Seaguy you didn't mention what Voltages you were looking at for your system but I'll assume it's probably 72V based on the dollar amount you mentioned for the batteries.
Some simple math.
My 16KW genset burns a hair over 1 gal an hour. with 40 gallons (in the tanks) on board that's say 38 hours. The genset puts out 170V DC for a max charge rate of around 164V to the 12 pack of AGM's. Note: AGM's and LA's are only good for 50% of their energy until you have to start thinking about recharging. Max Amps supplied is about 110A
At 144V my motors (2) use around 60A to maintain 7knots.
At 72V you need to double that to 120A (still 8.6KW)
So you have 2 diesel motors to spin your props and a Genset on board. Those 2 motors are going to double the fuel used plus the wife wants that hair drier running so you need the Genset on every so often. How is that more efficient?
So let's look at generators for a second. The new ones are lighter and much more efficient than mine is but it still weighs less than the 2 motors and sail drives your boat will have. I can carry the electric motors out on my own (about 120# each) by the way. So let's just say it's a 300# savings at this point. You have to maintain 3 motors were I have 2 of the 3 that require checking the torque on the connecting wires once a year and replacing 2 shaft bearings (at a cost of $20) every 300,000 hours of run time. No my fingers didn't stutter.
So we get to the BIG ticket item, the Batteries.
My AGM's are Northstar's, 210AH about $800 each x 12 = $9600. Probably the best LA batteries out there. Recommended a max 50% discharge (Northstar says 60%). They weigh around 140# each for a grand total of over 1700#! That is the best you could do 10 years ago. I've gotten 8 years of service
(Not sure but I think NS says about 500 full cycles) out of them and the one thing everyone has to do with Electric Propulsion is maintain their batteries religiously. For that matter everyone does but that's another string..
So today we have the option of LiFePo4
batteries, 180AH at $250 x 48 = $12000.
That might throw a few people into a tizzy but look at the advantages:
Life expectancy is 5000 cycles = they will out live me.
Weight 12.5# x 48 = 600#. a savings of close to 1/2 a ton.
1/2 the space needed.
A discharge down to 20% (10% if you push it) vs 50% = 30% more energy available. I would hope a run time on batteries only of around 7 hours.
Much faster charge time = less genset run time.
With these batteries I'll probably never exceed 1/2C which is pretty gentle on them. When you crunch the numbers the LiFePo4's win hands down.
All in all the system is pretty simple once you break down the components. There is some thought behind the way you approach the wiring
and then you are looking at an AH meter rather than a gas gauge. You don't need to shift the motors from forward to reverse, just throttle down and keep moving down to go backwards. I did put a noisy blower fan on one side to remind me the motors were on and even then have had the wife remind me I hadn't shut the keys off.
Quick story, I was under power coming into the creek when a 40+ mono comes across my stern under full sail. The Captain
says "you running electric?" I say "how did you know?" He responds "I'm not yelling am I?." So there is the cool factor.
I've burned more fuel getting rid of it do to concern of the fuel going bad then I've probably used to generate electricity by the way.
There are a lot of things I've probably forgotten to mention. In your case under your conditions it may not be the right thing. I've heard a lot of concern about headway, distance issues, spare parts
, something for nothing, costs, ect. ect. It works for me and I get to meet a lot of people who have "heard" about Electra Glide. There are a number of people on the Chesapeake running Electric, some good size sailboats and even a classic Skipjack under electric power. This isn't going away, in another few years there will be even more people on the Electric side of the fence.
One last note because you mentioned your boat was a "performance" cat.
Didn't Gunboat select Electric Drives for their 60' cat? Pretty much the same system I have onboard EG.
Steve in Solomons MD
Lagoon 410 S2E