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Old 25-08-2012, 15:55   #1
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Corbels for Sagging Cabin Top?

I have a slight mast compression issue which has caused my cabin top to sag around the compression post. The post is not compressed, the boss and plywood core under my deck stepped mast is sound.

The compression post also serves as part of the frame for two adjacent doorways. I think corbels will provide a practical solution and will be aesthetically pleasing.

After getting a rough estimate of $3000 from the onsite carpenter for the installation of four corbels (two doorways) to add support for the cabin top, I've decided to do it myself. I've almost no carpentry skills, but it doesn't appear to be that complicated. I found out that I can order hardwood corbels ready made online, or even buy them from Home Depot. Home Depot makes them in red oak. Is red oak a suitable wood for a corbel thats intended to be structural and used in a marine environment? What would the preferred wood type be for this use?

Anyone know of a trustworthy company from whom I can order corbels online?

Any suggestions or comments appreciated.
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Old 25-08-2012, 16:45   #2
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Re: Corbels for sagging cabin top?

Here is the link to a picture of the two doorways to which I'll affix the corbels.

https://picasaweb.google.com/1049009...29404012728658
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Old 25-08-2012, 17:59   #3
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Re: Corbels for sagging cabin top?

Hard to tell from the picture but it doesn't look like there is a compression member directly under the mast. If that is the case, corbels probably won't do any good. You'll need to run a laminated beam across the cabin and support it solidly to the keel or hull on both sides. If you are going to the hull, you will need to lay extra thick glass laminate under the support to keep from deforming the hull.

From the photo it looks like one corbel would be fastened to a plywood bulkhead and the other to a cabinet door. Not enough strength in either case. You have checked the plywood core under the mast to be sure it's not water logged and rotten???
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Old 25-08-2012, 18:16   #4
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Re: Corbels for sagging cabin top?

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Originally Posted by roverhi View Post
Hard to tell from the picture but it doesn't look like there is a compression member directly under the mast. If that is the case, corbels probably won't do any good. You'll need to run a laminated beam across the cabin and support it solidly to the keel or hull on both sides. If you are going to the hull, you will need to lay extra thick glass laminate under the support to keep from deforming the hull.

From the photo it looks like one corbel would be fastened to a plywood bulkhead and the other to a cabinet door. Not enough strength in either case. You have checked the plywood core under the mast to be sure it's not water logged and rotten???
It doesn't look like it in the pic, but there really is a substantial post (2x4 teak) directly under the mast, and another smaller, yet solid post on the other side of the doorway. The cabin top is sagging mostly between the two posts (toward center of doorway)

Can I use corbels made out of just any ol' hardwood?
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Old 25-08-2012, 18:32   #5
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Picture 8 may tell some of the story. There is significant water damage at the bulkhead. It makes no sense that the deck is compressed but everything that supports it is still okay. My guess is something that the post is attached to is shot. The bulkhead which I assume is tabbed to the deck is probably not supporting it's original load.
Great boat this should not be so hard to fix. Adding extra structure may not be the easiest method. Look at what is failing.
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Old 25-08-2012, 18:48   #6
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Re: Corbels for sagging cabin top?

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Picture 8 may tell some of the story.
Well, you had me scared for a minute there! Picture number 8 is someone else's boat. My boat is the clunker in picture numbers 43-66

No rot in my bulkhead
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Old 25-08-2012, 18:57   #7
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Oops good on you then. If the deck is compressing and the bulkhead is right there something is compressing that shouldn't be. Another thought how well tuned is your rig.not uncommon to have over tightened the rig and now you are driving all this down way to hard. Usually the deck will lift if that were the case though.
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Old 25-08-2012, 19:33   #8
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Re: Corbels for sagging cabin top?

I think the problem is simply that there isn't sufficient surface area where the mast support structure contacts the cabin top which has caused the post to push up into the glass over time. (It is possible that the rig has been overtightened)

The corbels won't reverse the damage but I hope they will stop the cabin top from sagging around the post even more.

....so what kind of wood to use for the corbels spruce? Oak? Mahogany?
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Old 25-08-2012, 20:22   #9
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Re: Corbels for Sagging Cabin Top?

The mast post should take the load directly to the keel or some sort of support underneath sole. If the mast is indeed directly on top of the post and the post is supported underneath the sole, there should be no sag. Assume the plywood bulkhead is attached to the post so it can't bow under load. That leaves the mast support settling. Have you/can you see under the sole to where the post is resting. My w32 had atwartship pieces fiberglassed into the hull that supported the sole and the mast step. If the 28 has the same arrangement, these supports could rot and/or sag allowing the post to compress the supports and deck to sag.

You are really going to have to attach the corbels solidly to the mast support post for them to have any strength to take the load on the deck. A piece of 3/4 inch plywood on each side of the support post thoroughly screwed into it and a filler in between lag bolted or through bolted to the post would probably be necessary. You asking these Corbels to take the downward force of the mast which is considerable.

There is some reason that the cabin top is deforming that shouldn't be. I'd find out what is causing it before I'd mess with Corbels which probably aren't necessary. The deck core sandwich should be thicker in the area of the mast step. That should prevent the deck from deforming around the post and pushing up into the deck. Check that first. If the deck is uniformly sagging, then the post and its support has to be at fault.

Measure that the mast is indeed on top of the post. Westsail installed the atwartship bulkheads that were supposed to be in line with the mast about 8" farther forward on my boat. Didn't discover this until the interior was fully finished, chainplates were in place for the as designed mast location, and the step was in place. It seemed to work out okay but always made me very nervous. If we'd kept the boat and taken it on another cruise, would have installed a SS post directly under the mast and through to the keel.

The workers at Westsail weren't shipwrights and had little understanding of the physics/reality of a boat going to sea. They did what they thought they were told to do. Most did it well but sometimes the QA didn't catch the errors.
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Old 25-08-2012, 21:09   #10
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Re: Corbels for Sagging Cabin Top?

picture # 60 indicates you have a serious de lamination problem, i think your problems go deeper than just ordinary cabin top deformation.
My money is on quite a bit of rot caused by leaking.
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Old 25-08-2012, 21:17   #11
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Re: Corbels for Sagging Cabin Top?

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picture # 60 indicates you have a serious de lamination problem, i think your problems go deeper than just ordinary cabin top deformation.
My money is on quite a bit of rot caused by leaking.
Picture 55 as well by the forward bulkhead.
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Old 25-08-2012, 23:51   #12
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Re: Corbels for Sagging Cabin Top?

I am no boat builder by any stretch of the imagination, but if it where me planning on doing a DIY repair, i would be looking at removing the Mast, and then introducing a new Wider Base Plate, as well as supporting under the new base plate and inside to the sole by four points.
If you are going to use corbels then think about distributing the load 360deg through the use of a truss and or beam,,then again supported by four equidistant points of contact to the hull.

Just my Nickle..Good Luck to you...
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Old 26-08-2012, 03:55   #13
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Re: Corbels for Sagging Cabin Top?

Hmm...I'm rethinking the use of corbels and am going to take a step back to identify definitively the cause of the sag. I really, really, really don't want the expense of taking the mast down, especially since I just paid big bucks to have it put up. Also don't want to go cutting into the glass and core under the mast step but maybe thats what its going to take. Dangit.

Thank you all for so generously sharing your expertise.
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Old 26-08-2012, 09:14   #14
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Re: Corbels for Sagging Cabin Top?

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Originally Posted by virginia boy View Post
I have a slight mast compression issue which has caused my cabin top to sag around the compression post.


Any suggestions or comments appreciated.
Describing it as a slight mast compression issue is like your wife telling you she is a LITTLE bit pregnant.
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