New Chain plates on CP-16

[Deebee]

Hi Bob,

I am just south of Boston. I think mine was metal fatigue,
nothing lasts forever.

Deebee

[philb Junkie19]

My first thought was the chainplate must have been wearing that slot over a long time and finally let go.  It'd hard to imagine how it could pull out suddenly from its original size and leave such a straight vertical slot with nothing twisted or distorted.  I still think it was worn to very little metal before it let go but that the wear and fatigue was both from friction and pounding that would happen once the hole got enlarged. Every other tack in windy conditions would see the pin beat against the s steel above hole, more so as the hole got larger. This would occur faster than just by the initial friction.  I like to hear Skip's take after seeing the picture.

[MacGyver]

This is a neat thread, I am not surprised as to the failure so much as to the shape of the slot there....

Typically we see this kind of thing when a wrong size pin is used, or the wrong material pin....

The small rust on the SS plate leads me to believe it is either a salt water boat, or a poorer grade of SS, which against a better grade of SS would succumb to the the pressures over time.
Every year, beginning of the season, all rigging should get a good eyeing up, to be sure it is in the best shape it can be for the season ahead. No sense getting hurt (possibly) when it is a hobby to have fun, not terror or pain over.

One thing this has made me rethink..... what am I going to rebed my chain plates with....... 5200 was planned...... that is what I typically use one other boats I work on....... hmmmmm.....

Mac

PS: I know I know, it is rare I use 5200... but that front trailer eye, the chainplates and mast step are typically the only ones I use 5200 on......

[Pacman]

5200 is great for permanent bonding, like hull to deck joints, because it bonds so strongly. 

That is precisely what makes it a "regrettable" choice for use with anything that might ever have to be removed.

If that broken chainplate had been bedded in 5200 by a well-meaning previous owner, the job of replacement would be ..hm....more difficult. (understatement)

Polysulfide sealer like Boat Life is the Rx for success on this one.

I would replace all three chainplates for peace of mind.

[skip1930]

Certain standards when talking about metal apply here. If one looks at the chain plates on a 20 foot Flicka, they are no less than 3/8 inch thick.

In determining the strength of the metal it doesn't matter if the metal is stainless steel or steel [black iron, same thing].
Stainless steel vs. black iron does matters in accessing resistance to corrosion. But that's an entirely different discussion.
The chain plate failure reads like a failure do to corrosion. Not ware.  No big deal, replace them. Quick and dirty reference ...

The ultimate working strength of steel is 10,000 lb times the metal thickness in decimal inches, divided by four. And no more.

Then all kinds of other factors color the equation. Like the drilled hole for the pin is x diameters from the edge of the chain plate. Like a rivet requirement.

And on and on and on... [ to remove items bedded with 3-M 5200 slow set, a lot of heat is needed~ like to remove the rudder casting once the four 1/4 inch bolts are removed from the transom. Those four bolts only had a drop of 3-M 5200 around each bolt, and it took a heck of a lot of heat and pounding with a rubber mallet to break that casting loose.]

skip.

[NateD]

Bob Perry (well known boat designer, http://www.perryboat.com/) just commented on a SailNet thread (http://www.sailnet.com/forums/general-discussion-sailing-related/99723-rigging-math.html):

"The biggest load on the chainplate is the bearing load where the pin loads the hole in the chainplate. That load determines the size of the chainplate in terms of thickness and cross sectional area. But below the hole the chainplate can be considerably smaller. You can see this on the Valiant 40 chainplates where I doubled the plate thickness at the hole. I think most builders just use a single thicknes because it's easy to do.

Be conservative with chainplates. Having your mast in your lap can spoil a nice day of sailing. "

Which makes it sound like ripping out the top of a chainplate isn't all that uncommon, which makes sense because the chainplate to hull connections are usually spread over numerous bolts while the shroud connection is a single pin.