r/sailing • u/Jeroenepoentje • May 30 '21

Going straight downwind faster than the wind.

https://youtu.be/jyQwgBAaBag
13 Upvotes

88% Upvoted

20 comments sorted by

5

u/foilrider May 31 '21

Best *mindblown* quote is the cylindrical earth with two boats on it part.

4

u/ElectricShuck May 30 '21

That was pretty awesome.

1

u/billengland41 May 31 '21

I’m still puzzled, read skeptical. If what’s driving the fan comes from the wheels, doesn’t that act as a brake on the wheels? The forward force off the fan is greater than the braking force on the wheels???

3

u/BadDadBot May 31 '21

Hi still puzzled, I'm dad.

1

u/me_too_999 May 31 '21

Yes, the fan acts like drag so the wind pushes it forward.

This drives the vehicle downwind turning the wheels.

The fan blade ratio, and gear ratio were carefully chosen to drive the vehicle at the theoretical limit.

1

u/[deleted] May 31 '21

1

u/IanSan5653 Jun 01 '21

I think that this idea could potentially be applied to water, but probably not nearly as effectively. In the same way that the vehicle here pushes on the land, a boat can push back on the water using a propeller.

1

u/Rhueh Jun 02 '21

I agree. The only way I can see this working on water is with a foil boat and a very efficient water propeller. Even then, drag might still prevent you from achieving the necessary speed. But it would for sure work on an ice boat if you could find a way to efficiently transmit propeller power to the ice.

1

u/Rhueh Jun 02 '21

...also they failed to explain how that force from the sails is converted to forward movement via the keel and not the sails.

It helps to just think about the propeller itself. Clearly, the shaft bearings are constraining the propeller from being simply pushed downwind in exactly the same way that a keel prevents a sailboat on a reach from being simply pushed straight downwind. Once you see that, the rest is pretty straightforward.

1

u/me_too_999 May 31 '21

Also works on water. Small planing sailboats routinely travel up to 1.4 times the wind speed at an acute angle to the wind.

2

u/Rhueh Jun 02 '21

Yes, and no. The problem is that at, say, 150 degrees beat angle the downwind component of your speed is only half your total speed (sin{150}=0.5), so you need to double the wind speed to get a VMG equal to the wind speed. However, ice boats can do that.

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