Secondary chain plate for lower shrouds.

[Greatest Lakes]

I have a 1976 Columbia 9.6. All 3 chainplates are made from aluminum, and welded to a reasonably substantial piece of C channel that connects all 3 under the deck. The forward two are attached with 5 bolts each to bulkheads, while the aft one is only attached to a little chunk of plywood that doesn't really have much load capacity. Over time, there has been indications that this aft chainplate has strained its little mount. What I would like to do, is either add another plate to the other side of the mount, or drill a single hole in the bottom of the existing chainplate, to add a secondary lower/inside shroud with a turnbuckle down to either a stringer or the intermediate bulkhead below it. If I go to the stringer, I can get a straight down pull on the chainplate and get perfect geometry. If I use a marine grade stainless U bolt with a backing plate, I think the stringer can handle the load. This is what I want the opinion on. Have any of you had to do something similar? I thought I had pictures of this, but I'll need to get better ones tomorrow. On the left side of the picture is the small cabinet containing the aft chainplate. You can tell its not very substantial, by no means is it going to pull out, but I'd prefer more rigidity. The small square patch by the speaker is a sample of marine weave carpet I'm going to be redoing the sides with.

[Don C L]

So I haven’t done this but looking at the photo I take it the chainplate pulls up on the deck and so any flex in the deck and/or hull to deck joint will be the weak links. And I am assuming the stringer is in the deck, is that right? I think those Columbias had hull to deck joint that was bolted together, but I’m not sure. Pulling up on that may lead to more leaks but I don’t know for sure. Considering the forces involved I’d be looking at something more substantial. That said I have seen U-bolts used as you describe in other boats. I think the rigidity you seek will come from bolting through a bulkhead or something well tabbed in or going out and bolting through the hull below the hull/deck joint.

[Greatest Lakes]

The hull and deck joint is bolted, sandwich style with the aluminum toe rail. It's not leaking, but under tension from the shrouds, it flexes just enough to make the cabinet doors start to bind. It's picky, and most would probably ignore it, but I think the design could have been better. I'd have liked to have seen the center chainplate be longer with a 6-7bolt pattern instead of 5 as well. Strength of some components has been my only real complaints with the Columbia. Cabin sole could have been designed better too. That's going to be completely replaced in the next 2 years or so hopefully too. Slowly but surely the whole boat is getting refurbished. I did new cockpit seats this winter from okume and mahogany. I just need good weather for a few days to do some brightwork now.

[Greatest Lakes]

Ok. I have a few photos. The chainplate itself, the C channel under the deck, and the lower stringer is directly below the easiest mounting point. I'll have new wall coverings next week. Custom Marine carpet of Michigan is cutting new ones for me.

[tkeithlu]

The central problem is that the after shroud's load is not carried through to the structure of the hull. The risk is running before the wind or wide, and getting hit by an unexpected gust, therby ripping the shroud out of the boat. Also, routine stress on the hull, which you are already experiencing. So, run the load to something that can distribute it to the hull. With no bulkhead available, two means come to mind. The first is running it to the stringer, as you suggested. That'll help, but has the disadvantage that the stringer is made to take a load ninety degrees from the direction of this load. The other is distributing the load on a larger plate that is bolted from the outside through the hull. The disadvantage then is bolt heads on the outside, and loading the skin in a direction it wasn't meant to take load, also.

There's a third option. A channel that runs from one bulkhead forward of the shroud to the one aft of the shroud, and then tie to it. That may not be possible given the inside of you cabin.

There's a fourth option. Glass in a plate on the inside of the hull and attach to it.

Be sure to do both sides.

OK, which way seems least problematic to you?

[Cheechako]

Well, the design has lasted only 40-50 years so far so you better change it.......
I'd be more worried about the aluminum where you cant see it as it passes through the deck etc.

[Greatest Lakes]

Fortunately, it's lifelong freshwater, the aluminum is in good shape.
There are two stringers there, if I determine where the mounting point is, then glass in a couple pieces of mahogany between the two and the hull, I will have the support of both stringers and a bit of hull, it will be effectively tabbed in. They are in the back of another small cabinet, so I won't lose much realty in there either. I dont see it getting ripped out as it is, but it is a concession on the original design to make it that way so that upper part could be considered a berth. Other owners have turned the whole area into cabinets, which I would do as well if I was planning extended cruising.

[Lionshooter]

That aft chainplate and attached shroud are not going anywhere ever.
The engineer's original design is adequately substantial to handle any load put on a boat of that small size. They will still be there long after you depart this earth, whether left as is or beefed up with additional hardware.

[Greatest Lakes]

Paranoia about the strength of critical components is a sailors prerogative. The expense is negligible compared to my peace of mind. The best time to make repairs is usually before something has the chance to fail. The minor flexing I'm getting is just enough to make me want to add this ounce of prevention.

[Lionshooter]

Originally Posted by Greatest Lakes

Paranoia about the strength of critical components is a sailor's prerogative. The expense is negligible compared to my peace of mind. The best time to make repairs is usually before something has the chance to fail. The minor flexing I'm getting is just enough to make me want to add this ounce of prevention.

If it would ensure your peace of mind, then go for it. I think it is over-engineering on a system that has proven worthy for 45 years. Keep in mind that strain on aft shrouds is minimal under sail unless one spends a lot of time running or on a broad, which is not reality. Also, the small sail area carried by that boat is not a major contributing factor when calculating strain on aft shrouds. If I saw that set upon a 36' or 40' boat, I would be concerned.