Rudder repair help

Hi,

We have sustained some damage to one of our rudders on our Woods Flica catamaran.

We are in marina that has mud berths and it would seem that there is something hard in the mud.
As the tide drys, sometimes with small waves, our rudder has be bashing onto it and now has about 200mm missing from the bottom. Only found this when lifted out recently.
The rudder has a 40 diameter stainless steel tube and is plywood sheathed in GRP. Maximum thickness about 40mm. Don't know what the internal build up is except the level of the bottom of the stock.

I am asking for some advice on how to repair it insitu since I cannot drop it due to the length of the stock tube and expense of having to hire a crane to lift the boat high enough to remove.

Thinking of using some foam core bonded to the bottom of the plywood and then sheathing with epoxy and cloth to a thickness of 6 to 8mm. Maybe add some Kevlar cloth to add strength and protection when drying out? This would be with about a 400mm overlap onto the undamaged plywood and sheathing.

Any advice welcome please.

Drawing below - orange is missing section!

 

Is it an option to dig a hole below the rudder to remove it? I know some people who did it, although for fairly smaller boats (~8 meters LOA), but I guess it is much cheaper than crane (or free if it is feasible manually, though this takes physical effort). But if you choose that route, mind the depth of the hole for mounting fixed rudder, not broken.

 

Sitting on concrete so no chance to dig a hole

 

I suggest removing a fair bit of FG from the rudder in order to determine how far water penetrated the wood.

No sense locking moisture into the rudder.

 

Unless you have a really compelling reason,avoid Kevlar.It is hard to cut and goes very fuzzy if you abrade a cured laminate in which it is embedded.

 

No moisture where the damage is but some around the stock.
Suspect it may be condensation inside the tube over many years but further investigation needed!

 

Moisture around the stock is a typical problem, it's coming from above where the tube enters the blade. The cure is a big countersink, filled with an elastomer (Sika, 5200, etc.) wherever metal meets glass. Cut the wet ply away completely and inspect the tube (stainless in wet wood is not a good combination). I would simply replace the tube, but that's your decision.
Scarf new plywood in, preshaped to rudder profile and glass over. If you replace the tube completely, make the core in two halfs and glue in place over the tube, then glass and fair.

 

Might have to change the tube at some point but not possible at the moment so need to repair what I have.
Can't get my head around trying to scarf airofoil shaped piece's of plywood together which is why I was thinking of foam core and stronger skins?

 

I would order solid glass or foam core or plywood and lotsa laminations and epoxy. Make sure to precoat the old plywood and use epoxy putty to avoid any weak spots. Putty is 2:1 or a bit more by volume cabosil to mixed resins.

Bond the core to the old first, let it cure one day in the clamps to keep it in right spot. Grind away rough stuff and sand with 40 grit, then putty to smooth and laminate smallest pieces first with say db1700 or 1708 and staggers say about 2cm or as needed to get enough layers.

You’ve got some fairing ahead of you. Make sure to not get too thick in a single go with epoxy.

I also like Rumars idea to ply each side of the tube..

 

Glue these halves in place over the tube. Once the core is positioned, glass over it and then fair the surface.