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Comment by jdonaldson | Hacker News Reader

Price aside, boats are a fairly compelling electric vehicle. Boats are one of the most efficient means of transporting large amounts of freight : https://en.wikipedia.org/wiki/Energy_efficiency_in_transport

In short, doubling the weight on a ship does not double the energy needs required for moving it. The same can't be said for wheeled transportation, let alone air. Batteries are very heavy, but on boats this doesn't have to be a big negative.

However, we're not talking large amounts of freight here, this is more or less a personal watercraft. Also, for personal watercraft most of the energy is spent on accelerating and decelerating, something that boats are not as efficient at.

So, good idea in theory, bad idea in application.

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> In short, doubling the weight on a ship does not double the energy needs required for moving it. The same can't be said for wheeled transportation...

I wonder how the physics of freight on a ship vs a train works.

Yes, the static coefficient of friction of train wheels would make it harder to run a longer train, but I see some awfully long freight trains with only one or two diesels at the front!

Trains are very fuel efficient. CSX says trains can move a ton of freight approximately 492 miles on a single gallon of fuel. https://www.csx.com/index.cfm/about-us/the-csx-advantage/fue...

Yeah, I'm surprised by these numbers. I think trains might end up being more efficient in an "ideal" sense. The rails really help remove friction from the equation. TIL.

I suppose you'd have to factor in other issues in the "real" world, like managing inclines, curves, and starts/stops for traffic. Boats generally have fewer constraints on those fronts. Also, boats can increase their size in more than one dimension, which lets them increase size more geometrically and efficiently (trains can only get longer, and hits practical length limitations faster).

Train wheels are pretty interesting. To keep good performance, wheelsets (axle and 2 wheels) are regularly removed and the wheels are machined on a lathe. This can also be done while the wheels are on the train. Then every couple of years the wheels are popped off the axle and new wheels are pressed on. The axles last like 70 years.

With such care, the rolling resistance is very low, but it takes a bit more power to start moving. Despite that, one strong person can push one rail car, usually starting with use of a pry bar.

A locomotive cannot actually move many rail cars from rest. That’s why there is slack between each car, and to start, the locomotive is first reversed to push the rail cars together, then moves forward, pulling a single car until the slack is taken up, which pulls the next one, and so on. That’s also why you see one locomotive pulling so many cars. If a train stops on an incline for some reason, it generally needs to reverse all the way down the incline and start again.

The static coefficient comes into play during braking and if the locomotive wheels slip under power. For that, sand is applied to increase friction.

I didn't know some of this, thanks! I used to live near an active train yard, and one of the warnings people gave me when I moved there was the sound caused by the trains "stacking" (which is the noise caused by each of the car couplings pulling out of their slack).

I thought it was a drawback at first, but now that I've moved away I kind of miss it.

I used to work in the industry, mainly designing the machines that handle wheels and I really miss it and miss being in the railyards.

Why would vehicle weight have any bearing on a vehicles efficiency traveling at a constant speed on level terrain?

If we’re going to compare boats to vehicles I would think that it’s at least fair to say that the vehicles terrain must be equally flat as that of the boat.

If I’m the engine pushing a large truck at 55 miles an hour across the highway, I have no sense of the vehicles weight.

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