PBO reader Philip Hodson wants to know which electric outboard he should choose for his 17ft sailboat. Our expert Ian Thomson has this advice…
Philip Hodson of Newmarket, Suffolk writes: “For years I have propelled my 17ft yacht Lysander, 1972 maywing with a powerful British Seagull 6hp Kingfisher outboard motor.
“But now, having reached the grand age of 70, I no longer have the strength to start the engine, so I am looking for an electric outboard motor to replace it.
“Torqeedo electric motors seem the closest alternative. The long shaft Torqeedo Cruise 2.0 with drawbar and claimed 5 hp will do. It delivers 2000W, operates on 24V, so at maximum speed, drains around 84A.
“The downside is the 24-3500 lithium battery recommended by Torqeedo, weighing 25kg (the Kingfisher weighs 27kg, about the most I can lift) – and its bulk. At 58x22x26cm, the 24-3500 battery is a great unit for a small boat.
“So I considered as an alternative two 100Ah 12V lithium batteries in series (to obtain 24V). Each will only weigh 12 kg, which is much more manageable!
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“The problem then is how their battery management system (BMS) works, specifically the cutoff system. If you pull the full 84A while it’s at full power to escape tides and rocks, it seems that many 100Ah lithium batteries cut out at lower amp draws.
“When a BMS cuts the power, does it automatically turn back on after a few moments? I compared the Lifos 105A lithium battery as a replacement to Torqeedo’s battery and made a comparison chart (top right). Am I on the right track?”
Ian Thomson replies: “The BMS is there to protect the battery cells from damage. Maximum output, and what happens if it is exceeded, will vary by battery capacity and brand. The BMS also takes into account variables such as temperature.
“The 105Ah Lifos batteries you mention seem to be rated at a maximum discharge of 120A (up to 30 minutes) so when wired in series as a 24V bank this should draw the maximum outboard power 2000W, or 84A at 24V.
“An 84A output is comfortably within their rating. However, notwithstanding that two smaller batteries would be easier to handle and install, there are other things to keep in mind:
- The combined capacity of the two 105Ah batteries is 2688Wh, the Torqeedo 24-3500 battery has 30% more. Thus the single Torqeedo battery would make it possible to drive 30% further, at the same speed.
- The weight per Wh is around 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.
- Torqeedo motors will run on alternative batteries, but with a fully Torqeedo system, data such as state of charge (shown on the helm display) will likely be more accurate.
“It’s also worth pointing out that Torqeedo recently discontinued the Cruise 2.0 motors, but their new Torqeedo Cruise 3.0 would use the same power at 2kW and (with good enough batteries) has 50% more maximum power if you really need it. .
“The Torqeedo 24-3500 battery could cope with the extra power, the Lifos batteries would most likely cut out after a short while.
“Finally, according to the Lysander Owners Association, your boat should weigh around 360kg empty, so let’s say a maximum of 650kg in cruising mode with two crew.
“So I would recommend that you first try one of the newer 1kW electric outboards – for example Torqeedo 1103 or ePropulsion Spirit 1.0 Plus – where the (removable) battery sits above the engine.
“Many customers with similar sized or even slightly larger boats such as the Westerly Nimrod, Shipmate Senior, Hawk 20 and countless Drascombes use the 1kW motors and find them more than satisfactory.
“I suspect you would very rarely need more than 1kW of power. Even with a spare battery, this type of outboard would be a much cheaper and simpler installation, the batteries (at less than 9 kg for 1,276 Wh for the ePropulsion for example) being very easy to take home to be loaded.
Electric outboard batteries compared
|Model||Torqueedo 24-3500||Lifos 105A x1|
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This feature first appeared in the May 2022 edition of Practical boat owner. For more articles like this, including DIY, money-saving tips, great boat projects, expert advice and ways to improve your boat’s performance, subscribe to Britain’s best-selling sailing magazine.
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