[Rhodes22-list] deck mounted chain plates

Sun Jan 11 15:38:01 EST 2015

Richard:
This matter of deck distortion is an excellent point. But unless my boat is
unique, all of the R22s have their lower (or "inner") shrouds attached to
the cabin top. Does this distort the deck? Yes it does. Because of some trim
I put on the forward bulkhead I could see the space between the bulkhead and
the cabin top widen perhaps as much as half an inch when on a heel. I just
figured that was normal for these boats. 
I have strengthened the cabin sides by filling the hollow core with plywood
and epoxy. See photo. At the time I intended to use strap chainplates bolted
to the cabin sides. Now, because of how I intend to redo the windows that
plan had to be abandoned but presumably the new core will still strengthen
the overall structure.

Graham Stewart
gstewart8 at cogeco.ca
613 389-1737

-----Original Message-----
From: rhodes22-list-bounces at rhodes22.org
[mailto:rhodes22-list-bounces at rhodes22.org] On Behalf Of The Rhodes 22 Email
List
Sent: January-11-15 2:40 PM
To: The Rhodes 22 Email List
Subject: Re: [Rhodes22-list] deck mounted chain plates

I believe the question should be not how strong my shroud is or how much
weight before my pin sheers.....it should be what is the load before my deck
distorts. Having worked on large yachts for many years one would never dream
of attaching a shroud directly to the deck. It is essential the load is
dissipated into the hull either via a direct hull attachment or via a
strong-point bulkhead attachment.

You may get away with it on the R22 as the loads are fairly light, but I
suspect you'll never get the shrouds truly tight as the deck will distort as
you tighten them up.

My 2c worth

Richard
S/V Waif R22
(and S/V Tembaya Tayana V42)

On Sun, Jan 11, 2015 at 11:03 AM, The Rhodes 22 Email List <
rhodes22-list at rhodes22.org> wrote:

> Hmmm. I guess the best I could suggest for an upper limit to shroud 
> tension is 2100 pounds, the breaking strength of 1/8" 1 x 19 stainless
steel cable.
> You could fire up a CAD program and doing a stress analysis of the 
> replacement chain plate to verify it can take that load, but that 
> seems a bit over the top.
>
> I guess I'd go back to the fact that the Rhodes' forestay is loaded 
> more heavily than the lower shrouds, both at rest and dynamically. The 
> pin for Magic Moments' forestay is 3/16", which will shear 
> considerably sooner than the 1/4" pin used by the part you're 
> considering at
>
> http://www.rhodes22.org/pipermail/rhodes22-list/attachments/20150110/e
> 0fd47a f/attachment.jpg. In addition, the new part appears to be a 
> doubled piece of stainless, 0.1" thick - which is thicker than our 
> more-heavily-loaded forestay tang. That leads  me to believe the 
> replacement part will take the load from the lower shroud to its 
> mounting bolts without a problem.
>
> The new chain plate has 2 - 1/4" mounting holes, and a 1/4" coarse 
> thread bolt has 1,750 pounds of tensile strength (if you believe 
> charts on the Internet). So the bolts should be adequate to take the 
> 2,100 pound load to the backing plate. Sooo... my vote is that the
replacement part seems OK.
>
> The last consideration is the size of the backing plate and / or epoxy 
> plug securing those bolts. To my thinking, this is the weak link in 
> the chain, but I don't have anything to reference for a suggestion.
>
> Good luck with it.
>
> Dennis
> Magic Moments
> Getting my sailing fix from books till the water softens
>
>
> -----Original Message-----
> From: rhodes22-list-bounces at rhodes22.org
> [mailto:rhodes22-list-bounces at rhodes22.org] On Behalf Of The Rhodes 22 
> Email List
> Sent: Sunday, January 11, 2015 8:56 AM
> To: 'The Rhodes 22 Email List'
> Subject: Re: [Rhodes22-list] deck mounted chain plates
>
> Thanks Dennis. However, the question related to the maximum tension 
> that the chainplates would need to withstand in use rather than the 
> matter of tuning the rig. I just don't want to use deck mounted 
> chainplates that are not sufficiently strong. That said, your 
> reference to tension gauges will be helpful should my boat ever see 
> water again.
>
> Graham Stewart
>
>
> -----Original Message-----
> From: rhodes22-list-bounces at rhodes22.org
> [mailto:rhodes22-list-bounces at rhodes22.org] On Behalf Of The Rhodes 22 
> Email List
> Sent: January-10-15 9:18 PM
> To: 'The Rhodes 22 Email List'
> Subject: Re: [Rhodes22-list] deck mounted chain plates
>
> Graham,
>
> The idea is to set the rig tension while the sails are down / furled. 
> Take a look midway down the page at 
> http://loosnaples.com/how-tos/tension-gauges
> to
> see Loos' suggested variation in shroud tension as the boat heels. I 
> suspect something similar happens in the forestay / backstays.
>
> The picture shows that a rig that's too loose loses all support in the 
> leeward shrouds, and notes that "The lateral stiffness of the mast and 
> the fore and aft stiffness of the spreaders is reduced by a factor of 
> 2 when the leeward shrouds go slack".
>
> Dennis
> Magic Moments
> Wondering if rig tension should be reduced in sub-zero temps.
>
>
>
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