Ned,
I had been letting the list answer your question about hull designs, but your combination of performance requirements just caught my eye. I know that you said that you had already been through the battery selection process, but I thought that this could be a good exercise for other people on the board.
Let's back into your request. You want less than 600 pounds of batteries, lead acid specifically. Trojan T-105 have a twenty hour rated capacity of about 22.5Wh per pound. This work out to 13,500Wh for 600 pounds of batteries. Since you're dealing with flooded cells, staying above a 60% depth of discharge is often recommended, this calculates out to 8100Wh usable. Factoring in the Peukert's Effect, using a Peukert's exponent of 1.25, this battery bank will last 184 minutes under a 2000W load. The voltage doesn't really factor into the calculation, you can pick the voltage to suit your needs, but the constant will be 2000W.
Since you're going to have a hard time finding performance figures for hull designs in electric drive measurements, we can translate this into ICE terms. Knowing that a gallon of gas is about 33kWh of stored energy and electric drives are about 3 times as efficient as ICE, your 600 pound FLA battery bank is similar to an ICE with a 0.6 gallon gas tank. So, you're looking for a hull design that can cruise at 10 knots while burning 0.20 GPH with an ICE engine. The FLAs should cost around $1500.
Alternatively, doing the same calculations with AGM (still lead acid) we get something like this: AGM are just above 20Wh per pound (lower energy density). 600 pounds equals 12,100Wh. AGMs can be run to 70% DoD which gives you 8450Wh usable. But AGM are more tolerant of high loads and factoring in Peukert's Effect, using a PE of 1.1, the battery pack will last 181 minutes at 2450W load. This 600 pound AGM battery pack is similar to an ICE with a 0.75 gallon gas tank and your allowance is up to 0.25 GPH with an ICE engine. These batteries should be between $2500 and $3000.
Finally, runnning the same calcs with LiFePO4, we get the following: Li batteries are 40Wh per pound. 600 pounds equals 24,000Wh. Li can be run to 80% DoD which gives 19,200Wh usable. Even more tolerant of high loads than AGM, after Peukert's Effect and PE of 1.03, the battery pack will support a 6000W load for 183 minutes. This 600 pound battery pack is similar to an ICE with a 1.8 gallon gas tank and you allowance is now 0.6GPH with an ICE engine. While these performance specs sound much more realistic, the batteries will cost close to $10,000.
All of these calcs were based on the 600 pound weight allowance. So if cost is a primary factor, you may have to accept less range or lower speeds. But building an electric boat with a highly efficient hull that meets your speed and range performance requirements does sound technically feasible, albeit still pretty expensive.
It's another example of a classic choice: high speed, long range or low cost; you get to pick two.
Honestly, these calcs came out more favorable than I expected, so I thought that I would share with the group.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "nedfarinholt" <nedfarinholt@...> wrote:
>
> I am interested in building an electric runabout and would like help on hull design. Here are the basic requirements:
> ....
> Cruising speed above 10 knots.
> Cruising duration 3 hours at 10 knots.
> Would like to accomplish the above with less than 600 lbs LA batteries....
>
> Any ideas or experience or can you refer me to some sites or literature. I have already scoured the web pretty thoroughly.
>
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