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Weight equalisation
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Grumpycat View Drop Down
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    Posted: 03 Jan 22 at 6:48pm
Originally posted by tink

Originally posted by iGRF

I've worked it out but you'll have to wait.

Mrs iGRF is not happy, I've tried to explain to her but she won't 



Please tell me Mrs iGRF is a professor of pure physics 

No she is a a saint in human form  Wink
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 6:47pm
No, the trapeze line (or tree rope) is more horizontal if the line is attached at the chest rather than the knees.  The man is no taller and has no more inherent leverage. 
I know the answer to this, but no one has yet explained it despite deriding GRF for asking the question.  


Edited by A2Z - 03 Jan 22 at 6:54pm
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Sam.Spoons View Drop Down
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Posts: 3401 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 6:45pm
Because to pull more horizontally/less down you would have to stand further from the tree (for the same attachment point on the tree). If you stood further from the boat (either on stilts or WHY) you would have a higher downward force not because of the angle of the trap wire but because you were further from the boat. But it's not a valid analogy because to pull the tree down you would be pulling sideways and the sideways component of your effort would be what pulls the tree over not the vertical component.
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Post Options Post Options   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 6:40pm
Sorry, but there hasn’t been an explanation as to why the trapeze wire coming out at a more horizontal angle if attached higher up the body doesn’t increase the righting moment. The only explanation given is that righting moment = body weight x distance from centre of buoyancy, which is correct but doesn’t explain why the trapeze angle method is wrong. After all, if you were trying to pull a tree over you would find it easier if pulling more horizontally and less down (from the same attachment point) so why isn’t the same true for dinghy stability?  
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Posts: 3401 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 6:19pm
Originally posted by A2Z

Interesting isn’t it?  Lots of people have told you that you are wrong about moving the trapeze attachment outboard increasing righting moment but no one has really explained why.  

I can't think of a simpler way to explain it...

Originally posted by Sam.Spoons

Tall people have more righting moment not because the trap hook is further from the boat but because their CoG is further from the boat. Moving the hook doesn't change that.

How high up the mast the wire attaches (almost always at the hounds IME) is also irrelevant WRT righting moment but the lower it is the more load on your legs to stay standing horizontally off the side of the boat

You have to have the trap hook at or slightly (but only slightly)* above your CoG to be able to move around the boat, if it's too far above and you feet slip off the gunnel your legs will fall in the water, if to low your head/body will fall into the water.

* In practice slightly above your CoG has proven to be most efficient.
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KazRob View Drop Down
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Posts: 245 		Post Options Post Options   Quote KazRob Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 6:12pm
Any basic mechanics book will explain it all. If there is a fundamental problem with these principles I'm sure they would have been noticed by now. As Tink said - all that matters is the overall moment and balance of forces. If you do a free body diagram and include the angles of the wires etc you'll see it makes sense.
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Posts: 789 		Post Options Post Options   Quote tink Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 5:33pm
page 181 of Frank Bethwaites "High Performance Sailing"
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Posts: 789 		Post Options Post Options   Quote tink Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 5:23pm
Originally posted by iGRF

I've worked it out but you'll have to wait.

Mrs iGRF is not happy, I've tried to explain to her but she won't 



Please tell me Mrs iGRF is a professor of pure physics 
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Posts: 789 		Post Options Post Options   Quote tink Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 5:19pm
Originally posted by iGRF

So why does the windward shroud go loose when I'm hanging on the wire?

I'm not suggesting a vertical force, is it not a sideways force?

And if the wind suddenly dropped without me getting back in the boat the weight of the wire will pull the boat over.
It goes slack because the trapeze wire is acting as a stay and taking the load off the shroud. The centre of gravity of the whole boat has not changed, the thing that is counter acting the sail force is the righting moment: that is the horizontal distance between the centre of gravity and the centre of buoyancy multiplied by the whole weight of the boat and crew.

Ignore dinghies think of a deep keel boat. When the mast is vertical the keel is actually doing nothing. Now imagine a heavy person steps on the gunwale. He will force the boat to tip towards him, as he does the the underwater shape changes and the center of buoyancy moves towards him, as this happens the keel and the centre of gravity moves in the opposite direction - at some point the movement of the keel will counter act the tipping momentum of the man on the gunwale. 
If the man on the gunwale pulled with all his might on the halyard nothing, absolutely nothing would change. His weight is still in exactly the same position and he weighs the same amount. 
If you took your Farr 3.7 on its trailer to a public weigh bridge and stood roughly where you trapeze from then actually trapezed or just sat in the boat there would be no change in the weight of the boat. 

It is very different for windsurfers as I have said because the mast pivots. 
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Posts: 382 		Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 5:18pm
Grf -I tell the youngsters that I work with that if they can’t explain something to their mum they don’t understand it ( in your case it would be your wife!)
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