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Weight equalisation

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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Topic: Weight equalisation
    Posted: 03 Jan 22 at 7:06pm
Originally posted by A2Z

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.  

Agreed... Enlighten us then?


Edited by Sam.Spoons - 03 Jan 22 at 7:08pm
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Post Options Post Options   Quote Mozzy Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 7:14pm
the tension (1000N) on the two examples above aren't the same. 

Increasing the height of the hook puts more weight on feet and less on the wire. 
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 7:17pm
But still exactly the same hanging off the boat?
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Post Options Post Options   Quote KazRob Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 7:17pm
In the diagram above, there is no net horizontal or vertical force and no net moment around the mast base in either case. In a boat the moment is balanced by the sail force / hull couple. In the diagram the moment is resisted by the mast base itself. The difference in tensions in the wires is irrelevant and can be calculated just basic trig and remembering that net forces and moments at any point must be zero if there is no net acceleration at that point.

Edited by KazRob - 03 Jan 22 at 7:32pm
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 7:36pm
Originally posted by KazRob

In the diagram above, there is no net horizontal or vertical force and no net moment around the mast base in either case. In a boat the moment is balanced by the sail force / hull couple. In the diagram the moment is resisted by the mast base itself. The difference in tensions in the wires is irrelevant and can be calculated just basic trig and remembering that net forces and moments at any point must be zero if there is no net acceleration.

Surely the forces are there but as the are perfectly balanced they create no movement, the tree roots are easily capable of balancing the mass of the 'sailor' so the force on them does not generate movement on the trunk. In a boat the force doing the 'pulling over is the power of the rig and the force doing the 'holding up' is the weight of the sailor, the analogy is not perfect as, because the boat can heel, they need to be in perfect balance. The position of the harness hook is still irrelevant though (excepting the physical effects on the crew Mozzy mentions)


Edited by Sam.Spoons - 03 Jan 22 at 7:38pm
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Post Options Post Options   Quote 423zero Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 7:44pm
The answer to the question of where to attach wire on the mast, is where it will do minimum distortion to the mast, thereby diminishing detrimental sail shape changes.
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Post Options Post Options   Quote KazRob Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 7:50pm
The basics remain the same. In simple mechanics with no significant member deflections, for no accelerations (linear or rotational), the net forces and moments at any point must be zero. In the tree analogy the tree reacts against the over turning moment by spreading the loads out through it's root system.
If you want to make it more accurate by considering member deflections, teh system becomes much more complicated to calculate but fundamentally the overall system remains in static equilibrium.
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 7:57pm
Lots of interesting stuff thanks all for contributing.
Now where I was before SWMBO dragged me screaming from the keyboard, was the following conclusion which is:
If my current hook position, is below my CoG, then a clear advantage should be gained by moving it above.
If not then it's obviously back to the platform shoes to push us all further out.

I do still dispute the magic replacement of a rack to the exact same position on a wire simply because I still believe in the weight transferance to mast via the wire is relevant, but at this stage I'm unable to prove why.

One question to Mozzy, re "Increasing the height of the hook puts more weight on feet and less on the wire. " do you think this might be because you guys in 800s are already a way out there on the racks? I'm contemplating flatwiring in the not to distant future and probably earlier than most so need to maximise what little weight and height I have available.
It is however true though, one of the issues we used to have in the early days of kitesurfing in controlling our descent from big jumps if our hooks were too low which they used to have to be in the days before depower in order to maximise pressure on the rails to control the kite, it is wny these days so many waist harnesses with high hooks now feature. Higher hook gives better control in the air if the support comes from above the CoG.with more weight below landings are easier to control.

As to my theory of out board weight from a wire rather than through a rack and the hull I shall have to research harder or build that bloody model to find out for certain.

Edited by iGRF - 03 Jan 22 at 8:05pm
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Post Options Post Options   Quote 423zero Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 8:52pm
Regardless of where you put hook on your body or attachment on the mast, you will not be able to equalise your height and weight to someone 6' 6" and fifteen stone, well not sailing a standard Contender.

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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 9:32pm
Originally posted by iGRF

Lots of interesting stuff thanks all for contributing.
Now where I was before SWMBO dragged me screaming from the keyboard, was the following conclusion which is:
If my current hook position, is below my CoG, then a clear advantage should be gained by moving it above.
If not then it's obviously back to the platform shoes to push us all further out.

I do still dispute the magic replacement of a rack to the exact same position on a wire simply because I still believe in the weight transferance to mast via the wire is relevant, but at this stage I'm unable to prove why.

One question to Mozzy, re "Increasing the height of the hook puts more weight on feet and less on the wire. " do you think this might be because you guys in 800s are already a way out there on the racks? I'm contemplating flatwiring in the not to distant future and probably earlier than most so need to maximise what little weight and height I have available.
It is however true though, one of the issues we used to have in the early days of kitesurfing in controlling our descent from big jumps if our hooks were too low which they used to have to be in the days before depower in order to maximise pressure on the rails to control the kite, it is wny these days so many waist harnesses with high hooks now feature. Higher hook gives better control in the air if the support comes from above the CoG.with more weight below landings are easier to control.

As to my theory of out board weight from a wire rather than through a rack and the hull I shall have to research harder or build that bloody model to find out for certain.

I wish I'd thought of the kite analogy*, the point is that the hook must be above your CoG or you'll land (on the board or in the boat) headfirst but it makes no difference to the power you can hold. Try moving it up (use a windsurfing waist harness maybe just to prove/disprove the concept?Windsurfing is different as you'll always be holding the boom and have your feet on the board so it doesn't relate to the trapeze dinghy dynamic. 

505's have always favoured big crews, tall and heavy, because they get the max weight as far from the boat as possible, so you would rarely find a big helm and a small crew doing well.

* Dinghy sailor since the mid '60s and windsurfer since '82 but I've never kite surfed.


Edited by Sam.Spoons - 03 Jan 22 at 9:33pm
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