Saturday, April 21, 2007


In the absence of any real sailing (by me) to write about recently, please indulge me by helping me to answer the question below that was emailed to me by one of my readers. It seems to me that this issue has all the potential to create as much confusion, misinformation and just plain bad science that I managed to stir up with my posts on How Sails Work early last year.

Good day to you sir,

The last 6 months I asked many off the same questions to myself that you also asked yourself. I have red everything that is to be red in regards to articles from A Gentry and I bought the latest version of C.A. Marchaj "Sail Performance". All in English so for a young Dutch guy a bit off an obstacle .... but I'm getting there.

I'm not addressing you in regards to "how or why sails work" but about the "Coriolis effect" and it’s part in Sailing tactics.

Nexus made instruments that can make the effect off wind gradient and wind shifts visible for us sailors. Very handy in regatta sailing. They claim that flowing air is influenced by the Coriolis effect.

With an overall tacking angle off lets say 75 degrees to apparent wind it can happen that on starboard tack the tacking angle is consistent 35 degrees and the port tack 40 degrees. Also on starboard tack (beam to port) sailors experience more need for twist high in the main then on the port tack.

I explain “beam to port” because here in the Netherlands we call a starboard tack a tack were the beam is on the starboard side and the wind is coming from port, I think in some other places they use it the other way around.

Could you give me your perspective on this question …… Why is the Coriolis effect influencing the wind gradient/direction?

Best regards


Carol Anne said...

Everywhere I've ever sailed, starboard tack means the wind is coming from the starboard side of the boat, and therefore the boom (I assume that's what you're talking about) is on the port side of the boat.

As for the Coriolis effect (the effect of Earth's rotation on moving fluids), I don't think it would have that much influence -- it's so small as to be almost unmeasurable.

It is popularly thought to influence water going down a drain or toilet, such that water spins one way in the Northern Hemisphere and the other way in the Southern.

But in actual practice, no matter which hemisphere you're in, it's pretty close to random which way the vortex actually whirls -- if there's an underlying current in the sink, or some slight assymetry in the water outlets into the toilet bowl, that will influence the direction of spin far more than the Coriolis effect will.

I just can't imagine any circumstance in which the Coriolis effect would influence the air flow over a sail. Yeah, it might influence the weather pattern in which the boat is sailing, but you take care of that as wind shifts.

Maybe you're actually thinking of some other factor, such as the Venturi effect?

JP said...

I would be surprised if the very small coriolis effect could alter the tack angle.

I'd agree with the comments above and suggest it might be worth checking the calibration on the wind indicator or maybe check for bend in the mast.

Tim L said...

I think I've seen reference to this in some older sailing books but just generally due to wind-shear/tide rather than coriolis.

The right hand shift of the sea-breeze (northern hemisphere!) is about the smallest scale weather phenomenon to be caused by coriolis - and even that is usually over many miles and hours and can be overidden by other factors pretty easily...

Litoralis said...

Flowing air is influenced by the Coriolis Effect but the Coriolis Effect acts on air flow in the atmosphere at a macro level. There would be no effect on air flow at the scale of a sail. The only possible micro effect I can imagine is a slightly difference in the tendency of a sail to produce tip vortices on different tacks.

Anonymous said...

Take a look ay the Brookes and Gatehouse site and read some of their instrument manuals. They ascribe very significant effects to the Coriolis effect and go to great lengths to correct for it in their sofware and systems.

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