How long should runner plank stud plates last?

[wnethercote]

Yesterday we had a short day of racing because few of us had slush runners, but I did have a set of angles and thrashed around in the slush all afternoon in a nice 10-12 kt breeze while the others retreated to sit around the wood stove and tell lies.  Today we started earlier, and I was feeling confident after a tune-up sail - but then one of my stud plates failed when the stud broke off.

It's the second time it has happened to me, albeit about 20 years apart.  Basically, the weld fails - probably after growth of a fatigue crack initiating at a crevice corrosion site.  I suppose (engineer speaking) we'd be better off using mild steel for these parts than stainless, whose primary virtue is not rusting, but at the expense of crevice corrosion susceptability.

Am I alone in having this sort of failure, or is there a general opinion about the useful life of a stud plate?  The one that died yeasterday was old and well-used.  It might have been a Sarns stud plate, but it could even have been one from Bruce Price (now that IS old!).

[Geoff Sobering]

I consider all the plank attachment hardware as "consumable" since they can have some pretty high loads on them concentrated on not a lot of hardware.
20 years for a stud-plate weld seems reasonable to me - as they say, YMMV...

I haven't had a stud-plate weld fail outside an extraordinary circumstance (i.e. a major crash), but I regularly find the screws holding both the plank and hull plates are bent at the head after a season of normal sailing.

Since I discovered the bent screws I check them about every year (at least after every time I may have over-stressed them). When I do that I also carefully check the stud/plate weld on both the top and bottom for any sign of cracking.

Aside: I've had fewer problems with the hull-plates after I started using a suggestion from Paul Goodwin and build up a wall of epoxy around the plates. That takes most of the sheer loads off the screws but still allows the plate to pull away from the hull if something dramatic happens. After a major spin-out this season I checked all the hardware and found bent screws on the plank, but much less on the hull (although some of the epoxy had been broken away, indicating the hull-plate had moved a bit).

[KB [us5219]]

There were many instances of premature failure of stud plate welds in the past.  (ask commodore Potcova!)
My understanding is Sarns has addressed and fixed this problem around 2-3 years ago.
However as, Geoff points out, 20 years is probably beyond the average life expectancy of these little welds.

I remedied one set by removing the studs from the plates, (or what was left of them)  then re-tapping the threads in the plates and putting in a bolt in place of the stud.  The bolt head was then ground down to almost flat.  I made sure the bolt head was an asymetrical shape.  When re-mounting the plates, I drilled a very shallow hole in the plank to accomodate the flattened head.  This was filled with just enough thickened epoxy to keep the bolt head (or whats left of it) from rotating when tightening/loosening nuts.
The short screws in the stud plates are the intended weak link, not the stud (in my opinion).

I have seen other systems that use simple wood cleats on the bottom of the boat combined with large rubber bands.  I think Don Jones has that type of attachement on his boat.  You have to look past the class legal flux capacitor to notice it though.

[eric_anderson]

Sarns had a bad run of them just after the company changed hands. I broke 3 off in one season,  2 durring the same north americans, both times rounding the leeward mark.    today there are better solutions,   Steve Duhamel makes double stud plates, If one weld fails, the other will hold it.

cheers,
Eric

[Bob Rast DN1313]

I have always used double stud plates, you can catch them before complete failure , check by tapping them with a wrench you can tell when they are loose.
I've made my own using heavier plate than sarns so you can get a better weld.
the double stud plates are 2 in on center  so you need hull plates that have 1in on center holes.
Also easier to level plank to hull by using washer on front or back stud to level plank to ice under load.


my 2 cents
Happy sailing
Bob Rast
DN1313

[wnethercote]

Thank you all for your comments!

As an aside, I have glued my stud plates on for many years,  in part because the one time I did drop a runner in a drain hole I damaged my plank beyond repair despite the weak link of the studplate screws - but I don't glue my hull plates down so I have a weak link there. If I travelled more I might see more of the alternative attachment systems (like Eric Anderson's ball and socket) for incorporation in my next build.  Or ... if my welding didn't look so much like duck poo I might try making my own stud paltes with thicker plate.  Of course, double stud plates offer a redundant, albeit not with respect to reworking the cockpit structure.

I suppose my best near-term approach is to start treating these things like consumables.

Warren

[Geoff Sobering]

... I don't glue my hull plates down so I have a weak link there. ...

Personally, I'm a big fan of having the plank/plate the weak-link. That way the stud stays attached to the hull and doesn't punch holes or rip up the bottom of the boat if the plank doesn't separate cleanly (ex. on attachment stays connected).

I use the longest screws I can to mount the hull plates. I even grind the tip off of 1" screws so they don't come through the cockpit floor.

[wnethercote]

Geoff - there is solid logic in your approach. 

To add another PS I just removed and examined both stud plates.  On the broken one the fatigue crack 'beach marks' extend three-quarters away around the hole in the plate before the ultimate strength failure at the remaining quarter.  (The final failure occured in lightwind when I decided to tack but the plank didn't!)   I can find no visual evidence of any cracking on the other stud plate; however, there is evidence of poor weld penetration on the broken one - but it is only apparant from the top surface of the plate after the stud broke off.  I guess a major factor is whether the welder went to bed early the night before ....