Chainplates

[Kelpie]

Chainplates make me nervous. They are such fundamental parts of our rigs, yet unlike the standing rigging itself their inspection and replacement is far from straightforward on many boats.
The commonest designs seem to involve cutting a slot in the deck, which inevitably becomes a source of leaks after a few decades- annoying at best, dangerous at worst as it could lead to corrosion.
Many chainplates bolt onto timber/ply bulkheads- obviously fine when the boat is new, not so good when it is 30+ years old and that bulkhead is getting a bit soft. Others are glassed into the hull, less potential for rot but no means of inspection and a good way of allowing crevice corrosion.

So, given all of the above... why don't all boats just use external chainplates bolted directly onto the hull? Obviously the layup of the hull would need a bit of strengthening at this point, and the through-bolts would probably want inspected every few decades, but you'd have the advantage of a wider triangle and thus stronger rig, you'd be able to see/inspect/repair/replace the chainplates with ease, and you'd not have problems with deck leaks.

Thoughts?

 

[Tranona]

They do now. However, everything is a compromise and most of the boats you will be looking at were built under the influence of the IOR racing rule which encouraged large overlapping genoas. So to allow close sheeting the shrouds had to move inboard and cutting slots in the deck and using the main bulkhead to anchor them was logical with in line spreaders and masthead rigs.

The weaknesses are leaks where the chain plates go through the deck, often inadequate attachment to the bulkhead and hiding the whole lot behind acres of teak. As a result more rigs (and no doubt a few boats) have been lost over the years because of this than AWBs affected by loose keels!

An improvement is to do away with anchoring to interior bulkheads or knees but using tie rods from deck to frames bonded in lower down the hull and linked to the hull grid structure. This means that only bolts holding the deck plates go through the deck and these are much easier to seal. You will find this method is used on many AWBs and with fractional rigs there is often only one plate and not lined up with the main bulkhead. My old Bavaria had this system and never had any problems with it.

The trend nowadays is back to hull mounted chain plates because jibs have shrunk and are now sheeted inboard as on my new Bavaria. The loads are taken by structural laminations down the hull interior. There is a good explanation of this somewhere on the Hanse website.

 

[William_H]

As said above much better for overlapping jibs to have shrouds inboard of gunwhale. Also makes any body moving forward much easier to get past shrouds. Yes we often see a saddle (a U shaped plate with legs and holes) on top of the deck with a similar saddle below deck with wires or plates down to structure although I have seen saddles or the bolts fail in this role, just too light for the job. Generally chain plates or equivalent should be mauch stronger than the wire so longer lasting. olewill

 

[vyv_cox]

For additional info there are case histories of chain plate problems on my website under 'sails and rig'.

 

[Seajet]

To my mind, from an engineering view chainplates on a grp boat ought to be suitably angled ' L ' shaped jobs combining deck and hull, both for keeping the rig up and keeping hull and lid together.

One could use dome head bolts on the hull sides so as not to be too obnoxious to boats alongside.


On my A22 the chainplates are bolted through the deck, and there are 72 bolts and self tappers holding that to the hull; on a larger boat with bigger loads I'd want them spread into and across the hull, ideally at the design stage by load spreading webs.

 

[vyv_cox]

To my mind, from an engineering view chainplates on a grp boat ought to be suitably angled ' L ' shaped jobs combining deck and hull, both for keeping the rig up and keeping hull and lid together.

Very similar to the Westerly ones reported in my case history. The bolts failed by stress-corrosion cracking.

 

[Seajet]

I'm thinking a few bolts in shear load and a few trying to pull the lid off are the best combination one can hope for .

 

[Tranona]

To my mind, from an engineering view chainplates on a grp boat ought to be suitably angled ' L ' shaped jobs combining deck and hull, both for keeping the rig up and keeping hull and lid together.

One could use dome head bolts on the hull sides so as not to be too obnoxious to boats alongside.


On my A22 the chainplates are bolted through the deck, and there are 72 bolts and self tappers holding that to the hull; on a larger boat with bigger loads I'd want them spread into and across the hull, ideally at the design stage by load spreading webs.

Read post#2 if you dare. The boats the OP will be looking at (Moodys, Oysters, larger Westerlys and many other boats of that size/type built in the 70 90s will not be as you described, but will be like I described and for the reasons given. Two recent articles in PBO show how they can fail, one on a Moody and the other on a Southerly.

Hull mounted chain plates are only suitable (if you want close sheeting) on modern boats if the clew of the jib is forward of stays, which it commonly is with a fractional rig and non overlapping (or the popular 105% or so jibs) used nowadays.

What makes you think that designers do not take into account rigging loads when designing hulls and load sustaining structures within them? Things have moved on a bit since 1970s - and even then designers new what they were doing even if the execution of some of the designs was suspect or limited by the materials or building techniques available.