My apologies - we've been burned before by vendors touting numbers (see the solar stick thread) that do not up to reality who essentially pulled a 'solidnav' on us (got down right abusive when questioned).
You mentioned using this system on a light Cat. I happen to have one, a 2007 Gemini 105Mc, with 1000 on the water a good representative. They are built specifically for cruising couples and are CE open ocean category A rated.
I think it still comes down to the numbers.
On my weight sensitive Gemini it just doesn't work; on a full keel mono where sailing performance and safety would not as deeply be affected it might.
I have a 27hp Westerbeke 30B, delivers at worst 20hp to the prop, a 15x12. Note the prop is exactly the type of high pitch prop touted as best for the system, and on my boat I have a Stillete drive leg that allows me to lift it out of the water for sailing speed; doing so adds 1 knt to my speed.
At 2800 rpm (it maxes at 3600) I get 7.2 knts in flat water, no wind using 1.12 gals per hour. I know this because I keep logs.
Completely ignoring the battery storage issues because batteries available today are simply not comparable (nor close to being) to diesel - let's just say batteries don't matter and go on with the discussion.
20hp = 14.9kw - plain and simple (nice conversion web site so I won't bore you with the math); Unit Converter
For the purist: Horse power (hp) ---> Kilowatts (kW): hp x 0.7457 = kW
Kilowatts (kW) ---> Horse power (hp): kW x 1.341 = hp
Reverse engineering the 8 kw system that is under development indicates it delivers 10.8 hp, at 100% efficiency. I'll bet solidnav claims it's equivelant to 50hp engines simply because the target boats for that system (34-44 ft boats) have diesels n that range.
For the math challenged, that's half my current hp, and at 9600 pounds of weight (light ship, dead empty of all tools, clothes, food, water, fuel etc, not half load) provides at best 1 hp per 1000 pounds - again, half what any modern NA recommends.
We can talk getting off the grid and being oil independent all we want, but I'm not giving up safety so I can feel good about my carbon footprint.
If I take off the 300 pounds of my engine I rounded way up) - remove the empty tanks, hoses, filters etc - I can save 350 pounds (again, light ship, so I'm not adding in fuel weight), then add back the 90 pound 8kw engine, I still - not counting batteries - need some heavy assed copper wires to handle the amp's the motor needs. I'll be nice and say 150 pound of weight total, saving me 200 pounds of weight.
To walk away dead even in weight I have a budget of 200 pounds for batteries, solar, wind and towed generator.
As I've said before, 900 pounds sinks my boat 1 inch. My full up cruising weight (due to design characteristics i.e., shortfalls) limits me to 1800 pounds of food, clothes, diesel, water and people. That's a arbitrary limit because if I say 1800 pounds I can hope for 2200 pounds in real life.
I have two 80w kyocera solar panels that deliver about 40ah a day at 12v, or 10ah at the 48v to drive the motor. If I bumped that up to 10 panels (totally ridiculous, no where to put them) they would weigh in at 500 pounds and give me only 50ah at 48v. I could motor about 1/2 hour on that; so that's no good. I could make bare steerage (1 knt) for longer, but so what?
Hmmm, wind gen - Practical sailor did a nice review on the systems out there today. I'll use their numbers - I can expect 100ah a day at 12v in most areas. going back to ridiculous installations add one on each corner of the boat (4) and I get 100ah at 48v and again get about 1 hour of motor time.
I'm completely ignoring the batteries because I said I would just so I could point on the one glaring deficiency that still remains in this system and it's marketing:
It's still not living up to the marketing claim of powering along through the doldrums now is it?
Solidnav marketing quote:
While diesel sailors are loading their decks with jerry cans and scraping through long doldrum days, you can motor through in silent bliss. As others are limited by range, you are limited only by desire.
The desire to have a back up motor powered by diesel?
Sure 4 windgen's dumping directly to the motor could turn the prop and with about 2ah (at 48v) - about 1/50 of it's normal - about the same as a guy with a kayak paddle. Not silent bliss in any reasonable stretch of the imagination.
We are talking doldrums so that wind gen isn't working so well now is it? We know solar, with the boat covered in panels provides half the power, during daylight - and nothing left over to store in batteries. Does it look to be feasible yet?
Kick in the aforementioned 16 lifeline AGM's, being able to use only 1/3 of ah so I don't damage the batteries doesn't really help - I still can't make the power it takes as fast as the motor consumes it. I would in fact need the batteries to store up the power so I could run the motor at all.
Serious guys with calculators take your hands off your keyboards, I know that solar panels wired in series can dump at higher voltages and make my numbers on the low side etc. I chose to keep it simple and not break out my table of formula's and calculators.
wow, I really went on for a bit didn't I ? I hate false marketing of false hopes.
Someone please prove me wrong !