EPROPULSION NAVY BATTERY
 
There's no escaping the fact that lithium batteries are expensive, but once you get past that there's also no denying that they're also by far the best solution for electric boat propulsion (just as with electric cars). Watt hour for Watt hour - Watt hours becoming the lowest common denominator of battery capacity - they are much lighter and more compact than any lead-based technology. And that's before you consider USABLE capacity... lead acid batteries should not be discharged below 50% (if you want them to last any reasonable length of time), whereas you can use 100% of a lithium battery's capacity.
 
Relating specifically to the Epropulsion Navy battery then, its 3042Wh capacity is nominally equivalent to four 60Ah capacity 12V batteries at 720Wh each (you'd need four 12V batteries wired in series to get the 48V the Navy motors require). Factor in the 50% discharge limit you should set yourself on lead acid batteries, and:
The Usable Capaciity of 1x Navy battery = The Usable Capacity of 4x 120Ah 12V lead acid batteries.
 
Lead acid battery weights are variable by brands etc but a reasonable working number is 30kg each, or 120kg for a usable capacity equivalent to one Navy 3 battery - which only weighs 31kg! So the weight saving is very significant, near enough 4 to 1.
 
You also save a considerable amount of space with lithium. A Navy lithium battery measures 60x21x28cm, or 0.035m3. A typical 120Ah lead acid battery measures 50x18x23cm or 0.021m3 and you need four of them so a total of 0.084m3. Lithium takes up significantly less than half the space (in this case .
 
Back to the elephant-in-the-room, cost. If we go with the above usable capacity calculations, and costing for reasonable quality lead acid batteries (say £150 each), the cost of lithium is about four times higher. But don't forget the life/charging cycle expectancy is at least double for lithium, so the cost per charge cycle use is then "only" about double. Put together with the weight and space savings, the price difference doesn't look as crazy as the raw figures first suggest.