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Weight equalisation

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JimC View Drop Down
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 22 at 9:37pm
The position of the hook/trapeze wire makes no difference to righting moment, but it makes all the difference in the world to the loads on the gear, the ability of the sailor to move in tune with the boat and the effect on the mast bend. So the intuition that its crucial is correct, but the reasoning as to why is wrong.

This is all rather allied to the lee bow effect discussion, where what IGrf called a lee bow effect was generally a real thing, but it was most definitely not the fallacious lee bow effect that used to be touted in some books.

Its an internet lesson really, before two people are to have any chance of agreeing on an answer they both need to have exactly the same understanding of the question. Something that has been proved in the last year or so by the US physics professor who made a complete fool of himself by claiming the "Downwind faster than the true wind" phenomenon cannot work.
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Rupert View Drop Down
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Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 04 Jan 22 at 9:13am
GRF, bung a spring balance in the trapeze system, go sailing, take photos and prove everyone wrong. Simple.
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iGRF View Drop Down
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 04 Jan 22 at 10:04am
Originally posted by Rupert

GRF, bung a spring balance in the trapeze system, go sailing, take photos and prove everyone wrong. Simple.


Not sure that would prove anything other than the loads passing through the wire.

I already know, that one minute you can be standing on the gunwhale leaning back even and the boat just trucks along fast displacement, but if you can just by virtue of a big enough power increase to counter your weight, get right out, the weight comes off your feet is pretty much all transferred through the rig and the boat quite literally leaps forward (I'm talking little Farr 3.7 which I always found a tad underpowered) I'm now beginning to wonder here wether it was my harness hook in the wrong position and wether had I positioned it higher I could achieve that breakout point earlier. You even lengthen the trap lines to drop lower to increase that angle, the flatter you are the more unweighted the boat becomes and the faster you go, is all I know. At that point it's just like a windsurfer raceboard, but what I don't know 100% is why and I want to, those Contenders are heavy ass MFs.

Edited by iGRF - 04 Jan 22 at 10:07am
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iGRF View Drop Down
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 04 Jan 22 at 10:26am
Here check this one, the rig is even canted slightly to weather yet I know for a fact that the lee shroud aint helping that much because it had come adrift at the start of that particular race and I was playing catch up at the time.
All my weight is coming down through the wire and at that point the boat was trucking well, one of my better days. The harness hook is already quite high, I also know this because the bloody thing had come adrift and was riding up, those useless new QR zhik models do that from time to time.

So had the hook been lower and the angle of that wire narrower (acute?) would the rig have been working as efficiently? I know things slow down as you come in but that is usually because you head up at the same time.



Edited by iGRF - 04 Jan 22 at 10:28am
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423zero View Drop Down
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Post Options Post Options   Quote 423zero Quote  Post ReplyReply Direct Link To This Post Posted: 04 Jan 22 at 10:30am
How was your back? You must have good core strength.
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Post Options Post Options   Quote ohFFsake Quote  Post ReplyReply Direct Link To This Post Posted: 04 Jan 22 at 10:34am
In terms of the physics, what this all boils down to is a single mass (comprised of boat, rig and sailor) which are all fixed in relation to one another, once the sailor is on the wire.
This mass has a centre of buoyancy, typically somewhere between the centre line and the leeward chine, and a centre of gravity which is typically somewhere to windward of the hull for a trapeze boat.
The righting moment is the difference between those two. How the sailor is attached to the boat has no effect whatsoever on this equation, whether they are hanging on a wire or sitting on a rack or a sliding seat. All this affects is how the forces are resolved within the boat, the righting moment is dictated simply by the how far away from the centre line their weight is.

Looking at the trapeze system in more detail, if the wire is hooked on to the sailor higher up their body this will increase the tension in the wire as it is now supporting the same mass but with a greater horizontal component. This is balanced by an increased force acting inwards via the sailor's feet pressing on the gunwhale, but again this is purely all about resolving the static forces within the boat and has no effect on the overall righting moment.

So yes it will feel different, and indeed it is quite intuitive to grasp that the more vertical the wire the more you are hanging from it, whereas when the wire becomes more horizontal there is more force trying to pull you back into the boat. But once all these forces are resolved the righting moment is the same
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 04 Jan 22 at 11:25am
Originally posted by iGRF


I already know, that one minute you can be standing on the gunwhale leaning back even and the boat just trucks along fast displacement, but if you can just by virtue of a big enough power increase to counter your weight, get right out, the weight comes off your feet is pretty much all transferred through the rig and the boat quite literally leaps forward (I'm talking little Farr 3.7 which I always found a tad underpowered)

This is the key, the power increases and you push out a bit further to balance it and you go faster.

I'm now beginning to wonder here wether it was my harness hook in the wrong position and wether had I positioned it higher I could achieve that breakout point earlier. You even lengthen the trap lines to drop lower to increase that angle, the flatter you are the more unweighted the boat becomes and the faster you go, is all I know. At that point it's just like a windsurfer raceboard, but what I don't know 100% is why and I want to, those Contenders are heavy ass MFs.

The boat is not 'unweighted', it still weighs the same, but it has passed 'the hump' and started planing. The flatter you are the further from the CoB your CoG is so the greater the leverage and the more power you can balance so you go faster.
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 04 Jan 22 at 11:43am
I know I've read somewhere the wider area the weight is displaced over, the earlier to plane, it was the physics behind formula boards, it's why wide goes earlier than narrow, and I'm sure the fact that c of g of that out fit moves from the centre line further to weather and the bigger the weight component of the crew of the whole package the earlier it all goes.
Now I'm not disputing all your schoolboy physics bollox, but I swear to God there's more going on with this weight transference, now wether it's the wire angle, the helm angle, but nothing has changed with all your calculations except at some point everything knotches up, and we breakout.

Edited by iGRF - 04 Jan 22 at 11:45am
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Post Options Post Options   Quote ChrisI Quote  Post ReplyReply Direct Link To This Post Posted: 04 Jan 22 at 11:52am
Love the discussion and Happy New Year to all.
Nice picture GRF and glad to see you've been out over Christmas/New Year.
Here's a suggestion on your original question (from another engineer ) if you have a spare moment in a coffee break.
Make up a model but make it with one mast and a trapeze (with weight etc) EACH side so it is exactly balanced.
Then without changing the position of the weights change the attachment point on one of them and see if the thing unbalances.
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Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 04 Jan 22 at 11:53am
Originally posted by 423zero

How was your back? You must have good core strength.


Yes, luckily I have, formed years ago water ski race training then windsurfing and mountain biking help. I've also been lucky with my back and knees given the punishment I've handed them all these years and learning this dinghy sailing sh*t is probably the worse, but each year a little better than last, which at my age is something to treasure.
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