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Comment by Jedd | Hacker News Reader

Sailing faster than the apparent wind[0] has been going on for a while, especially on v.high performance land and water yachts. [1]

The physics involved still twist my brain, but there's nothing terribly new here.

[0] https://en.wikipedia.org/wiki/Apparent_wind

[1] https://en.wikipedia.org/wiki/High-performance_sailing

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No, the issue in question here is sustained sailing faster than the wind directly downwind, which is not what water yachts and sailboats are doing. The connection (belt, gear) between the wheels and the propeller is necessary to do the former.

Yeah. Two boats could sail faster than the wind such that their center of mass is moving directly downwind faster than the wind.

This is not what they are talking about. It's always step one to get past to understand what's going on here.

I don't believe DDWFTTW has been achieved on water.

You are mistaken. It has been achieved, and the fastest race boats today do it routinely. This is one of the reasons you don't see spinnakers on boats like the America's Cup AC72s, AC50s, F50s, huge open class trimarans, etc.

I'm not sure the first boat to achieve it, but it wouldn't be surprising if it was done during the Clipper era. Those boats held 24 hour records from 1860 something all thee way to the 1980s, where modern racing yachts finally eclipsed them.

I think we just have different definitions. It's about the "Directly"

You need a propeller to go Directly Downwind Faster Than The Wind as far as I know. I think on water it has to be very large. I expect there are other ways other than a propeller but haven't seen them.

A half way definition (This isn't DDWFTTW to me either), I'd like to know if an America's Cup boat could beat a balloon? So it would be allowed off the 'race track' but it has to come back to meet/beat the balloon at the narrow finish line. So it would cover a much larger distance than the balloon and still beat it to the finish line. Is this possible in practice?

The Veritasium video doesn't actually answer this, it kinda goes around it.

[edit] If it's symmetrical which it should be, as long as they are going downwind faster than the balloon, they just reverse it at the halfway point and get back inline with the balloon but ahead.

I know what you meant. You are mistaken. Look at the 180 degree data on the polars for AC72 or Bank Populaire. They're more efficient at a broad reach, but they can do it a few knots faster than the wind.

Why does the Blackbird use a propeller, why not a simple high-performance land sail boat like the Greenbird, a design that fits with the original idea from the sailing community?

There is nothing online that says a sail boat can go DDWFTTW. DDWFTTW is at least a 4 decades old term in the sailing community looking at magazines.

Here's a design from 1985 using a propeller https://issuu.com/latitude38/docs/latitude3894apunse/162

Sorry, you are mistaken.

the sails on those sailboats act like a wing even downwind . That's only usable with racing boats which use light foils fly over water instead of heavy keels , but yes they can move faster than the wind . Curiously i can't find a good explanation of their design.

The Blackbird cannot use a land sail to go faster because it needs something to act like the keel on a sailboat and provide a counterforce. The propeller is an elaborate way to provide that force

You are agreeing current sail boats can go DDWFTTW which is directly downwind not a degree either way, faster than the wind, for a indefinite period of time? Tacking is not allowed, since that's not directly downwind.

This should also mean going directly upwind is practically possible, not a degree either way, directly upwind for a indefinite period of time with sails.

Do you have a link for this?

Here's a link to a working directly upwind rotary sail boat model - https://www.youtube.com/watch?v=j1bR5hb8RCQ

I dont know much about sailboats but i think they mean this: https://en.wikipedia.org/wiki/Wingsail . when they are at a slight angle and in combination with lightweight foils pointing the other way it seems they are able to go faster than wind in any direction, without anything like a propeller. In any case i think talking mixing wings, sailboats and propellers makes it confusing to understand what the blackbird does

You can't go DDWFTTW with sails.

No one has gone DDWFTTW on water. It will take a propeller to do when it happens.

If you want to contradict this you need to post a link of someone agreeing with you, else you misunderstand DDWFTTW.

(The link on Wingsail leaves out 0 and 180 degrees for the Points of Sail which are the only two we care about, directly downwind and directly upwind, the wiki article makes no comment on DDWFTTW. This thread is just people saying random words like AC72 or broad reach. If your ideas correct someone else will sum it up for you and you can link it. )

There some speed graphs like this online: https://www.catsailingnews.com/2013/07/ac72-polars-penalties...

I think what you re saying is that you can't be at 0 degrees to the wind, which i guess is true

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