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-   -   Faster on Starboard than Port. Why? (http://www.sailnet.com/forums/racing/18764-faster-starboard-than-port-why.html)

IrishMistRacing 07-31-2006 06:06 PM

Faster on Starboard than Port. Why?
 
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 :)

astraeus 07-31-2006 06:11 PM

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.

sailingdog 07-31-2006 07:30 PM

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.

hellosailor 08-01-2006 01:02 AM

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.

Blue Eagle 08-01-2006 01:15 AM

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...?

sailingdog 08-01-2006 04:33 AM

Quote:

Originally Posted by Blue Eagle
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.

hellosailor 08-01-2006 12:38 PM

"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>

sailingdog 08-01-2006 02:31 PM

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???

Sonofasonofasailor 08-01-2006 04:21 PM

My money is on the prop being offset to port.

TrueBlue 08-01-2006 04:57 PM

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


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