Knowing the ropes
Which rope for which task? It pays to know the difference.
If sails provide the muscle power for yachts, then ropes are surely the sinews. They hoist our sails and then provide the means to control them. They tether us to the shore, tension our guardwires, and are central to the workings of such mechanical devices as reefing gears, davits, and the recoil starters on outboard motors. We rely on them utterly.
I should apologise at this stage for using the word ‘rope’ rather indiscriminately in this article so, for those who would hang me up by my thumbs for any terminological imprecision, I hereby tighten up the definitions with the following:
- Cord: A small laid up rope less than about 8mm diameter, anywhere between twine and rope.
- Rope: Any cordage with a diameter greater than 8mm and less than 40mm. Ropes can be made of fibres both natural and synthetic, and of wire.
- Hawser: Anything bigger than 40mm diameter and about as far as we need go, even on a superyacht. Above there is ‘cable’ but who gives a darn?
Anyway, to continue. Ropes are one of the earliest inventions of man and, until a mere fifty years or so ago, were made of natural fibres such as sisal, hemp and cotton. These performed relatively poorly and were susceptible to rot. Now, thanks to the miracle of polymerisation, they’re unashamedly synthetic and even more useful and durable than they ever were. Incidentally, the word ‘rope’ is used here loosely and include such terms as ‘line’, ‘cord’ and ‘cable’. When we come to it, the words ‘fibre’ and ‘filament’ are interchangeable.
Ropes use many of the same types of filaments found in woven and laminated fabrics. Think sails. Each bestows its own properties, with those that perform the best usually -- and often unfortunately -- being the most expensive.
Ropes are diverse in both type and purpose. A rope that will perform well in one task may be useless in another. For example: halyards should be made of rope with the most resistance to stretch, mooring lines quite the opposite, sheets should be supple and easy to handle, lifebuoys and liferafts should be tethered with lines that float and you can string up your fenders with just about anything. It’s therefore essential to match the rope to the job, perhaps saving costs by accepting lower levels of performance when the demands aren’t too critical.
Before we move on to the different forms of construction, let’s take a look at some of the materials available -- both the relatively old and the amazing new.
This workhorse of the polymer clan is by far the most widely used fibre found afloat. Indeed, on most cruising yachts, polyester ropes make up every part of the running rigging, from the halyards and sheets to the tiny luff lines.
With a specific gravity (SG) of 1.38 (meaning 1.38 times the weight of fresh water) it’s quite a heavy material. It offers high strength and reasonably low stretch -- with an even greater resistance if it’s ‘pre-stretched’, a process we’ll touch on later. It’s also quite good at withstanding the ravages of UV light, and holds any colouring well -- ideal for ropes which are colour coded to make them instantly identifiable. Add to that an inherent resistance to flexural fatigue and it’s small wonder that polyester is the first and most economical choice for so many applications.
Developed by DuPont in the 1930s, nylon was the first fibre to be synthesised entirely from raw materials -- coal and water being basic ingredients. And the water is appropriate as it continues to serve us sailors to this day. Chemically described as a polyamide -- a polymer of amines -- the name ‘nylon’ became instantly generic when it emerged because, following an unsuccessful wrangle over the exclusivity of the word ‘cellophane’, DuPont declined to register it as a trademark.
Unlike polyester, nylon ropes are valued for their stretchiness -- an extremely useful property when it comes to absorbing the shock loads imposed on mooring lines and anchor warps. The material is naturally strong but that strength diminishes by up to 15% when the ropes become wet. And, curiously, because the individual filaments swell when they’re soaked, the ropes will temporarily shrink a little in length. Other characteristics are good resistance to abrasion, UV degradation and flexural fatigue. Nylon has an SG of 1.14 so is lighter than polyester when dry.
A down side to nylon’s absorbent nature is that impurities will be drawn into it over time, discolouring the rope and making it feel stiffer. This affects the handleability but not its strength.
High Modulus Polyethylene (HMPE)
More commonly known by the trade names Spectra and Dyneema, HMPE is the high-tech fibre most likely to make the leap from racing yachts to the humbler world of cruising. If one ignores its cost, it comes bristling with what seem like irresistible qualities. These include high strength, very low stretch, and a resistance to both weathering and abrasion even better than polyester. With a SG of only 0.97, HMPE is also much lighter. On the down side, it has a tendency to ‘creep’, which means that it extends slightly under constant loads.
In practical terms, HMPE is ideal for halyards. For instance a 10mm rope with an HMPE core and a braided polyester outer cover is stronger than a 12mm rope made entirely of polyester -- the breaking loads being 4800kg and 4300kg respectively. What’s more, by virtue of both its inherent lightness and lesser diameter, the HMPE halyard will be 40% lighter. To put this in context, my own boat Shindig has five halyards and a topping lift -- all in 12mm polyester braidline. In total they contribute 18kg (about 40lb) to the weight aloft. If I switched to
HMPE I would save 7.2kg (16lb) and there would be small but useful gains in both stability and windage. And, I could take things further. By stripping the braided cover from the length of halyard that lies within the mast once the sail is hoisted, that part would be reduced in weight by 65%, while retaining all of its strength and resistance to abrasion and weathering. Weight for weight, HMPE is the strongest fibre in existence. This leaves a polyester, comparable in strength, nearly seven times heavier than the naked HMPE -- a quite astonishing reduction in the burden aloft.
Such antics might seem rather extreme to cruising sailors but one application has obvious merit. In the lightest conditions feather-light HMPE spinnaker sheets would allow the sail to fly when heavy polyester ones might lead to its collapse.
With stretch characteristics very similar to polyester, polypropylene is a hard wearing and relatively inexpensive material. Its main claim to fame is that it floats, which makes it ideal for such tasks as MOB rescue device grab-lines and dan buoy tethers. Unfortunately, it’s also susceptible to fading and UV attack, though improvements have been made in this regard. It’s a quite a good choice for dinghy sheets -- particularly the more capsizeable of the breed -- but its comparatively low resistance to weathering prevents it from being widely used on yachts.
Stronger but even more expensive…
Right at the top end of the fibre hierarchy, and in advancing order of performance, are aramids such as Kevlar and Twaron, the liquid crystal polymer Vectran, PBO -- the acronym standing for polybenzoxasole – and is 20% stronger than HMPE. They all boast exceptional strength and stretch resistance but are not without their problems. For instance, Vectran is a bit of a wimp when it comes to facing the sunlight, and both it and PBO have limited flex life. Not, I’m sure, that you won’t lose much sleep over such deficiencies, for these muscular prima donnas really do belong in the sailing stratosphere where pockets are as deep as the water sailed upon.
Twist or plait
Every rope starts with individual filaments lying parallel. From there the manufacturing process proceeds thus:
- The parallel filaments are twisted into an initial yarn.
- A number of initial yarns are twisted into a final yarn.
- The final yarns are twisted into strands or plaits.
- The strands or plaits are brought together to form the rope
In each of these successive stages the fibres are twisted in the opposite direction to the stage that preceded it. This is done to reduce kinking.
Before we move on, let’s go back to those parallel filaments. Since much of the stretch in ropes arises from each component’s tendency to straighten under load -- much in the manner of a coiled spring -- wouldn’t it make sense to somehow gather together a bunch of filaments and leave it at that? Being straight to start with, each would immediately take up the load and the stretch would then simply be a matter of the material’s elasticity.
Unfortunately, it’s not as simple as that. Quite apart from the practicalities involved in containing those filaments, there’s a more serious problem. A rope’s strength depends on sharing the work between every strand. Parallel filaments work fine when the tension is applied in a straight line between two points but turn that load around, say, a block and those on the outside of the radius become loaded disproportionately while those on the inside will be relieved. Unable to take the load on their own, the outermost fibres could break, the rope will be weakened, and a few repeats of this process will see total failure. Clearly, the bigger the rope, the greater will be the radius and the worse the problem.
To ensure that every single part works equally, it’s therefore essential that each filament crosses from one side of the rope to the other, transferring the load as evenly as possible from the outside to the inside of any turns. This is achieved by twisting or plaiting the yarns -- often both -- and it’s the way in which this is done that largely determines the performance of the rope and the way it handles.
Let’s look at some different forms of construction.
Cheap and cheerful and utterly traditional, such ropes have been around since the dawn of mankind and continue to serve us well. The best way to learn how a three strand rope is constructed is to take a short length, unlay it and lay it up again. You will see that its final helical form can only be maintained by inducing lots of twist into each of the three strands. It’s almost as if you’re nearly kinking the strands as you re-establish the lay.
Now twist the rope against the lay to open up the strands. Persist and you’ll end up with something like the mess shown above. If it can’t twist around its neighbours, each strand will twist around itself, resulting in kinks or ‘hockles’. Look closely and you’ll see that there are even hockles on the hockles. And if the rope is old and stiff, there’s no way back from there for it will be almost impossible to relay. This particular rope was attached to one of my fenders, which rolled round and round as the boat moved backwards and forwards.
In any situation where twisting can occur, it’s much better to use a symmetrical, more balanced form of construction. The simplest examples are ropes where eight -- sometimes twelve -- loosely twisted strands are plaited together to form a soft, pliant rope with a characteristically square section. Solid braided (as opposed to cored) ropes of this type are almost invariably made of nylon and are therefore very stretchy. This makes them ideal for anchor warps, mooring lines and mooring risers.
For the cruising sailor, the joy is in the stowing. Solid braided anchor warps can be flaked quite casually, with little fear that they will emerge as a tangled lump. And, for rodes made up of rope and a short length of chain, the splice that joins them is reasonably simple to master.
Braided and cored
While the stretch in solid braided ropes is actually beneficial when mooring or anchoring, it’s a menace where most of the running rigging is concerned. Yes, some elasticity can help absorb shock -- the mainsail kicker in a gybe, for instance -- but here we’re talking tiny amounts, not gross elongations. The quest for a balanced, non-kinking rope, constructed in such a way as to be inherently stretch resistant, has led the rope makers to develop a family of ropes made up of braided covers over cores of different types -- essentially a two-part rope with the components running concentrically..
Superficially they all look much the same, with little indication on the outside of what’s going on within. And, on the matter of appearance, at this point it might be worth mentioning the finishes which can either be smooth or matt and hairy. The matt effect is achieved by ‘staple spinning’ the yarns used to braid the outer cover. This technique harkens back to the old days when natural fibres came in short lengths (the staples) and had to be spun together to form a continuous yarn. This lack of filament continuity means that for polyester ropes of any given size, hairy ropes won’t be as strong as smooth ones, though they’re easier to handle and work better in rope clutches and jam cleats.
The most common cored rope is called ‘braid on braid’. This aptly describes a rope having a braided core inside a braided cover. If there was ever a universal yacht rope this is it. Although it might not be the absolute best choice for every job, it does acceptably well as halyards, and is ideal for sheets or other control lines. I carry a spare coil of 12mm diameter on Shindig, just in case we need replacements.
Then there are ropes with a ‘low twist’ 3 strand core which are claimed to be 40% stronger than braid on braid and will stretch half as much. These properties would make it a very good choice for halyards but, unfortunately, they are more difficult to splice.
Mix it with the big boys
Lastly, we come to the exotics -- the HMPE cored ropes and upwards. The reason this group isn’t included with the others is that their braided covers contribute nothing to their mighty strengths. Indeed, as I’ve already touched on, the cover can be -- and often is -- discarded in many circumstances.
As the one high performance fibre with a really legitimate place on cruising boats, let’s allow HMPE to represent the others. To illustrate why the cover is so puzzlingly redundant we need to consider the first two letters of the abbreviation HMPE -- namely HM. This stands for 'high modulus' and is an indication of how little the material will yield for any given load. The higher the number, the less it will stretch. Now, the modulus for a good quality polyester is about 100, while the HMPE weighs in at a hefty 1,400. As load is applied, the polyester would therefore continue to stretch long after the HMPE is fully loaded in short, it wouldn’t even notice it if the polyester wasn’t there. This disparity is further accentuated by the more linear braid pattern of the core, which aligns the fibres more directly with the load.
This sometimes creates problems when HMPE ropes are being held by clutches or jammers. Because the cover is simply that -- a cover -- the clutch jaws grip the non-loaded part of the rope. The danger is that the core could slip inside the cover, thereby releasing the load with possibly disastrous consequences. Design improvements in both ropes and hardware have gone some way to minimising the risks.