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Sailing in tide..quiz.
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    Posted: 03 Oct 17 at 6:43pm
The bias is very real. It won't feel like a shift though, it will just feel like committee boat is favoured. 

Say the line was 100m. With a bias of 9.46 degrees the committee boat end starters would be 16m 'upwind' of the pin end starters. 

The conveyor belt analogy is correct. All boats will be lifted the same amount on starboard, and headed the same on port (from the true wind). If the conveyor belt keeps on moving at the same speed, nothing changes and the boat will finish the beat with same 16m difference they started it.

Now, if the conveyor belt changes speed... then you have yourself the next question...

Take the same scenario as I've posted already (9 knots true wind, 1.5 knot cross tide at the start, line set square to the true wind).  The line is 100m long (so I've already told you the bias in meters). Boat A starts at the CB, Boat B at the pin. 

Both boats are perfectly matched in angle and speed. There are no shifts in the true wind. 

Both boats sail on starboard tack, as they progress the tide slackens to 0.1 knots then B tacks. When they cross which boat is ahead and by how far?

 


Edited by mozzy - 03 Oct 17 at 9:13pm
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iGRF View Drop Down
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Impossible scenario, both boats start on port?
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Posts: 356 		Post Options Post Options   Quote Dougaldog Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 17 at 8:41pm
I'm sorry to spoil the earlier comments about how the line was laid, but any RO worth the title takes his wind readings from a 'free floating' boat and not one that is moored. At the OK's a fortnight okay, sailed over the highest springs of the year, we delayed final position whilst taking wind readings when drifting. Once we anchored the Committee Boat the course looked 'orrible and skewed - but as soon as the boats came out to start sailing and you could see them close hauled on both tacks, the beat looked spot on.
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Posts: 6496 		Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 17 at 8:44pm
Any race officer b'stard I knew would have skewed the line to make sure the tide was backing them off..
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Posts: 1459 		Post Options Post Options   Quote sargesail Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 17 at 9:12pm
Originally posted by Dougaldog

I'm sorry to spoil the earlier comments about how the line was laid, but any RO worth the title takes his wind readings from a 'free floating' boat and not one that is moored. At the OK's a fortnight okay, sailed over the highest springs of the year, we delayed final position whilst taking wind readings when drifting. Once we anchored the Committee Boat the course looked 'orrible and skewed - but as soon as the boats came out to start sailing and you could see them close hauled on both tacks, the beat looked spot on.
D

And there are too many that aren't worth the title....the number of times I see an RO with a compass and a bit of string at anchor.  As I moaned about it at the weekend one of the group commented that a friend of his used tidal stream data in a computer to work out where his marks should be....and that's fine.  But all too often you end up with a WM where the string was pointing!

Used to love Ken Falcon as RO - would jump in a RIB and then 'borrow' a boat to get a feel for conditions.

Did you up hook routinely between races to test that your feel for the tidal apparent was still in date?
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 17 at 9:15pm
Originally posted by iGRF

Impossible scenario, both boats start on port?

You got me! I've edited to starboard. Both boats start on starboard and sail on starboard until the tide slackens and B tacks. 
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Assuming the tide slackens across the whole course? No windshifts? And they were sailing at exactly the same speed? B would have to take avoiding action to avoid hitting A (still on Starboard)?
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Posts: 356 		Post Options Post Options   Quote Dougaldog Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 17 at 9:30pm
Sam - as soon as the line was closed we were off....Of course, taking readings from a moored boat works fine on a pond....

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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 17 at 9:46pm
Dougaldog, I love your optimism and faith in race officers! But the scenario is only a bit of fun to illustrate a point. Out of all the unrealistic things about it (no shifts, same boat speed, no gust etc etc) a race officer setting an unintentionally bias line is about the most realistic. 

Sounds like you did an excellent job at the event you ran though. 

When I'm race officer at my club I just guess the course by eye. As long as they can cross the line it's all fair; everyone has the same opportunity to choose the correct end. 

Once I did lay a finish mark that dried out... oops, I did get a fair bit of stick for that. But there is no rule that the course has to be on the water!

I doubt anyone takes floating wind readings for club racing, for instance, and lots of club will be working off fixed lines anyway!
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 03 Oct 17 at 9:49pm
Originally posted by Sam.Spoons

Assuming the tide slackens across the whole course? No windshifts? And they were sailing at exactly the same speed? B would have to take avoiding action to avoid hitting A (still on Starboard)?

Spot on! A would have a one meter lead and B would have to avoid. 


Edited by mozzy - 03 Oct 17 at 10:06pm
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