New Posts New Posts RSS Feed: Sailing in tide..quiz.
  FAQ FAQ  Forum Search   Register Register  Login Login

Sailing in tide..quiz.

 Post Reply Post Reply Page  <1234 11>
Author
iGRF View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 07 Mar 11
Location: Hythe
Online Status: Offline
Posts: 5733
Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 17 at 9:08pm
Seen that video, at last, the bit from ten minutes where he talks about a cross tide half way up the course and draws a similar vector illustration to yours, which I have to think of a method to disprove it to you both so you get where I'm coming from.
Back to Top
Sam.Spoons View Drop Down
Really should get out more
Really should get out more


Joined: 07 Mar 12
Location: Manchester UK
Online Status: Offline
Posts: 2540
Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 17 at 7:49pm
But will either of those boats be able to make ground against the tide (which means sailing 'downwind')?

Edited by Sam.Spoons - 11 Oct 17 at 7:50pm
Spice 346 "Flat Broke"
Blaze 671 "supersonic soap dish"
Back to Top
Guests View Drop Down
Guest Group
Guest Group
Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 17 at 7:44pm
Yes; but what you are describing is a fast boat and a slow boat. Not really anything to do with the tide.  

A low drag boat with efficient rig will go faster in a 3 knot tidally induced apparent wind than a high drag boat with inefficient rig. But the speed difference between the two boats will be same as in a 3 knot true wind with no current. 
Back to Top
craiggo View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 01 Apr 04
Location: United Kingdom
Online Status: Online
Posts: 1806
Post Options Post Options   Quote craiggo Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 17 at 7:19pm
[QUOTE=mozzy]
How quickly the boat reaches equilibrium will depend on it's weight and how 'draggy' it is. 
The point is, shortly after coming off the anchor, that force (the anchor) is no longer acting on he boat and it slows through the water to zero. Once it's off the anchor the only forces pushing the boat through the water is the sheer between wind and water. 
The tide doesn't move a boat through the water, because the tide is the water. So how fast a boat moves through the water does not depend on tide, but on how much power the sails have and how draggy the hull is, exactly the same as a lake. 

[/QUITE]

Correct, which means a low drag canoe body hull with highly efficient rig, thats in a tidal flow with no wind, will experience an apparent wind initially equalling the speed of the tide, which in turn will allow the boat to start making some forward progress at an angle to flow of the tide. At some point the hydrodynamic drag will equal the forces generated by the apparent wind and the boat will have achieved its maximum speed. A draggy hull with inefficient rig will accelerate slower due to rig and will stop accelerating earlier due to the increased hull drag however it may be able to point higher!
OK 2129
RS200 411
Back to Top
A2Z View Drop Down
Far too distracted from work
Far too distracted from work


Joined: 10 Oct 16
Online Status: Offline
Posts: 342
Post Options Post Options   Quote A2Z Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 17 at 9:41pm
Originally posted by sargesail

No it doesn't. 

The hydrodynamic drag will affect the acceleration....the change in velocity.  

Given 2 boats of different profile both end up accelerating......and as they approach the speed of the current the flow past the hull tends to zero....so that the hydrodynamic drag tends to zero so that the difference in hydrodynamic drag tends to be even smaller than that.....

A 'fascinating' race during that very high pressure week in June.....drifting up and down the Solent with the same piece of flotsam alongside us.
LOL
Back to Top
Guests View Drop Down
Guest Group
Guest Group
Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 17 at 9:35pm
How quickly the boat reaches equilibrium will depend on it's weight and how 'draggy' it is. 

The point is, shortly after coming off the anchor, that force (the anchor) is no longer acting on he boat and it slows through the water to zero. Once it's off the anchor the only forces pushing the boat through the water is the sheer between wind and water. 

The tide doesn't move a boat through the water, because the tide is the water. So how fast a boat moves through the water does not depend on tide, but on how much power the sails have and how draggy the hull is, exactly the same as a lake. 


Edited by mozzy - 10 Oct 17 at 9:37pm
Back to Top
sargesail View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 14 Jan 06
Location: United Kingdom
Online Status: Offline
Posts: 1356
Post Options Post Options   Quote sargesail Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 17 at 9:35pm
No it doesn't. 

The hydrodynamic drag will affect the acceleration....the change in velocity.  

Given 2 boats of different profile both end up accelerating......and as they approach the speed of the current the flow past the hull tends to zero....so that the hydrodynamic drag tends to zero so that the difference in hydrodynamic drag tends to be even smaller than that.....

A 'fascinating' race during that very high pressure week in June.....drifting up and down the Solent with the same piece of flotsam alongside us.
Back to Top
A2Z View Drop Down
Far too distracted from work
Far too distracted from work


Joined: 10 Oct 16
Online Status: Offline
Posts: 342
Post Options Post Options   Quote A2Z Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 17 at 9:12pm
Originally posted by mozzy

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. 
Your anchor scenario is exactly my point.  Here you have hydrodynamic forces acting on the hull and tension in the anchor cable is holding it still.  Cut the cable, the hydrodynamic force now accelerates the boat until the drag of the hull equals the hydrodynamic force. At this point the boat moves at constant velocity, but the magnitude of that velocity surely depends on how draggy the hull is.
 

Back to Top
Guests View Drop Down
Guest Group
Guest Group
Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 17 at 10:55am
Originally posted by iGRF


Now finally that other point about boards, the reason the tide is very important is that the planing threshold for a board is around 8 knots wind, give or take board volumes widths etc, so if you have only 5 knots of wind, but find 3 knots of tide, you get to plane which then increases the created wind and you plane even better, you can even pump yourself onto the plane and stay there, it's not something you can easily describe to fixed rig sailors, but like ice yachting, it works.

Pretty similar planing speed to the 49er I used to sail. Not only would getting in tide take you to the mark 3 knots quicker, but you would be sailing in 8 knots breeze. 8 knots was enough for you to plane so you would see a significant increase in boat speed through the water. 29er was the same, but you needed more like 10-12 knots. 

Interestingly, for moths, if it is wind against tide, they are better sailing against more tide downwind. The increase in boat speed from the additional apparent more than outweighs the additional distance they have to sail through the water. Only works for boats which have VMG greater the wind though. 


Edited by mozzy - 10 Oct 17 at 10:56am
Back to Top
iGRF View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 07 Mar 11
Location: Hythe
Online Status: Offline
Posts: 5733
Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 10 Oct 17 at 10:12am
I think it's time for another anecdote.

Once upon a time they used to hold a race up the Thames, there were hundreds of boards. The race went ahead wind or no, because we sailed with the tide.

So with no wind whatsoever we criss crossed the river the water movement creating a 'true' wind and the movement we generated forward producing a 'created'wind so our apparent wind was exactly where it might be if the true wind were the couple of knots that the current generated.

But what was wierd, as you might normally do, watch up wind for wind obstacles, trees, bushes etc, it didn't matter, even under bridges, no change the only thing that was important was to judge where the tidal flow was greater and play the shorter routes the bends in the river allowed, from Putney up to Barnes as I recall.

But,here's a thing even as the river bent around the bend, no change in the appearance of a wind direction, both tack were equal, very little difference the only place changes that occurred were thanks to tidal flow points and being boards with a free sail rig, you could feel them.

Now finally that other point about boards, the reason the tide is very important is that the planing threshold for a board is around 8 knots wind, give or take board volumes widths etc, so if you have only 5 knots of wind, but find 3 knots of tide, you get to plane which then increases the created wind and you plane even better, you can even pump yourself onto the plane and stay there, it's not something you can easily describe to fixed rig sailors, but like ice yachting, it works.
Back to Top
 Post Reply Post Reply Page  <1234 11>

Forum Jump Forum Permissions View Drop Down

Bulletin Board Software by Web Wiz Forums® version 9.665y
Copyright ©2001-2010 Web Wiz
Change your personal settings, or read our privacy policy