Mast compression

OK, thanks.
So, with enough pulleys and masts, I could destroy Darth Vader AND the Death Star just by pulling a rope.
I'll probably never understand magnets though.

 

OK then lets ask the next part of the question, if we want to bend the mast as we obviously do on a beach cat by applying downhaul, typically 12:1 but sometimes 16:1 we have a force of 12 kgs for every kg pulled on the downhaul operating at the base of the sail. Now downhauls at 12 / 16:1 are a real pain to mount and fit on the outside, most A cats are now operating a 12:1 cascade system internal of the mast with the last cascade split so that it can be operated on either side of the boat.

Now would it not be easier to simply use the halyard at 3:1 ( looped around the top pulley around another pulley at the bottom ) and then a very easy 4:1 pulling that halyard giving 12:1 compressive forces on the mast (2 x on the upward and 1 downward x 4 ). You would need to lock the base of the sail at the bottom of the sail rather than the top.

One of downsides of 12 -16:1 directly on the sail, is it does stretch the sail pretty quickly, would using the halyard as part of the method to reduce the amount of actual tension on the sail by putting some of the tension directly on the mast rather than the sail achieve the same results ?

Somewhere there has to be a problem, either the halyard tends to want to try and pull the top of the sail out of the track as the mast is bent perhaps or something I'm not thinking of. Any clues

 

The principle as to how a pulley system operates is based on “work” which is defined as force x distance moved.
Ignoring friction, the work is constant.
By using a pulley system you can use less force but the distance moved at the pulling end is increased by the same factor.

To bend a mast you need 2 things, force and eccentricity. Eccentricity is the distance of the force from the centroid of the mast.
With a straight mast the eccentricity at the luff is usually small, so it is difficult to bend.

Two other methods which are much more efficient to produce bending are the kicking strap/vang and when close hauled the mainsheet, as the eccentricities are huge.

Running the halyard up and down the inside of the mast will not help bending as the eccentricity will be zero.

I am not familiar with modern rigs but I would assume the increase in halyard or downhaul tension produced by the powerful pulley systems is used to stretch the sail rather than bend the mast.

 

Kicking straps are not used as most modern rigs have very lightweight booms or are heading toward no booms at all.

Masts control the shape of the modern fat head sail, I start with about 40mm ( taking a line from the top to the bottom measured to the edge of the luff tube ) prebend, with downhaul at 16:1 ( because being an oldie I like to pull rope rather than weights ) I can increase that to about 80mm but using the fulcrum of the sail leach and mainsheet at 12:1 combined with successive downhaul pulls then that is increased even further ( I've never measured it ) but the mast does start to look like a bow.

The sail takes all that tension and does stretch over time but the laminate material now used in the sail is so good at resisiting stretch that its shape is pretty much what it is regardless of tension on it, unlike Dacron. One of my sails actually has carbon fibre tow from the hook at the top to the downhaul eyelet at the bottom just purely to stop stretch, which is sort of saying that we bend the mast to shape the sail rather than stretch the sail.

Now if we could use pulleys located strategically, to use the halyard as part of the method of bending the mast rather than just the sail ( we now have line stronger than steel weighing 1/3rd ), I reckon I would get much more longevity out of the expensive mainsail.

 

Theoretically, but you would have to pull a lot of line thru the system. Practically at some point the friction in the blocks gets to high and doesn't efficiently transfer the force, you are just pulling against systemic friction, not applying load.

I guess you could use a really, really big cascade system... Anyone know the tensile strength of the metal used in the Death Star? We could model it.