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Lee-Bow..... Windsurfers...etc (Dons tin hat)
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    Posted: 26 Sep 19 at 1:51pm
Originally posted by mozzy

Originally posted by Sam.Spoons

the vector diagrams/conveyer belt explanations don't stack up in the real world. Nobody on here is going to deny the existence of gusts and shifts so why would we believe that current is the same in speed and direction across the course (obviously we know it is not)? If it was then the absence of LBE would be explained. But the OP did observe something happening and it's interesting (to me at least) to discuss what caused it, maybe if he hadn't used the expression in the thread title then it wouldn't have got so heated.
Sam, I linked to the thread with the vectors. really, take time to read that. The vectors i showed illustrated a situation where tide did change across a course. It showed what the tidal lift would be and what the gain would be. 

No one is saying the tide doesn't change across the course. If that's what you think the argument against lee bow pinching above the current is, then you have woefully misunderstood the premise. 

The conveyor belt is used to explain in very simple terms how there is no induced flow over the foils for a boat that is within the flow. In really life there can be belts twisting and turning and going in different directions and speed right next to one another. That is clear from my vector diagram. 

No, actually I'm arguing the opposite (I think, not maybe getting the correct words), not that the flow over the foils is altered by the current but that there is some kind of explanation to why GarethT saw what he saw, just think of it as an interesting puzzle that I'm curious to solve.
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 19 at 1:53pm
Originally posted by Sam.Spoons

  
edit :- Sorry mozzy that has nothing to do with ferry gliding, the second, red boat shows a neat example of a ferry glide. 

 


Sorry Sam, I never said the video I posted had anything to do with ferry gliding. If you read my post I said I'd never heard of it as ferry gliding. But I had seen people use rope to pendulum cross a river, which is the action talked about by Gareth, but not illustrated in the thread. 

I am also sorry your video of kayaking has nothing to do with rope thing Gareth is talking about. But top marks for bringing up yet another dissociated use of the same term.  

The kayak thing you show I have heard to referred to as ferry gliding. It's more similar to what I referred to earlier in the thread where you turn the boat into the current as you approach the dock / shore. But also, what kayakers do is go directly across the current than try and crab across fight ing it. They then wait until the other bank when the current is slower, or even there is an eddy to paddle back up to somewhere which was originally opposite from their crossing point before they were swept down tide. 

But again, this stuff is not even wrong. 

P.s. love your tom scott video. Love his stuff. That's the same thing as the pendulum crossing. But different to the kayakers? No?

But, neither are relevant. 


Edited by mozzy - 26 Sep 19 at 1:56pm
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Posts: 3401 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 19 at 2:07pm
I'd say the kayaker and the 'reaction ferry' are doing exactly the same thing, the ferry relies on the wire to buck the current, the kayaker 'paddles like feck' Big smile. A boat sailing directly up current and just bucking the current could achieve the same effect bu pointing her bow in the desired direction. The pendulum thing is related but gets you to point downstream not level with the starting point so I don't see it as useful WRT the sailing analogy as the 'proper' ferry glide.

As I said I'm not taking a stance on the LBE thing, just trying to find an explanation for the effect GarethT observed.

Never come across Tom Scott before but, yes, it is a brilliant video. Will look for more from him.


Edited by Sam.Spoons - 26 Sep 19 at 2:19pm
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Posts: 6662 		Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 19 at 2:25pm
Originally posted by Sam.Spoons

But the OP did observe something happening and it's interesting (to me at least) to discuss what caused it

Langstone Harbour Mouth has the most extreme tidal current variations of any place I have ever sailed. No explanation at all is needed for strange things happening. I've sat there of an evening watching people painfully beating in at the edge of the channel against the tide, and every now and then one of them would tack a fateful couple of feet too far out and be swept half a mile back out to sea and have to start all over again. Boats feet apart can be in utterly different tidal stream.
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 19 at 2:42pm
Originally posted by Sam.Spoons

I'd say the kayaker and the 'reaction ferry' are doing exactly the same thing,

Well, they're not. The kayaks are using variable flow across the river and varying their effort accordingly. Basically, don't try to fight the current too much, allowing it to sweep you downstream a little, then recover that distance in a back eddy on the other side, where the current might even be reversed.

The pendulum crossing / reaction ferry are attached to the bank via a rope and are using the flow of the river to drag / swing them across. The ferry is far more elegant, but it is using a rope to shore to 'pull' them through the water. 

What this even has to do flow over foils in a boat not attached to shore is the real question. The information being presented is disparate and so irrelevant that the conclusions you're all drawing from them are not even wrong. They can't even be described as an argument.

I like you Sam, you seem genuine. Are you trying to say what if instead of a rope to shore, you have sails that are pushing you along through the water instead? Well, the rope will always pull you in the same direction, no matter how you the steer the boat. The wind and drive in your sails will not.  If you had a rope pulling you towards the windward mark, then yeah, sure, head up and point straight at the mark and cross tide will push you there. But, we don't. The wind isn't an ever present driving force like a rope to a bank. It demands we align our sails correctly and sailing the correct VMG angle. If you head up, the sails will stall and you'll lose way. You'll be floating around in the tide.  So that is why the rope to shore argument is laughably bad. It's like it's come from your mate who's never been sailing... you know the one... why can't we just sail straight at the mark?


Edited by mozzy - 26 Sep 19 at 2:46pm
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  Big smile And I like you too.
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Posts: 6499 		Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 19 at 4:00pm
Originally posted by Sam.Spoons

  Big smile And I like you too.


I'm going off him, must have been a difficult child.

I agree the kayaker neutralises the current by paddling hard but puts the current on one bow side just enough to guide him across the stream, good illustration.

The Rope on the Ferrygliding example I trust is a fixed rope, and the current is used, with a foil device presenting an angle of attack sufficient to power it in one direction or the other.
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Posts: 3401 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 19 at 4:42pm
The rope is a wire fixed across the river with a tail from a sliding car on the wire down to the ferry. The clever bit is that it's attached about ⅓ from the bow to a large lever which can be moved from side to side so the pull is from the gunnel. Presumably the boat has a long straight keel and the 'tow rope' take off sets the boat at an angle to the current. When he's ready the ferryman just flips the lever to the new upstream side and off they go.
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 19 at 5:07pm
Originally posted by Sam.Spoons

The rope is a wire fixed across the river with a tail from a sliding car on the wire down to the ferry. The clever bit is that it's attached about ⅓ from the bow to a large lever which can be moved from side to side so the pull is from the gunnel. Presumably the boat has a long straight keel and the 'tow rope' take off sets the boat at an angle to the current. When he's ready the ferryman just flips the lever to the new upstream side and off they go.
Yes, very elegant. The basic principle is the same as the pendulum though. It's using the energy of the flow to push itself across the river whilst attached to a bank.  

I mean, it looks nice, but it's basically no different than a yacht swinging on it's mooring when the tide changes. It's using the differential between the ground and the water to move it in relation to the ground. 

The energy source is the water moving relative to the land. A sailing dinghy the energy source is the air moving relative to the water. The sailing dinghy has no contact with the land and therefore it can't access that energy, which is why bringing it up is so insane and irrelevant.   
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Posts: 3401 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 19 at 5:35pm
I can see we're going to have to agree to disagree here WRT it's relevance or not  Cry 
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