A question for our Antipodean cousins

[StuM]

A question about speed variation on opposite tacks comes up about once a year. But it's usually a while before someone mentions coriolis.


Here's the last two loooong discussions



http://www.cruisersforum.com/forums/...ck-188414.html


http://www.cruisersforum.com/forums/...-197605-6.html

[Cliffhanger]

Interesting theory, I'm glad you suggested it.

So, for those who don't know, the Coriolis effect makes objects traveling in the Southern hemisphere deflect to the left and in the Northern hemisphere deflect to the right. So you would gain extra lift on Port tack in the south and Starboard in the north. The question, is, how much? Is it significant? This depends on your latitude, your speed and the mass of your boat. I'll use my boat as an example. With a 6000kg boat doing 8 knots at 27deg S latitude there will be roughly 2 Newton force providing lift from the Coriolis effect. The driving force in 15kts of wind is 1200 Newtons for my boat. So that is 1/600th of the driving force or a 1:600 ratio. Ignoring water dynamics this would give us a theoretical 0.1degree lift on port tack and a negative lift on Starboard. So, a difference of 0.2 degrees between the tacks. So, it is real but with variable winds, tidal flows, waves, uneven ballast etc etc. I don't think it could be detected on my boat, in my part of the world. Faster, lighter boats closer to the equator would have a greater effect of course.

[Bruce K]

Nothing to do with coriolis!! The speed differential is obviously caused by the sun light and solar wind creating pressure gradients unequal on each side of the sails, this causes molecular flow of the lighter elements in the air toward the bow or stern depending on the hemisphere both north/south or east/west. this is so obvious that I don't understand why anyone poses the OP's question.

[Christian W.]

Yes, it's known by racing sailors, and has been documented, that a windspeed lift or gust in the Southern Hemisphere will lift you higher on port tack than the same condition met while on starboard tack. We learnt this as dinghy racers back in the day.

[StuM]

Originally Posted by Cliffhanger

Faster, lighter boats closer to the equator would have a greater effect of course.

Quite the opposite. Coriolis effect is zero at the equator. It increases with latitude.

[billgewater]

OK. Here are some way out options.
1. A boat regularly moored in a pen, may have a different marine growth pattern (possibly not even obvious to the eye) on the two underwater hull sides. This could be a consequence of different sunlight exposures due to adjacent boats or buildings.
2. Local surface wave patterns do not necessarily follow the prevailing wind direction. Wave patterns can also be modified by refraction in shallower water (although this is more likely with gravity ocean waves). Surface wave patterns are also altered by reflection from land masses. This could make the surface waves have different velocities or patterns relative to the hull on different tacks.
3. If pointing is measured via GPS instruments, then any tidal effect would be conspicuous. The water surface and the boat would have relatively different velocities to the wind depending on the tack. Basic high school vector stuff.
4. The lift generated from the sail foils when pointing depends on velocity differentials on either side of the sail foil. The consequent velocity difference leads to a pressure difference which is what we pursue when pointing. Now the pressure difference can also be (theoretically) affected by heating due to sunlight orientation on the sail panel. The sunlight may raise the exposed surface leading to a lowering of the air density near the exposed surface. Different tacks mean that the sunlight orientation changes. This effect may be negligible and would hardly be expected to have any dramatic consistent effect, but I don’t know of any studies that confirm or rubbish the suggestion. Maybe the effect is dependent on the sail colour/cleanliness as well. Also, after a tack, the exposure of the head-sail or main to the warming sunlight may be altered due to changed shading from the other sail.
5. Measuring pointing is a tricky affair and may have a subjective element. However, when pointing we all know that fine tuning is important and the stability of the tracking is easily upset. Maybe very seemingly irrelevant matters can become important. Boat asymmetry issues would certainly seem to be the conspicuous culprits though. Exposed halyards, radar reflectors, etc. could also have an effect. Objects on a mast, could have distinctly different effects on different tacks.

[Rough Magic]

Most sailboat hulls are designed to have more wetted area at a certain angle of heel. Most crusers about 15 degrees. So, if not at the same angle on both tacks, there might be a small difference in speed. Weight difference on each side of the center line would affect.

[44'cruisingcat]

Originally Posted by Djarraluda

Sailing instruments may reveal the result but not the cause.

Sailing instruments may BE the cause. Just a couple of degrees out of calibration could make the difference between pinching on one tack and sailing easily on the other.

[MartinAdelaide]

It's because the world isn't really flat. It's higher in the centre and the slope affects boats more in the outer parts (what you call south).

[RaymondR]

My previous boat sailed a lot better on one tack than the other. I did not bother to seek out the reason and just assumed that the builder could not read a tape properly and that consequently the hull form was not symmetrical.

THREAD DRIFT ALERT

If coriolis force causes weather systems to rotate why do high and low pressure systems in the same hemisphere rotate different directions?

[hhansen]

Unfortunately the link for the article didn't work.
New try: Ocean Sail Articles: Sail Twist and Wind Shear

[hhansen]

The direction of the airflow is from high to low pressure, so in a high pressure system flow is outwards and in a low pressure system inwards. Coriolis shifts the direction of the flow. On the Northern hemisphere to the right. Therefore on the Northern hemisphere high pressure systems rotate clockwise and low pressure anti clockwise. Coriolis shift on the Southern hemisphere is to the left.

link in earlier post was not correct:

http://www.oceansail.co.uk/Articles/...istArticle.php

[boatman61]

Originally Posted by RaymondR

My previous boat sailed a lot better on one tack than the other. I did not bother to seek out the reason and just assumed that the builder could not read a tape properly and that consequently the hull form was not symmetrical.

THREAD DRIFT ALERT

If coriolis force causes weather systems to rotate why do high and low pressure systems in the same hemisphere rotate different directions?

"For every action, there is an equal and opposite reaction."

[44'cruisingcat]

Originally Posted by RaymondR

My previous boat sailed a lot better on one tack than the other. I did not bother to seek out the reason and just assumed that the builder could not read a tape properly and that consequently the hull form was not symmetrical.

THREAD DRIFT ALERT

If coriolis force causes weather systems to rotate why do high and low pressure systems in the same hemisphere rotate different directions?

Because in a high pressure system the air moves away from the centre, in a low it moves toward the centre.

[sailstoo]

Tune your rig using a left handed Loos gauge. Available on Amazon.