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Comment by qume | Hacker News Reader

I really want this to be a thing, but my rough calculations given the total cross section area of the sail and the average wind conditions on shipping routes come out to as close to nil as makes any difference for ships these size.

I posted a very rough calculation on the last HN post about this and got downvoted for some reason. I guess everyone else wants this to be a thing so much they are happy to set aside physics through sheer force of will. HN people and the investors / instigators of this project.

This is one time I'm desperately wanting to find out I'm completely wrong.

My last post in a nutshell - these things are order of magnitude the same size as the sail on my own sail boat. Forget about the type of sail. Even if 100% of that wind energy was converted to forward motion it's going to do essentially nothing in the context of a big ship.

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You were downvoted because your calculations were for regular sails, and sailboats, instead of for Magnus effect.

Wikipedia says:

    F = L*rho*v*2*pi*r^2*omega

where L is the length of the cylinder, rho is the fluid density, v is the fluid velocity, r is the cylinder radius, and omega is the cylinder angular velocity.

What angular velocity are you assuming in your calculations?

My back of the napkin assumed all energy from the wind over the entire cross sectional area.

Magnus effect or not, i'm just dealing in orders of magnitude and for the average wind speeds on shipping routes, I just don't understand how this works even if 100% of the wind energy is extracted from the cross sectional area of the sail.

[Snipped after looking at how it works again]

No, I don’t think that’s what it’s doing. It’s actually powered - using electricity to make it spin. As a result of the spin, it creates vortexes that push 90 degrees to the wind.

Yeah I think I was still confused about how it works, but I've just decided to snip out my comment instead.

After looking at the math a bit (including the one helpfully provided above), I think you're missing alot by ignoring the effect of the rotational velocity when thinking about this effect. The spinning rotation has a multiplier effect on the air movement similar to a how a much larger/fuller sail could redirect the air flow to produce more forward motion in a traditional sail.

[Edited] The effect is similar to how a rotational wing aircraft also produces lift that is not strictly proportional to the area of the wing (although for different reasons).

Without seeing your back-of-napkin numbers it’s hard to know where you might be going with this, but the idea that sail efficiency has something to do with the amount of energy in the wind in a particular cross section seems in need of justification. You’re probably better off considering air/sail interactions as a momentum transfer - the goal of the sail is not to extract all of the energy from a packet of air (presumably leaving blocks of frozen nitrogen in the sail’s wake), but rather to extract as much forward momentum as possible out of the interaction, leaving the air with more rearward momentum than it started with.

These things are not vaporware [1], and the claim (by experienced operators) is a fairly conservative 7-10% fuel savings.

[1] https://en.wikipedia.org/wiki/Rotor_ship#/media/File:Uni-Kat...

I understand that, in fact probably uniquely to people on this thread I've been on a rotor sail boat (not sailing though, in dock).

They do work when the ratio of the sail area to the boat is appropriate. In the case of the one I went on it was a very efficient hull too - very light weight catamaran.

The ships in the article you link have huge sails on reasonably small ships. The ones suggested by Norsepower are absolutely tiny compared to the ship. They are, like i've said, about the same cross sectional area of a small pleasure sailing yacht on huge container ships.

Do you have numbers to share? Link to your previous post?

It looks like it's being installed on the Maersk Pelican - a "small" tanker. https://www.marinetraffic.com/en/ais/details/ships/shipid:73...

I think I've heard that in, general, wind gets quite stronger the higher the sail is. Is this in your calculations?

No I don't, my calculations at this stage are as follows:

Sail area - same as my 10 ton boat. Gets me to 7kts in a good strong wind.

Boat they intend to install it on: 110000 tons!

My point is that for this to have any impact at all, a massive amount of energy has to be extracted from the wind. An amount of energy that just doesn't exist in the volume of moving air they are talking about.

I'm still baffled by this.

This is more complicated - you ignore hull speed for one. Amount of force needed to move ship through waves is non linear: https://en.wikipedia.org/wiki/Hull_speed

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