Chainplates on a Cobra 850

[BigStu]

Hi all

As part of my winter maintenance I've had the standing rigging renewed. I've not taken the chainplates off to examine, but will be doing this (it was a case of the rigger having a slot and me not being ready!). However, on a Cobra 850 the chainplates are essentially U bolts through the deck. They are then fastened under the deck through a piece of wood glassed into the deck and then a wood block backing and large washers. I was surprised that they are not then fixed to the hull or bulkhead.

There are no stress cracks etc that I can see of- but has anyone experience of the security of this type of arrangement as to me- it does not seem that strong! The boat has survived 40 odd years like it though...

What are peoples thoughts?

 

[johnlilley]

The long term risk is of water leakage into the encapsulated plywood which would cause softness and lack of load spreading allowing high localised loads on the grp deck. This is not uncommon and obviously the older the vessel then the risk of water leakage increases. As a result of slight water leakage which may or may not be present it is also possible to develop crevice corrosion in the U bolts. This cannot be seen above deck and is usually localised on the hidden shanks between the deck and the nuts below.
The method of attaching the U bolts just to the deck has been used by a number of boats but there are much stronger methods and many builders transfer the loads to webs bonded to the hull and or bulkheads using various brackets. Many of the smaller Cobras do get deck problems caused by the deck lifting.
Lay a straight edge along the deck at the U bolts and see if there are any bumps. Originally the deck would have been flat.
John Lilley

 

[BigStu]

Thanks so much for the reply! There is no deflection in the deck and no sign of water ingress around any of the holes etc. So the wood ply in the deck seems fine. I want to replace anyway- but this seems to be more difficult that I thought. I can get stress tested U-Bolts to fairly hefty loads, but a centimetre narrower inside the 'U' than what is there- does that matter? Also, I can't angle the new U-bolts in line with the shrouds either. I can't help feeling that the set up does not strike me as particularly strong- but as I said, there are no stress cracks or deflection etc and it's been fine for 40 odd years. When you see other yachts of similar size, they have plates through the deck bolted to the hull or bulkheads. How come on the Cobra just through deck is fine- are they laid up like tanks? I'd like to improve the situation- so I think fabricating a large stainless backing plate would do the trick. Ideally I'd like to get a bracket made to go through the deck and then bolt to the hull, but the angles mean this may be hard to get right- hence maybe why they did not do this is the fist place! Any help on possible improvements would be gratefully received!

 

[alahol2]

I found this page helpful when I was investigating my U-bolt shroud plates... http://www.bmse.co.uk/articles/decking-u-bolts/

If you search 'cranked u bolt' you will find the type that I think you require. I eventually found a set that were cranked and that fitted the holes I had.

 

[Rhylsailer99]

Bluewave 10mm angled A bolts have matching centres but the holes need to be bigger.

 

[smithy]

If it's lasted 40 years it's obviously strong enough to do the job so why change it? I do understand why you would like to check the condition of the u bolts though.

 

[Rhylsailer99]

If it's lasted 40 years it's obviously strong enough to do the job so why change it? I do understand why you would like to check the condition of the u bolts though.

I changed mine as the backing plates was shot an the mast was coming down so it seemed like a good time to do that. It was easier to go to 10mm u bolts instead of original 8mm as i could not source 8mm with 50mm centres but the 10mm u bolts have 50mm centres.