What is the strongest sailing knot?

When it comes to ropework, no cruising yachtsman shares the same opinions as any other, but look closely and there are just a few areas of near-universal agreement. Bowlines are best for bending sheets onto genoa clews, for instance, and fenders are best hung from lifelines with a clove hitch. And that’s about it.

Why is there so little consensus on the best knot to use in various situations familiar to us all? Let’s look at mooring: the RYA recommends a round turn and two half-hitches for fastening a mooring line to a pontoon cleat because it can be tied and untied with load in the line. I’ve always used a pre-tied bowline dropped over the cleat, then bounced some slack into the mooring line and unhitched the loop to cast off. Others prefer spliced loops, which can’t be untied, regardless of load. Fairly recently splicing nuts have arrived on the market. They can handle a line up to a maximum of 10mm diameter, so they’re not much use for mooring, but how sturdy a loop do they create? Rock climbers use figure-8, or Flemish, loops. Do they know any better?

The same divergence of opinion is evident when bending two lines together. If you’re taking lines ashore while anchored, extending a towline or running out a kedge, you’ll probably need to bend at least two lines together. But which knot is strongest and most reliable? Is a reef knot stronger than a sheet bend? How does a Carrick bend fair against a double fisherman’s knot?

We wanted to remove any doubt by finding out which knots performed best in the two familiar scenarios described above. We went to visit Marlow Ropes at its Hailsham headquarters, Ropemaker Park.

How does any knot affect break load?

A knot is the weakest point of any line, that’s well known, but exactly how much weaker? We tied overhand knots in the Dyneema and Marlowbraid to see how it affected their known strength, and found that the break load of both lines was reduced to a startling 35 per cent of their ISO specification.

Before anyone starts panicking, according to ISO2307, lines must be proof-loaded up to 50 per cent of their theoretical break load before testing. Our lines had not been proof-loaded but onboard, use alone would serve that purpose. But this does serve to illustrate that knots can significantly lower the breaking strain of cordage.

How we tested

We used 12mm Marlowbraid, the cruising yachtsman’s favourite; 10mm Dyneema to get a glimpse at the future of knot-making; and 14mm three-strand to find out the best solution for mooring. Using Marlow Ropes’ 30-tonne strain facility, we tested seven loops and five bends to destruction to find out which knots could bear the most load for each of our three cordage types.

First we tied a simple overhand knot in the Marlowbraid and Dyneema to see what difference a knot makes to the break load of any line. Then we looked at loop knots. For three-strand we looked at a round turn and two half-hitches, a bowline and a spliced loop, all popular mooring options. For the Dyneema and Marlowbraid polyester we tested a bowline, splice, figure-of-eight, anchor bend, double fisherman’s loop and a splicing nut.

The bends we tested were the reef knot, sheet bend, double sheet bend, fisherman’s knot, double fisherman’s knot and Carrick bend. Our findings were very surprising.

Loops

Wherever a rope needs to be fastened to something, you’ll need a loop, but which one is strongest?

Round turn and two half-hitches

Touted in the RYA’s Knots, Splices and Ropework Handbook as ‘one of the safest, secure and reliable knots you will ever use’, this is a general purpose knot used for fastening a line to a fixed loop, hoop, bar or bollard. Simple to tie and release on a line under load, it’s the RYA’s recommended knot for mooring.

In our test, the 14mm three-strand stretched but held well, right up to 2.6 tonnes when two of the three strands parted at the knot. The load represented 67 per cent of the line’s specified break load. Bear in mind that a simple overhand knot reduced the breaking strain of Marlowbraid and Dyneema to 35 per cent of their specified breaking strain.

Bowline

The ‘king of knots’ works with any diameter of line. It’s the go-to knot for bending sheets to sails and attaching lines to harnesses, but it can’t be tied or untied under load. The tail should fall into the loop, not outside like our three-strand bowline, though it performed well enough.

It’s not one for bearing serious loads, though. The Dyneema line’s jacket broke just behind the knot and the core pulled through at 1.9 tonnes, a mere 35% of its rated break load. The Marlowbraid snapped in the same place, at 2.1 tonnes, 47% of rated break load, and the three-strand failed at 2.4 tonnes, or 62%.

Eye splice

Many of us can splice three-strand, but splicing an eye into Marlowbraid cores is a lot harder. But it’s a handy skill to have – our test shows that spliced loops, with good long splices, are the strongest option.

The Dyneema splice withstood 4.4 tonnes, 81 per cent of rated break load, before the core snapped at the base of the splice. The Marlowbraid snapped, again at the base of the splice, under 3.7 tonnes or 82 per cent of its break load. There was a much shorter splice on the three-strand but it also endured 3.7 tonnes, 95 per cent of its rated break load, before two strands snapped at the eye.

Flemish loop

Also known as the double figure-of-eight, this is a knot frequently used by climbers. It’s easy to tie and puts a fixed-length loop at the end of a line. It tends to jam after heavy loading but if you make sure the standing line, and not the tail, forms the outside of the first loop, you’ll be able to pick it apart.

The Dyneema line’s jacket snapped just above the knot and the core pulled through under 1.9 tonnes of load, or 35%, the same as the Dyneema bowline. The Marlowbraid snapped entirely just below the knot with 2.4 tonnes of loading, 53% of its rated break load.

Anchor bend

So-called because it’s useful for fastening cables or trip lines to anchors and kedges, this is a round turn and two half-hitches with the first turn taken through the round turn. This makes it more secure but it can’t be tied or untied under load.

Again, the Dyneema line’s jacket failed, allowing the core to pull through. Failure happened at 1.9 tonnes of load, again 35% of its rated best, matching the figure-of-eight and the bowline. The Marlowbraid withstood 3.2 tonnes, an impressive 71% of rated performance, before the line snapped about 6in below the knot, which was jammed solid.

Double fisherman’s loop

This loop is also known as a poacher’s noose because it’s a sliding knot. It’s
strong and easy to tie, but once it’s under heavy load, you’ll have no chance of
sliding it back again or untying it. You’ll need to cut it off.

The Dyneema jacket failed again, a few inches below the knot oddly, and the slippery core pulled through, but it withstood 2.1 tonnes of load. That’s only 39% of rated break load, but it’s the second best performer after the splice. Incredibly, for Marlowbraid, this loop equalled the load borne by the splice, 3.7 tonnes and 82% of its break load.

Splicing nut

These have been on the market for a few years, but could they match the strength of a splice? The black core has barbs in channels that grip the standing end going into the bight, and the bitter end twice as it wraps around the black core. The grey shell is screwed on to hold it all in place.

As the largest rope they can handle is 10mm, we used our 10mm Dyneema and 8mm Marlowbraid (rated break load 1.58 tonnes) for the smaller nut. They were disappointing. At just 0.5 tonnes both lines pulled through, deforming the barbs as they went. That’s 9% of breaking strain for Dyneema and 32% for the Marlowbraid.

Bends

When you need to fasten one line to another, you’ll need a bend, but which one?

Reef knot

So called because it is the knot used to secure reefs, using the gaskets running through the mainsail’s reefing points, the reef knot is generally perceived to be useful for joining two lines together but it’s not secure. Pull one bitter end perpendicular to the line and the knot capsizes. Nor is it a particularly strong knot. The Dyneema jacket snapped at just 0.9 tonnes, 17% of its rated break load, and the Marlowbraid managed only 1.2 tonnes, 27%, before the knot pulled through. It’s useful as a binding knot, if you ensure both bitter ends are on the same side of the knot, but as a bend it’s hopeless.

Sheet bend

This is more useful than the reef knot, particularly for tying two lines of differing diameters. Also known as the weaver’s knot, as it’s used to make fishing nets, its main drawback is that it can fall apart if there is no tension in the standing parts. Make sure both working ends emerge from the same side of the knot, otherwise you have a ‘left-handed’ knot, which is less secure.

Again, Dyneema seems to buck convention as the knot just slipped through. We stopped the pull before the two lines slipped free, but there was only 0.9 tonnes of strain on the gauge, just 17% of its rated strength and the same as the reef knot. Marlowbraid fared much better, with 1.8 tonnes, 40% of its strength, before it exploded.

Double sheet bend

This, as its name suggests, is more secure than the single sheet bend, particularly with lines of different diameters, because the extra turn with the thinner line adds more friction, making it less likely to fall apart. Again, the working ends must emerge on the same side of the knot.

Dyneema continues to beat convention. It gripped until the jacket broke, at which point the core pulled through with 1.2 tonnes on the clock, 22% of rated break load. The Marlowbraid fared better, ‘bouncing’ the tension into the knot before exploding – there’s no other word – at 2.0 tonnes, or 44%.

Fisherman’s knot

Popular with anglers, hence ‘fisherman’s’, and it’s called a ‘knot’ to distinguish it from the fisherman’s bend (anchor bend), but it’s still a bend. Simply, it is two lines, overhand-knotted to each other, and used for bending together two similar lines.

The Dyneema held briefly until the jacket once again snapped and the core pulled through. The jacket failed with 1.1 tonnes of strain, just under 20% of its strength, while the Marlowbraid snapped completely at the pinch point where the standing end meets the knots. There was a creditable 1.8 tonnes on the gauge, 41% of its theoretical strength.

Double fisherman’s knot

This serves a similar purpose to the fisherman’s knot but the working ends are tied over the opposing standing parts in a double overhand knot. This makes it more suitable for slippier line, like new cordage with a shiny jacket or slime-covered lines.

The double fisherman’s knot proved to be the best bend for Dyneema, withstanding 1.6 tonnes of load or 30% of its rated break load, before, once again, the jacket broke and the core pulled through. It was also the strongest bend for the Marlowbraid, withstanding 2.6 tonnes, 58%, before failing in the same way as the single fisherman’s knot.

The results

Which loop proved to be the strongest, and which bend outlasted the rest? Let’s analyse the results

Conclusion

For all three types of cordage, the eye splice was the strongest loop, provided the splice is very long and tapered, because splices tend to fail at its end. A spliced loop is fine for mooring lines and it’s the best solution for fastening shackles to halyards, provided the splice is served with whipping twine. Splicing nuts didn’t fair at all well. If you can’t splice three-strand, you haven’t actually tried because it’s fairly easy, but you should choose the round turn with two half-hitches.

Of the other loops, the double fisherman’s loop is the strongest. If you can’t splice Marlowbraid or find a rigger to do it for you, that’s the knot to choose. The bowline, the ‘king of knots’, fared marginally worse than the figure of eight, suggesting a shackle spliced onto your jib sheets and taped round with Velcro for protection is the best option.

Of the bends, the double fisherman’s knot was the pick of the bunch for both Dyneema and Marlowbraid, with the double sheet bend a fairly distant second. The single version of both performed better than the reef knot.

In general, it’s clear that a knot in any load-bearing line weakens it significantly so anything but a long line with a splice at the end is seriously compromising the line’s strength. On the other hand, it’s reassuring to know that you will have fair warning that a knot is about to fail because there will be plenty of creaking and jumping as the tension hauls any stretch out of the line.

Which knot is best for what?

Mooring

A decent, long eye splice is the best option for security and strength – handy virtues when it comes to mooring up – but the loop will wear in the same place. If you use Marlowbraid for mooring but can’t splice it, the round turn and two half-hitches is best. It’s stronger than a bowline and it can be untied under load.

Sheets and shackles

For fastening sheets to sails, the data says you’ll be better off with a double fisherman’s loop – however, that’s assuming you don’t mind cutting them off the clew at the end of the season – the bowline is much easier undone. For fastening shackles to halyards, that’s not a problem so if you can’t eye-splice a shackle, choose the double fisherman’s loop.

Bending lines together

Again, going from the data, the strongest bend for joining two lines of the same, or similar diameter was the double fisherman’s knot. For lines of different diameter, the double sheet bend is the most suitable knot – It’s the most secure and among the quickest to tie.