Silent 80...

Hi all, first post here. I searched, and found no thread on this boat, a few mentions of a 55.
I contemplated posting in other topics, but those have fewer posters on them. And noticed several multihull threads in this general purpose design one,hopefully in the correct place.

There's an electric catamaran (with diesel back up of course) that doesn't make sense to me- 80 feet length, 36 foot beam, 1.25 meter draft,60 tons, cruising speed 6-8 knots (claimed), has 26 kw (peak) of solar, 240 kwh lithium battery pack, etc.
We know that 26 kw of solar really isn't, unless you have a perfect sunny day, and the sun is perpendicular to the panels at all times. Otherwise it drops off precipitously.
We know that the rated output and the actual output of a solar panel,are two different numbers- with the actual being often 5-10% less. We also know that 26 kw is about 36 hp.

From the first link below:
Q:"The battery has a capacity of 240 kWh. What is the battery chemistry and how long can the vessel cruise at 5 knots using only the battery?
A: We use Lithium-Ion batteries from Victron Energy. Under normal sunny conditions, the boat can cruise day and night at a constant speed of 5 knots.

Seems to me he is implying it'll run 24/7,and their website claims "unlimited cruising".
Basically they are saying that (even ignoring the inverter,line,controller,and motor losses,and ignoring the solar losses and lower actual output) that 26 kw peak/35 hp from the solar will run it all day, which in my mind is a stretch.
And then run all night- say 12 hours at 20kw/h (26 hp). And there's certainly not enough solar to run the boat AND charge the batteries to any extent.

And then the next question,he says it'll take 2 days to fully charge the dead battery off of solar...so how can it do the above?
The website says 6-8 knot cruise, this interview says 5 knot cruise.

Background: boating since a kid (decades ago) on the British Columbia coast, power and sail,heavy waves/tides and currents. One Trans At on a 50' mono, Australia (and all points in between) to Hawaii on a 55' cat, thousands of hours of fishing,crabbing,prawning,exploring,ocean kayaking,PWC,etc. So I have a pretty good idea what it takes to move around.

The question is to all of the expert naval architects,designers,and multihull wizards on here (along with an obvious one which I won't ask)- solely electric,will 20 or 25 or so hp push an 80', 60 ton cat,on anything rougher than a millpond lake "day and night"?

Thanks in advance for your thoughts.

Electric-Hybrid Yacht With 26 kW Of Solar Power Under Construction | CleanTechnica https://cleantechnica.com/2019/11/14/electric-hybrid-yacht-with-26-kw-of-solar-power-under-construction/

SILENT 80 | Electric Hybrid Catamaran | by SILENT-YACHTS https://www.silent-yachts.com/silent80/

 

Also, the 55' I was on had two 100 hp engines,and we'd run 2/3 throttle to make 7 or 8 knots and it was maybe 20 tons.

 

Not an expert, just interested in solar powered yachts. 26kWp is something like 122m² of solar panels. That's quite a lot. To make reasonable estimates you'd need to know what area and season you want to operate the boat. Then you'd need to factor in the buffer. I figure for most use cases you'd be cruising for one or a few days where the battery can buffer so you recharge while on anchor. So how efficient this is depends on your use profile and location and area. With a generator you could buffer unusual cases so there is on inconvenience, but it makes less sense if you need to rely on that. If you just use the yacht over a weekend with enough time for the battery to fully recharge it works. I think the idea can work but I think it needs to be a much much lighter and more efficient yacht.

Even in the ideal case with very high efficiency solar panels and Mediterranian summer you'd get 174kWh a day or only 7.25kW all day round. Is that enough to move a big yacht like that at 5 knots?

All Sky Insolation Incident on a Horizontal Surface (kWh/m²/day) NASA POWER | Data Access Viewer https://power.larc.nasa.gov/data-access-viewer/

Code:

Solar PV Surface    122.00 m²    1464 kg

                          kWh/m²/d    kWh / day
North Sea Summer                4.75    117.9
North Sea Spring / Fall        2.72    67.6
North Sea Winter                  0.97    24.1
Mediterranian Summer            7.01    174.0
Mediterranian Spring / Fall     4.63    114.8
Mediterranian Winter               2.28    56.6

PS: Sorry for the bad formatting haha. It's just the solar insolation multiplied by efficiency and surface area of the solar panels.

 

50' cat, 24 tons, pulled about 80 amps @ 12 volts at 3am when sailing with one human awake, just nav and misc house loads during delivery. So multiply that by three, I guess. Then add hotel loads like watermaker and A/C and galley and laundry and guest usage to that. Take that out the batteries. Apply round-trip efficiency figures, and and deduct from solar charger capacity and see what you have left for propulsion.

Then compare this to the actual drive train fitment. If the boat is propped and shafted for 2 X 250kW at 1200 rpm (something like a pair of 28 x 20 props), it's going to take in the neighborhood of 6 horsepower to just to tick the shafts over at no thrust at 5 knots. So you can subtract another 144 hp-hour from the system. You still haven't moved the boat yet. Now calculate the actual resistance of the boat, and figure a prop efficiency no greater than 50% at these speeds, and see what kind of power is required. Compare that to the residual solar.

That shafting deduction is a big reason why these economy cruise calcs tend to be wishfull thinking. You still have to overcome the friction of the shafting and gears that are sized for much more power. That and the fact that added drag from waves and wind are not adequately allowed for.

Soooo -

Estimated daily house load for normal 24h cruising - 100 kWh
Estimated shaft friction @ 3% installed max torque times actual rpm for 5 knots for 24 hours - 100 kWh

Estimated daily power available from solar - 150kWh - It's not looking good. You are 50kWh short and you haven't made any thrust yet. I'm pretty sure I would recommend the 2 X 130 kw gensets for anyone wanting more than a sunset cruise.

Maybe Alik or Ad Hoc can supply some reasonable resistance estimates for this sort of craft in smooth water and in 10' seaway/25 knot wind @ 5 knots boat speed, but until then, lets say you need an additional 40 kW propulsion under nice cruising conditions - so about 1 MW from generators each day.

Let's say it makes sense to run alternate gensets at 100 kW average load. You need ten hours of runtime. It looks like you can manage about five hours in the dead of night on batteries before having to start the genset. Run the genset from 5 am to 10 am, then let solar take over topping up. Then run genset 7 pm to midnight. Run both sets anytime most of the guests are ashore on excursions.

I don't have a problem with this at all as an engineering solution, but the owner should still be prepared to buy diesel by the ton from a delivery truck.

 

Thanks for the reply Dejay, it still doesn't add up to me.

Thanks for the detailed write up, Phil.
I also left out house loads, shaft losses just to try and give them even more benefit of the doubt. I guess that 800 gallon fuel tank, with optional tankage to be over that, is not for decoration purposes.

Spent a little time on yachtworld, looked at some sailing cats in the 75 foot range, from 50 to 60 tons and they all have at a very minimum (just one) 400 hp,with the rest 500 to 600 hp. What the cruising speed would be at say 5% (25 hp- a bit over idle?) load I have no idea.

Nice to know my suspicions are not misplaced,and as well, the price is $5.5 million.

Edit- spelling