Breaking load, stretch properties and construction of polyester polypropylene aramid nylon HEMP MANILA SISAL ROPE FOR TACKLES sheets and halyards splicing rogue yarn marker cable laid
click to return to Home Page
Translate this site

Details
Parts of a smack
Photos - details
Photos - sailing
Rope
Tackles
Shackles
Beaufort Reefs
Tollesbury List
Research old owners
List of Smacks
For Sale/Wanted
Bitza this'nthat
CSPS news
CSPS Newsletters
Match Smacks
Line Drawings
Line Downloads
Find sail areas

Smack in Church

A 44' smack requires over 700m of rope and at £5 /m or more for modern hi-tech synthetic rope it is worth considering the options available.

Performance figures are available for synthetic ropes but I had to dig a bit to find information on natural fibre ropes.

CONSTRUCTION

Natural fibre rope is mainly made of manila, sisal, hemp or coir.

Manila is glossy, smooth and pliable and good.

Sisal is stiff, harsh, short fibre and not so good.

Hemp is the best fibre, of great strength and durability, flexible when wet and wears to the last rope yarn. Best is Italian then European, New Zealand and St. Helena. The Admiralty dismiss Indian hemp as not suitable for reliable cordage.

Coir is light and floats, does not absorb water, stretches before parting but very low strength.

Size for size, all natural fibre ropes have a lower breaking strain than synthetic ropes, need more care, can shrink and put extra strain on gear and nowadays are more difficult to source (some can be obtained from Chatham.)

But natural fibre ropes can be cheaper and well, there is nothing quite like the feel and the smell of a bit of manila. Some of the Thames barges are using sisal again from the cost angle as they use miles of rope.

Three main groups of rope construction;-

3 strand. Three strand is cheapest and easy to splice. It's resistance to sheave abrasion is up to three times better than braided rope.

Braided. Nice to handle, more flexible and less prone to kinks and twisting up than 3 strand when used in a tackle. The braided cover can be quite worn with little loss of strength to the rope.

Plaited. Can be neatly spliced to chain, flakes down well and hardly ever kinks. Nylon, designed for warps and not covered in this article.

Synthetic fibre ropes are made from four main yarn groups and as they are man made, copyright trade names are used a lot to confuse the unwary but less wieldy than the full chemical names. With the exception of nylon, the more expensive the fibre, the stronger the rope with less stretch.

ARAMIDS. SD3 (Spectra- Dyneema), Kevlar. Many trade names, very expensive, very strong, very low stretch properties close to flexible wire rope. A braided rope, often in lurid colours. Normally stocked in small sizes. Sensitive to being nipped, difficult to make a strong splice and parts with little warning.

POLYESTER. Dacron, Terylene. Good all-round rope. Strength can vary by 30% depending on construction. Can be pre-stretched to reduce stretch in use. Gets stiffer with old age and overloading. (like me!)

POLYPROPYLENE. Hardyhemp, Hempex, Nelson, Marstron, Sturdee. About 80% the strength, more stretch, but 60% of the cost of polyester. 'Greasy' feel; choose a good knot to make secure. More prone to ultra-violet degradation than the other synthetics. Uses different types of filament, continuous (cores), multifilament (smooth) and spunstaple (hairy)

NYLON. A little stronger than similar polyester lay-up but nearly twice the stretch. Priced in between polypropylene and polyester. Used for kedge anchor warp and mooring lines.

STRETCH

The age old problem with trimming sails. Sails trimmed to perfection then a bit of a blow comes through and goes and the sheets and halyards have stretched and need trimming again. And with natural fibre rope, the added consideration of the rope getting wet and shrinking. The problem can be reduced by choosing the right type of rope; size, material, construction.


Chart A .load/stretch graph_main

Chart A. This chart, copied from Marlow Ropes leaflet (thank you Marlows) shows the stretch of various ropes they manufacture. Load in this graph is the average breaking load. As one would normally select a rope so that it has a Safety Factor between 6:1 and 12:1 (about 16% to 8%) it is the bottom left corner of this chart (below) that is of interest…...

Chart B .detail of above graph

Chart B. This chart combined with the breaking load of the chosen rope (see table below) gives one the stretch. ie. 100m of 24mm nylon with a load of 1.426 tonnes on it (10% breaking load) will stretch to about 109m. Note that the 8 plait ropes are only made in small sizes and don’t find much of a place on a smack. Also Marlows Doublebraid was marketed after this chart was published so I’ve guessed it’s relative position.


In the polyester group, which is the type most common on sailing craft today, the braid covered three strand core lay-up, like Marlowbraid, is the strongest with 16 plait matt cover the nicest rope to handle but alas, nearly the most expensive. It is available in white with a discrete black marker (or rogue yarn).

Braid covering a braided core is even more flexible than the above and slightly more stretch but 20% cheaper. It's strength is similar to three strand which is about 75% of braid on three strand core. In my humble view, the best compromise for price/strength/flexibility/stretch.

Standard three strand polyester has twice the stretch factor of braid on three core; similar stretch to polypropylene but with 50% more strength. A shiny finish makes three strand harder to grip than 16 plait matt but costs nearly half as much and is the cheapest polyester.

Pre-stretched three strand costs the same as 16 plait matt, has only 10% less strength but half the stretch of an already low stretch rope. The biggest size listed is 14mm and the factory pre-stretching process reduces the diameter such that it looks like 12mm. 14mm pre-stretched has 80% the strength of 10mm 7x7 galvanised wire with about twice the stretch. Not as flexible as the other polyesters but good for bobstay and bowsprit shroud tackles and deadeye lanyards.

With modern ropes, strength is usually not a problem. I have not heard of a synthetic fibre rope in good condition breaking in the running rigging of a smack. The sheaves of the old blocks on Alberta could take up to 2 1/2" (20mm) rope with best Italian hemp having a breaking load of under 3 tons, weaker than 12mm polyester, so brute strength is not a problem. Stretch is the problem and is directly related to strength. The stronger the rope, the less it will stretch for a given load.

For example, Alberta's (44' smack) 4:1 peak halyard is 163' long (85' when the gaff is hoisted) with 70' of 4:1 hardener on the standing end, or in tackle parlance, a luff tackle to advantage (hauling on the hardener) upon a two-fold tackle to disadvantage (hoisting the gaff up the mast)

What is the load? The article on tackles (also on this website) gives a general guide. Assuming good blocks with only 10% friction:-

Ma (Mechanical Advantage ) of tackle 1 x Ma tackle 2 x effort = load so,

3.08 (luff hauling) x 2.86 (2-fold hoisting) x 80kg crew hauling = ~700kg.

Bear in mind that the ‘two fold tackle’ up the mast does not have it’s sheaves paired on two axles but splayed between the mast and the gaff spans therefore loosing mechanical advantage but the fall of the hardener can be sweated in so gaining mechanical advantage. For this exercise one can be assumed to cancel out the other. Given enough room, two crew can sweat it while a third tails giving a theoretical force of 1400kg. Add to this a component of the force on the leach of the mainsail generated by the mainsheet, also 4:1. In a bit of wind it takes two crew to trim it. Again, from the article on tackles, a man can be assumed to haul with a force of 55kg so say 100kg for two men multiplied by the mechanical advantage of the tackle, 3.57, equals 357kg. Say 300kg is added to the peak making a nice round tonne.

Look at the possible load another way; trial and error, experience or what I call the ‘Brunell Formula’. Smacks have sailed for decades and the gear ended up doing the job. What we are looking at here is only changing natural fibre rope to synthetic so what did the old boys do.

Assuming a safety factor of 6:1 the load on the old 20mm dia. manila rope would be 1/6th of 2.75 tons breaking strain = 0.46 tons, say 470Kg. Compared with the tackle result above, the old fishermen worked to a lower safety factor, about 2.5:1 or worse in a gust! If it works it must be right! To go to larger rope requires bigger blocks and more expense.

Stretch on the manila with a force of 1000kg or 40% of breaking strain would be in the order of 15% of 85'(length of peak haly’d to hardener when raised) = 12.75'. To hoist the main with it’s gaff, which on Alberta weigh about 150kg or 6% of manila breaking strain, will absorb 5% of stretch, about 4’, as it goes up and sweating in hard on the fast peak halyard (for diagram of the tackle see the peak halyard set-up on the Details page on this site) would double the force taking up another 4’ of stretch leaving over 4’ to be taken up by the hardener. A couple of foot or more could be trimmed in later making the initial fleeting between the blocks of the hardener needing be about 6’ plus the height of the crew. (you want to be hauling down on the top block.)

If new 16 plait matt rope is used, raw strength is not a problem and the haly’d can be sized for comfort. 14mm is a reasonable size to handle and it has a breaking strain of 5.13 tonnes, more than twice that of 20mm manila, and much lower stretch characteristics. With a 1000kg load, or 20% of breaking load the stretch is only 4% of 85’ equalling less than 4’, a three-fold improvement over manila. Of greater benefit is the reduction in trim required while sailing.

However, not all modern fibres have such good characteristics. Fake hemp look-alikes appear correct on an old smack but are usually made of stretchy polypropylene. The mistake is often made that as modern fibre is much stronger than natural fibre, all modern ropes can be sized on ease of handling. A look at the figures proves this error.

14mm Hardyhemp has a breaking strain of 2020kg, the same 1000kg load is therefore 50% of breaking load and the stretch for a polyprop. rope at 50% load is about 16%, so on 85’ of peak haly’d = 13’, and we are back to the same stretch problems as 20mm manila. If not enough fleeting is allowed in the hardener, gradually through the day, the peak is tensioned to take up the creeping stretch till the hardener is block-a-block and the main still looks like a bag of spuds.

If the manila was replaced with Hardyhemp of the same size, which is probably what Marlows intended when they designed the rope, then the stretch comes down to a more manageable 9’.

In conclusion, it is better to use a modern braided rope for halyards.

STRENGTH

.ROPE STRENGTH – AVERAGE BREAKING LOAD Kg

Dia. mm

6

8

10

12

14

16

18

20

24

28

32

Circumference ins.

7/8

1

1 ¼

1 5/8

1 ¾

2

2 ¼

2 ½

3

3 ½

4

Coir

.

.

.

200

.

350

.

550

750

1000

1350

Manila & Sisal

300

450

700

1100

1350

1750

2250

2750

4000

5100

6650

Italian hemp

450

600

750

1150

1400

1850

2350

2900

4200

5600

7250

Hardyhemp

390

640

1030

1470

2020

2870

3330

4470

5940

.

13600

Nelson polyprop.

670

1210

1760

2410

3250

4200

4840

6210

8430

10800

18840

3 str. std. p/ester

760

1420

2360

3200

3930

5090

7100

7770

11210

14640

.

Doublebraid

900

1590

2210

3180

4330

5660

7160

8840

.

.

.

3 str. pre-stretch

1100

1830

2800

3550

5250

.

.

.

.

.

.

16 plait matt p/e

.

1490

2890

3770

5130

6700

8410

.

.

.

.

Marlowbraid

1070

1820

3190

4320

5970

7840

9040

10190

.

.

.

3 str. Nylon

.

1910

2720

3750

5100

6640

7920

9790

14260

18640

22600

Spectra/Kevlar

1480

3180

4510

6810

10000

11470

.

.

.

.

.

7x7 galv. wire

2330

4150

6310

.

.

.

.

.

.

.

.

 

Thanks to the following for information used above:-
The Admiralty 'Manual Of Seamanship' Vol. 2 1951
Marlow Ropes Ltd.

6/10/1999

Click to GOTO top of page.
Based on an article first published in the Colne Smack Preservation Society’s Journal Issue No.20