from Falmouth and on a sunny Sunday when I was committed to garden maintenance I filled a spare 50 gallon water butt for a controlled test. I mounted the motor on piece of timber with the propeller well under water, set my countdown timer to 30 minutes and started the motor on its middle speed setting. The water swirled in the butt and I set to my allotted tasks. Three hours later I had checked the motor every half hour and it was still swirling the water nicely. I turned it off for lunch then carried on. I was surprised to find it still running after another three hours, and gave it a burst at full power which finished the battery off in the next half hour interval. The following weekend I replaced the battery connectors, which were getting ominously hot, and ran the motor at full power on the recharged battery. This time two and a half hours of power resulted with another half hour dying away to nothing. Timing the motor taught me that two and a half hours is quite a long time, and six hours is tediously long.

My confidence in the motor/battery combination had greatly increased but how far would six hours of battery power get me? For my next test I put the boat on the water at the sailing club and measured the distance between two parallel jetties. With very little wind I impressed the racing fraternity by gliding silently under bare poles, and timed the boat over the measured distance at all five forward settings of the motor. I took the average of a run in each direction to allow for the effects of wind, and used the race officers calculator to convert times and distances to knots. This gave me a theoretical range of 13 nautical miles at 2.2 knots on setting 3, and nearly 8 nautical miles at 3.1 knots on setting 5. The boat was normally rigged but not equipped for camping, probably weighing in at about 450 lbs(200 kg’s).

I now feel reasonably happy with the motor. The endurance should be adequate for a cruising weekend, and my experience is that the power is sufficient for the conditions I would wish to sail in. From the performance in the water butt I believe the problem at Waldringfield was the connector clips, which I have now changed to a quick release clamping type with insulated covers, probably familiar to caravanners. The new connectors run much less hot than the old ones. I have no easy way of knowing how much charge is left in the battery, and charging the battery in mid cruise remains a difficulty. However I shall put thoughts of smelly petrol aside for the moment and glide silently over the water instead.

Some technical details:

My motor is a Thruster 12V T2800 made by Mercury Marine and fitted with a clear plastic propeller. It has five forward speeds and two reverse. I use speeds 1 and 2 for manoeuvring at jetties, speed 3 for pottering along, and the two higher speeds as required for more arduous conditions. The motor clamps to the transom or bracket like any other outboard. The shaft can be adjusted for motor depth, and pivoted to the horizontal position when not in use. I find the Cruz motors best with a little centreboard and the crew weight forwards to reduce the effect of the wind on the bow. The motor weighs approximately 16 lbs/7 kg’s on the bathroom scales.

The battery is a lead acid leisure battery. A leisure battery is designed for steady discharge and recharging rather than the high currents needed to start cars. At 100 Ampere Hours (Ah) capacity it is larger than recommended (90 Ah) which increases the endurance. The battery takes one to two days to charge fully using a car battery charger. The battery weighs 52 lbs/23.5 kg’s, heavy and awkward to move and install.

My apologies to Mercury Marine if the above information is available by sending them a stamped addressed envelope.