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Sailing in tide..quiz.
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Post Options Post Options   Quote Guests 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.
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Posts: 1810 		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!
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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. 
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Posts: 3393 		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
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Posts: 6496 		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.
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 11 Oct 17 at 10:29pm
But here's what I don't get GRF. The situation and he describes is exactly the reason you get a triple bonus in your original situation. Not only is the tide tacking you up to the lay line, but it's also increasing your breeze and giving you a lift. 

The starboard boats get swept toward the layline, and they get the increase in breeze, however, the shift in wind is a header for them, which is why you won out coming in from the left. 

So you start a thread describing a situation where you get a tidal lift, and now, ten pages later your going to think of a method to disprove it ever happened. 
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Posts: 288 		Post Options Post Options   Quote bustinben Quote  Post ReplyReply Direct Link To This Post Posted: 18 Oct 17 at 2:00pm
Your commitment to this argument is admirable mozzy  LOL  many have tried...

On the plus side the clear explanations of tidal effects are probably very helpful to people who don't already know this stuff, so it's probably worth it from that point of view!
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Posts: 6496 		Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 18 Oct 17 at 2:06pm
The only bit we are in disagreement over is the difference between a tidal lift, and a windshift lift, on your position in a fleet, when you understand that, you'll be fine. I can't help anyone who doesn't get that.

Edited by iGRF - 18 Oct 17 at 2:07pm
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 18 Oct 17 at 4:53pm
It would give me hope if iGRF could understand. But, i can't think of any more ways to explain it. 

I showed a situation where the wind was veered for sailors on the water compared to the true wind felt by the RO.

The video I posted showed how as the boats went from no tide (feeling true wind) to the current, where the tide backed the true wind, favouring boats which had created leverage of the left (just like a regular wind shift). 

I can't think of another way to frame effect tide has on apparent wind. 

This sort of tidal knowledge is useful. You will know that not only when you hit a cross current will it sweep you sideways toward a side of the course (and layline), but it will also shift the wind lifting you up toward the layline as well. 

Hopefully the thread has been useful for people and some have learnt a few things. 
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Posts: 3399 		Post Options Post Options   Quote 423zero Quote  Post ReplyReply Direct Link To This Post Posted: 18 Oct 17 at 5:07pm
Would a gybing board work with cross current?
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