Sizing u-bolt for shrouds

Moonshining

Moonshining

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We had an interesting moment coming back from Poole on Monday in our Jaguar 27. After a fairly bumpy journey we were approaching Hurst Castle when the lower forward starboard shroud went flying with a loud bang.

The U bolt which holds the shroud to the deck (held in place by a stainless steel backing plate) had snapped just above the nuts. Looking at it, it was fairly heavily corroded. Scarily, one leg of the u bolt on the rear lower shroud had also sheared.

We motored to Cowes and I replaced the failed bolts with Wichard 6mm u bolts with a working load of 1280Kg. These are actually slightly smaller than the ones which failed. Eyeballing them, the failed ones look like 8mm but the 8mm bolts I tried wouldn't fit through the backing plate holes. The 6mm ones did, so were an easy get-us-home solution.

I'm obviously going to have to assume the port U bolts are in danger of breaking too and replace those. However, I can't find any information on what size/working load I should look for.

Does anyone know have any information on sizing deck fixings like these? What would be usual on a 27' boat like mine?
 
Poole Triangle

I had a very similar experience years ago, also off Poole !

A lower port shroud U bolt failed, breaking cleanly off the base plate; i put it down to having stressed that U bolt the year before, lashing it to a pontoon to keep the boat upright with someone up the mast.

I also went the Wichard route; I find it very hard to imagine these breaking, the old saying was 'could you hang the boat from the standing rigging / chainplates', and I'm sure you and I could.

Interesting you mention corrosion on the failed U bolt, was this as in low grade stainless, or some other material like mild steel ?
 
Seajet said:
I had a very similar experience years ago, also off Poole !

A lower port shroud U bolt failed, breaking cleanly off the base plate; i put it down to having stressed that U bolt the year before, lashing it to a pontoon to keep the boat upright with someone up the mast.

I also went the Wichard route; I find it very hard to imagine these breaking, the old saying was 'could you hang the boat from the standing rigging / chainplates', and I'm sure you and I could.

Interesting you mention corrosion on the failed U bolt, was this as in low grade stainless, or some other material like mild steel ?
Click to expand...

They're definitely stainless steel, although I'm not sure of the provenance. I think that this was crevice corrosion caused by water getting under the upper plate of the U bolts and sitting between that and the lower backing plate. Certainly the upper side of the rear backing plate was wet when I took it off.
 
Yes its crevice corrosion.

Essential to make sure that no water can penetrate between the bolts and the deck. Good sealing is the answer.

We (I) resealed the Ubolts on one side of a friends boat because they leaked into the locker I kept my stuff in.

On the other side one leaked into the wet hanging locker and one into the galley sink so were ignored.

The one over the sink failed (lower shroud). The one in the hanging locker (cap shroud) broke off when i tried to undo the nuts.

We replaced all four but the ones I had re-sealed several years previously were perfect.
 
Sometimes the U bolts are not perfectly aligned with the shroud load. Actually some on smaller boats make no effort and just stick straight up! They therefore flex slightly under load and eventually snap where the thread meets the plain shank as it's a stress riser. This is obviously aggravated by any crevice corrosion in the damp, deck thickness.

The U-bolt is a pretty unsatisfactory way of making a chainplate. They are frequently the culprit when you hear of boats, including the likes of Westerly, drop their rigs. However they are curiously popular in the UK, on the grounds of cheapness I guess. Then they sort of became accepted as an 'industry norm' even when they weren't saving very much in the bigger scheme of things. Bit like cast iron keels.
 
TimBennet said:
Sometimes the U bolts are not perfectly aligned with the shroud load. Actually some on smaller boats make no effort and just stick straight up! They therefore flex slightly under load and eventually snap where the thread meets the plain shank as it's a stress riser. This is obviously aggravated by any crevice corrosion in the damp, deck thickness.

The U-bolt is a pretty unsatisfactory way of making a chainplate. They are frequently the culprit when you hear of boats, including the likes of Westerly, drop their rigs. However they are curiously popular in the UK, on the grounds of cheapness I guess. Then they sort of became accepted as an 'industry norm' even when they weren't saving very much in the bigger scheme of things. Bit like cast iron keels.
Click to expand...


a few comments here as well :D
http://www.ybw.com/forums/showthread.php?t=326667
 
:)
VicS said:
Yes its crevice corrosion.

Essential to make sure that no water can penetrate between the bolts and the deck. Good sealing is the answer.

We (I) resealed the Ubolts on one side of a friends boat because they leaked into the locker I kept my stuff in.

On the other side one leaked into the wet hanging locker and one into the galley sink so were ignored.

The one over the sink failed (lower shroud). The one in the hanging locker (cap shroud) broke off when i tried to undo the nuts.

We replaced all four but the ones I had re-sealed several years previously were perfect.
Click to expand...

Another job!!! Is there no end? :(:)
 
botheras said:
A mate of mine used to have a Jag 27 and was active in the association. I recall him saying something about chainplate issues... Try a search of the Jag assoc website?
Click to expand...

Since I sent the original post, I found that there's been a longstanding issue with Catalina 27 lower shrouds which originally had an undersized eyebolt deck fastening. It's possible to buy a retrofit kit containing u bolts and backing plates but, from the photographs of the kit, the u bolts look the same as the ones that just broke so I'm a bit suspicious of it.

I think I'll finish up getting the 8mm Wichard Watertight U bolts. (2400Kg breaking strain and a rubber collar to prevent water ingress). I think that I'll be able to keep the existing backing plates with a little judicious filing of the existing drilled holes. 4x2400Kg breaking strain bolts, plus the main upper shrouds on big chainplates should give me some confidence next time I'm bouncing around between Hurst Castle and the Shingles bank!
 
Do Whitchard do canted versions? I didn't think so.

It's awhile since I've seen the side decks of a Catalina 27, but if the axis of the U Bolt is fore and aft, then a canted version for the lowers will have to endure less bending moments than a straight one.

You might want to consider these: http://www.southernriggingsupplies.co.uk/a-bolts-angled--straight-stainless-steel-aisi-316-428-c.asp

Remember, the ones you have failed! Don't replace like with like unless you are certain you have totally eliminated the cause of the failure. The rule in designing rigs is the chainplates should NEVER fail! Upgrade them all - don't just replace failed items.
 
TimBennet said:
Do Whitchard do canted versions? I didn't think so.

It's awhile since I've seen the side decks of a Catalina 27, but if the axis of the U Bolt is fore and aft, then a canted version for the lowers will have to endure less bending moments than a straight one.

You might want to consider these: http://www.southernriggingsupplies.co.uk/a-bolts-angled--straight-stainless-steel-aisi-316-428-c.asp

Remember, the ones you have failed! Don't replace like with like unless you are certain you have totally eliminated the cause of the failure. The rule in designing rigs is the chainplates should NEVER fail! Upgrade them all - don't just replace failed items.
Click to expand...

The bolts are oriented so that the long axis is on an imaginary radial line from the mast, so they shouldn't need to be canted ones. I'm not smart enough at visualising vectors to work out if the loads are uneven on the two legs though.

I'm certainly going to replace all four bolts. I have to assume that the amount of corrosion is the same on the port side.
 
Westerly failures

FWIW, I do recall being told that part of the problem is that U bolts are much more prone to fail compared to a V bolt type, due to the high stress placed on the middle of the 'gentle' U shape by the toggle clevis pin. I understand that early Westerly corsairs had U bolt arrangements, some if which failed, and so westerly switched to the V type.
 
Not sure if Westerly changed from U to V bolts but they certainly changed to cranked ones so that the U was bent to match the angle of the shroud. The U bots were aligned fore and aft.
 
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