Shock absorbers put to the test: These 12 models take the strain off the line and the boat
Sailors are familiar with the situation: swell is rolling into the harbour, the wind is also pushing in from the side, and the boat is jerking violently against the mooring line. Every jerk creates peak loads that are not only unpleasant but also put considerable strain on the materials. Comfort suffers massively as well. When the jerks are severe, it’s almost impossible to sleep on board. Every jolt is felt throughout the whole boat, crockery clatters in the cupboards, and restful sleep is out of the question. Anyone who has spent a night in the harbour in swell knows that it takes its toll on the mooring lines and on your nerves, and the next day you lack the energy for relaxed sailing or shore excursions.
But even when the boat is moored in the harbour without a crew, there remains a concern that the mooring lines will eventually be unable to withstand the increased wear and tear, causing the boat to drift and sustain or cause considerable damage.
Shock absorbers as a simple solution for sudden spikes in load
The solution is shock absorbers: rubber components that are threaded through or wrapped around the mooring line. They stretch under load and absorb the energy. The peak load is reduced, transforming the sudden jolt into a gentle, controlled movement. The line can absorb significantly more energy because the shock absorber allows for additional stretch. This not only protects the material, but also makes mooring in the harbour much more comfortable and significantly reduces wear and tear.
Many shock absorbers are designed in a bone-shaped configuration with eyelets at either end. The mooring line is threaded through a hole at one end, wound several times around the elastic rubber body, and then threaded through the hole at the other end. This compresses around 40 to 60 centimetres of line down to 20 to 30 centimetres. The key feature is that the rubber is stretched, pulling the line wrapped around it taut – significantly increasing the effective length of the line. Good mooring lines already have as much stretch as possible by nature. Unlike sheets and halyards, this is desirable in this type of rope. However, the stretch values within the working range are in the low single-digit percentage range. With a shock absorber, the mooring line can stretch considerably further under load and absorb energy. Instead of jerking, the movement is, ideally, gently slowed down. The system is simple but effective, and particularly helpful in mooring situations where the mooring lines are rather short.
We tested twelve shock absorbers of various designs and from different manufacturers. To do this, we visited the test laboratory of the rope manufacturer Liros. On the test rig, we simulated and measured the load on the mooring line with each shock absorber.
An overview of strain values
The models range from classic bone-shaped shock absorbers to innovative clip systems and integrated solutions where the shock absorber is fitted directly into the line. Prices range from around 20 to 80 euros per unit. It’s certainly an investment, but very reasonable compared to the cost of a snapped mooring line or damage to the boat.
It’s all about the music when it comes to small loads
The first surprising finding: maximum elongation occurs at significantly lower loads than one might expect. Mooring lines have breaking loads of over two tonnes; high-quality models can handle three tonnes or more. One would therefore expect shock absorbers to deliver their maximum performance in the range from several hundred kilograms to over a tonne.
In reality, however, the critical stretch occurs for most lines weighing under 150 kilograms, at around one-tenth of the breaking load or even less. Anyone who has seen a shock absorber on board that is already stretched very far, with the line creaking under load, would assume the loads to be significantly higher. On the test bench with measuring equipment, this exact scenario was observed at loads as low as 100 to 200 kilograms. That may not sound like much, but it is precisely the range in which boats pull on the line when there is swell in the harbour. In stronger swell and wind conditions, this load may be even higher, but the critical range for comfort and to protect the material definitely lies at the lower end of the load spectrum.
Products and data at a glance
From a load of around 300 kilograms onwards, most shock absorbers do not respond significantly. The extension increases only slightly. The rubber has practically reached its maximum extension. This is actually an important insight: shock absorbers are not emergency energy absorbers for extreme loads, but rather provide comfort and wear protection for normal everyday harbour operations.
To ensure better comparability, a high-quality mooring line was tested first. At 1,500 decanewtons – equivalent to approximately 1,500 kilograms of tensile force – the Porto mooring line from Liros exhibited 7.5 per cent elongation. Now for the comparison with the test winner, the U-Cleat 16 from Lindemann: at 100 decanewtons, the system comprising the line and shock absorber already exhibits just under 30 per cent elongation. Many times that of the line on its own! At 200 decanewtons, the figure is 45 per cent. This is a dramatic improvement over the line on its own, and precisely within the load range that is relevant in harbour conditions. From around 300 decanewtons, the curve flattens out significantly. With the U-Cleat 16, elongation rises to over 60 per cent at 1,600, but the greatest increase clearly occurs in the low load range. This is precisely where boats pull on the line in swell. Other high-performing models also exhibit this characteristic behaviour: massive elongation at low loads, followed by a pronounced plateau where the elongation values increase only slightly.
Which designs are effective – and which are not?
In addition to the classic bone-shaped design, there are also the perforated strips and two completely different designs. In terms of the measured stretch values, the perforated strips were in the bottom third of the test field. Due to their design, threading the line also takes a little longer: the rope simply has to be threaded through more holes. However, as these shock absorbers are only installed once, this disadvantage is not particularly significant. Nevertheless, the bone-shaped U-Cleat models offer the advantage that they can be threaded through much more quickly, without having to thread them at all. The perforated strip models cannot compete in this respect.
The special designs are the Smart Snubber and the Miami. The small, S-shaped Smart Snubbers are simply clipped onto the line. Furthermore, several can be linked together in series. However, due to their compact design, they simply do not offer as much stretch as the long, bone-shaped models.
One special case is The Snubber Twist: bone-shaped, but with a ball of rope at each end. This model can also be simply looped through. On the first attempt, as the load increased, the rope and ball slipped out of the shock absorber. The second test involved tightening an additional screw. This prevented the rope from slipping out, but at 500 dekanewtons the shock absorber broke and the screw protruded as an unsightly point from the broken piece. Unfortunately, this meant the model failed the test.
The Miami model is visually appealing; the integrated shock absorber is visible only as a slight thickening in the line. However, the elongation figures were not particularly impressive, especially in the lower load range. At 100 decanewtons, the line-damper combination showed a meagre 3.5 per cent elongation. At 1,400 decanewtons, the line broke.
A quieter harbour – with marked differences
In conclusion, however, a positive assessment can be made for all models (with the exception of the Snubber Twist with the screw): every shock absorber improves the stretch values of the mooring line. This means that every shock absorber will also make unpleasant jolts in the harbour somewhat more bearable. Nevertheless, the differences are still very significant. A trend can be discerned from the measurements: the larger the model, the greater the stretch. This is hardly surprising, but it can play an important role in the purchasing decision. This is because a range is always specified for the line diameters. For a 12-millimetre line, two sizes are often suitable. Opting for the larger one can provide even greater stability in the harbour when there is swell.
All in all, it’s a worthwhile investment; it enhances both safety and comfort.
An old banger with upgrades
The classic design for shock absorbers is the ‘bone’. The mooring line is secured at both ends and looped round the centre three times. Under tensile load, the shock absorber (as manufacturers call it) can be stretched by the length of the rope used for the loops. Under load, the line then appears straight and the rubber bone is looped around it. Despite having the same basic design, the models differ in size and method of attachment. The differences in size are variants designed for different line diameters. Attachment using clips, as with the U-Cleat models from Unimer, or using the rope loops on the Snubber Twist, makes it easier to attach them to the mooring line. This can be an advantage if they are only to be attached when needed. Those with separate mooring lines fitted with shock absorbers may also be able to cope with the one-off (more time-consuming) process of threading them through.
Cleverly threaded: the perforated strip
As well as the bone-shaped design, the perforated strip design is also very common. Here, the extra length of the mooring line is integrated into the shock absorber in the form of S-curves. The disadvantage of this design is that the line has to be threaded through a large number of holes. These mooring springs are therefore not suitable for quick retrofitting, but are effective once they are in place. Two distinctive designs were offered by Smart Snubber and the Miami model. The former are very compact and clip onto the line. The latter is a mooring line with a shock absorber integrated into the braided sheath.
Quick to install and modular
The Smart Snubber is simply clipped onto the line. This makes it easy to fit retrospectively, even once the boat is already moored. Two are recommended per line, but you can also combine more. Its compact design is a major advantage. Depending on the distance to the cleat, longer snubbers may not always fit. With rope diameters ranging from 10 to 16 and 12 to 20 millimetres, they cover a wide range. The test suggests that it is better to choose the larger size.
Concealed damping
The Miami model combines a permanently integrated damper with a braided sheath. Energy is absorbed as the loaded line compresses the rubber at its centre. This worked in testing, but did not deliver convincing results. Nevertheless, although other models offer greater stretch, the Miami is significantly better than a mooring line without any additional shock absorber.
This is how we carried out the test
Each shock absorber was tested on the test rig at Liros. To ensure that the results were comparable and reproducible, each shock absorber was secured using a new section of the same Liros Porto mooring line. This line has a breaking load of 3.8 tonnes, although this was reduced to 1.8 tonnes in the test setup due to the use of two clove knots. Each shock absorber was pre-stretched three times to 50 decanewtons using the line to allow everything to settle. The load was then increased continuously until breakage. In the vast majority of cases, the mooring line broke at one of the knots. The Snubber Twist is an exception here. The Miami model was tested using its own line.
Hauke Schmidt
Test & Technology editor
