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Comment by oh_sigh | Hacker News Reader

Notably, this ferry is driven on a chain which crossed the channel already.

I suspect they would have gone for batteries if the route wasn't already anchored in

Yea, they only use 150-200 kWh per day, that's not a lot of batteries to store on a large ship like that (it's like ~5-10 EVs worth of batteries maybe? not the whole car, just the batteries!), even if you couldn't charge between sailings (which you could). I think it was probably a cost thing, too, as this system with a cable is probably way cheaper than batteries.

edit: I was trying to reinforce here that, although this application could easily have used batteries due to the low energy needed per day, it didn't because there was a better solution due to the pre-existing cable infrastructure. Certainly this cable method isn't useful for most ferries, and most ferries will be traveling further and faster and require way more batteries.

They use so little because the use the (steel) cable for traction (as I understand it) and not water. If they had to use a propeller in the water to cross a river, the consumption would be much more.

So in essence, there's symbiosis between electricity and running a cable-guided ferry. Otherwise you'd need more energy, therefore bigger batteries, etc.

I really dont think so.

There are just crossing 500m at 2-3 knots several times a day.

This is basically less than a semi rolling resistance which is maybe 1/5kwh/km?

EDIT: even so, x10 batteries 2mwh are still very reasonable for a boat which cycle daily

When he mentioned the steel cable, I wondered why they don't use an arrangement more like cable cars. It sounded like it is in case they break loose during a storm or emergency, so it useful to have the engine on board along with the back up generators.

Are the cables actually used for traction, or just a countermeasure against having to fight the cross current?

Yea, they pull themselves along the cable. The commenter above you was correct, it is way more efficient than props, plus it's way more efficient for high currents like this river. My comment was simply that they could have used batteries, but for this particular application a cable was perfect. I was reinforcing that this was a great confluence of constraints that enabled this.

I was also wondering this, you beat me to asking. If there is precedent for two cables why not connect them to make a long narrow loop of them like on a ski-lift; place an electric-powered winch on one end and an idler on the other?

Was neat to see the video, and I’m sure they have their reasons.

One reason may be that the cable sinks to the ground, which makes it possible for other boats to cross. If it was like a ski lift, the cable had to be more tensioned I guess, thus obstructing?

> it's like ~5-10 EVs worth of batteries maybe? not the whole car, just the batteries!

Batteries are heavy, batteries need recharging, batteries can explode, batteries wear out, batteries cost a lot to replace.. or you can have a cable that might snag, but probably won't if you keep tension on it.

And batteries in marine environments age much more quickly than those in dry-land applications. Salt loves anodes and cathodes.

Some teslas have 100kwh batteries. The Hummer EV has ~200kwh battery. Depending on how much extra capacity is needed to optimize for longevity of the pack, and charging, it might be far less than 10 cars worth of batteries. Potentially as little as 1.

Makes you think that there might be quite a few river ferries that could be converted quite economically.

The Ford F150 Lightning comes with either 98kWh or 131kWh of batteries depending on the package.

Wait a minute... in theory, there could be a ferry that depends on power from an electric vehicle driven onto the ferry. That would solve the logistics problem of moving heavy batteries on and off the ship. The ferry could initially use consumer vehicles and switch to customized vehicles when the economics make sense.

Unless something changes, I think the F150 lightning will be my next vehicle purchase. The electrical grid in my area can be spotty, and one of the big selling points for me is the ability to tie the truck into your house for use as a battery backup. The larger capacity truck would be the equivalent of over 10 powerwalls!

Furthermore you could charge them during the middle of the day and at night, and use the batteries during morning and evening commutes, which is pretty ideal.

As for the additional mass and displacement, the video mentions boats having to push water out of the way, but water also fills in behind the boat, so there isn't as much physical work involved as it sounds like.

Also safety, I'd guess. Fire has much worse consequences on ships than on land...

Worth noting is, the subtitles said "The ferry uses, each day, about 150-200 kWh" whereas the speaker said "the ferry uses today between 150 and 200kW". I'm guessing Tom Scott had him clarify later, but the units, and what could be assumed to be a timeframe or not, is not the same.

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