Welcome to the group! I can see that you’ve done your homework and come up with a plan based on traditional sources. What we (the people in this group) have found over the last 10 years, is that the traditional powering models and calculators aren’t quite right for electric conversions. The Dave Gerr book is a great resource, and I respect him greatly, but we’ve seen time and again, that our end results are not close to his predictions (typically we need much less power than his formulae suggest). In conversation with Dave, he accepted that his work is more applicable to power boats operating at faster speeds.
What we have found is that displacement is the most accurate indicator of how much power a boat needs. A rough rule of thumb for a properly designed electric marine drive is that 1kW/ton of displacement will deliver 90+% of hull speed, 1kW/1000# of displacement will deliver hull speed. Another observation is that increasing speed by 1kt (while staying under the 90% hull speed, the flatter part of the speed chart) requires twice the power. Conversely, halving one’s power should slow one down by about a knot.
Since you did not provide a displacement for your boat, I did a quick search and found that many 55’ sailboats come in between 40,000 and 55,000 pound displacement. Given that, I did some quick calcs based on an assumed displacement of 50,000 pounds, or 25 tons. Personally, I am in the 1kW/ton camp (my boat achieves 95% hull speed at that power), so that would indicate a 25kW system for your boat.
Working further with my displacement assumption, I would predict the following results:
- 25kW = 8.5kts
- 12.5 kW = 7.5kts
- 6.2 kW = 6.5kts
- 3.1 kW = 5.5kts
- 1.6 kW = 4.5kts
Given these numbers, we can start sizing the motor, batteries and wiring. Most of the folks here have stayed with 48V systems for safety, ABYC considers up to 50V as low voltage, and greater than 50V as high voltage, High voltage systems should follow the additional requirements listed in ABYC TE-30. But with your max power draw of 25kW, a 48V system would pull 520A which isn’t very practical or safe. Going to 96V brings your max load to 260A, getting better. And choosing 144V should keep your max power below 180A. 144V components are readily available, i.e. motors, controllers, chargers, the DIY electric car guys have been running around this voltage for decades. Given your boat, I would choose 144V too.
Personally, I find that 100Ah traction banks are pretty much the minimum for electric boats, and you may find that a pack that size gets worked harder that you might like. My pack is 160Ah and seems pretty good. But going with your planned 14.4kWh bank, we can see that 90% discharge could drive your boat for 8 hrs at better than 4kts, giving a battery only range of more than 30 nm. Even at 6kts, you should be able to run for more than 2 hrs on batteries alone.
Ok, this response has turned out longer than I expected, but it should give you some new info to digest.
I converted my boat to electric in 2010 and have been very pleased with how it has turned out. I have posted here extensively with detailed performance observations and predicted performance of other boats.Feel free to contact me here or off-list for more, if you like.
Fair winds and following seas,
Eric
1964 Cheoy Lee Bermuda 30, 5.5kW drive, 8kWh LiFePO4 battery bank
Marina del Rey, CA
PS. I am not a vendor, just a hobbyist/satisfied owner.
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