Tacking Angles

[davel]

When reading reviews of a yachts performance, tacking angles are often referred to.

What is the definition of the tacking angle? Is it the difference between the compass bearing before and after the tack (ie no account of leeway or tide), does it factor leeway but not tide or both?

Dave L.

 

[Guest]

I've always thought the tacking angle is what the boat can lay to windward. ie: sail at 45 degrees to the wind the tacking angle is 90 degrees. If you are an America's Cup boat make that 18 degrees and a tacking angle of 36 degrees. NB this is all apparent wind not true.

 

[jamesjermain]

In the context of YM reports, the tacking angle is the angle between the conmpass heading on one tack and the compass heading on the other tack. We do not make an allowance for leeway because it is very difficult to assess and will, in any case, vary depending on conditions. Tide is, by and large, not relevant.

We also, usually, give the angle at which she sails to the apparent wind, which is self defining.

JJ

 

[davel]

Thanks for that.
Dave L.

 

[Guest]

I often wonder what are the guidelines for rating good and bad performance. I find i can sail at 28 deg off the wind, and be making fair progress if not brilliant. Ease her off to 35 and she's happy as a lark.

Conversely, ifI am sailing at 30 deg off the wind I will still tack round 90 degs and then harden up becuase that seems to give faster overall performance - but maybe I'm just a rotten sailor.

At what point does sailing close to the wind become "as close as you report"? When the sails start to luff or when all reasonable forward progress is lost?

 

[chippie]

As a sailor of a trimaran, I am very aware of the different windward sailing angles of different types of boats. You may find that your GPS has a feature called course made good (CMG). I think this can be used to determine the most efficient course to a destination. I am still playing around with it though as I dont use the GPS much, as I mainly do day sailing.
I would be interested to know any rules of thumb which says whether one is pinching before the sails tell you.

 

[bedouin]

Not only the GPS - my nav instruments also have a 'VMG' facility that is calculated from the wind angle and the boat speed. It's very useful while trying to trade off speed against close pointing.

As far as telling when one is pinching - on Bedouin I can tell by the feel; if I sheet the genny right in she begins to sit more upright and slow down a little, even with all the telltales flying (you do have telltales don't you?).

Easing the genoa sheet an inch or two and bearing away a degree or so gives much better VMG

 

[vyv_cox]

I would have thought that using the GPS to measure tacking angles could give a false value, as it is measuring COG. Presumably what one loses to tide on one tack is compensated by a gain on the other, but some theories hold that lift is obtained by "lee bowing". I doubt it myself, but I always measure tacking angle on compass alone.

 

[chippie]

Yes, I have telltales,but heeling is insignificant on my boat and as she is lighter than a similar length keeler deceleration in light airs is more rapid.(As well as acceleration). In all honesty I end up pinching it when I am not paying enough attention eg looking at dolphins or somesuch. It always seems to be in light airs. In stronger airs trimarans get in the groove and you can tell how you are going a lot easier, there is a lot more happening to keep me focused.
OK I admit I have only myself to blame.

 

[chippie]

I think that the GPS calculates the course made good or the velocity made good which is strictly speaking more about achieving a course over ground as you say. It would show the most favourable angle to reach a given destination whatever the external forces were(tides,winds etc.)

 

[peterb]

Make it true, not apparent, and you would be right.

Imagine a steady wind with a boat able to sail at a true 45 degrees to it on either tack. Then in tacking the boat would turn through 90 degrees. But when on the tack the apparent wind would appear to draw forward, so that on the boat you might only see an apparent wind angle of 35 degrees. The tacking angle, though, would remain at 90 degrees.

One point not always recognised in pinching is the effect of keel stall. As you get closer and closer to the wind the ratio of forces generated by the sails in the forward direction compared to the sideways direction will fall. The sideways force on the sails has to be balanced by an equal and opposite force generated mainly by the keel. As the water speed drops the keel angle has to increase in order to generate this force, until finally the water flow round the keel breaks down, drag increases and the leeway angle shoots up. Sailing the boat a little more free keeps the speed up, avoids leeway, and may actually result in a water track (as opposed to heading) that is closer to the wind.