Rounding up under pressure

I think I disagree entirely with your point. If the boom is in a fixed position (i.e. sheeted in for beating to windward) then the centre of effort of the sail is fixed. Simply moving the main sheet position forward along the boom serves only to increase the sheet loads in proportion the the reduction in sheet length! The heeling and "weather helm" effect of the sail will not change. Better to either: Ease the sheet, flatten the mainsail or if the weather is set, take a reef. One of the causes of weather helm is the heel of the boat - as the boat heels, the centre of effort in the sails move to leeward of the centre line of the boat - as a result the driving force of the sails is now translated into a turing motion about the centre of lateral resistance of the hull.

Regards, Jeff.
 
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Your point about increasing loads is taken,but they are taken by the boom more than the mainsheet.

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Let me put it this way. The amount of sideways force on the boom is determined by the amount of sideways pressure exerted by the wind in the sails. In order to counteract this force and stop the boom moving sideways, the mainsheet has to transmit an equal and opposite sideways force. The amount of this force as a turning moment is determined by the pressure of the sails on the one hand, and the grip of the stern of the boat/rudder etc. in the water on the other, and does not vary depending on the mainsheet.

Exactly this turning moment will be transmitted through the mainsheet to the boat, and it makes absolutely no difference whatsoever where the mainsheet is attached either to the boom or to the boat.

If you move the mainsheet from the end of the boom to half way along it, then you are doubling the tension on the mainsheet, but the turning moment on the stern and on the boom is the same. Equal and opposite.

The exception is for example where the boat is actually turning - for example where the amount of force on the end of the boom becomes greater than that on the stern, in which case the forces are unequal and the boat rounds into the wind.

The mainsheet is just a mechanism for transmitting the turning moment.
 
Perhaps a clearer explanation

A boom is a kind of lever.

Imagine a child's see-saw. One on end of the see-saw is a kid. On the other end at the same distance from the centre is a kid of the same weight. The see saw is in balance, so it doesn't move.

Now move one kid half way towards the pivot. He will shoot up in the air. You need to find a second kid of the same weight to sit in the centre with him (the equivalent of doubling the tension on the main sheet). The see-saw is now balanced because the turning moment exerted by the two kids half way along is equal and opposite to the turning moment exterted by the one kid at the other end.

Even though you have moved the kids half way along, the turning moment needed to counter the kid at the other end has stayed exactly the same.

A boom is like this, except that instead of a kid at the other end, we have the turning moment created by the sail, and instead of the two kids sitting to counterbalance this turning moment, we have the forces created by the rudder etc. acting on the stern of the boat.

The mainsheet is just a bit of rope we use to tie these forces together. A bit as though, instead of sitting the kids on top of the see saw, we dangled them underneath by a rope (not a bad idea in the case of certain kids, so long as their feet can't touch the ground...)
 
Let me clear up a point: the mainsheet attachment to the boom is only moved forward a bit to keep it more or less vertical relative to its attachment on the boat.a) to stop it cutting a swathe across the cockpit b) to ease friction in the tackle by keeping it all in line.
and C the main point here,Simoncr,your last sentence cuts right to the nub of this
-"the mainsheet is just a mechanism for transmitting the turning moment"
Exactly
Taking for the sake of argument the mast base as the pivot point of this turning moment, the further away the mainsheet attachment point on the hull is from this pivot point, the greater the mechanical advantage of the pulling forces applied to it. By moving the hull attachment nearer the pivot centre ,albeit with the same applied force, the simple mechanical advantage is reduced, in this case,its ability to pull that end of the hull down to leeward.
The same effect occurs if the boom attachment point of the mainsheet tackle is moved forward, but less so.The loads on the tackle and mountings do increase but not by much if moving only a foot or so.
Take the case, seen on many yachts, where the mainsheet traveller is mounted on deck forward of the main companionway hatch. The loads are handled easily by a couple of winches (double ended mainsheet), and a few blocks to share out the load on the boom. This kind of arrangement can transmit very little turning moment to the hull.
Hopefully all these pent up forces will be appled to moving the boat along.
Pete.
 
Point of attachment has nothing to do with it.

These transmissions of tensions are 'internal' matters, and are of interest only in their magnitude relative to the breaking strain of things transmitting them. We all know and recognize the pretty diagrams showing the forces generated (off centre-line) by the sails and foils and how these forces balance each other and hull forces... etc. These 'diagram-forces' are the external forces which drive the boat forward, cause it to heel, imbalance causing it to round up.
 
Taking your splendid example of the see-saw:
Now hang it upside down, with the pivot anchored to the ceiling. Put one child of equal weight on each end.It balances. The child at the right end represents the bow ( in this perfectly balaced boat ) and its tendency to carry straight on, ignoring many other factors.
The child on the left`s weight represents the `pull `of the mainsheet, in this case downwards. Move this child one foot along the plank towards the centre. The balance is lost. Although her weight is the same it exerts less downwards force to oppose the constant force on the other end. In the case of our perfectly balanced boat, the bow has now moved downwards i.e. lee helm.
Note that the total weight or downwards force on the pivot remains the same. None has been added or taken away.
To bring this example into line with the problem which started this splendid thread, namely heavy weather helm:
Put one child on the right end, then put two on the other end.No balance. Too much weight, downwards force, is applied to one end ( weather helm) and the other end has shot up ( to windward)
Now move the two children on the heavy end up the plank towards the pivot. At a certain point balance will be achieved. The total forces are unchanged. By moving the heavier weight towards the pivot ( or the mainsheet attachment point towards the pivot point of the mast) we have reduced its EFFECTIVE weight,( its turning moment)
I know that all this is a gross oversimplification to what happens on a boat with the slight problem of the pivot point being anchored not to the ground or the ceiling but sloshing around in a liquid, but the essential balancing of forces is the same.
Another way for AICHTW to improve his boats handling is to simply increase the size of his rudder. This will increase the lateral resistance, far out from the pivot point on the lateral plane, to achieve a similar effect. This also will increase drag and may have to be balanced to avoid heavy tiller loads.
Cheers
Pete
 
I think Cobra 25 (or whatever he calls himself now) will be dismayed at your cavalier treatment of children! /forums/images/graemlins/laugh.gif
 
I always had a similar problem with a 20 foot boat I owned baout 15 years ago, I was only able to point 45 degrees into the wind easily, when I got to between 35 and 40 degress it would round up and stall. the major difficulty was tacking in light winds, let alone taking forever to get from a-b in light winds.

I never worked out why this happened, ie no experimentation to fix it, but I had my theories. I think that the shape of the sails was although not baggy they were not very flat. The main was not as flat as the genoa, however the genoa car was too far off to the side. there are 3 things I thought I needed to correct to rectify this, but could not be to sure, as I never tried to fix the problem and sold the boat instead.

1. Get flatter newer sails.
2. Bring the genoa traveller inboard slightly
3. be able to haul the main sail in I could only get it to 10 degrees off the centre line, partly because the boom was too small and and too much curve in it.
4. I could never get the boom 100% flat.
5. Get a more heavy duty boom vang, again too much bend in the boom
6. A larger rudder, but I think the sail shape and position was more important

Another note, when sailing at 40 degrees into the wind, the rudder/tiller was at 25 degrees and very very very heavy. I think this was a factor in stalling the boat speed and rounding up to wind. But I think this was due to sail shape.
 
RE: Lakesailor.
I did rather avoid the point that the children were proportionately starved to ensure they were exactly the same weight!
Cheers
pete
 
I can agree with some of what you say. My sails aren't too bad, my booms fairly stiff, I've got a moveable car on the mainsheet, I've got a pretty decent size rudder. I've tuned most of the weather helm out. The spreaders and shrouds make moving the headsail car inboard tricky.
But the symptoms (apart from the weather helm) are the same. I think it's mainly an underwater thing.
 
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Simon cr.- You misunderstand - its where the mainsheet is attached to the boat hull thats important.

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Ah, now I understand...

So if you move the shroud chainplates inboard the heeling moment will be reduced.

In fact I have no shrouds and my boat doesn't heel at all /forums/images/graemlins/crazy.gif
 
In case you're not: a see-saw works in the same way whether it's upside down or not! And whether the kids are hanging off it or sitting on it. And the bows of the boat are not the other end of a lever formed by the boom... Though it would be fun if you could lift the bows of the boat out of the water just by tensioning the kicking strap...
 
Seaboots....so theoretically one doesn't need a main sheet and one could just use the kicker...impractical I know, but just wondering. can someone explain sail twist please?? I haul my sheet to windward when beating and want to point higher, and free it to leeward when it gets too much...but where should it be in light winds when beating??
 
Mainsheet attachment ....

Thew amount of force at boom is same at mainsheet attachment point at other end .... My race boat had a traveller that divided the cockpit ..... I hated it as it was dangerous to fingers and also carried a lot of force ....

I can understand the argument of moving the point - with my set-up the sheet not only pulls boom toward the centre-line, but also tries to pull it aft giving it a twist ... If the point was moved forward then the boom would be more squarely pulled in ...

But as said before - I don't fancy the beefing up of the structure to carry that load ... and also the dividing up of the cockpit by bridle, traveller or whatever ....

So its back to dumping wind from sails ... or reefing of course !!
 
I seem to have explained this badly.
The WHOLE BOAT is the seesaw,pivoting ( for the sake of argument ) around a theoretical point ,roughly at the base of the mast.
If you wish to manoevre a stationary boat in a marina, using warps and the wind is blowing her off so that she is bows on to the pontoon with the stern sticking straight out, where do you attach the warp to the boat to pivot it round so that its lying alongside the pontoon ? to the end, at the stern cleat. You can attach it to a winch 6feet forward of the stern if you like, but you will have to apply more force or weight to haul in the same boat, against the same weight of wind ,or LOAD, than you would have to apply if you attached it to the stern. Conversely,trying to haul the boat in from the forward point using the same force used at the stern point would be much more difficult. In other words, the same force, applied to the same resistance with the mechanical advantage reduced, has less effect.Incidently, those pulling would instinctively postion themselves to get the best ANGLE of pull (90 degrees). Another reason to position the boom attachment point of a mainsheet vertically above its attachment point on the boat.
This is why door handles are placed near the edge of doors rather than a foot towards the centre of the door. Its easier for the applied force to pull or push the resistance of the door. Conversely again, you can put the handle further in if you like, but it will require more force applied to pull or push the same door. Imagine this with some resistance applied to the door, like a too thick carpet.
Changing the attachment pint of your chainplates would make no difference to your angle of heel, but shortening your mast would. You hint therefore at the basic theory here - the length of the lever applying the force. Look at the pic of Nigel`s boat. The length of the lever from the BOAT`S theoretical pivot point at the base of the mast , is the length of the boom PLUS the horizontal distance on deck from the end of the boom to the mainsheet`s attachment point, in this case, right at the stern. By moving his attachment point forward, for instance, to a point vertically under the end of the boom, the length of the lever is plainly reduced, so the same applied force is working at less advantage, so less effect.
Over to you /forums/images/graemlins/cool.gif
Pete
 
Isnt the force actually generated from the front part of the sail, rather than the main sheet.

The main sheet is where we feel the force, but the boat itself is driven from the sails.

The majority of the balancing you are discussing is infact related to the mast position, the rudder position and the keel position.
 
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The majority of the balancing you are discussing is infact related to the mast position, the rudder position and the keel position.

[/ QUOTE ] Unless you have a mainsheet there will be no forces to transer into the keel and rudder. The forces will be divided between the mast (and/or windward shrouds) and the mainsheet according to the position of centre of pressure on the sail.
 
OK- you're serious....

Let us say that you have a mainsheet with a tension of 100kg at the end of a 5m boom.

The moment of force acting around a pivot in line with the mast is 100 x 5 = 500 kg m

Now replace that with a mainsheet with tension of 200 kg half way (2.5m) along the boom. The moment is now 200 x 2.5 = 500 kg m. And that's the key: if you move the sheet forward it has to have more tension.

Sorry. The position of the sheet makes absolutely no difference at all to the forces on the boat. You could even take away the sheet altogether and just have a rigid joint at the gooseneck and the behaviour of the boat would be exactly the same. Elementary mechanics.
 
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