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Sailing in tide..quiz.

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sargesail View Drop Down
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Post Options Post Options   Quote sargesail Quote  Post ReplyReply Direct Link To This Post Posted: 02 Oct 17 at 10:45pm
Originally posted by iGRF

Originally posted by sargesail


Originally posted by iGRF



So this is actually what happened.

Blue arrives at island before red tacks to cross channel.

OK - so talk me through the curved laylines.  Why does blue gradually get lifted on port?
Why does red gradually get headed?
And where can I get one of these windsurfers which point so high that the tacking angle looks like 40 degrees?


The curve indicates the strength of the tide 'lifting' the board higher, tidal lee bow, as classic as that, is twofold, not only does the board physically go faster thanks to the energy provided by the tidal flow, but the direction in which the tide is headed lifts the craft upwind.

The reds stayed out of the tide.

You can buy race boards for next to nothing on eBay and I'd strongly recommend any sailor trying one out if for no other reason, than to actually feel all that knowledge you obviously have, in the palms of your hands, you'd find it exhilarating.

OK so blue, once on port, gets an increasing shove to the east from the tide, which has a greater east going component as she heads south.

What about red?  I'm not sure I understand how they stayed out of the tide.  Surely they crossed the main flow on starboard.  And what they experienced on the west coast they should also experience on the east?
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Post Options Post Options   Quote Time Lord Quote  Post ReplyReply Direct Link To This Post Posted: 02 Oct 17 at 10:50pm
Hasn't anyone accounted for the rotation of the earth yet? Must have an effect surely.   
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iGRF View Drop Down
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 02 Oct 17 at 10:58pm
I'm not sure why he (the coach) sent them so far along the coast, I often wonder wether he might have had a nightmare experience at Portishead (Windsurfers used to sail there every year) and if you want a truly aw (ful or some) tidal experience, try sailing there, I swear to God at some stages of the tide if you lassoed a fixed mark you could waterski behind it.

But one of the things about Portishead they used to set a wing mark right out in the channel and if you didn't beat far enough up the bank and set out for it too soon, you'd never be seen again.

Getting back to the energy thing again, something back there someone said was wrong, inland on a lake, there are only the two components, the true wind and the created wind which only happens once the board/boat moves and they combine, so there is nothing else for the boat and it's foils and the wind to 'know' other than the static water we sail on.

The boat or it's foils don't 'know' anything on the sea either, the thing if anything that 'knows' the difference is the true wind which reacts against increasing or decreasing resistance from the water because it also is flowing either with or against the strength of the wind and so delivers more or less energy to the foils.

Edited by iGRF - 02 Oct 17 at 11:00pm
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 02 Oct 17 at 11:02pm
Originally posted by Time Lord

Hasn't anyone accounted for the rotation of the earth yet? Must have an effect surely.   


Coriolis effect? Come on, just as i was beginning to make some progress...

Edited by iGRF - 02 Oct 17 at 11:03pm
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mozzy View Drop Down
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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 17 at 10:52am
So, simple tidal question. In the scenario above, which end of the line is favoured and by how many degrees (upwind start)?  


Edited by mozzy - 03 Oct 17 at 11:25am
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 17 at 12:05pm
Port end, on Starboard tack, CBA doing the maths but around 10%? The apparent wind caused by drift (i.e. boat stationary in the water but moving 1.5kn at 090 over the ground) would be around 010 and slightly more than 9 knots so a good lift in heading plus the drift would take you towards the windward mark. You would, in fact, be 'lee bowing' the tide  Clap
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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 17 at 12:07pm
Almost, but you've called the wrong end of the line. 

Maybe you're accounting for the inevitable pile up at the committee boat; but I didn't actually say there were any other boats, just simply which end was favoured. 


Edited by mozzy - 03 Oct 17 at 12:15pm
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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 17 at 12:30pm
The tide will veer the true wind by 9.46 degrees, lifting starboard tack. And creating a starboard end bias of almost 10 degrees. 
The race officer (moored up, so not moving in current), will measure true wind direction and believe their line is perfectly fair. If a fleet start the result will be a pile up at the committee boat as not only will people be aiming to start at the starboard end because of the bias, but will also be swept toward that end as they rack up. 
Bonus points if you also calculated the 0.12 knot increase in wind speed! 
See below vector calculation. A is true wind speed, and B is current induced apparent.  Ignore the angles (they’re different because of the way we describe wind coming from and tide going to then tide creating apparent from the direction it is pushing you towards). You can see the grey resultant vector veered from the true wind. The angle change is highlighted at the bottom.

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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 17 at 12:40pm
That's a very interesting calculation and perfectly logical and in all my time I've never seen it explained like that and which goes some way for me to understand why you don't 'get' my explanations, but...

Real world

You haven't quantified the length of the beat, therefor the duration, one of the other reasons for choosing the port end, is the liability that going right will cause possible overstand scenarios at the windward mark.

Questions, love to see the maths on your vector calculations, also where that chart thing comes from? Is this part of the same deal that gives those 'polar' things you guys often crack on about.

We (windsurfers) obviously never have or had access to all this, so it's actually cool to see it all written down and explained logically, thanks, lets have some more..
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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 17 at 12:55pm
I didn't include length of beat, length of line or any other details as they are not needed to say which end is favoured. I thought it would complicate things adding gain factor which would come in to play later in the race. I understand there are valid reason not to start at the favoured end if there is a bigger gain to be made by course positioning later up the beat. 

I just wanted to show that when the race officer sets this perfect line, the sailors will feel an apparent (and very real for them) bias of almost 10 degrees.  If you want to lead this race from the start, you need to be at the starboard end. 

What's also interesting, is the above information is pretty much redundant to sailors on the water. The bias will be apparent to them. Whatever technique they use to assess bias (crudely sticking the boat in irons or sailing the line on each tack and subtracting your tacking angles) will tell them the starboard end is favoured. 

The only person likely to be fooled by this is the race officer. Measuring his line against true wind, for boats sailing in apparent he will be unaware of the 10 degree starboard bias. He may even get cranky and go in to black flags when the resultant pile ups and general recalls ensue. 

A smart race officer will set a true wind port bias to account. He will also offset the windward right, to account for the apparent right shift and the fleet being swept right as they progress up the beat. Offsetting the windward will off course offset the run as well, so it's a balancing act. 
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