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Old 12-08-2013, 07:30   #1
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Bobstay Plate / Backing Plate Question

Hi,

I removed the bobstay plate this weekend for inspection. While I found the actual bobstay plate to be in good condition, the backing plates were a mess. As near as I can tell, they were made of mild steel and showed significant wear.



There was one large piece as seen in the pics then the yard used several smaller pieces stacked on top of each other like spacers.

Question is: Is there an engineering reason why they used one piece backed by several smaller pieces? Would it be ok to simply order one thicker piece from mcmaster carr? I see no reason why a thicker piece wouldn't fit.


Thanks!

- z
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Old 12-08-2013, 07:44   #2
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Re: Bobstay Plate / Backing Plate Question

Use ss 316, and there is no reason to pack 2 backing plates with each other, i guess this are the backing plates for the bobstay chainplate? Dont be tempted to use Aluminium.. Go thicker instead and problem solved, btw : the reason they use individuals pieces i guess is to contour the hull inside.Cheers.
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Old 12-08-2013, 08:09   #3
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Re: Bobstay Plate / Backing Plate Question

Yes many small pieces sometimes used to fit the inner shape of the hull. On our boat it is a SS, V shaped slab.

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Old 13-08-2013, 07:18   #4
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Re: Bobstay Plate / Backing Plate Question

I took our bob-stay fitting out as well. The backing plates were aluminum. I think aluminum is sometimes used as it is sacrificial to the stainless. Mine were pretty corroded, but are over 25 years old. This fitting is loaded in shear, so the backing plate is not loaded as highly as it might otherwise be. The stem of our boat is over 5 inches thick, so I'm not too worried about spreading the load. It's already spread. The biggest problem I had was access to the bolts. The forward water tank is right up against it.
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Old 13-08-2013, 08:58   #5
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Re: Bobstay Plate / Backing Plate Question

Quote:
Originally Posted by Saltyhog View Post

(...) I think aluminum is sometimes used as it is sacrificial to the stainless. (...)

It is sacrificial and that's why it should not be used as a backing plate for bobstay attachment.

But you could possibly use bronze. (?) Easier to work with.

SS easier to buy though.

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Old 13-08-2013, 09:24   #6
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Re: Bobstay Plate / Backing Plate Question

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I took our bob-stay fitting out as well. The backing plates were aluminum. I think aluminum is sometimes used as it is sacrificial to the stainless. Mine were pretty corroded, but are over 25 years old. This fitting is loaded in shear, so the backing plate is not loaded as highly as it might otherwise be. The stem of our boat is over 5 inches thick, so I'm not too worried about spreading the load. It's already spread. The biggest problem I had was access to the bolts. The forward water tank is right up against it.
To your point... I have attached some pics for other CR38 owners in case they are interested.

I'll probably stick with SS for the backing plate. I had not considered the sacrificial aspect. It wouldn't make sense though because why would you want your backplates to be sacrificial.

Maybe I'll sandwich an AL plate between sets of bolts (SS Plate>Bolt>AL Plate>Bolt) to use as a sacrificial.

I also noticed that this backplate was not bonded like my thru hulls, although both are below the waterline. Maybe this should be bonded as well? I'm confused about the bond/no-bond topic. With that said, in the no-bond camp the SS seemed like it was in pretty good shape even though it sits underwater 1/2 the time.
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Old 13-08-2013, 13:45   #7
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Re: Bobstay Plate / Backing Plate Question

That is *exactly* what our's looked like. It's the same fiberglass triangle that I had to cut out to get to the nuts on the bob-stay fitting. I could not remove the nut on the bottom bolt as the bottom 2 bolts are actually a u-bolt welded to the fitting to make a tow point or for attaching an anchor snubber. I had to cut the upper bolt to get to the lower one. Hope that makes some sense. The top bolt, an actual bolt, came out --but with a fight. Do you have now or are you planning on adding a floor for the anchor chain to sit on?
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Old 13-08-2013, 14:28   #8
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Re: Bobstay Plate / Backing Plate Question

If you use aluminum backing plates, sand and paint with Zinc Chromate primer. This virtually stops the corrosion.

The smaller plates spread the load of the fasteners over a larger area increasing the strength. On high load fittings, I'll use a fender washer with a regular washer and lock washer over the backing plate. The larger the area that the fastener bears against the backing plate, the stronger it is. Putting fasteners directly against the backing plate puts the force just at the fastener. Under severe strain, the backing plate will have a greater tendency to deform just around the fastener spreading the load over a smaller area.
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Old 13-08-2013, 15:12   #9
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Re: Bobstay Plate / Backing Plate Question

1. Use 316 SS
2. it does not need to be super thick - 1/4" would do it
3. make it in one piece
4. clean the fiberglass up underneath where it goes with a 36 grit sanding disk
5. Cover the backing plate with plastic wrap and put some grease on the bolt threads
6. put thickened epoxy on the (now clean) glass under where the backing plate goes, put the plate on (with the plastic wrap on it) and bolt very lightly in place just so the epoxy squeezes out evenly all around the sides - making sure the plate is even and parallel to the hull surface
7. let the epoxy harden over night - this provides a nice smooth loading on the hull.
8. take off the backing plate, remove the plastic wrap, cover with an a good underwater caulking, and then bolt firmly back in place.
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Old 13-08-2013, 21:59   #10
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Re: Bobstay Plate / Backing Plate Question

Awesome Info!

Truth be told, when this is all said and done, I will probably just epoxy the triangle area I cut out back into place once we have established it won't leak. In the meantime, I'll just put it back into place and pile my rope on top of it.

I'm thinking I will use 1/4" SS backed by thick SS washers for good measure and to distribute the load, then follow your advise estarzinger.

BTW - I had no idea that the u bolt was for a snubber. We have been using an ABI chain gabber with great success although I will probably increase the line length. I have the original CR documents but didn't see any mention of using the u bolt in that regard. Also great info.

Question: should I be using some kind of corrosion inhibitor?
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Old 14-08-2013, 07:07   #11
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Re: Bobstay Plate / Backing Plate Question

I'm surprised that you were able to remove the fitting without destroying it. On mine, the forward water tank comes right up to the fitting. Clearly the tank was installed after the fitting. No way to get a wrench in there around the the upper leg of the U-bolt, so had to cut the the upper leg. Even then, the lower leg is against the tank, so I had to loosen the nut as far as I could and then pull the fitting out a bit before I could remove the nut entirely. I don't think an Anti-corrosive will help much here. The best thing to do is as Evans said. Good solid bedding will prevent movement and working of the sealant, thus preventing water from getting in to corrode the buried parts.

That said, since I have to have the bob-stay fitting remade, I'm considering having it made of AL-6XN rather than 316L. AL-6XN is a super-austenitic stainless steel that is supposed to be better than 316 in every way but price. May have the chainplated made from it as well, depending on material cost. Anybody have any experience with this material?
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Old 14-08-2013, 07:33   #12
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Re: Bobstay Plate / Backing Plate Question

My water tank does not go right up to the forepeak. In fact, there is probably 3-4 feet between the water tank and the fitting. With that said, there is a bulkhead that cuts across right up against the bolts; there is still several inches available. I was able to get a ratchet right onto the bolts starting with the top and working my way down. FYI, they were 19MM/.75" in case anyone is interested.

Its funny you mention this because I was curious as to what is under my forward bunk; I can see all the sides of the water tank through the inspection port, I know where the second tank is, and I know where the diesel tank is, so that space must be "empty".

I'll probably unscrew the bunk floor and take a peak below. If there is room, maybe it would make a good space to store clothing.

The only thing that is below the water tank is the in-hull depth sounder which has crapped out [and good luck getting to that]. I have to imagine that is was a very tall man with very long arms that installed it or it was installed during construction of the interior.
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Old 14-08-2013, 08:32   #13
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Re: Bobstay Plate / Backing Plate Question

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Originally Posted by zboss View Post
My water tank does not go right up to the forepeak. In fact, there is probably 3-4 feet between the water tank and the fitting. With that said, there is a bulkhead that cuts across right up against the bolts; there is still several inches available. I was able to get a ratchet right onto the bolts starting with the top and working my way down. FYI, they were 19MM/.75" in case anyone is interested.

Its funny you mention this because I was curious as to what is under my forward bunk; I can see all the sides of the water tank through the inspection port, I know where the second tank is, and I know where the diesel tank is, so that space must be "empty".

I'll probably unscrew the bunk floor and take a peak below. If there is room, maybe it would make a good space to store clothing.

The only thing that is below the water tank is the in-hull depth sounder which has crapped out [and good luck getting to that]. I have to imagine that is was a very tall man with very long arms that installed it or it was installed during construction of the interior.

Beware your boat anchor chain locker is not the kind with the draining inside of the boat to the bilge, many are designed like that, because you mention store clothing, , just figurate!!
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Old 14-08-2013, 08:55   #14
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Re: Bobstay Plate / Backing Plate Question

Hmm... Our boats must be more different than I thought. What year is yours? On our '87, the forward water tank goes from the fore peak back to about two feet forward of the bulkhead separating the head/shower and v-berth. Ours has the v-berth offset to starboard. Not a true v-berth. There is some (small but deep) storage aft of the water tank that is sealed from the bilge. Also access to one of the diverter y-valves for the holding tank. The depth transducer is just accessible by removing the lower drawer from under the v-berth. I can just reach it. I'm about to replace it as it was removed while the bottom was being done.
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Old 14-08-2013, 09:50   #15
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Re: Bobstay Plate / Backing Plate Question

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Hmm... Our boats must be more different than I thought. What year is yours? On our '87, the forward water tank goes from the fore peak back to about two feet forward of the bulkhead separating the head/shower and v-berth. Ours has the v-berth offset to starboard. Not a true v-berth. There is some (small but deep) storage aft of the water tank that is sealed from the bilge. Also access to one of the diverter y-valves for the holding tank. The depth transducer is just accessible by removing the lower drawer from under the v-berth. I can just reach it. I'm about to replace it as it was removed while the bottom was being done.
Same layout... maybe mine is smaller. Not that I would EVER admit that in public

This is probably better reserved for another thread but I will follow with great interest your installation of the sounder. I also can barely get my hands on it but I can't imagine how a new one could possibly be installed without access to sand and clean the area first.
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