# Faster on Starboard than Port. Why?



## IrishMistRacing (Jul 31, 2006)

I just started racing my C&C 34 and have discovered that it is faster by a half a knot on starboard tack. I've adjusted the stays (rod rigged) so the sail track up the mast is straight. I'm not sure how tight to make them, so I tightened them up reasonably tight.

What are things I should be looking for?

Thanks


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## astraeus (Jan 30, 2006)

That sounds like a tuning issue. Have you sighted up the mast while you are sailing on each tack? The mast may be straight dockside, but when you put it under load you may find that the stays are not at the same tension. Be careful, it is possible to overtune which will greatly reduce the lifespan of your standing rigging.

I watched the Brian Toss video on rig tuning and found it pretty informative. If it's within your budget I'd recommend a professional tune with someone who is willing to teach you about what they are doing.


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## sailingdog (Mar 19, 2006)

You also might want to check the distance from the chainplates to the masthead on each tack, using something like a spare halyard. They should be the same distance away, but it sounds like it may not be the case on your boat. 

One other thing that could be causing it is a badly calibrated wind instrument. If it is off by two or three degrees, on one tack you'd be sailing 35 degrees off the wind and pinching, and on the other you could be sailing 40 degrees off the wind, and that could make a that much difference.


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## hellosailor (Apr 11, 2006)

The first question to look at, is whether the boat is faster on one tack (many simply are) or whether you've got an instrument error. With currents and leeway, even using a GPS it can be hard to determine that. During slack, check and recheck to make sure it isn't instrument error.

And check that using the GPS versus the knotmeter, because often a knotmeter has been mounted offcenter so it reads differently on both tacks because it is "shallow" on one tack and "deep" on the other, and the water flowing past the different sides of the hull and keel is moving at different speeds.

There are also a surprising number of boats that have the keel put in a bit off center. (Ooops. Blush, Yes, don't look too closely down there.<G>) You'd think C&C would be careful about that...but then again C&C's are generally lightly built and it is possible your keel was damaged, smacked a rock and shifted, or a PO recaulked it and didn't quite set it right--or FAIRED it wrong.

If you really want to perfect the keel, you get a full set of keel templates and fair and sand the keel until it is uniform and a perfectly symmetrical shape, while it is hauled.

Could also be a prop--is yours offcenter? That would pull the boatspeed on one side.

And then there's the rig, something may not be symmetrical and careful measuring with a steel tape measure (fiberglass sometimes stretches) from the masthead, make sure the mast is centered, check all the rigging, etc.

Then there's instrument error...If your masthead gear really is perfectly centered or not. That's a hard one, because there aren't many ways to make sure "IS this thing straight?" when you're up there. So I'd suggest looking into all the other stuff that literally is below that, and progressing up to the masthead last--even if that's a more likely place for the error.


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## Blue Eagle (Jun 29, 2006)

I read a piece about this a while ago that I think put the blame at the feet of coriolis forces and the different angles of attack that this creates with the wind hitting different heights on the sail.

Basically, assuming that you tune the sails to draw cleanly at the point of greatest belly in the sail - or between 1/3 and halfway up the sail, on one tack the slight difference in wind direction at the top of the sail will give you a beneficial increase in lift, whereas on the other tack it'll do the reverse - there's a diagram, but I don't remember just now where it is - does anyone else remember seeing or reading this?

I guess the effects are small, but with all else being equal, and particularly for a taller rig, perhaps it could create this 1/2 knot extra...?


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## sailingdog (Mar 19, 2006)

Blue Eagle said:


> I read a piece about this a while ago that I think put the blame at the feet of coriolis forces and the different angles of attack that this creates with the wind hitting different heights on the sail.
> 
> Basically, assuming that you tune the sails to draw cleanly at the point of greatest belly in the sail - or between 1/3 and halfway up the sail, on one tack the slight difference in wind direction at the top of the sail will give you a beneficial increase in lift, whereas on the other tack it'll do the reverse - there's a diagram, but I don't remember just now where it is - does anyone else remember seeing or reading this?
> 
> I guess the effects are small, but with all else being equal, and particularly for a taller rig, perhaps it could create this 1/2 knot extra...?


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.

The issue lies in a combination of the hull/keel shape, differences in the underwater profile of the boat on the two tacks, rig tuning, or in the instrument error or miscalibration.


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## hellosailor (Apr 11, 2006)

"What a load of CRAP.... the coriolis force has nothing to do with this."
No, honest! In the Northern hemisphere all water drains down a hole clockwise, and all sailboats will go faster on a clockwise tack because of the same thing.
In the Southern hemisphere, the water drains widdershins (counterclockwise) and all boats go faster on a counterclockwise tack. That's how DC lost the cup to the Ozzies the first time, he forgot how things work differently down under!

Gee, Saildog, I thought everyone knew that! <WEG>


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## sailingdog (Mar 19, 2006)

I don't believe that Coriolis force affects things on that small a scale. I've seen water going down a bathtub drain both counter-clockwise and clockwise. Also, the Coriolis force effects would be less on a smaller boat, as the surface friction effects are greater at lower heights. 

Last point I'd make is if the Coriolis force were in effect, wouldn't it make him sail slower on a starboard tack rather than faster? Isn't a starboard tack having the boat to sail counterclockwise???


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## Sonofasonofasailor (Feb 22, 2006)

My money is on the prop being offset to port.


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## TrueBlue (Oct 11, 2004)

Or the rudder post not plumb with the centerline - creating increased drag on one side of the hull.


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## CharlieCobra (May 23, 2006)

I agree with the last two posters. KISS, it works.


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## Blue Eagle (Jun 29, 2006)

sailingdog said:


> 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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## Blue Eagle (Jun 29, 2006)

*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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## Blue Eagle (Jun 29, 2006)

*Wind Shear / Boat Speed*

Also, you might find the following link instructive:

http://www.bom.gov.au/weather/nsw/amfs/Wind Shear.shtml

Cheers,

Blue Eagle


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## Sailormon6 (May 9, 2002)

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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## Blue Eagle (Jun 29, 2006)

you're welcome Sailormon - let us know how you get on...

Blue Eagle


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## TSOJOURNER (Dec 16, 1999)

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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## TSOJOURNER (Dec 16, 1999)

Just make sure that you are always on starboard tack as you approach the finish line.


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## TSOJOURNER (Dec 16, 1999)

*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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## sailingdog (Mar 19, 2006)

nataletka-

don't spam the forum. You'd have to be quite stupid to not understand that what you're doing is against the rules here... It is also quite rude as net etiquette goes.


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## IrishMistRacing (Jul 31, 2006)

*Thanks to all - IRM Took First in our Last Race*

The bottom line of everyone's input is that there is no single quick fix and the solution was to check rigging tension, be aware of sail shape and generally keep it simple. At this point Irish Mist is no longer faster on starboard, and the team is now concentrating just going fast!!!

Thanks again


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