Power consumption of electric drives in Catamaran

[Saltifinch]

We're interested in building a 35' power catamaran, primarily used as a coastal cruiser. Would love if anyone has approximate power consumption figures or charts, specifically in the 6-10 knot range, which will be our primary cruising speed.

We'll be using electric drives, with energy coming from solar power and either diesel or fuel cell. The hybrid system would be in serial. I've seen estimates that it takes 10kw to move a 35' catamaran at 6 knots. Any more data would be greatly appreciated.

[Macdoug]

I can’t offer you any help on this as I have no experience, but I will be very interested in the answers. I have looked at the option of electric drive on a sailboat, but my feeling from limited research is that it isn’t quite ready for mainstream yet, unless, as you are doing, you have a back up generator (I was considering it without the generator).

[Saltifinch]

Much of this is technology and build dependent. With the build we are using, and with OxfordPV tandem solar cells coming out in 2022, we could possibly generate 25 kw of power. The question is, will that be enough for cruising at 9knots. We've looked at hydrogen as well, but the infrastructure is almost nonexistent to support it. So either a complex system that uses a fuel cell as well as an electrolyzer, or just a simple old fashioned generator as backup

[mcarthur]

For our power cat - bigger and heavier than yours - an estimate from a vendor is:
8.5kW battery draw for about 5kn,
22kW for 6.3kn,
30kW for 7.4kn
83kw for 8.5kn
124kW for 9.1kn
and then it gets a bit silly (if not already!).

I've read about other examples:
- 25' cruiser, 4kW for 6kn
- 30' cruiser, 8kW for 6kn
- 29' Uma, 6/7kW for 6kn

[mcarthur]

...and I'd say if you kept your 35' fairly light, it's underwater shape is good, and have a 10:1 LB ratio or more, then 6kn may be "gettable" by 5-6kW, 8kn by 20kW, 10kn by 50kW. At 6kn or so, you don't need a fancy CS shape or anything too much, but by 10kn it's probably starting to make a difference (I seem to recall Malcolm T stating that CS only comes into itself in the early-mid teens....)

[StuM]

Originally Posted by Saltifinch

Much of this is technology and build dependent. With the build we are using, and with OxfordPV tandem solar cells coming out in 2022, we could possibly generate 25 kw of power.

Peak output? So you are looking at maybe 100kWh per day.

Neglecting house loads:
If you motor for 10 hours, your maximum sustainable output from solar (and a large battery bank) would be 10kW. (On your figures that would be 60NM at 6 knots)

Motoring continuously for 24 hours, your maximum sustainable output from solar would be about 4kW.

[Saltifinch]

These numbers are exactly what I was hoping for. Saving 100kw a day is almost 10 gallons of fuel we'd be saving per day. And silent running for the most part. Seems like a smart idea to me. We're not interested in cruising fast. We want an efficient trawler basically, but with more open liveable space. So a power cat seemed the best.

[Dsanduril]

25kW of generation from 30% efficient cells requires 83 square meters of panels (nominal ratings at STC and no dead space). On a 10.5m cat that’s going to require an 8m beam and 100% coverage of the entire vessel with no shading problems. Not saying it can’t be done, but is that the boat you want for “more open liveable space”?

I’d certainly be interested in your proposed power train, I like the idea of a hybrid solution on a cat, but unless you want to dedicate every bit of deck space to solar panels (a la Tûranor PlanetSolar) I think the estimate of available power from the sun may be stretching it a bit.

[Saltifinch]

I think the formula is roughly 1kw per square meter. If the vessel is 10.5m x 4.5m, then to generate 25kw of power should only require approximately half the approximate space of the vessel. We picked a plan specifically that has a very large cabin specifically for this.

[StuM]

Originally Posted by Saltifinch

I think the formula is roughly 1kw per square meter. If the vessel is 10.5m x 4.5m, then to generate 25kw of power should only require approximately half the approximate space of the vessel. We picked a plan specifically that has a very large cabin specifically for this.

Totally insolation at sea level is roughly 1000W/m² (clear sky)
Assuming that your OxfordPV tandem solar cells do achieve 30% efficiency in a panel (which is doubtful in the near future - they are getting close to that for a cell), you will still need 83m² of panels over your 47.25m² vessel to generate a peak 25kW, so still nearly twice the boat's area.


By way of comparison, the CEO of Solaryachts has stated:
"the Solarwave 64 has 2 x 60 kW Motors, topspeed 11 kts, 160 kWh batteries, 15 kWp on the roof with a production of about 80 kWh per day"
And that's a lot bigger than a 10.5m x 4.5m vessel (at 19.5m x 10m = 195m², it's over 4 times the area).


So if you look at the same percentage coverage and solar efficientcy as the SolarWave, you are likely to achieve about 4kW and 20 kWh per day (If you can up the panel efficiency, you may stretch it to 5kW and 25kWh per day.)

[StuM]

Solarwave 64:

[Saltifinch]

Oh dang I made a fundamental mistake in my math. I see it now

[valhalla360]

Originally Posted by Saltifinch

We're interested in building a 35' power catamaran, primarily used as a coastal cruiser. Would love if anyone has approximate power consumption figures or charts, specifically in the 6-10 knot range, which will be our primary cruising speed.

We'll be using electric drives, with energy coming from solar power and either diesel or fuel cell. The hybrid system would be in serial. I've seen estimates that it takes 10kw to move a 35' catamaran at 6 knots. Any more data would be greatly appreciated.

A lot will depend on weight and hull design. If you keep her light and have a very slippery design (think narrow hulls with little load bearing capability), it may be viable.

On our Gemini 3400, the 10kw for 6kt is about right. The problem is 7.5kt is 20kw and it's mostly just making a big wake at that point. Of course, the hull design isn't intended for motoring at 10kts.

Getting 25kw of solar on a 35ft boat seems an overly ambitious assumption. Assuming a 15ft beam and 14w/sft, that's only 7.3kw if the entire area is covered. I seriously doubt these new magic solar panels are going to beat current technology by a factor of 3.5. Probably need closer to a factor of 4 when you figure house loads and losses storing that power for later use.

That said, if you travel slow, it will be technology viable (financial viability not so much). Doing the Great Loop, it's mostly day sailing and most days, we would travel 20-30miles in a day typically and 40-70% of days, we were stationary. Add in some marina stops where you can get a boost from shore power and a small generator and some planning should make such a trip viable.

[Ifitsworthdoing]

Hi Saltifinch - we are planning the same thing as you: We are commissioning a non-sailing version of the Broadblue 346 (34ft, 5 ton lightship) catamaran with plenty of solar electric on board.

Primary propulsion is going to be 2 x 9kW rim driven thruster pod motors with possibly a NeanderShark 50hp diesel outboard (installed in the centreline cockpit floor lazarette) as secondary/back-up propulsion and a small/portable 3.2kW diesel gen set in the aft cockpit floor storage space (where the gas bottles would normally live except we will do without LPG).

OceanVolt kindly ran the boat's design numbers through their proprietary software and came up with the following performance figures:

Speed (kn) / Shaft power delivery (kW) / Battery power draw (kW)
2.4 / 0.3 / 0.4
3.4 / 0.8 / 0.9
4.3 / 1.7 / 1.9
5.3 / 3.6 / 3.9
6.3 / 5.3 / 5.7
7.2 / 11.3 / 12.3

So 6.6kn is roughly the unity point for speed vs. power draw. I trust this helps somewhat.
Best wishes with your enterprise!
Kai (IfItsWorthDoing)

[Adelie]

The unity point doesn’t seem very important except maybe for marketing purposes.

The better question involves how much solar will be mounted, how much battery capacity you will have and what speed you can maintain 24/7 under perfect insolation conditions.

I expect that speed to be about 3.0-3.5kt.