I think it all depends on what you want and expect for a particular boat. In the case of my 26 ft, 4 laden tons displacement, 21.25 ft LWL giving around 6 knots hull speed - my 2.3 kW calm conditions max power measured at the battery (not static pull) which is around 2kW shp based on 85% efficiency from battery to shaft; that then gives me a max top speed in calm conditions of 5 knots or about 85% hull speed. My chosen constant cruise speed is around 4 knots being around 1kW measured at the battery. So in my case 0.5 kW shp per long ton of laden displacement is suffice.
As for voltage and efficiency, again it depends what you want. I run 2 X 12.8 V (25.6V nominal in series) lithium iron phosphate (LiFePO
4) batteries, which have great C rates compared to lead acid variants and a very stable voltage under load with little or no Peukert effect. By running direct drive with no reduction gearing and by using a high torque/low RPM per volt motor this gives me 40.5 RPM/volt, 0.22 Nm/Amp and a max shaft speed of 900 RPM under load. By choosing an appropriate prop I'm able to run the motor at cruise speed at around 89% efficiency compared to around 92% if using a reduction ratio but of course there would be around a 3% or more loss using a reduction ratio compared to direct drive. Another bonus is I only require 2 batteries instead of 4, therefore costing less. My range at cruise is around 16 to 20 miles, which is sufficient as it is a sailboat! I carry a 1kVA Honda generator permanently fixed and ready to supply the full 900 Watt continuous charge power. This is cheaper than buying more batteries.
As ever it is horses for courses and your power and cruising requirements might be different. In my case I simply wanted to replace my 10 hp diesel sail auxiliary with around the same effective thrust, for the same weight or less and the same cost or less when compared to a new diesel. I started out with lead acid and progressed to Lithiums. In my view the cost per nautical mile plus all the additional benefits of Lithiums, even though high initial capital outlay, outweighs dead lead. Why buy more batteries for a 48V system when two at 24V is just perfect for ones needs?
By starting simple you can always upgrade as finances permit. In my case this has been a generator, Lithiums, an AGM house bank with an inverter/charger with power assist, enhanced onboard monitoring with 24/7 data logging via a 4 G router to a website, 100% charger redundancy on both propulsion and house banks, with a total of 1,700 Watts on the propulsion bank and 700 on the house bank using a 230 V 16 or 32 Amp shore supply for faster charging. My point is that there is no one right way, it depends what you want and expect of your system, along with available finances.
John Rushworth
SY Elektra
http://elektra-yachts.co.uk
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