Many times we are being contacted with the question whether a yacht should go for polyester or dyneema mooring lines.
We try to give a bit more information on behaviour of both the dyneema and polyester materials to clarify which rope is the best for your yacht.
Polyester is the basic yarn which works great in many ways. It has a good abrasion resistance, can be supplied in various colors, provides stretch and is well priced.
The polyester mooring lines are normally used in a double braided construction. This means a 12-stranded core and 32-strand jacket. By weaving it a bit loose the rope provides stretch whilst still remaining a good breaking strength. The outer jacket is woven more tightly and has more twist in the yarns to ensure a good abrasion resistance on the fair leads, bollards and winches.
When we talk about Dyneema mooring lines only the core is made out of 12 stranded Dyneema, and the cover remains being made out of 32 plaited polyester. Dyneema is simply to slippery to get a grip on bollards, therefore we need the polyester cover.
Dyneema is a great modern yarn. It is very strong, light, and has got virtually no stretch. The fact that dyneema is strong and light makes it great for a mooring line. Obvious you could go for a smaller size rope, and even with the same diameter the rope is much lighter.
The specific mass of polyester is 1,42 and dyneema is 0,97.
In theory you could downsize from a 40mm polyester to a 28mm dyneema. It will remain the same strength and the line will be much lighter.
In practice it’s not always that easy.; now we come to the disadvantages of dyneema mooring lines.
| M-ropes M32 Polyester | M-ropes D32 Dyneema | |||
|---|---|---|---|---|
| Yacht length | Ø Polyester | Ø Dyneema line | Ø Matching stretcher | weight saved |
| 30 to 50 meter | 36 | 24 | 36 | 45 |
| 50 to 80 meter | 40 | 32 | 40 | 53 |
| 80 to 100 meter | 48 | 36 | 44 | 97 |
| 100 to 120 meter | 60 | 40 | 56 | 153 |
The dyneema core is very solid. Specially under tension it’s quite hard. Therefore the polyester cover will get squeezed quite hard in the fairleads causing the polyester cover to abrase or melt. Next to this since there is no stretch in dyneema lines the shockload will be higher, again specially in fairleads this damages the cover quite hard.
Another thing when going down in diameter is that the surface of a 28mm line is much smaller compared to a 40mm line. The load per square inch becomes much higher which again is not great for the cover.
Basically you need to understand that Chafe Protection is required when going for dyneema mooring lines.
Another item to consider is the lack of stretch in dyneema lines. In choppy or windy conditions it’s good to have stretch in the lines in order not to have massive peak loads on the deck equipment. With dyneema lines we can solve this issue by using stretcher loops between the dyneema line and the shore. These loops are about 4-5 meter long and provide the needed stretch in the rope. However it will always be less then stretch in a polyester line.
When the crew is getting used to dyneema lines they will work great. It’s important to get the slack out of the lines so the yacht remains stable without moving. If the yacht does not move at all, it can’t gerenate any peak loads.
Fortunately with our M-ropes dyneema mooring lines we are able to re-braid the polyester cover if it is damaged. This gives you a virtually new rope for a small percentage of an entirely new line.
The biggest benefit of dyneema mooring lines is obviously the handling of a thinner and much lighter rope.
In general when going from 40mm polyester to 32mm or 36mm dyneema the weight is about half. In practice we see customers with yachts above 70 mtr are choosing for dyneema lines since the polyester lines simply get to heavy