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Sailing close to the wind

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NickA View Drop Down
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    Posted: 03 Apr 20 at 9:58pm
Here is an amazing piece of footage:

https://www.yachtsandyachting.com/news/227910/21-knots-upwind-in-a-Moth


But if he's managing 22knots and getting an apparent wind of 35 knots, then my GCSE level trigonometry says he's not making much ground to windward.  If the boat can beat at 35 degrees to apparent wind, then it will make about 18o to windward (ie 18 degrees higher than a true wind beam reach).  If the boat can beat at 20 degrees to apparent wind, then it can make 42 degrees to windward, which is useful!

How high can these boats point to apparent wind?  The A-Class cat (which can also do this) has a massively high aspect rig, but a moth has a short rig with (presumably) lots of end loss.  Does heeling it to windward and hooking the top of the sail make enough difference?

I guess the trick is to have a very flat sail that develops little drive but can sail very high, and a hull with minimal drag, that doesn't need much forward drive to keep going.




Edited by NickA - 03 Apr 20 at 10:00pm
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Rupert View Drop Down
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Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 04 Apr 20 at 7:10am
In the real world, he just wants to get to the windward mark first. As always, there is the trade off between height and speed, whether in an Oppie or a foiling Moth. I obviously missed trig in my O levels, though. Possibly slept through it. I hope someone clever can explain.
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Post Options Post Options   Quote A2Z Quote  Post ReplyReply Direct Link To This Post Posted: 04 Apr 20 at 11:09am
The angle (to the apparent wind) that you can sail is the sum of the aerodynamic and hydrodynamic drag angles.  These angles are the arc cotangent of the lift:drag ratios of the aero and hydro parts.  So if the aero and hydro elements both have lift:drag ratio of 6:1, the drag angles are about 9.5 degrees each, or 19 degrees in total.

On the other side of the equation, your pointing angle is the angle of attack of the rig + sheeting angle + leeway.  The best angle of attack is roughly between 15 and 20 degrees, with flatter and higher aspect ratio sails being towards the lower end of that range. So, if you sheet to the centreline and make, say, 4 degrees leeway, you can expect to be able to point at 19 degrees to the apparent wind BUT ONLY IF the aero and hydro drag angles allow that to happen.

Moths and A class cats obviously have low hydro drag and also low drag rigs (flat sails, rotating or luff sleeve rigs, long luffs, attempts at end plating the foot), so there overall pointing ability is much better than a Laser or Enterprise. 

By my sums, if you are doing 22kts in 35kts apparent wind at an angle of 19degrees to the apparent wind, you are 45 degrees to the true wind
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 04 Apr 20 at 12:22pm
What we tend to miss is that its usually possible to pinch up way higher than we actually sail on a beat, in two sail boats especially if you let the jib go or manually sheet it in close. The boat damn near stops, but it does make some ground to windward.
Given a sufficiently low drag boat its possible to sail effectively in that regime, and of course the rig and foils are also optimised for it, whereas the setup on a higher drag boat is optimised for how it gets to windward best.

People who sail high performance boats on random courses get used to the phenomenon that when transitioning from a shy reach to a beat the sail settings barely change, its just a question of slowing down so you can point!
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Post Options Post Options   Quote Do Different Quote  Post ReplyReply Direct Link To This Post Posted: 04 Apr 20 at 12:55pm
"People who sail high performance boats on random courses get used to the phenomenon that when transitioning from a shy reach to a beat the sail settings barely change, it's just a question of slowing down so you can point!"

Love it, slowing down to point up. Sounds wrong but makes sense how you explain it.  Thumbs Up



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NickA View Drop Down
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Post Options Post Options   Quote NickA Quote  Post ReplyReply Direct Link To This Post Posted: 05 Apr 20 at 12:54pm
Sitting with the other old codgers on the club terrace, watching the kids bombing up and down in their WAZPs it seems that they are always close hauled upwind and down; to the extent that we realsed that they never actually gybe only tack, as they're going through head to wind even downwind.

Flat sail, sleeved luff, deck sweeper main (plenty of end loss at the head of the sail, but little at the bottom).  These things are getting more like ice yachts than dinghies! 

And yes if he is really achieving 19o to apparent wind, then he's getting get 45 degrees to true wind (if that 22knots boat speed really equates to 35knots apparent) .. indicating btw a true wind of just 16 knots, which is perhaps the most amazing thing of all as the boat now has an up wind VMG that is nealy equal to wind speed! 

Even in my heavy Javelin with its 1970s sail plan; we can still gain VMG by footing off, especially if the boat starts to plane (which it will despite weighing around 130kg) .. at which point we are close hauled again, but going much faster and pointing a few degrees lower .. though as another sailor noticed, going nearly as high up wind because the foils work harder (less leeway!).  This does need 20knots or more of true wind though.
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Post Options Post Options   Quote turnturtle Quote  Post ReplyReply Direct Link To This Post Posted: 06 Apr 20 at 7:16am
Originally posted by JimC

What we tend to miss is that its usually possible to pinch up way higher than we actually sail on a beat, in two sail boats especially if you let the jib go or manually sheet it in close. The boat damn near stops, but it does make some ground to windward.


and this is before there's any tidal effect on the le....  LOL
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