Laurent Giles 'Jolly Boat' Exeter |
29er GBR 074 Tynemouth |
J24 (Sail No. 4239) Dartmouth |
List classes of boat for sale |
Sailing in tide..quiz. |
Post Reply | Page <1234 11> |
Author | |||
iGRF
Really should get out more Joined: 07 Mar 11 Location: Hythe Online Status: Offline Posts: 6496 |
Post Options
Quote Reply
Topic: Sailing in tide..quiz. 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.
|
|||
Sam.Spoons
Really should get out more Joined: 07 Mar 12 Location: Manchester UK Online Status: Offline Posts: 3398 |
Post Options Quote Reply 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" |
|||
Guests
Guest Group |
Post Options Quote Reply 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.
|
|||
craiggo
Really should get out more Joined: 01 Apr 04 Location: United Kingdom Online Status: Offline Posts: 1810 |
Post Options Quote Reply 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 |
|||
Guests
Guest Group |
Post Options Quote Reply Posted: 10 Oct 17 at 9:41pm | ||
|
|||
Guests
Guest Group |
Post Options Quote Reply 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 |
|||
sargesail
Really should get out more Joined: 14 Jan 06 Location: United Kingdom Online Status: Offline Posts: 1459 |
Post Options Quote Reply 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.
|
|||
Guests
Guest Group |
Post Options Quote Reply Posted: 10 Oct 17 at 9:12pm | ||
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.
|
|||
Guests
Guest Group |
Post Options Quote Reply Posted: 10 Oct 17 at 10:55am | ||
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 |
|||
iGRF
Really should get out more Joined: 07 Mar 11 Location: Hythe Online Status: Offline Posts: 6496 |
Post Options Quote Reply 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. |
|||
Post Reply | Page <1234 11> |
Forum Jump | Forum Permissions You cannot post new topics in this forum You cannot reply to topics in this forum You cannot delete your posts in this forum You cannot edit your posts in this forum You cannot create polls in this forum You cannot vote in polls in this forum |