Decking U bolts

U-bolt type shroud attachment fittings: A Case Study by Mike Buggy


Many production sailing yachts from the late 1970s to the early 1990s used stainless steel hoop/U-bolt type fittings on deck as shroud attachments. These are usually, although not always, backed up by large plates below deck and by chainplates from one or both legs of the fitting. The chainplates usually run down and are fastened through a bonded knee or structural bulkhead.

Manufacturers differed on how to allow for the inward and/or fore and aft pull angles of shrouds relative to the deck plane. On some vessels the hoop/u-bolt is not physically bent or angled, but the fitting is installed on deck at a suitable angle to the fore and aft line so that the pull is in the same plane as the hoop.

Where the hoop/u-bolt is installed in the fore and aft line, it is often bent inwards at the deck plate during manufacture to allow for shroud angle and deck camber. Some manufacturers however used fore and aft hoops/u-bolts perpendicular to the deck plane but at least most had the common sense to use oversize bar, so few problems were apparent in the early years.

Fractured U bolt

This case study is based on later medium to large Westerly models (early 1980 to early 1990s) which had potential shroud attachment problems arising from this type of U-bolt/Hoop design and placement. Although not proved conclusively, some rig losses were also attributed to the problem.

In the years before its demise, the company was aware of the problem andhad issued advice and arranged for owners to buy new modified ‘cranked-in’ and stronger shroud deck fittings at a knock down price (no pun intended!). Many, but not all, took up the offer. One of the problems was that the rigging not only imposed a pull at an unfair angle on the fittings, but that the metal bar can ‘work’ in the short space between the underside of the fittings’ deck plates and the chainplate bolts, as the rigging was alternately stressed and relaxed under load. This problem is exacerbated by corrosion if moisture is trapped aerobically in the core around the u-bolt holes.

The view amongst class associations for vessels with these fittings is that they should all be checked to ensure they are angled inwards correctly in line with the shrouds. Below decks, the chainplates and bolts/nuts should be checked regularly and the U-bolts themselves inspected every 10 years and ideally be changed if over 20 years old.

When the first replacement is being made, the bar size should be one grade (e.g. 12 for 10mm) larger than the original, and the deck plate should be at least 25% bigger. This is to reduce any risk of the fittings ‘working’ and also ensures that the ‘crush area’ of the smaller original deck plate is covered.

Specific Case Study: Recent U-bolt replacement task – 1990 Westerly Oceanranger (38ft, 8 tons).

This relatively large displacement class used U bolts as described above for the Aft Lower shrouds only. On this particular vessel, the U bolts had already been replaced with modified, cranked in, type in about 1992. As this boat was intended for ocean work, and it is impossible to predict the life of the fittings, it was decided that another renewal was the safest course of action.

Cardboard templates were used to check the angle between the shrouds and the deck planes in both the fore and aft planes and athwarthships. The composite cranking angle was found to be 70o. The templates were taken to the same stainless steel fabrication company who had produced the earlier modified fittings for Westerlys.

The diameter of bar (12mm) used in the existing hoops was carefully measured in several places by calliper and mic. The hole spacing, thread type, and length was also measured carefully.

The design of the replacement fittings was discussed with the fabricator. It was agreed that as the fittings being replaced were not the originals (and they were already cranked inward and oversized) the new ones should be similar, but with a much larger deckplate area. The crank angle would be exactly as measured. It was also decided not to use hooped u-bolts but to use ‘A’ shaped fittings. This was to avoid any tendency of the shrouds to ‘straighten’ the conventional U shape.

Carrying out the job

The babystay was loosened off at the same time as the aft lowers. Forestay, backstay, cap shrouds and intermediate shrouds remained in place. A non stretch line was taken up to the first spreaders and around the mast. The ends of this were taken to the cockpit winches, hove in tight and made off. The aft lowers were then removed.

Removal of the old deck fittings was difficult as furniture and facings had to be removed to expose the saloon aft bulkhead chainplates. Access to the under-deck U-bolt nuts was difficult as they were shrouded by the reinforcement webs of the chainplates.

Conventional spanners were too thick to use, while the long threaded shanks prevented use of conventional sockets. Thin walled ring spanners were fairly successful. Long sockets of the plug spanner type worked well, and the older type thin walled box spanners were invaluable. Final tightening would not have been possible without these.

Once the nuts had been removed, the fittings were levered out on deck by using long strong screwdrivers with blocks of wood as fulcrums.

As the steel deck plates on the new fittings were much larger, the Treadmaster deck covering had to be marked and then chiselled out to allow the new plates to sit snug to the deck.

A certain amount of clearing had to be done in the holes, and the new fittings required a little fine tuning to get them to fit the deck hole spacings.

Before installing the new fittings, marine silicon sealant was applied around the initial parts of the deck bolts and also over the deck plate seating area.

It is important that the long threads of the deck bolts are cleared and that the nut pairs intended for each shaft are each tested in advance by running them up and down the full length of the threads. This requires a spanner and some effort as the threads are new. There is not room to do this from below decks once the fittings are in place.

The fittings were tapped home with a wooden mallet.

Note that any work on the chainplate bulkhead bolts should take place BEFORE tightening home the new deck fittings. If possible, as many of the chainplate to bulkhead bolts and nuts should also be changed. At the very least all these should be checked and tightened.

The fittings were taken up by single nuts on each leg using a box spanner.

It is important that NEW nuts are used, when replacing these deck fittings, and also when the fittings are removed for survey and re-installed.

Ideally the highest spec Stainless nuts should be used. There should be A2 and A4 markings on good quality marine grade nuts. A2 is frequently used by manufacturers below decks but A4 is the better quality for this task.

Once the fittings had settled in place and filler fully exuded around the deck plates, a second locking nut was run up each shaft and tightened using a ring spanner on the bottom end of a box spanner.

Finally, the shrouds were reconnected once all chainplate bolts had been checked and tightened.


Original WESTERLY GROUP LTD advisory note on shroud deckplates:



“Westerly Group actively pursues a policy of continual product and production improvement and development. From this process and in conjunction with the WOA, our attention has been drawn to a potential problem with some of the earlier designs that could be potentially dangerous. The problem has been identified as the aft lower chainplate of the U-bolt type which have not been cranked. Due to the loads on this fitting and as it does not fully align with the shroud, a levering effect can cause the fitting to break at the thread. As this is below deck, and therefore not apparent from a cursory inspection, it is only possible to see a potential failure by the removal of the fitting from the deck.

The WOA has advised its members of the problem. However this information may not have reached all owners of affected designs. It is therefore hoped that this notice will rectify the situation.

Westerly Group have now designed a fitting which has a larger diameter rod and deck plate and is cranked to the correct angle. This is available from our spares department at Waterlooville at a special cost of £50 +vat and can be fitted by any boatyard or competent person.”

Explanatory Note in a later Westerly newsletter

From the discussion in the other magazine articles, it appears that the aft lowers on these boats do not drop vertically down to the deck, but slope outwards towards the toe rail. This angle to the deck is exacerbated because the deck also slopes towards the toe rail. This means that the shroud does not approach the deck at 90 degrees, but at angles between 70 and 77.5 degrees (depending on class). Since the U-bolts are mounted fore and aft (not athwartships), the shroud puts a considerable fatigue on the fitting if it is not cranked (bent) towards the shroud.

The solution appears to have been supply of new U-bolts available through Westerly Spares (which no longer exists) which were made from 12mm rod, have a larger deckplate attached to them, and are cranked appropriately.

Note on A2 (304) and A4 (316) stainless steel fastenings

A2 and A4 are both Austenitic stainless steels with A4 grade having a higher corrosion resistance than A2. They are both non-magnetic.

Grade A2 (AISI 304) has excellent resistance to atmospheric corrosion, except in severe industrial and marine environments. Also suitable for most non-saltwater marine domestic applications.

Grade A4 (AISI 316) has a higher corrosion resistance than A2 grade in many chemical environments, including contact with dilute sulphuric acid, and acetic acid. Suitable for marine conditions, but not recommended for immersion in sea water. Typical applications include fasteners for boat deck fittings

Mike Buggy

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