Arby,
This is the type of conversation that I was hoping would develop. We can already identify a couple of constraints that need to be acknowledged. Lead acid batteries weigh about 60-65 pounds per kWh. Therefore, a 10kWh batteries pack will be around 650 - 700 pounds with cables, platforms, and tie downs. Lithium batteries are about 35% of the weight of lead acid batteries.
My boat is around 5 tons displacement and I'm targeting a 48v system that can pull up to 125A, resulting in a 6kW drive. I hope that the boat would perform sufficiently running at around 2.5kW with the rest of the power held as reserve. Given a 24' LWL, the estimate of 500W per ton would result in a cruise speed above 4kts at 2.5kW.
I'm interested how you would dyno the shaft power of an installed system. I'm pretty sure that you're on the eastern half of the country, I wonder if there's anyone in SoCal set up to perform a similar measurement. If we could collect this type of metrics, it would go a long way towards objectively measuring the efficiency of some of the various drives available. While total thrust measured with a bollard pull may be more relevant to the end user; the variations from hull shape, shaft angle, and propeller design would skew the results towards the installation and away from the drive's performance.
Given that the weight of the battery pack seems like a limiting factor to me, and that displacement keelboats already carry thousands pounds of ballast, the elegant solution would be to make some of that ballast "active". I will attempt to do that with my second battery bank after the drive is installed and running. I think that the best solution would be a boat built with vertical battery racks running into an internal keel or a battery filled bulb at the bottom of a fin keel. A power bulb my be able to be retrofitted, but I would want to remove some of the original ballast to keep the boat sailing as designed.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@
>
> Keith has raised an interesting question...How much power should a boat have to consider it safe? I've read that 2.5kW per ton will allow a boat to reach hull speed. For your Hunter, that would be a 10kW system. 70% of hull speed requires but 500W per ton, however, so a "3hp" (2 kW) system would move your boat just fine.
> Please suggest a standard, in shaft power, not controller input. I'll be glad to dyno your system if you would like an actual curve for your Mars/Sevcon system. Knowledge is power.
> I will agree that 3hp on a 10,000lb boat can provide manuevering power, but would be hard pressed to buck heavy seas and headwinds. By the same token, installing a 15kW motor and expecting to support it with a 100kWh battery pack is not reasonable.
> If other members would care to suggest power to displacement ratios, and the batteries to match, we could help to build a database of working, marginal, and disfunctional boat repowers.
> By creating a standard, we help not only folks like ourselves, we help the boating industry in general to accept a greener option.
>
> Be Well,
> Arby
>
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