ePropulsion Navy Electric Outboards

ePropulsion Navy Electric Outboards are quiet, smooth and efficient. Their immense torque
makes them suitable for powering displacement-hull vessels of up to about 5 tonnes.

Unlike other electric outboards at this size (including Torqeedo Cruise), the Epropulsion Navy electric outboards all use direct-drive brushless DC motors. This means that the motors are directly connected to the propeller shaft, with no delicate/noisy gearbox in between. And the motor is housed underwater, in the “pod” at the bottom – so there’s no driveshaft or belt or cooling system running up the leg.

The main benefits of direct drive motors are very quiet (all-but silent) running, mechanical robustness/simplicity, and greater efficiency (meaning longer run times from your batteries).

These electric outboards are also incredibly easy to use – you just press the on button and twist the tiller handle or push the remote control lever, and off you go. No putting your back out (or knocking other crew out!) pulling the starter cord, no choke to fiddle with, no waiting around for it to “warm up” in case it stalls.

And being so mechanically simple there’s just less to go wrong, which you might have guessed means reduced servicing/maintenance costs. But it’s better than that, in fact no servicing is required (other than very basic DIY greasing etc). And unexpected repair bills are extremely unlikely, for example there’s no carburetor to gum up (the most common cause of petrol engine problems in recent years).

The direct translation of kW (kilowatts) to hp (horsepower) is 1.34. That would suggest the Navy 3.0 is equal to a 4hp petrol motor, and the Navy 6.0 to 8hp. BUT… electric motors have much more torque, from very low revs. To utilize that torque they turn relatively large propellers, relatively slowly, and that’s a very efficient way to drive a displacement hull. (Much more efficient than a small propeller spinning fast, as you get with a petrol motor.)

So in our experience the horsepower translation is about right if you want an outboard for a small fast dinghy, but underplays it if you want an outboard to drive a heavier displacement craft at displacement speeds. That’s how Epropulsion (and other brands) came up with saying their 3kW motors are “equivalent” (note not “equal”) to 5-6hp, and their 6kW outboards to 10hp.

Another factor that comes into play if you’re replacing an old petrol outboard is that most people never run them flat out, because it’s too unpleasant! That 10hp engine on the back of a small yacht might well never have been used above 6hp, for example.

We have now supplied Navy outboards for several “real world” installations, including small yachts and commercial users such as hire boats, passenger ferries and fishing boats. In all these cases, all requiring propulsion for displacement speeds, we’d say that 1kW replacing 2hp seems to be about right.

Using a Navy 3 on a fairly slippery (Swallow Yachts Bay Cruiser) 26ft yacht weighing two tonnes, that used to have a 9.9hp four stroke petrol outboard, we got:

• 4 knots at 750W output (surprising, but remember all that torque comes immediately)
• 4.5 knots at 1000W output
• 5 knots at 1500W
• 6 knots at full power 3000W

NB that last extra knot requires the same power as the first five, so if the previous 9.9hp engine was any faster it wasn’t much, we are up against hull speed by this point.

And on a 3.6m inflatable dinghy:

• a Navy 3 will achieve 6-7 knots even quite heavily loaded, but not quite planing speeds when lightly loaded
• a Navy 6 will achieve similar speeds heavily loaded, because you come up against hull speed again. It will get the boat on the plane with only one person on board, but not for more than an hour or so (as otherwise the weight of batteries makes it too heavy to plane!).

In summary then:

• for displacement-hull boats a Navy 3 will replace up to about 6hp, and a Navy 6 will replace up to about 12hp
• for light planing boats it’s more like what you’d expect, ie a Navy 3 is about the same as a 4hp petrol, and a Navy 6 about the same as 8hp.

This depends entirely on what capacity battery or batteries you fit – Navy motors are not supplied with batteries.

Many of us are used to “measuring” battery capacity in amp hours (for our car, boat or motorhome 12V systems), but it’s only a useful measure of battery capacity if you also know the voltage. The more useful – or “lowest common denominator of battery capacity” – is Watt hours or Wh, also sometimes expressed as kWh (1000Wh). You can find out the Wh for any battery by multiplying together the amp hours and the voltage.

If you have more than one battery wired together (in series or parallel) you add the Wh together to get the “bank capacity”. Please note that with lithium batteries – which is really what you want for electric propulsion – you can usually only wire together identical batteries.

Once you know the Wh rating of your battery or battery bank it’s very simple. If you’re using a 3kW motor at 1kW output, it will use 1kW or 1000W per hour. If your battery/bank capacity is 9kWh then it will last 9 hours at 1kW. But if you run the same motor at full power (3kW output) your 9kWh will only last 3 hours. Similarly the 6kW motor will also run for 9 hours at 1kW output, and 3 hours at 3kW, but only 1.5 hours flat out (because that’s drawing 6kW).