Speed difference between Port and starboard tack

[Rocinante33]

Frank,
Another helpful bit of information to guide you in rig tuning is located in the Specs & Docs section for the E-33. It is in the #2 owner's bulletin included there. Like your 30+, my 33 is a fractional rig. This bulletin came from an expert at the factory (Don Kohlman) at the time of production. It does not list specific tensions you should use, but does provide a stepwise process toward achieving the proper tensions on the fractional rig. Make sure to look at the diagram on the last page to be clear on the terminology he uses.
These documents were aboard my boat & Sean was kind enough to add them to docs. It probably is not where you would think to look for your boat, but I think it is relevant.
Good luck,

 

[Frank Langer]

Thanks, Keith. Very helpful--you're right, I wouldn't have thought to look there.
Frank.

 

[evm]

....

Sorry to not look through the entire thread. Perhaps this was brought up.

Was the speed measured via GPS or knotmeter? My knotmeter is offset and reads differently based on heal angle and tack.

--Ethan

OK I went back and looked. I see that GPS vs meter was brought up.....

 

[msc1212]

I don't know about the more experienced sailors but the Raymarine speed transducer I installed this spring is just about useless. I put it where they recommended and calibrated it correctly (I think) but it is consistently off my GPS. I've gone into the software and given it a correction factor of 125% which brings it closer to the GPS. The only time I use the transducer is when trimming. I can see changes in the speed almost immediately and in 100's of a knot rather than the 10's which the GPS gives.

I went down to the boat again this weekend and loosened all shrouds and then began to tune them according to the Rigger's Apprentice. If you google mast tuning there are a number of guides that show up for various boats. Scanning through them gives you a very good idea of the principles behind mast tuning. There is no difinitve text as far as I can find, nor is there an absolute answer on tension with the exception of One design boats like the beneteaus. Of course, as we all know, evertthing has to be reduced to simple equations for Beneteau owners because- after all- they are Beneteau owners.

Sailing again this weekend. Frost on the boat in the mornings now. Should be interesting.

 

[corkhead]

Read this today.....

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 instrument say you are!

 

[Kevin Johnston]

South Hemiphere Port Tack Efficiency

So, does that mean in the South hemiphere the opposite tack will be more efficient? I find that interesting. Can anyone in the Southern Hemiphere confirm this?
KJ

 

[NateHanson]

That's very interesting.

I'd like to know from someone whether this is a practical difference though. Is this really something that we can notice on our boats? Or is this such a minute discrepancy that it's effectively unnoticeable?

Maybe we should speed up the earth's rotation, and all go out sailing on starboard tack in the spring! (except for our down-under mates - they can whiz along on the port rail)

 

[Seth]

Good one, but

It is true that the Coriolus effect is responsible for some elements of wind shear in a "big picture" sense, but it has little or nothing to do with any of the windshear sailboats experience-especially those with rigs under 90-100 feet off the water.

Wind shear-or changes in wind direction between the surface and up to about 50' above the surface is more local weather related-air temperature, density, winds aloft as created by weather systems combine with and affect surface winds in very inconsistent ways.

Although fairly rare in the summer, there can be enough shear to cause significant differences in wind angle from one tack to the other to the point where the genoa leads must be different from one side to other, but you typically see this in spring or fall-usually just after a frontal passage. When the shear is this strong, it could be port OR starboard tack-it depends more on where you are relative to the weather system than anything else-and if it is this strong-it will ceertainly overpower any effect caused by earth rotation-especially this close to the surface.

The bigger reason a racing sailboat (or any sailboat) will set up one lead fwd or aft of the other has more to do with the seas and the angles you are hitting them-and this is the more typical reason one tack may be faster or slower than the other-not wind shear. On one tack you are pounding directly ino the chop and not so much on the other.

Shear effects from earth rotation alone are so minor we don't consider it in terms of boat performance, and if it were noticable at all, the diff would certainly not approach what was reported in this thread.

Shear CAN impact performance and trimming from side to side, but basically this is a local weather phenomena-and only when it is extreme will it impact performance to such a notiocable degree.
Cheers,