Deck mounted chainplates on a cal 20

[cyrano138]

I am trying to choose between two sets of replacement deck-mounted chain plates for my refit. I went to size up on the rigging and need to size up the chain plates as well. I found bronze chain plates on two different sites and the price difference is pretty significant. I'm wondering if anyone has experience with either of the two sites and their hardware.


Spartan gives a breaking strength of 10,000 lb, and I'm not sure what percentage of that would be considered a safe working load limit, and rig right doesn't give any info about breaking strength or composition other than to say cast bronze.






I need to purchase six or seven so the difference of $20 a piece would be significant enough to make me consider the less expensive Spartan hardware.



Anyone have experience with anything like this?

[tkeithlu]

Rule of thumb is a breaking strength of 2.5 times the working maximum.

It would not surprise me to find that these two chainplates are the same casting out of the same foundry somewhere in China. I can't find a difference between them. Both use 3/8" bolts to attach.

In terms of working strength, think in terms of the load necessary to roll your boat to 90 degrees - a flat knock-down. That puts the maximum load on the chainplates.

In your position, I think I would be most concerned for how the chainplates transfer the load to the hull. Backing plates? It doesn't matter what their strength is if they pull out of the hull.

Best of wishes for your successful project.

[cyrano138]

Originally Posted by tkeithlu

Rule of thumb is a breaking strength of 2.5 times the working maximum.

It would not surprise me to find that these two chainplates are the same casting out of the same foundry somewhere in China. I can't find a difference between them. Both use 3/8" bolts to attach.

In terms of working strength, think in terms of the load necessary to roll your boat to 90 degrees - a flat knock-down. That puts the maximum load on the chainplates.

In your position, I think I would be most concerned for how the chainplates transfer the load to the hull. Backing plates? It doesn't matter what their strength is if they pull out of the hull.

Best of wishes for your successful project.

I think the standard move is to use an identical chainplates as backing plates, connected to a turnbuckle and then to the interior of the hull (see photo).

I plan on doing that as well as soon as time and budget allow. The original design included no reinforcement -- the load was transferred to the hull via the hull to deck joint (riveted).

My understanding is that the reason for this arrangement was because one of the sets of lower shrouds was removed leaving you with an extra chain plate. I plan on leaving both sets of lower shrouds but this seems like as good a solution as any.

I had considered, and really would have preferred, to use external chain plants bolted to the outside of the hull, but I would have had to change the placement and angles of some of the shrouds. Honestly I just don't know enough about sailboat ringing to know if it's okay to do that. The general consensus seems to be that the relevant loads are relatively small on a cal 20.


Thank you for the advice!

[tkeithlu]

That's one hell of a good design for the load transfer to the hull, and overcomes the OMG problem I have with the idea that the builder used the deck/hull rivets to do the job. I had visions of the deck and hull parting company when you jibed. Spoil your whole day. Go for it!

If your hull/deck attachment is similar to the drawing - joined on the outside of the hull, then using external plates would be difficult. The design you've got achieves the same end - the plates are internal rather than external. It is a bit complicated. If you want to simplify it, consider fabricating a strap that does the job of the turnbuckle and the plate against the hull.

[Cheechako]

I would think 10000 strength is more than enough for a 20 ft boat. Spartan used to be Cape Dory, I used a lot of their bronze fittings on a 31 footer I built in the 80's. Seemed to be good stuff.
You are probably ok using either brand though. Or order one of the cheaper ones and see how it looks. In castings you dont want a lot of porosity. (like air bubbles in the casting) Some will show on the surface, some will show where the countersink is machined. (if it is)

Regarding external chainplates; moving outboard is actually a better angle. A small change fore or aft is not a big deal.

[Bill O]

I'll ask why/who told you should go up one size of rigging when the previous rigging was designed by a naval architect specifically for that boat?

Many years ago I did something similar and went up one size of wire on a rig I designed. When the NA from Offshore Spar checked over my calculations and rig design, he told me it was unnecessary to do so.

On our current boat, someone oversized the rigging and chain plates from the original sizing. When Bruce Bingham (the designer) was on our boat, he immediately noticed the oversized standing rigging. He remarked that the smaller specified rigging would be capable of lifting the whole boat by the mast and more than enough for offshore conditions.

Moral of the story is save the expense of over sizing the standing rigging unless you have a NA tell you it needs to be done. Over sizing feels like the right thing to do, but fight the feeling since it typically isn't needed.

[Bobby Lex]

[QUOTE=cyrano138;3750835]I am trying to choose between two sets of replacement deck-mounted chain plates for my refit....Spartan gives a breaking strength of 10,000 lb.... [QUOTE]

I'm certainly no engineer.....BUT.....

Is that the breaking strength of the tang or the breaking strength of the 2 bolts securing the chainplate to the backing plate?

Seems to me those bolts could be the potentially weakest part of the assembly. Due attention should be given to their role in keeping the whole thing intact.

Bob

[cyrano138]

Originally Posted by Bill O

I'll ask why/who told you should go up one size of rigging when the previous rigging was designed by a naval architect specifically for that boat?

Many years ago I did something similar and went up one size of wire on a rig I designed. When the NA from Offshore Spar checked over my calculations and rig design, he told me it was unnecessary to do so.

On our current boat, someone oversized the rigging and chain plates from the original sizing. When Bruce Bingham (the designer) was on our boat, he immediately noticed the oversized standing rigging. He remarked that the smaller specified rigging would be capable of lifting the whole boat by the mast and more than enough for offshore conditions.

Moral of the story is save the expense of over sizing the standing rigging unless you have a NA tell you it needs to be done. Over sizing feels like the right thing to do, but fight the feeling since it typically isn't needed.

There are several reliable sources that recommend it, admittedly a few that caution against it, but the owner's manual itself at least circumstantially endorses it.

[thomm225]

Cal 20 Black Feathers the boat that sailed in the Single Hand Transpac Race (SHTP) from San Francisco to Hawaii used the standard rigging that was recommended for it's class.

http://digitaleditions.walsworthprin...articleBrowser

[Bill O]

Originally Posted by thomm225

Cal 20 Black Feathers the boat that sailed in the Single Hand Transpac Race (SHTP) from San Francisco to Hawaii used the standard rigging that was recommended for it's class.

http://digitaleditions.walsworthprin...articleBrowser

Good article for the OP if the boat is being rigged for offshore use.

[cyrano138]

Originally Posted by thomm225

Cal 20 Black Feathers the boat that sailed in the Single Hand Transpac Race (SHTP) from San Francisco to Hawaii used the standard rigging that was recommended for it's class.

http://digitaleditions.walsworthprin...articleBrowser

Like I said, some people advocate for, some against it. I've already made up my mind on that. Here's another guy who sailed one from CA to HI.

Taken from "Expand Your Pocket Cruiser's Horizons" by K.P. Chin for Sailing World


"The stainless steel headstay, backstay, all shrouds and turnbuckles were upgraded to the next larger size, with the old ones kept aboard as emergency spares."


John Vigor also makes the recommendation in one of his books.



I'm reasonably sure there isn't a question alive that doesn't spark a lot of healthy debate, and their are probably very few that have universally agreed upon answers. I might have clarified that the rigging intern buckles need to be replaced anyway and the cost difference of sizing up has not proven to be too much. I do appreciate everyone's input. I have a copy of the book black feathers but I don't know if I have that article. Thank you for the link!

[Jim Cate]

What is the breaking strength of the larger wire that you intend to use? If it is less than that of the chain plate (and I suspect that it is), then all is well.

And do use good quality fasteners when you assemble the rig. Cheap Chinese bolts don't have rigorous QC and a failure there could be catastrophic!

Jim

[cyrano138]

Originally Posted by Jim Cate

What is the breaking strength of the larger wire that you intend to use? If it is less than that of the chain plate (and I suspect that it is), then all is well.

And do use good quality fasteners when you assemble the rig. Cheap Chinese bolts don't have rigorous QC and a failure there could be catastrophic!

Jim

The largest will be 1x19ss x 5/32", which I think clocks in at about 3300 lbs. Or, that might be the working load limit. I forget.



Could you recommend a source for good quality ss bolts? Or I guess bronze if that's a better choice.

[tkeithlu]

That's the breaking strength.

[thomm225]

So your little boat has a comfort ratio of 12.12 and a high capsize screening formula of 2.24 which are both indicators that it isn't an offshore boat.

My little boat has a comfort ratio of 29.08 and capsize screen formula of 1.71 which shows it's a very seaworthy and good boat even for offshore sailing. (but will work you hard in rough conditions)

What you need to figure out is after you over build the rigging on this boat would you ever even have needed to do it?

My boat is hard to handle in winds say over 25 knots offshore unless I can head down wind.

My boat displaces 6600 lbs. and yours displaces 1950 lbs.

Just something to consider.

My rigging is 1 x 19 wire 3/16" thick with 3/16" thick chainplates 12" long

Chainplate bolts are military grade SS

BTW , riggingonly.com is a good place to get anything having to do with rigging.

I'm getting 3/8" clevis pins and SS cotter pins delivered from them today. Next will be 1 x 19 wire and Sta-Lok fittings. Already received SS turnbuckles

Rigging Only