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 :

Skeletal Traction

31-O10_NonOp_i110_L

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.

rig4-4

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)

DSCN3760

DSCN3764

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

“Very good explanation, mmm probably going a bit far to have a jack for the mast 🙂 I pity the ‘patient’ with the 2 to one tackle “oh sorry I did not meant to pull your leg off” !!
The soft rigging would be good I am not sure what the UV life of PBO etc is mind you it won’t get baked like here with the UV
I assume the frames are “grown” ones I think you talked about replacing them with laminated ones.”
And…
“Yes the runners will cut the max load down as they are a much more efficient angle I just had a picture of pulling up on the aft deck pushing down on the mast and spreading the gunnel so to speak.”
Stephen Mundane said :
A lot clearer than mud now Steve — learned a lot there — thanks. If I ever need any rigging advice, I know who I’m asking. Same goes if I ever need to put myself into traction 😉”
New section.
I thought i would answer the questions/comments with a whole new section in the actual post rather than writing a new one.  This post does demonstrate a problem for me in that i wrote it very quickly in-between slapping on coats of some nasty 2 pack paint primer to my new food crate that i am building in the workshop.  Rather than writing the post reasonably cohesively in one session and then doing an intelligent edit it’s gone out essentially raw and unedited which is how i used to write and very much what i have since learned not to do.
Rigging generally and yacht rigging specifically is the only technical area of boats where i can sometimes do something that the average yachtie can’t and that’s because i learnt the basics over 3 years of doing the job .  After that i was lucky to meet people like Jaws who is a much better rigger than i am.  Nowadays i keep an interest in the subject although ironically my last boat had almost no rigging to play with at all !.  I like going to the French boatyards and ports because there are often the bigger radical race-boats like IMOCA 60’s and big multihulls that clearly sail with huge loads and have to employ all the latest tech and tricks but most of which hark back to much simpler sailing ship techniques.  I was extremely lucky many years ago when i was working at Dickies that i went to a London boat show and met the last 2 qualified and working Royal Navy riggers doing a demonstration there….making up new bits and pieces for HMS Victory.  Their techniques of making up big spliced strops for some of those enormous multi-part blocks translated into modern materials would look perfectly at home on a modern IMOCA or super-maxi.  Like many of those boats but on a much smaller scale i am also trying to take out unreliable mechanical ‘fasteners’ like shackles, rigging screws, even blocks and replace those with simple lashings, old and modern ‘knots’ and simple eyes/deadeys where i can.  If you want to do a little bit of home learning take a look at the ‘Lute’ hitch to attach your main halyard to the head of the sail rather than an expensive shackle.
Rig loads…deck spreaders and lashings suddenly make sense.
017
 tack-loop
Here is one of my favourite bits of kit to play with.  One of these was employed to lead my anchor warp aft on the little Liberty and i am designing a couple into the running bobstay/soft forestay idea for Inanda.
Range-of-hard-eyes-2015
Today’s rigging theme is mainly about changing the direction of force, load or pull and that’s something we do all the time….just think about your genoa cars and turning blocks for example.  As is my usual routine i’m going to begin and end with a couple of stories….the last one even ‘surprised’ me.
Today i don’t have a single photograph of Dickie’s boatyard as it was when i worked there with its rickety old wooden sheds and slipways.  Today it’s just another modern housing estate on the outskirts of Bangor.  Back then it was a busy and expanding boatyard in the early 1980’s.  Outside my rigging shack, which was directly under the boss’s office we had the long slipway where we could haul big motor yachts and fishing boats and at the head of that was the winch-shed with i think something like a ‘100 ton’ electric winch.  One of the first proper jobs i was involved with was on the slipway helping to build a cradle for a big (to my eyes) for a fishing boat that we were going to haul out on the main slipway.   The slipway itself comprised three i think railway tracks set into the concrete and on top of that we started the construction with a pair of long bogies that had multiple rollers that ran on the tracks.  We built the cradle for the boat starting with long and extremely heavy ‘green-heart’ oak beams lashed across the bogies with bearer-blocks on those to take the keel.  We then had to set up temporary strong uprights at each side and exactly the width of the boat to hold the boat upright until we get her partway up the slip and get the bilge blocks on.  One of the first problems was setting up the vertical frames as they had to be not ‘vertical’ but perpendicular to the bogies and it didn’t quite work to do that by eye.  The foreman was extremely surprised when his rigger knew how to calculate the sides of a right-angled triangle using Pythagoras’s theorem…ok i had to borrow the yard secratary’s calculator  to do the numbers and i didn’t know how to do the lashing properly to hold the verticals in place but the foreman was an old merchant seaman and he showed me how to do that.  Here’s one for you…..once we had floated the fishing boat exactly over the cradle we had to lash the uprights together across the deck to pin the boat upright and centred, the foreman rigged a ‘Spanish Burton’ to do that and that was the first time i met one of those in practice.   Once we had her in the uprights the haul-out went easily as the huge winch was directly centred on the slipway and she came up smoothly once we had greased the bogie rollers.
In the main part of the yard there was a completely different set-up for hauling bigger boats in and out of the main shed using a similar but smaller winch as that winch was set at about 90 degrees to the main axis of the shed and wasn’t even near the doors.  To set up the winch cable to pull a boat into the shed the wire had to be led first across the yard to one 90 degree turning block, be led up the shed to a 180 degree turn and then come back down the shed to the boat’s cradle trolley.    The turning blocks as i remember them were huge steel ‘snatch’ blocks with an opening cheek such that the winch wire could be led through them and the snatch blocks were then chained to hard points set in the ground.   Like this but much larger.
il_fullxfull.321392049
The total change of direction of load in that set-up was 270 degrees and of course the trolley actually had to go over the wire at one point and the winchman couldn’t see what was going on most of the time but had to rely on hand signals from the foreman.
At that time i knew nothing about the loads and hazards involved with highly loaded winch cables running around snatch blocks.  What none of us had seen was that one of the less bright boatbuilders was standing with his heels against the winch cable just inside the bight of a turning block. What we all saw was the perfect back-flip he did when the snatch block shackle failed and the winch wire whipped his ankles out from underneath him !.  The foreman’s language was a bit ‘salty and forthright’ shall we say (he was a scouser) and livid at the boatbuilder’s stupidity for standing inside the bight of a loaded cable.  Later he told me the story of literally seeing someone decapitated in a similar situation when a tow-rope (wire) failed.  The boatbuilder was lucky in that he was standing directly against the cable….a couple of feet away and it would have probably taken him off at his ankles.
Today’s technical post then is to consider the loads involved at turning blocks and this is where you get to do a bit of homework.  Consider this situation.   This is a very common set-up, the halyard comes down the mast and turns through 90 degrees at the mast-base block and goes to a winch.  Ok so lets say there is 10Kn (kilo-Newtons) load in the halyard, and therefore at the winch, how much load is there on the turning block.
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And then this one : the turning block attached to the rail for the jibsheet.  Lets say there is 10 KN load at the genoa clew…how much load is there at the turning block ?
OLYMPUS DIGITAL CAMERA
For your bonus question i was once asked if as a rigger i had ever acted as a ‘Nawashi’ in ‘Shibari’……go find but be prepared for a surprise…..as i was.  Have fun riggers !

8 Comments

  1. Very good explanation, mmm probably going a bit far to have a jack for the mast 🙂 I pity the ‘patient’ with the 2 to one tackle “oh sorry I did not meant to pull your leg off” !!

    The soft rigging would be good I am not sure what the UV life of PBO etc is mind you it won’t get baked like here with the UV

    I assume the frames are “grown” ones I think you talked about replacing them with laminated ones.

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  2. A lot clearer than mud now Steve — learned a lot there — thanks. If I ever need any rigging advice, I know who I’m asking. Same goes if I ever need to put myself into traction 😉

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  3. Info is good and stored away until… This is out of context but probably of help in general with a Deben 4 Tonner. Go to Classic Boat December 2016 where there an interesting and potentially useful article about the boat. There is even a solution to keeping the lead ballast in place and off the boards. Enjoy.

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  4. A further thought, if I may. As you are modernising the set up I presume that you will do the same with the blocks, going for either ball or roller rather than staying with modern plain bearing, to reduce as far as possible friction within the systems?

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    1. Actual use of blocks will be reduced to the minimum. What i am working on, right now in fact, are blocks that look like traditional ash blocks but are anything but…..future post spoiler.

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  5. Nawashi in Shibari eh! I’m sure there’s a dark corner of the internet where you could make some money demonstrating your skills 😉

    As for the halyard load on the turning block, do you use L X A =S (L=load, A=angle factor determined from a table, S=Stress on the block)?

    If so, the answer is 14.1 Kn as A=1.41 for 90 degrees.

    The same methodology for the jib sheet rail block gives a stress of 8.4 Kn on the block, assuming an angle of 130 degrees where A=0.84.

    Once again, learned something new there Steve — cheers.

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    1. It would be great except they have got their table written out completely back to front….at 180 degrees the load on a turning block being double….internet eh !

      As for the Shibari question asked by a young lady…..my actual response in those younger and (slightly) more innocent days was “a do-what”. Nowadays of course the answer is merely a click away on wikipedia or as you say in some dark corner of the net. In those days i didn’t find the answer for several months until i ‘just happened’ to be trying to find a copy of easyriders magazine in a ‘specialist, shall we say, magazine store. M response i think was ‘crikey’ and i never had quite the same relationship with my climbing rope ever again.

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