Faster on Starboard than Port. Why? - Page 2 - SailNet Community

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  #11  
Old 08-01-2006
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I agree with the last two posters. KISS, it works.
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  #12  
Old 08-02-2006
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Wink

Quote:
Originally Posted by sailingdog
What a load of CRAP.... the coriolis force has nothing to do with this. The coriolis force would vary, depending on the direction of the wind and what point of sail you were on, and should, if it exists at such a microscopic level, even out overall. If you think that coriolis force is going to be strong enough on such a small scale to effect boat speed by a significant percentage—hull speed on this boat is about 8 knots, so half a knot is about six percent—you're hallucinating.
Thanks for your kind words SD - but really, was it necessary to be quite so vehement? I only put it out there for discussion as something I'd read - and perhaps I didn't explain myself clearly:

What I'd *read* is that the coriolis effect - which is what makes cyclones rotate anticlockwise in the northern hemisphere and clockwise in the southern hemisphere - also has an effect on less dramatic wind patterns. And that this, when combined with the surface drag at sea level is sufficient to create a difference in the true wind angle of a few degrees between the bottom and top of the mast. The conclusion that the paper I read came to was that these few degrees' difference were sufficient, when beating to windward, to create an measurable difference in boat speed. All other things being equal, it seems at least *plausible*, especially when you reflect that wind speed and hence the abiblity to generate lift increases the further you move up from sea level. I'll try to find the original piece so you can all read it for yourselves and form an informed opinion

Blue Eagle
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  #13  
Old 08-02-2006
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The coriolis effect and wind shear.

Right, found it - or at least a similar reference at last... from the advanced setup manual for the Silva NX2 Race Software - if you're not re-trimming the amount of twist in your sail between tacks, or if, like mine, your sail is a little old and baggy, and you can't flatten it properly on port, you may be getting more boatspeed on starboard than port. See below:

http://www.silva.se/marine/fdx/nx2advanced_manual.pdf

1.5 Wind Sheer
When there is wind-speed, there is wind shear!
What is wind shear?
The main reason for wind shear is due to the Coriolis force. The wind will rotate counter clockwise on the North Hemisphere around the centre of the low pressure.
Then, there is a friction between the sea surface and the free air above. This friction will slow down the airspeed at sea level and gradually up into the free air above. This friction will also decrease the effect of the Coriolis force, so the wind will shear (to the right) from sea level up to the mast top (and above). The wind transducer will only measure at one altitude, so you need to understand and consider this sheared angle to adjust the sail accordingly down to deck level.
Note! Several meteorological effects will have impact on the size of the sheared wind. When cold and warm air is mixed with faster winds from higher levels, gradients and sheared winds may locally change very fast.
Generally, by applying sail trim according to the size of wind shear, you can get more power out of the wind on starboard by adding more twist to match the shear (on starboard only). This will reduce the top force from the wind, move the pressure centre downwards and allow for a more forward pointing and efficient wind force.
The wind instrument will "show you" that you are sailing lower than port side, but it is only a relative illusion since the reference is from your average attack angle and sail trim including wind shear.
On port side, the sheared wind is "negative", and it requires more flat sail trim. The instruments will tell you that you are sailing high and fast on port tack, but this is also a relative illusion, but opposite from starboard tack. You have less wind force in the top of the sail, so the efficient wind pressure centre is moved downwards. Then, for a given heel angle, you will have a wider wind angle, which mean that you are actually not sailing as high as the instrument says!
************* End Quote *************

cheers,

Blue Eagle
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Old 08-02-2006
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Wind Shear / Boat Speed

Also, you might find the following link instructive:

http://www.bom.gov.au/weather/nsw/am...%20Shear.shtml

Cheers,

Blue Eagle
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Old 08-03-2006
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Sailormon6 will become famous soon enough
Recently, my C&C 35 was significantly faster on port tack than on starboard, and, after messing around unsuccessfully with sail trim, the first thing I checked was the rig tuning, but it was right on the money.

The Kenn Batt article describes exactly what I was experiencing on my boat, so I'm going to try his suggestion to use less twist on the headed tack and more twist on the lifted tack. I don't know if it's correct, but it sounds logical, and, if I can't find a solution, it's going to drive me nuts!

Thanks for the suggestions Blue Eagle!
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Old 08-04-2006
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you're welcome Sailormon - let us know how you get on...

Blue Eagle
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  #17  
Old 08-07-2006
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It likely is the placement of your spedometer transducer, if you are going by the speedometer. If you are using GPS it is likely current. My boat is the same way.
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Old 08-07-2006
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Just make sure that you are always on starboard tack as you approach the finish line.
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Old 08-08-2006
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faster on one tack?

I have found that it is not that easy to determine why a boat is faster on one tack than the other. But here is a list to get started.

-Instraments are not calibrated. probably #1 reason
-Wave angle
-all of the under water reasons you could think of, keel not faired evenly, or not centered and straight, rudder not straight, transducer off center, etc.
-Rig not properly tuned.
-Sails not properly trimed. A lot of bigger boats have different people trimming from side to side and that can be a huge problem. Wind Sheer is a real and powerful force not to be forgotten.
-I have even seen drivers who could not drive as well sitting on different sides of the boat, left or right handed drivers???

My biggest question is, is the boat really different side to side? When racing agaist other boats do you really see the difference. That should be the first test, never blindly trust the instraments, go find a buddy boat and test tack to tack.
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  #20  
Old 08-18-2006
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