Philip Hodson of Newmarket, Suffolk writes: “For years I have been powering my 17ft, 1972 Lysander yacht Maywing with a stalwart British Seagull 6hp Kingfisher outboard motor.

“But now, having reached the grand age of 70, I no longer have the strength to pull-start the engine so am looking for an electric outboard motor to replace it.

“Torqeedo electric motors seem the closest alternative. The Torqeedo Cruise 2.0 longshaft with tiller and a claimed 5hp will do the job. It provides 2,000W, works on 24V, so at top speed, drains about 84A.

“The downside is Torqeedo’s advised 24-3500 lithium battery, weighing 25kg, (the Kingfisher weighs 27kg, pretty much the very maximum I can lift) – and its bulk. At 58x22x26cm the 24-3500 battery is a large unit for a small boat.

“So I have been considering as an alternative two 100Ah lithium 12V batteries in series (to get 24V). Each will weigh just 12kg, which is much more liftable!

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“The issue then, is how their Battery Management System (BMS) works, especially the cut-out system. If drawing the full 84A while on full power to escape tides and rocks, it seems many lithium 100Ah batteries will cut out at lower amp-draws.

”Where a BMS cuts the power, does it cut-in again automatically after a few moments? I have been comparing the Lifos 105A lithium battery as a substitute for Torqeedo’s battery and have made a comparison table (above right). Am I on the right track?”

Ian Thomson replies: “The BMS is there to protect the battery cells from damage. The maximum output, and what happens if that’s exceeded, will vary by battery capacity and brand. The BMS also considers variables such as temperature.

“The Lifos 105Ah batteries you mention appear to be rated at max 120A discharge (up to 30 mins), so when wired in series as a 24V bank it should be OK to draw the maximum outboard power of 2,000W, or 84A at 24V.

“An 84A output is comfortably within their rating. However, notwithstanding that two smaller batteries would be easier to handle and fit, there are some other things to also bear in mind:

• The combined capacity of the two 105Ah batteries is 2,688Wh, the Torqeedo 24-3500 battery has 30% more than that. So the single Torqeedo battery would allow you to motor 30% further, at the same speed.
• The weight per Wh is approx 7g per Wh for the Torqeedo, and closer to 9g per Wh for the Lifos. The Torqeedo is therefore lighter for its capacity, and only heavier overall because it has more capacity.
• The volume of the single Torqeedo battery is only 7% more than the two Lifos batteries, while the Torqeedo has 30% more capacity. The Torqeedo battery is more space efficient for its capacity, and only slightly larger overall.
• The Torqeedo motors will run on alternative batteries but with an all-Torqeedo system data such as charge state (shown on the tiller display) is likely to be more accurate.

“It should also be pointed out that Torqeedo have recently discontinued the Cruise 2.0 motors, but their new Torqeedo Cruise 3.0 would use the same power at 2kW / 2.7hp and (with sufficiently capable batteries) has a 50% greater maximum output if you really need it.

“The Torqeedo 24-3500 battery could cope with the extra output, the Lifos batteries would most likely cut out after a short while.

“Lastly, according to the Lysander Owners Association, your boat should weigh approx 360kg empty, so let’s say a maximum of 650kg in cruising mode with two crew.

“I’d therefore recommend you try first with one of the newer 1kW electric outboards – e.g. Torqeedo 1103 or ePropulsion Spirit 1.0 Plus – where the (removable) battery sits on top of the motor.

The ePropulsion Spirit 1.0 Plus is a light and powerful electric outboard engine

“Many customers with boats of a similar size, and even a bit larger like the Westerly Nimrod, Shipmate Senior, Hawk 20, and countless Drascombes, are using the 1kW motors and find them more than satisfactory.

“I suspect you’d very rarely need more than 1kW power. Even with a spare battery this type of electric sailboat outboard motor would be a much cheaper and simpler installation, with the batteries (at less than 9kg for 1,276Wh for the ePropulsion for example) being very easy to take home for charging.”

Electric sailboat outboard batteries compared

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