Feel the force diagram.

My post ‘Ping’ got quite a few comments, at least one of which was along the lines of not understanding what i was talking about.  Well that’s situation normal for me whenever i am talking or writing .    I did realise that not everyone will not intuitively grasp what i am talking about with rigs and the compression/staying forces created by the standing and running rigging so i thought to re-visit that whole subject.

Before i begin a simple technical explanation about rigging forces i thought i would also use this post almost ‘live’ to pass on today’s news which also has a neat tie-in with the whole rigging thing but with a completely orthopaedic flavour.  My news is that i have just had my long appointment with a second orthopaedic surgeon at the treatment centre where the operation will be done and also been seen by the pre-operative assessment nurse and their physio. The news is generally good in that surgeon 2 agrees with surgeon 1 that a partial knee  replacement is the way to go and everything is now in place to have surgery sometime in mid/late July.   It seems as though i will have very few limitations and that it will be very much up to me to work at it to get a really good range of movement back.  It seems that i can even do some work kneeling and the only proviso there from the physio is that maybe i should set up some work pants with knee pads…that wouldn’t be a bad idea at all.  I am a bit more overweight than i realised and a lot of that is simply because i’m not throwing a loaded barbell around and rucking loads of firewood so i really have got to attend to the diet a bit and crack on with upper body work to get rid of some of the fat.

There is a strange link today with orthopaedics and the subject i am talking about and to explain that i need to tell a little story from my first year as a student nurse which came immediately after my 3 years of being a rigger.  I don’t know how it worked that way but i seemed to be able to grasp intuitively exactly what was going on in the forces applied to boats by their rigs and the forces within the rigs themselves.  Years later i even returned to the subject and got all mathematical with the subject when i had to do some problem solving for one boat owner.    Anyway, back to being a scruffy first year student nurse and a lecture given by an orthopaedic registrar about traction for broken bits.  Simple idea here, when long bones get broken the muscles tend to go into spasm and the whole structure, say the leg, shortens and goes out of shape.  If at that point you simply pull on one end and let the bone-ends meet up properly again they will heal again quite quickly.  That job can be done by heaving on the bottom end of the leg while keeping the top end from moving….we call that traction and there are several ways of doing it.  One way, and my apologies to the squeamish, is to drill through the bottom foot bone and stick a long pin through that, put a device like a big hoop (stirrup) over the pin and use that to heave on with essentially some blocks, some weight and some cordage. Often these systems look complicated to the uninitiated but made perfect sense to a rigger.

Something like this :

By the time we had that lecture i had already done 8 weeks on an orthopaedic ward and of course 3 years as a rigger/sailor so nothing about the simple rigging of a traction system confused me in any way at all.  In the lecture the orthopaedic registrar had a model of a system like the second picture except that the pulling part started with a standing end, ran to the block attached to the pin ‘stirrup’, went back to a turning block attached over the bed end and then went to a weight.  Now, the reg said something like “this is how we apply x weight”  quoting the amount of weight marked on the actual weight plates and i immediately said something like “hang on that’s a 2:1 tackle and you actually have double the weight (force) on the stirrup, and therefore the bone.  Now of course in those days senior registrars didn’t take to being corrected by the lowest of the low (first year student nurse and male) so a bit of an argument ensued and of course i got ‘the look’ from our tutor which meant STFU and ‘words will be had later” and i wasn’t at all surprised when she kept me back after the session.*

A sailor will almost immediately understand that if we apply a given force on the running end of a 2:1 tackle then we get double the force at the running block….the downside being that we only get half the distance of movement.  There is even a simple mathematical formula for that which i used to be able to quote.  We can apply that all the way through the many tackles we use, and yes i should quote loss for friction thus in a 6:1 we would never get 6 times the force production.  This sort of thing is very related to the things i was talking about in ‘Ping’ because most of Inanda’s running rigging will involve tackles to increase the force of pull while also sharing the standing loads out better and where i can reduce standing loads by having less heavily pre-tensioned stays and use runners/preventers instead.   I should also say that despite having quite a large rig she doesn’t even have a single sheet winch so i am designing purchase systems for the jib and stays’l as well.

Some of the basics.

The second side to this is the more difficult aspect because it requires a basic understanding of trigonometry as applied to the standing rigging and what we then call force diagrams .  We can play around with some simple examples of boat rigging problems in working out how the forces work with some common rig types.

In rig B for example you can see that the forestay is exactly balanced by the backstay, one pulls forward (and down) and the other pulls backwards (and down).  That obviously creates a strong down-force directly down the mast which the boat would ‘experience’ as a compressive force.  We should know that the harder we tighten down on the backstay the harder we pull on the forestay and the more compression we put on the mast.  The slightly tricky part is that there are 2 resulting force directions from say the backstay…..one being back and the other being down.  The relationship between the 2 depends on how steep the angle is. If we take very tall rigs with high aspect sails we often get triangles of force that give us very high compression loads compared to the amount of useful pull (aft in this case) that we get.  It’s one of the problems of tall and skinny IOR rigs and partially explains why those rigs need multiple spreaders because without them nearly all the force would be compression and very little would be applied sideways to hold the rig up.  It’s also the case that the more compression forces we put into the mast the stronger that has to be and that the structure of the boat then also has to resist those loads.  Crucially the compression loads have to be matched by the ability of the hull to hold the shrouds and stays without coming apart. Old wooden boats can literally be pulled apart by high loads in the rig….especially where there are damaged frames in the load bearing area near the chainplates.

With Inanda my rig layout is much more like the catamaran in the above picture in that the forestay load pulls against the shrouds which have their attachment points aft of the mast base.  I had a similar set-up on my Wharram Tiki 26 and with that boat to get a tight forestay i first cocked the mast back a bit more by tightening both shrouds against a slack forestay and then really heaved forward on the mast using a complex tackle and only then doing the forestay lashings….my Tiki was one of the few with an acceptably tight forestay.  The end result for Inanda if i took the same approach would be over-tight shrouds and a leaky boat.

Inanda’s problems are twofold in this respect.  First that she does have cracked frames and secondly that she doesn’t have a rig in which the forestay loads are directly balanced by a standing backstay but instead rely in tension on the aftermost shrouds. Those shrouds aren’t very far behind the mast in the fore and aft plane so the resulting force diagram would show lots of compression and very little aft pull for the amount of load in the shrouds and forestay.  The trick i am thinking of applying is having less tension in those shrouds and more in a new running backstay/preventer stay set-up which will pull from all the way aft….much wider force triangle and much more effective aft pull on the forestay.  It will get even neater with the outer forestay and bobstay in that i am going to re-rig the bobstay with a tackle instead of a lashing and only have it under load when the jib is up.  Ultimately the solution will be of course to repair those frames and maybe sheath the hull….then i can pull on the runners until she creaks just like the Volvo 60 i sailed did !

Running bobstay. (not my photographs)

That system can also have a soft stay rather than the heavy chain and it stops the anchor chain chewing on the stay.  It could also have a ‘soft’ forestay as the jib won’t be hanked to it and the primary tension will be in the jib luff.

*My tutor thought that he was a bit of a ‘cock’ (technical expression in nursing) but was fascinated by how i knew, what i knew about rigging.

Comments and new section 05.05.

Alan said