Contessa 26 / Oversized topside chainplates

nickrj

nickrj

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Hi all,

I'm not sure what the technical term is for strips of stainless that are used as chainplates, mounted vertically from the top of the topsides, however I'm considering making the modification.

Are there any downsides to this? I'm getting new standing rigging made, so the additional length is not a problem. I'm in a fortunate position where I can more or less get anything cut and built out of high grade stainless for free, and think this would be a worthy upgrade.

It means I can get rig strain off of the deck, and onto the topsides, which I believe is a much better & stronger place for them to be. The inside chainplate could be a large single piece of stainless.

Any comments or thoughts?

Cheers, nick
 
Depends on whether or not the hull was designed to take that particular load. This is a question for the designers I would have thought but I for one wouldnt go ahead without a meaningful comment from someone that really knows.
 
Nick,
Hi. I have external chainplates on my early Contessa 26, added by a previous owner who was planning a circumnavigation (but he never started) so this was one upgrade I did not need to perform on the boat.

I've never 'raced' the boat against other Contessa's so I cannot comment on any impact on sheeting angles or relative upwind performance but I don't believe there is any negative impact.

A good view of them is shown on the revolving slideshow on the homepage of my website below or detailed at:

http://www.contessa26moonshine.me.uk/photos/117-1710_IMG.html

The chainplates extend through the hull bulkheads and provide the extra strength I wanted for the Jester Challenge 2010.

If your considering upgrading your rigging my advice (which the rigger would say to me everytime he phoned me up to authourise another peice of work) was ... *** IT'S ONLY AS STRONG AS THE WEAKEST LINK ! ***

So if your considering upgrade to chainplates, think about how strong your masthead fitting is; consider how you can strengthen your backstays (mine are adapted to also be thru-deck - attaching to the hull lip); if adding a removable storm rig consider the strengh of the thru deck fitting etc.; use quality fittings throughout.

Having made the following changes to my boat I find she sails as beautiful as ever ! .... I replaced original rotostay with Furlex 200s RR on 7mm forestay (up from 5), 6mm backstays (up from 5), new terminals throughout, Inner storm forestay added with highfield lever; new storm forestay deck fittings, new masthead SS fittings to take larger forestay and backstays:

The details and photographs of the upgrades can be found on my website below.

http://www.contessa26moonshine.me.uk/job07.htm


Hope this helps.

All the best, Andrew
 
Narrow sheeting angles are less important in a seaway than sheltered waters; a corner is mechanically much stronger than a flat plate; and moving the shrouds away from the centreline just a few inches is going to make a significant improvement to the rigidity of the mast. It all makes sense to me. /forums/images/graemlins/ooo.gif
Especially as my own Pandora had been modified the other way round - by bringing the shrouds inboard from the edge of the deck to half way between the coachroof and topsides. The result was awful. The middle of the deck bowed upwards and the coachroof sank downwards despite the provision of a compression strut under the mast, and left me with an eighteen inch split to repair in the deck. /forums/images/graemlins/mad.gif
You'll get there a lot quicker with a whole mast compared to half a mast or none at all, whatever your sheeting angle./forums/images/graemlins/grin.gif
Robin.
PS Nice pics, Andrew. You've been busy! Should be two transatlantic contenders in Liverpool before very many more months have passed. /forums/images/graemlins/cool.gif
 
I once went to look at a Medusa. A 26 foot Maurice Griffiths Bilger Keeler.

Someone had beefed up the shrouds and they were attached to chainplates as far outboard as possible. Not sure if this was original.

Probably through overtightening, the topsides had horizontal ripples. Didn't buy it!

I guess this is about original design and thinking carefully about what and where you beef up.

Nice website
 
[ QUOTE ]
...about original design and thinking carefully...

[/ QUOTE ]
Completely agree. Strengthen one part and the load is transferred somewhere else, in the worst case, to a stress concentration point.
Robin.
 
But surely the load is only transferred elsewhere when the part you've strengthened would otherwise have broken? (or maybe stretched).

Suppose there's a stress concentration point at the top of the mast. There's no way for it to know you've strengthened the chainplate and feel extra stress until things get to breaking point. Let's say the chainplate used to fail in condition X. In all conditions up to X, everything is the same. In conditions above X, chainplate holds, but masthead is experiencing extra stress. At some point (X+Y), either masthead goes, or the strengthened chainplate. Here's the decision - which would you rather fail and when? Either is nasty. Ability to withstand stronger stresses must be good - but Y might not be very big. Either way, there's no difference until we get to condition X.

But it's different for different improvements. Hard to see the downside of stronger chainplates unless you specifically want them to be the weakest link. But thicker shrouds leads to extra weight aloft & more stresses before you get to breaking point.

And moving the chainplates (as Nick wants to do) does seem to be a mod that requires careful thought. (go for it, Nick!)

Disclaimer - I'm not a boat designer. But I am considering options such as bigger chainplates for my Golden Hind 26.
 
[ QUOTE ]
Disclaimer - I'm not a boat designer.

[/ QUOTE ]
Nor me. Point well made. What I was getting at was the strength of the sapling. Strap a bit of (weightless) four by two to the top half and it snaps in the middle.
That's why we reinforce GRP by tearing the mat and overlapping the layers. Especially where bulkheads are fitted.
I share concerns with the mast because it seems relatively common for them to break. Old GRP is not as strong as new GRP - it bends more easily. Over the years, my deck and coachroof became distorted. In part this was because a previous owner had moved the shrouds inboard. This caused increased tensions in an area the designer had not made provision for. So I put them back where they belonged, close up against the toe rail, where they were originally.
However, one of the reasons I added a full width bulkhead was to stiffen the hull/deck/coachroof by way of the mast step. Lee shrouds go slack easily enough in a blow. In the event of, say, a knock down, just how much slack is there? And how much will the rig tolerate? I just don't know. But I felt it was something I ought to pay attention to, to the best of my ability and pocket.
Yours and Nick's suggestion is in part, as you point out, to do with removing the weakest link from the chain, which is a sound strategy. Mine was more to do with the attachment point - the corner is intrinsically strong. But as Paul pointed out, we do need to think things through. We need to consider the possiblility of unforeseen side effects.
The craft of the designers and the resilience of the materials have proven themselves over many years. But one look at my patio door sized cabin windows makes me realise that transatlantic crossings is not what Mr van de Stadt had in mind when he designed my boat. So some changes are inevitable. I see a lot of sense in what you and Nick are proposing - with the proviso I don't know exactly how you are going to do it and even if I did I've got no special knowledge or experience!
Good luck!
Robin.
 
Interested in your modifications re chain plates,i have had some stainless plates made for my IP 24 motor sailer due to deck mounted being unsuitable.
I have researched and one way they suggest the three plates actually exiting the deck from below just inside the toe rail.They are joined just below the point they pass through the deck,by a piece of stainless rod and also a strip welded across the bases of the plates.
The whole assembly was then to be glassed into the hull.
Looked quite impressive but i must admit i prefer the external sandwich method!
A really interesting forum,i shall be looking in again!
 
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