This may be an answer to those occasional posts by sailors who complain they always go faster on one tack that the other. The following quote is from an e-mail today by Performance Yacht Syatems in Annapolis, MD. I would be interested in getting Don Guillette's comments on this e-mail "Understanding wind sheer...Wind sheer is a phenomenon that causes the wind speed and direction to change between the surface of the water and the top of your mast. The main reason for wind shear is the Coriolis effect caused by the Earth's rotation. Then, there is a friction between the sea surface and the free air above. This friction will slow down the airspeed at sea level so the wind appears to gradually speed up into the free air above. This surface friction reduces the effect of the Coriolis force, so the wind in the Northern Hemisphere will shear (change direction) to the right from sea level up to the top of your mast. Generally, by applying sail trim according to the size of wind shear you can get more power out of the wind on starboard tack by adding more twist to your sails to match the shear. On port tack the sheared wind is "negative", and it requires flatter sail trim. Your instruments might tell you that you are sailing high and fast on port tack, but this is a relative illusion. You have less wind force in the top of the sail so the efficient wind pressure center is moved downwards. For a given heel angle, you will have a wider wind angle down low on the sails which means that on port tack you are normally not sailing as high as the instruments say you are!"
Understanding wind shear
- Thread starter jprieto
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Someone needs to hit the books.
Someone needs to hit the books. The reason that apparent wind changes as you get higher is, as they said friction slows the wind closer to the surface but the difference in angle of attack is just because the wind speed relative to your boat speed changes as you go higher. So the vector result of the boats speed and wind speed is a change of direction as you go higher. You can see this effect most dramatically on square rigged ships. All the yards are trimmed differently as you go higher up the rig. The Coriolis Effect is what causes wind and ocean currents to revolve in clockwise or counterclockwise directions in the north or south hemisphere. It is a “global” effect and has little to do with local wind. All the best, Robert Gainer By the way I think wind sheer means something when you talk about wind and this isn’t it. But I don’t know this for sure. Added 7:50 Interesting read on wind sheer. www.geo.mtu.edu/department/classes/ge406/jmedward/windsheer/how.htm
Someone needs to hit the books. The reason that apparent wind changes as you get higher is, as they said friction slows the wind closer to the surface but the difference in angle of attack is just because the wind speed relative to your boat speed changes as you go higher. So the vector result of the boats speed and wind speed is a change of direction as you go higher. You can see this effect most dramatically on square rigged ships. All the yards are trimmed differently as you go higher up the rig. The Coriolis Effect is what causes wind and ocean currents to revolve in clockwise or counterclockwise directions in the north or south hemisphere. It is a “global” effect and has little to do with local wind. All the best, Robert Gainer By the way I think wind sheer means something when you talk about wind and this isn’t it. But I don’t know this for sure. Added 7:50 Interesting read on wind sheer. www.geo.mtu.edu/department/classes/ge406/jmedward/windsheer/how.htm
port tack problem
Not to mention there is a glaring problem with trim on a port tack under this scenario...You're close-hauled. Assuming the boom is trimmed optimally for its wind, then the top of the sail must be at least in constant luff or even backwinded. I can't recall that ever happening under normal circumstances.
Not to mention there is a glaring problem with trim on a port tack under this scenario...You're close-hauled. Assuming the boom is trimmed optimally for its wind, then the top of the sail must be at least in constant luff or even backwinded. I can't recall that ever happening under normal circumstances.
Chesapeake 'Wind' shear
I'd raise an eyebrow at the interpretation of a global effect on the microcosim of a small boat on the Bay. A much greater effect and one not always understood is how the tidal current differences affect the boat, her pointing, sail trim and resultant course over the bottom. Sailing or racing in the Chesapeake, we have these little 'low tech' current indicators all over the place called Crab Pots showing you the localized current direction and speed. It would be more appropriate to factor in the tidal current direction and strength to a favored tack than a supposed coriolis efffect. I do concur with the increased wind speed at higher elevations, but since I only have a 41' mast, I'd rather factor in the 1.5-2 knot current and its differing speeds due to changes in water depth.
I'd raise an eyebrow at the interpretation of a global effect on the microcosim of a small boat on the Bay. A much greater effect and one not always understood is how the tidal current differences affect the boat, her pointing, sail trim and resultant course over the bottom. Sailing or racing in the Chesapeake, we have these little 'low tech' current indicators all over the place called Crab Pots showing you the localized current direction and speed. It would be more appropriate to factor in the tidal current direction and strength to a favored tack than a supposed coriolis efffect. I do concur with the increased wind speed at higher elevations, but since I only have a 41' mast, I'd rather factor in the 1.5-2 knot current and its differing speeds due to changes in water depth.
Local conditions vary greatly
in the Puget Sound, more from mechanical turbulence than other factors. A look at the wind patterns amonst the islands is a good indicator. Currently there's a 20+ Westerly coming down the Strait of San Juan de Fuca. This is being turned into a 12 knot Southerly in the San Juan passes and channels. This same wind is at 15 knots from the South in the Admiralty Inlet which joins the SJdeF and the Rosario off Fidalgo Island. It makes for some interesting sailing to say the least.
in the Puget Sound, more from mechanical turbulence than other factors. A look at the wind patterns amonst the islands is a good indicator. Currently there's a 20+ Westerly coming down the Strait of San Juan de Fuca. This is being turned into a 12 knot Southerly in the San Juan passes and channels. This same wind is at 15 knots from the South in the Admiralty Inlet which joins the SJdeF and the Rosario off Fidalgo Island. It makes for some interesting sailing to say the least.
I guess that explains why we can't seem to get the
Cup back permanently from those southern hemisphere sailors. They're flagrantly beating us on port tack with our own Coriolis! Why haven't we thought of that before?
Cup back permanently from those southern hemisphere sailors. They're flagrantly beating us on port tack with our own Coriolis! Why haven't we thought of that before?
That info may be useful to a 747 captain but
to a vessel that has 2/3s of its sail area at less than 30 ft in the air I don't think it matters much if any.
to a vessel that has 2/3s of its sail area at less than 30 ft in the air I don't think it matters much if any.
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