yachet rigging, force distrebution

A forum set up for physics questions from students in the courses PHYS1121, 1131, 1221 and 1331 at the University of New South Wales. It is intended for questions that cannot readily be answered in class,
either because they fall outside the main syllabus and therefore would be distraction (however interesting) or for other reasons.

Moderator: msmod

Post Reply
Posts: 3
Joined: Fri Jun 06, 2014 10:17 pm

yachet rigging, force distrebution

Post by sammit »

I have been playing a bit with yachet rigging over the break and have been thinking about how the caps and lowers tranfer torque, intuetivly I thaught that the caps to the the top are more critical and lowers half way up are just ecomimising on haveing lots of wire, and support the middle of the marst a bit, but after thinking about it further, and drawring a free body diagram I now think this is wrong my interperitation of my diagram is that moast of the torque from the top of the marst is transferd by the caps threw the spreder and onto the lowers that then transfer most of the torque to the boat, with the caps transfering significantly less as the force they apply is at a much more obtuce angle to the axis or rotation of the boat. and hence the caps primerelly transfer force to the spreders, and hence the lowers, rather than directly to hull of the boat and hence less tensile force is requierd for a given torque, it also means that the lowers are crutial.
is my analeses more or less right?
User avatar
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

Re: yachet rigging, force distrebution

Post by joe »

For non-sailors, see http://www.thecoastalpassage.com/petrearigging.html for a diagram.

A free body diagram is always a good starting point!

Let's think of a tack or reach, where the shrouds on the lee side carry no load. So, on a boat with only one spreader, and thinking of only lateral forces, only three cables are under tension: the cap shrouds, which go from masthead to spreader, their continuation, which goes from to spreader to chainplate, and the lowers, which go from the junction of mast and spreader to the chainplate.

On the windward side, the cap shroud prevents the upper mast from bending to lee. It does this:
- provided that the spreader does not bend upwards, which is why the cap shroud continuation goes down to the chain plates, and
- provided that the mast doesn't bend to lee in the middle, which is what the lower shrouds are to prevent.

Post Reply