Apparent wind and my A-Ha! moment

[kpgraci]

While I ran across the concept of apparent wind I found it interesting but rather trivial, it’s adding vectors, put them end to end and the diagonal is the result, I learned this in high school. So I get the benefit when traveling into the wind of increased wind speed, great.
Now when I’m out sailing I’m often on a close reach, with my vane pointers set to about 60 degrees apart, a bit optimistic for my boat so I keep the vane pointed just outside the mark, I’m guessing about 45 degrees to the wind. I’ve always assumed I could therefore tack 90 degrees to hit a mark, but in reality, according to the GPS track, time and time again it’s more like 100-110-120 degrees or worse. Until yesterday I thought this resulted because of my misreading of the wind vane, what I thought was 45 degrees was really more like 50 degrees, so the tack would result in a 100 degree turn instead of 90. Why sometimes the tack would be 120 degrees was a mystery.
But then I started thinking again about apparent wind. As I travel 45 degrees to the wind on a close reach, the apparent wind moves forward, so according to the apparent wind calculator, at 6kts boat speed, with a 10kt true wind 45 degrees off the bow, the apparent wind is 14.86kts at 28.41 degrees off the bow.
Well, in response to that I would of course dutifully steer the boat so the wind was maintained at 45 degrees, so my angle to true wind is increased by 45 - 28.41 = 16.58 degrees. No wonder my tacks always exceed the 90 degree turns I expected.
So does this sound right or do I need to go back to high school?

 

[Clark]

I believe your thinking is right but your example is a little off. Close hauled with the apparent wind at 45* puts the true wind at a somewhat wider angle and when you tack (or at least most of us) you slow down which will put the apparent angle a little wide of 45*. That is OK because you need to build speed back up; trim the sails and then gradually point higher until you get your apparent back to ~ 45* on the opposite tack. Now a really good team can maintain most of their speed through a tack and have sails set pretty quickly on the new course for maximum pointing.

 

[Barnacle Bill]

There are a number of reasons opposite tacks can be different. One is waves and how they differ as they hit the hull on opposite tacks. Another is you get wind shifts (lifts and headers) that are temporary, another is maybe your mast is not plumb, leads for genoa cars can be different from side to side, etc.

 

[Todd Smith]

Yes, if you are sailing at a 45 deg apparent wind angle, you could be sailing at a 55-60 deg true wind angle. Thus your tacking angle could be 110-120, instead of the 90 you expected.

But, you should be able to sail close hauled much better than 45 deg apparent wind angle. More like 30 deg apparent.

Leeway is another factor that will increase your tack angle, but not by that much. ( unless you have the centerboard up while close hauled. ).

 

[kpgraci]

Right, some good points. I have a shoal keel but I don't think leeway is a big issue. Wave and tides I'm sure account for some of the difference. I've been playing around drawing vectors and it's really quire dramatic, 110-120 seems to be about right and matches my experience.

On more than one occasion I've waited for the harbor to be 90* abeam to tack only to find I could not steer to the point I wanted, requiring additional tacks. Why this concept didn't dawn on me earlier I don't know.

 

[Ed Schenck]

Excellent, you are very analytical and went looking for answers. I always experience the same thing. But instead of thinking about it I just cuss to myself and sail on. And do the extra tack that I thought I shouldn't have to do.

 

[jibes138]

There are a couple of factors you need to include. They are called "set" and "drift."
The windage of the boat will make it slip sideways through the water some amount. If you take a sight on land and hold your apparent wind angle constant you will see the sight moving sideways. The currents in the water will also either aid you or hinder you depending on what way you are going. Putting a keel sideways to a three knot current will result in a very considerable change in position. Always remember that the forces on the hull from the water in the form of currents is much greater than the forces from the wind. This is why entering a slip it is more important to understand the current situation than only worry about the wind.

 

[Bill Roosa]

true wind realities

You are right on track but here is the way I think about it. Perhaps this will give you some additional epiphanies.

As the boat speed increases or the wind speed decreases the AW comes forward and vica versa.
So really fast boats (ice racers) can go almost any direction in any wind due to their high speed and slow boats are severely limited in light airs. In fact when talking about real boats with lots of drag and low hull speeds there are wind speeds below which you really can't sail in except down wind and even then not very fast.

So if the wind is 3 knots and you try to get your 5 knot hull speed boat to perform you are always sailing close hauled. You get the boat going as fast as you can but your AW is so different from your TW that you can't even turn into the wind and must go perpendicular to the TW or even downwind (tack > 180 deg).

Conclusion, when the wind speed is at or below the boats hull speed you WILL be sailing close hauled and have really bad (wide) tack angles. The higher the TW speed the narrower the tack angle and conversely the slower the boat goes the better the tack angle.

So now you have to get out your trig tables or your airplane pilot E6B calculator and figure out best VMG. Is the faster and longer route better or is the slower and shorter one? There is always a “best” direction to sail given the conditions and direction to destination. It is normally not the most fun one.

 

[kpgraci]

So if the wind is 3 knots and you try to get your 5 knot hull speed boat to perform you are always sailing close hauled. You get the boat going as fast as you can but your AW is so different from your TW that you can't even turn into the wind and must go perpendicular to the TW or even downwind (tack > 180 deg).

This is exactly what forced me me think about this further. 100* tacks are one thing, but yesterday in light air after tacking then settling on the same relative wind direction and speed I had turned fully 170*. Crazy.

 

[Ed Schenck]

[FONT=Bauhaus 93, fantasy]But you are Sailing!![/FONT]

 

[COOL]

.
So does this sound right or do I need to go back to high school?

I agree with Clark above, I think you are grasping the concept but are
confusing things in your explanation.
If you are sailing at a true wind angle of 45deg on both tacks,
then you would tack through 90deg on your compass. Your Windex
however would show you pointing just inside your indicator tab with
your apparent wind angle at 28deg, which is likely pointing just a bit to
high for your boat, and an Oday 22 will not be doing six knots
at that AWA.
If your windex showed you sailing at a 45deg AWA in 10Kt True wind
then you would actually be pointing at a 70deg TWA with 13 knots
of apparent wind, if you were sailing at six knots.
A realistic scenario for your boat upwind in smooth water in
10 knots of true wind would be a boat speed of 4Kt,
an apparent wind angle of 35 deg, tacking through just
about 90 deg, but losing 5 or more degrees to leeway.

 

[kpgraci]

[FONT=Bauhaus 93, fantasy]But you are Sailing!![/FONT]

It will NEVER look like that where I park, I did need to wear a jacket and some gloves later in the evening, however. 51 degrees, brrrrr.

 

[DougM]

It will NEVER look like that where I park, I did need to wear a jacket and some gloves later in the evening, however. 51 degrees, brrrrr.

HOWEVER:
We, on the other hand, don't have to be concerned about Hurricanes. Its obvious that there are trade offs.

 

[kpgraci]

Ah...It's called Tacking Angles.

 

[JVB]

Can anyone post a tacking angle graph where you select a couple of parameters, like wind speed and hull speed or some calculated parameter, and read the expected tacking angle ?

 

[Joe]

Forget high school math... you've made it too complicated

Your mistake is that you are using apparent wind to measure tacking angle.... Or how close you can sail to the wind... Forget the windex and simply install a set of tactical marks on your vessel and compare them to TRUE WIND direction. True wind direction can be determined by averaging the compass direction of starboard and port tacks. You can extrapolate your boat's compass headings (not your windex angle) to determine your tack or jibe angle... but you can also use tactical cards stuck to your coaming near the helm.

You can make these marks up on heavy cardstock and tape them to the coamings on either side, lining them up with the center line of the boat. 0deg to 180deg... with marks at 45, 90, 135. Use the cards to record your tacking (and jibing) angles. Make sets for various wind conditions. Use the cards like a pelorus.. when the mark lines up with the appropriate indicator it's time to act... forget using the windex to determine tacking angle. Once you've created the cards you can use them to determine the lay line, the crossing angle and whether you're ahead or behind your competitor.

 

[Don12364]

We presume you are using tell-tales on your jib?

 

[Stu Jackson]

Joe, I was thinking along the same lines. Forgetting about all the complexities incurred with leeway, strength of wind, etc. It just seems simpler to make a "design criteria" that says forget about the complexities, and just work with tacking, assuming jib fairleads are equal and all that. Also 12 knots, nice breeze, wind and waves in exactly the same direction. Got it?

You're right: true wind is the average of your compass headings. Simple. Apparent wind is forward of that on each tack.

You end up tacking 120 degrees based on your compass, less against apparent wind. That's why all of the slick sailing rags say: "Tacks smoothly within 90 degrees" all the time!!! Guess what they're talking about.

While I don't use cards, they certainly are a great idea. I've learned just to find "that right spot" over my shoulders and off the pushpit on either side to "call the tacks" - same concept. I aim for 120 degrees, checked by the autopilot and Ritchie compasses.

 

[kpgraci]

We presume you are using tell-tales on your jib?

I do, but I'm not making the connection.

 

[Bill Roosa]

For the geek in you

You can also hit that true wind button on the wind instrument.
There is also a "tack" button that will tell you the new course on the opposite tack for the same (but opposite) apparent wind angle. (using this in light winds is truly depressing)
Then you sail till the "new tack" reading is in agreement with the direction you actually want to go and you get the perfect single tack to destination. Course it helps to do the long leg first and take advantage of wind shifts. These become VERY apparent if you leave "tack" on for a while.
I use this much more than my GPS BTW