Developement in the lithium battery technologies has made it viable option to retrofit electric propulsion as an alternative to diesel engines on sailboats. On sailboat diesel engine is an auxiliary power source and often used mainly for creating electricity. Improvements in battery technology with LiFePO4 battery chemistry and decline in solar panel price make it viable option to convert to electric propulsion when time to repower comes. There are some caviats though. Most economic solution would be to repurpose electric vehicle power train to boat but 300-400V battery voltage with salt water environment creates some risks that are better avoided. Also prevalent 12V power systems in boats today limit amount of practical power that can be used.

Our approach is to create a motor for 48V direct current system. 48V system allows to implement 30-40kW electric propulsion while having reasonable currents flowing from battery to engine. Out approach to this is to use two 15-20kW 48VDC brushless motors with belt drive to create drop in replacement for diesel engine. This motor is powerfull enough to power 45 feet medium displacement cruising sailboat. Motor needs also battery bank to power it and for this purpose we use out 5kWh 48V batteries in parallel. To keep drain currents reasonable we need minimum of two per motor so four batteries making total battery capacity 20kWh. In our reference implementation we use 30kWh battery bank (six batteries). This allows 30-60 minute full power use. While this may seem like a way too short time, full power is usually needed only few seconds at the time. It allows time to escape lee shore in heavy weather and more than enough for normal operation. To complement the system can be also fitted with generator providing 3-4 kW of continuous power.

Energy system is planned with three sources of energy and three power lines. Battery bank and main loads are 48VDC system (motor, bow thurster, fridge etc) while most DC loads are behind DC/DC converter in 12VDC bus. While DC/DC converter ads inefficiency, those loads are only used sporadically. We also have 220VAC power line that powers galley systems (induction stove, combination microvawe and convection oven), diving compressor, etc. Our chouse of inverter is Victron Mupltiplus that also provides 48VDC battery charger.

48V battery bank can be charged from three different sources. We have shore power via inverter as well as 3.6kW generator which both can provide 70A charging current to 48V system. Then we also have solar arrays that also provide charge to the batteries. Solar arrays should be sized so that average daily consumption can be covered with solar power.