How much electric power is needed to power a 40 - 45' cat at 5 knots?

[senormechanico]

Solar panel efficiency is improving, but imho you should wait until it approaches 300%.

[Pizzazz]

So, one post says, a 12-ton catamaran, with 2x 8kW motors can motor all day long with 65 sq meters of solar at 5 knots economical speed. I think this is not bad actually, if true.

If they have 10kW of solar, and you choose the longest days (say 30% in July), this will give you 30 kWh of power, which means, you have at best 1.5 kW available. I doubt this can give you 5 knots. May be 3-4 knots max. The point though is that for a relatively small investment (small motors, panels and batteries are not that expensive), you have unlimited range. Also, if you are crossing an ocean, you will get another knot of current, so you will make on average 100 nm/day or 30 days to cross. Not bad actually and not different from many sailboats.

I would probably want to couple this with a generator. I am convinced now that a diesel generator driving an efficient electric motor with a properly sized propeller is probably just as efficient as the direct drive... only because all the components work at optimum speed. Also, a diesel generator is much cheaper than a marine diesel engine. So, get the setup, start with the generator and migrate to solar panels. Then you can motor for 10 hours and run silent for 14 hours. Kind of weird way to minimize the noise of an engine.

I could see the use of this for may boat. Say I wanted to move it across the ocean. One way is to ship it ($80k). Another way is to run it slowly at 7 knots optimum speed, on one engine (constant noise, wear and tear on the expensive engines but not so bad overall, since it will be 200 hours/engine). Alternatively, I could put a small electric motor (5 kW) with a properly sized propeller and run the generator at 5 knots. It would take a month, 600 hours on the generator but it will be relatively quiet. Fuel would not be a problem because you can get 10 nm/gallon on the generator (the big engines will probably use double to power just to turn themselves over).

[Jim Cate]

Also, if you are crossing an ocean, you will get another knot of current, so you will make on average 100 nm/day or 30 days to cross.

What a wonderful ocean you have discovered. None of the ones that I have crossed have such a benevolent current lurking about. More likely their evil twin, the adverse current which seeks us all while at sea.

Jim

[mcarthur]

Originally Posted by Pizzazz

So, one post says, a 12-ton catamaran, with 2x 8kW motors can motor all day long with 65 sq meters of solar at 5 knots economical speed. I think this is not bad actually, if true.

If they have 10kW of solar, and you choose the longest days (say 30% in July), this will give you 30 kWh of power, which means, you have at best 1.5 kW available. I doubt this can give you 5 knots. May be 3-4 knots max. The point though is that for a relatively small investment (small motors, panels and batteries are not that expensive), you have unlimited range. Also, if you are crossing an ocean, you will get another knot of current, so you will make on average 100 nm/day or 30 days to cross. Not bad actually and not different from many sailboats.

Hmm, with no shading (motor boat, not masted) 65sqm of area should give (1000W/sqm @ 20% efficiency) about 13kW at about a daily 4x multiplier = 52kW/day.
5kn for a relatively efficiently hulled (beam-length) and decent length cat, especially at that speed, should be around 1:1, so about 5kW (assuming no huge wind against you).
52kW/5kWph = 10hrs @ 5kn = 50nm before you are drifting for the next 14 hours on passage.
Of course for 1-2day passages you could supplement with pre-stored battery power: 50kW of usable battery giving you another 10hrs and another 50nm. Then they are depleted too.
So the only good options for longer would be a) go slower, say 3.6kn instead of 5, cutting power to about 2kW = a whole day's power & 85nm; b) use a DC diesel genset to go direct to motors or better to re-fill the batteries, and then it's just a choice of genset size and diesel storage.
(a) would of course require batteries to store the power during the day and let it out slowly at night.