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Sailing in tide..quiz.

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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: 10 Oct 17 at 9:22am
I can't see that link, but it's pointless to argue with him, he's done his maths, he can't possibly be wrong.
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 17 at 9:21am
I agree with mozzy. The only thing causing the boat to move at less than the speed of the current is aerodynamic drag, in the hypothetical situation of the wind exactly matching the current in speed and direction the boat would quite soon exactly match the current (and wind) speed and would appear becalmed. If there was truly no wind (which is actually much more likely than the wind exactly matching the current) the boat would not appear becalmed but would experience an apparent wind with a vector exactly opposite to the current. 

The interesting question is "would that apparent wind be sufficient to allow a boat to make progress against the tide?" (I think no but I could be wrong)  Wink
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 17 at 9:08am
Originally posted by A2Z

 
However, if the water is moving due to a current, there are hydrodynamic forces in play. 
What are these 'hydrodynamic forces' you talk of? if the boat is stationary in the water (albeit moving in relation to the land)?

Originally posted by A2Z

Each boat will then accelerate until they achieve a steady velocity (in the direction of the current) and with a magnitude such that the drag of the boat moving through the water is equal to the force of the water (current) on the boat. .  

If the boat achieves a steady velocity in the direction of the current there is no force from the water. 

Newton's first law. Once the boat is accelerated up to the speed of the current and reaches a steady speed it will remain in that uniform motion. I.e. it needs no additional water force to keep moving. 

Put it another way:
The water on your lake is moving at 1000 miles an hour and yet when you put a boat in it, and it is becalmed, it doesn't move through the water. 

Or, imagine a boat on an achor:
In 2 knots of tide a boat at anchor (fixed to the land) will experience 2 knots of flow past it's hull. On the boat it will appear as if your anchor line is pulling you through the water at 2 knots. Relative to the land you will be stationary. 

Release the boat from it's mooring and it accelerates up to 2 knots until the water is now longer flowing past the boat, on the boat you will be stationary in the water. From the land the boat will now be moving at 2 knots with the current. 


Edited by mozzy - 10 Oct 17 at 9:09am
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Post Options Post Options   Quote A2Z Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 17 at 8:00am
Originally posted by mozzy


All boats are affected the same by tide, here is why:
A2Z; ask yourself this. When it's a flat calm on the lake, do the boats move through the water? The boats aren't rigidly attached to the water on a lake, but if they are becalmed they don't move. You don't see water flows past and hydrodynamic forces moving boat through the water on a lake do you? 

So if you accept all boats becalmed on a lake sit still,  then why would they move through water when becalmed in a current? If there is no force to propel the boat through the water they will sit still, apparently attached to the water and from the shore the boat will appear to move with the tide. 

Now, add in wind. All boats are still affected just the same by tide (this is where iGRF is wrong about boards being more or less effected by being lighter or having less hull in the water or whatever). The tidal vector is the same for all boats. 

Boats sit still on a becalmed lake because there are no aerodynamic or hydrodynamic forces to move them.  However, if the water is moving due to a current, there are hydrodynamic forces in play. Each boat will then accelerate until they achieve a steady velocity (in the direction of the current) and with a magnitude such that the drag of the boat moving through the water is equal to the force of the water (current) on the boat. This requires that the boat speed must be less than current speed, because if the boat moved at the speed of the current there would be no hydrodynamic force. The boat would have to slow down until such time as the hydrodynamic force equalled the drag again. This is true even ignoring aero drag.  So I maintain that boats drift downtide at a speed less than the current. This doesn't alter the basis of the argument that a shift in apparent wind occurs, but the size of that shift might not be as big as expected.  
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 09 Oct 17 at 12:21pm
Watch from 10 minutes in... 



Edited by mozzy - 09 Oct 17 at 12:22pm
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 17 at 3:09pm
Oh dear, that is not proof. 

Stating something is the case in one location, but not describing why, isn't proof. Show me why both tacks are lifted on the river. 

Also: Jonathan Mckee says the left is favoured by a wind shift and the right by more current. http://https://www.cgra.org/McKee-interview-long.pdf

You still don't understand the start line situation. I showed the vectors. I can't make it any clearer. 

You seem to be arguing against yourself. One minute you say you beat 'squadies' because you got a lift from the tide, next you say it's an 'illusion'. 

The tide going across the course does collectively move everyone that way. In doing so it produces a very real shift in apparent wind. 





Edited by mozzy - 04 Oct 17 at 3:22pm
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 17 at 2:51pm
Originally posted by mozzy



]Like I said iGRF, even a broken clock is right twice a day. It sounds more and more like you lucked out and experienced a tidal gain one day and beat some decent sailors. 



Yep, I must have lucked out, then again and again and again and again year in year out, I guess I must be just lucky.
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 17 at 2:46pm
Originally posted by mozzy



[QUOTE=iGRF]Tide can and does lift on both tacks.


Hilarious. Prove it. 


Easy.
Point 1, favorable tide say up river, wind blowing downriver, like say Columbia River Gorge Both Tacks lifted.


Point 2. Your gammy start line prediction.

All boats off the line, relative to the wind will stay at the same angle to the wind yet the course they collectively sail on is moving sideways creating the illusion of a lift, but not altering the attitude of the boats against one another as would be the case with a windshift favoured starboard end.

I take it I don't have to point out to you what happens when wind lifts a fleet as it departs a start line.
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 17 at 10:57am
Originally posted by iGRF

Tide can and does lift on both tacks.

It just doesn't lift in the same manner as a windshift lift which is what you and your maths fail to grasp.

Hilarious. Prove it. 

But, bear in mind whilst you're proving this, its also going against why you had that one decent beat and got the better of the 'squaddies'. For if it's true that tide lifts you on both tacks then both the starboard boats in you scenario and you on port would be lifted. So if this was the case, how did you get an advantage? 

Like I said iGRF, even a broken clock is right twice a day. It sounds more and more like you lucked out and experienced a tidal gain one day and beat some decent sailors. 

There are plenty of times in sailing when people adopt the best strategy for all the wrong reasons. It doesn't prove they know what they are talking about, or are good sailors. 


Edited by mozzy - 04 Oct 17 at 1:09pm
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 04 Oct 17 at 10:48am
You need to work on your problem setting iGRF. There are so many things left out or wrong. 
Originally posted by iGRF

 
I have another scenario for you to consider, in your {doesn't make any difference which angle the boat is to the tide) assumptions.

Lets take the typical triangle with a cross tide, but the 1st leg has been set broad and the return leg of the triangle is tight.
Okay, so by triangle course you mean a course that start on a reach? 1st leg a broad reach, second leg a tight reach and third leg presumable a beat? 

Originally posted by iGRF

 
The advantageous cross tide assists on the 1st leg, but is disadvantageous on the 2nd.
Okay. I get this, that's fine. But really, the extent to which a boat has to crab across the tide to stay on the rhumb line is determined by:
  • the strength and direction of the tide 
  • the strength and direction of the wind
  • the boats speed through the water over various headings and wind speeds (speed polar) 
  • distance and heading between marks.
You provide none of these key bits on information which would allow a definitive answer to the problem. You are either leaving this information out because you don't realise it is required, or because you don't understand how to solve the problem and want it to be ambiguous so you can blag your way out of it later.

Originally posted by iGRF

 
It is still a reach though but the wind is failing
So, you're saying the wind is dropping, but not so much as they have to start tacking to get over the tide and up to the mark. I think I understand that. But again, you don't say what the wind started out at, or what it drops to.  

Originally posted by iGRF

 
Boat A gybes and sails for the mark with the tide still on the weather side of the foils.
Sails for the mark... so ambiguous. Do you mean points it bow at the mark? Most people in this situation would sail for the mark by taking a transit to show them where the rhumb line was and crab the tide. AS you can tell from my bullet point list above calculating the true heading needed is difficult, so just taking a transit and keeping the boat on it is a very practical way of solving the problem. 

Tide on the weather side of the foils? Ha. The tide is coming from the weather side of the boat, the foils, the rig and everything. It is a single factor, moving the whole boat in one direction as one. Meanwhile, the foils will experience flow in from the direction the boat is moving (through the water). In terms of flow on the foils where the tide is coming from makes no difference. The boat isn't moored to the bottom. 

Originally posted by iGRF

 
Boat B gybes higher and presents the bow directly into the tidal flow.
So, boat B sails higher, directly in to the tide. But, the tide is still slightly across the reach (although we don't know for sure as you haven't provided the information). If they only sail directly in to the tide, they will never reach the mark. (unless the mark is directly in to the tide as well... in which case boat B is doing the same as boat A, which is sailing directly to the mark as well).

There must be a second part to B's strategy. I.e. they sail directly in to the tide, then bear away later on the reach to cross tide and drop down to the mark. 

Sailing high then dropping low could be a good strategy in a failing breeze. You'll spend more time fighting the tide when you have breeze to do so, then can ferry glide down to the mark later on when the breeze dies... none of that is in your description of the strategy though. 

Originally posted by iGRF


Who gets to the lee mark first?
Well, unless boat B bears off, he'll never get to the mark, so I'd say boat A. 



Edited by mozzy - 05 Oct 17 at 9:24am
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