Apparent wind, true wind, and sailing faster than the wind

[Peter Taylor]

In the thread I started on "How Water Ballast Works"
( http://www.swallowyachtsassociation.org/smf/index.php/topic,1094.0.html )
we strayed off onto discussing boats sailing faster than the wind, and the effect of apparent wind. When I looked at explanations of apparent wind etc. on the web I was surprised how confusing they seemed to be.  I thought I could do better but having tried,  now realised it's not so easy!

However my attempt so far on the subject is attached as a pdf file.  If anyone cares to read it I'd be happy to have comments, particularly where something is not clear or could be improved. As a scientist (retired) I'm very happy to accept criticism - the "scientific method" consists of writing down what you think and then other scientists telling you why it is rubbish!  If we end up with something that's helpful I'll turn it into a library article.

Peter

[Andy Dingle]

I've been over your paper several times now Peter, and I think I am getting there!
Not being a scientist (or mathematician) of any description I can't offer informed constructive criticism. Interestingly (to me) I have been ploughing through my collection of sailing books, even from some notables such as Uffa Fox, Reg White, Bob Fisher, Brian Phipps (authors who have obviously made their names by using this phenomen to their advantage) and can find little or no detailed explanations of apparent wind and how to make good use of it.
But to me, now more a sedentary cruising man (as befits my girth), one of the most useful points to take on board is the explanation of why we can point up higher in gusts - I must admit that until I read your work, I would have said the apparent wind moves closer to the bow in a gust - the increased speed bringing it onto the bow, not further away. But now it makes more sense, the apparent wind in the gust is pulling off the bow, putting the boat more on a reach, it's most effective point of sail... Shan't worry too much about the possible planing effect in my Baycruiser23!

Yet again many thanks for taking time trouble to explain all this magic to us. It is very much appreciated.

Kind Regards


Andy
Baycruiser 23 No.25 'Equinox'

[Andy Dingle]

Peter.

I've been studying all this again.. and re my last post.

In the paragraph pertaining to sailing in gusts you say - 'so the apparent wind direction will move further from the bow, the boat is freed up' which I thought I understood..
Then you say, when talking of sailing faster than the wind that - 'The faster the boat sails, the closer the apparent wind direction is to the bow'..

I must confess to being confused again. I had originally thought, in a  gust, the boat will sail faster and bringing the apparent wind closer to the bow? I thought I had grasped the fact that in a gust the boat will speed up, putting it more onto a reach (close) making it sail faster and more effectively, now that the apparent wind had swung off the bow, allowing the boat to be more effective on this reach as you explained.
But then you say that as the boat speeds up the apparent wind is brought closer to the bow, (which is what I originally thought!) ..  surely this would actually slow the boat, as the apparent and the true wind were more in line with each other..?

I'm probably wrong..

Andy



     

[Michael Rogers]

Andy, I salute your determination actually to UNDERSTAND all of this! 'Maths' and all things related being difficult for me, I do tend to take the easy way out, accept that I'm unlikely to understand why, and simply accept that it is so (whatever 'it' may be). To quote Hilaire Belloc (who, incidentally and irrelevantly, was an enthusiastic if somewhat gung-ho sailor) - 'But scientists, who ought to know, Assure us that it must be so'.

Peter, thank you for your really valiant attempt to make this subject comprehensible to the likes of Andy and me. The SailNet forum article suggests four practical points which we can take from the maths, parallelograms of forces etc, and can bear in mind when relevant while we are sailing. I wondered whether the final version of your article for the library might incorporate them (I'm sure they are there somewhere in what you have written!), perhaps as a 'Practical Summary' or some such. They are -

1) The direction of the Apparent Wind is always ahead (in terms of boat heading) of the True Wind direction.
2) On a beam reach or close hauled the AW strength is greater than that of the TW, and increases the closer to the wind the boat is sailed.
3) As the TW direction moves aft from being abeam, the strength of the AW progressively lessens compared with that of the TW.
4) When the TW direction is well aft, small changes in TW direction produce much larger changes in AW  direction.

If I've got those right, I'll accept that It Is So, and try to ruminate on the practical implications for my sailing.

Michael

[Tony]

Luff up in the gusts.  End of.

(End of my critical analysis of the topic, anyhow.)
 Never did bother to find out what a Cosine is so in no position to pontificate in my usual annoying fashion - a great loss to science, I'm sure - but anything that helps me pinch a few yards ( Sorry. No SI units available. ) to windward is welcome. This from a bloke who once tried lee bowing - on a reservoir.

[Andy Dingle]

Lee bowing eh..?

Now that is an interesting discussion point.. or perhaps maybe best left until after a few pints of 'Old Gibberer'..
or whatever the French equivalent is..

[Peter Taylor]

Thanks for the comments...

Michael:  the points you mention from the SailNet article are all there (apart from no.4 which I should have mentioned)... following your useful suggestion I'll add a "Practical Summary".

Andy: in gusts it's a case of the time sequence of events.  When the gust first hits the true wind increases rapidly but the boat needs time to speed up so the initial effect is that the apparent wind direction shifts further aft.  If you are working your way upwind you can take advantage of the change in apparent wind by allowing the boat to point higher - the gust has probably made the boat heel so it will be trying to do that anyway.  If the gust continues to blow then the boat will speed up and the apparent wind will then start heading you again because you are now going faster.  If you previously let the boat turn towards the wind then you will now have to bear away again to avoid ending up in stays.  However by doing a swerve to windward during the gust you will have stolen a bit of ground upwind.

Tony: many reservoirs have a wind driven circulation within them so the occasional bit of lee-bowing might well be useful!

All: reading your comments I wonder if it was a mistake putting in the equations.  They were meant to be there for anyone who prefers equations, but they are not necessary, and I was assuming most people would ignore them. The diagrams alone should be enough to show what's going on (probably easiest to understand if you draw your own versions). In a library version I think I'll put the equations in little boxes to indicate where to stand your gin and tonic while reading the rest of the paragraph!

Peter

[Peter Taylor]

Following a (very minor) operation I've been banned from sailing or even doing boat modifications for two or three weeks (not that I've missed any sailing during the last day or so given the weather!).  So I've reverted to being a computer nerd and tried revamping my explanation of apparent wind and related topics,  taking into account the comments on my last attempt.  The new version is in colour, so it must be true.  A pdf version is attached; unless someone points out that it is all rubbish, I'll prepare a version for the library.

Peter

[Graham W]

It's like attending a particularly useful Open University course.

[david]

Excellent work Peter. A well thought out and presented  paper. I can almost  understand  now!
 Just one question, I did not see any mention  of what the  tell tales measure! Ok, bad attempt  at humor.