Catamaran crossbeam rot repair

[Rhino91]

Hello

I have a 35ft Shawn Arber glass over ply sailing catamaran. The forward crossbeam is boxed ply and it has considerable rot in a section measuring approximately 30cm by 15cm.

The previous owner attempted to repair this by laying glass matt and epoxy over the area but this appears to have only made the situation worse by trapping in the moisture. The rot is so bad that a screwdriver can easily be pushed through it. It is essentially mush.

I have attached a photo and the approximate area is in the part in the red rectangle (it also extends on the back side of the beam where the net attaches to the beam). It has hard to demonstrate with a photo though as the paint is covering the rotten wood. I have tapped over the beam with a hammer and everywhere else feels solid

Are there any suggestions on how I might go about repairing this? One idea I had was to cut the rot completely out in a square section, glue additional stringers in and then place fresh ply back over the top and epoxy it in place.

The beam operates primarily in compression, resisting the hulls from pushing in toward one another.Any ideas would be greatly appreciated.

Thanks for your thoughts.

Cheers

Ryan

[Spot]

Hello Ryan,
Ahoy and welcome to CF.
Quite a kick-off post and problem there.
Fore-stay and anchor rode bow roller would indicate mid-span forces up and down as well.
-Have you identified how the water is getting in?
-Do you have access to build plans or the naval architect?
-What is your comfort level with composite work?
The more pictures and details you can supply the less general the answers will be.
All the best!

[Rhino91]

Hi Spot

Thanks for the quick reply.

The majority of the bending load by the forestay is transferred to a compressive load via the seagull-striker. As I understand it the load from the anchor is supported through the forestay and rigging as well as bridle transferring anchor load to hulls.

I found the attached image helpful in understanding the loads on the crossbeam.

I suspect that the water has found its way in due to people standing with their heels on the beam while observing the anchor and this has caused a depression, paint to crack, and subsequent water ingress. It may also be through the bolts which will be removed and re-sealed.

I do not have access to build plans or a naval architect at this stage but the designer resides in my state and I am able to take it to his yard. I will most likely end up doing this but need the time and money to move the boat there first so wanted to get some ideas/see if anyone had experienced similar issues and how they approached it before going down that path.

My father is happy to help me with the work, he has a considerable amount of building and wood working experience and is confident we could do the work required.

Thanks again

Cheers

Ryan

[NevilleCat]

Have you spoken to Shawn then?
No idea really, but I would have assumed you would be better off replacing the whole beam as it is structurally pretty important with the rig loads etc. Shawn may have better ideas?
He's probably the guy I'd be asking no matter what design it was anyhow.

[Rhino91]

Thanks NevilleCat, i'll try and get into contact with him.

[Bean Counter]

Originally Posted by Rhino91

Thanks NevilleCat, i'll try and get into contact with him.

If it was me, I would look at an alloy or CF beam replacement.


If you can not get hold of Shawn Arber try Geoff Cruse, he is a marine surveyor and custom catamaran builder, searchable with Google.

[psk125]

Sounds like that in any case you will need to open up the beam and replace the rotted plywood. Your suggested fix: opening it up, putting in stringers and putting in fresh plywood is essentially what you will need to do. Doing it is something else. The plywood repair will need to be scarfed into good material at a very flat angle because the strength of this crossbeam is critical. 20:1 comes to mind. Sistering the beam with splints might also be possible. Talk to the designer. Neville's idea of replacing the entire beam starts to sound reasonable if you find other problems once you open things up.

[Rhino91]

Originally Posted by psk125

Sounds like that in any case you will need to open up the beam and replace the rotted plywood. Your suggested fix: opening it up, putting in stringers and putting in fresh plywood is essentially what you will need to do. Doing it is something else. The plywood repair will need to be scarfed into good material at a very flat angle because the strength of this crossbeam is critical. 20:1 comes to mind. Sistering the beam with splints might also be possible. Talk to the designer. Neville's idea of replacing the entire beam starts to sound reasonable if you find other problems once you open things up.

Hi psk, thanks for the help. This is broadly the solution ive been getting on the boatdesign forum as well. Interestingly one of the members there said its "not a major structural and critical beam" and is a "relatively straight forward repair" which I was surprised by and heartened by.

Nevertheless im going to arrange to slip it latter this year and carry out the work under the guidance of a professional.

[Spot]

Ryan, thanks for the nice graphic.
Are you able to look inside the beam and confirm that the damage is localized?
Hopefully you can converse with the architect and make a good plan going forward.

[Franziska]

Hi.

We had a similar issue at the lower side of the beam where water collects also when the boat is ashore.
I did remove the beam, stripped it completely from paint, routered out the damaged area (turned out a lot smaller than anticipated) and glued new wooden battens in at the affected areas.

Than glassed the whole beam over with heavy UD.
It's stronger than before now, especially because initially the UD had not been asked for in the drawings.

Good luck with yours.

[tkeithlu]

Rhino91, I do hope that you can get in there and repair the beam, and am in complete agreement with the advice you've gotten here. That said, it's a big beam for a reason - there are bunches of forces on it, from the weight of what lies between the hulls to rigging load. What I'm seeing are bending loads, ergo a big box beam.

I think that here may be some confusion in your use of "compression." Compressing a beam on its side is not compression. Compression is equal forces in the line of the beam applied at the ends. Any force that twists, bends, or otherwise is applied to the sides is not compression.

[Cadence]

Had you considered having an aluminum replacement prefabed?

[Ken Z]

Not seeing the amount of rot or how intense it is hard to say how much should be replaced. All new work should be scarfed and when done combined with the amount of rot you may have a large project and larger one to fair it in and not a good thing in the future resale and inspection.
I would replace and internally use aluminum on edge as wide as possible to increase rigidity. I have seen multis with 1/2” x 8” aluminum on edge bolted with ss between the hulls and zero flex.

[tkeithlu]

If you are getting into replacement with aluminum, I would recommend an "I" beam, wide flange beam, or box beam rather than a flat plate. Flat plates on edge can buckle long before any of the others, weight for weight. Consider an I beam cantilevered from one hull and weighted down by the other hull. The top flange is stretched and the bottom flange is compressed. There actually is no force on the center, which is why you can drill a hole in it without losing strength. A load in the horizontal direction stretches and compresses the flanges in that dimension. Thay are not as good with torque, but you have two beams, fore and aft, which solves that problem. Wide flange and box beams have a lot of the same properties, with minor differences.

Your current beam I gather from the text is a laminated plywood box beam. If three sides of the beam are good, or you can construct an aluminum angle of the right dimensions to bolt to good wood, then a patch as suggested by Ken Z makes a lot of sense. At least that part won't rot again.