How would you redo this Mast Base?

[geem]

Indeed.
I would try to avoid removing a big area of the top skin of the deck. A few ~8mm holes maybe. Start by enlarging existing screw holes. break up any rotten wood with a sharpened allen key in a power drill. Get it dry. Stuff it with chopped glass. Flood it with epoxy.
If the top skin looks badly abused/splintered etc, add a couple of layers of glass cloth and epoxy over a slightly bigger area. Or maybe as WH suggested, build up the top to replace the wedge in fibreglass.

If you use an angle grinder with a super skinny disc you can cut a nice neat square under the mast base. Lift out the skin and you will have a good look at the core. If all is good then epoxy the skin back on to the core and nothing is lost. No harder to repair than a few 8mm holes and no damage to the core if you are careful.
I have replaced the ply core under a mast base on my old boat and had a new mast base made. The core was rotten. I cut out the glass skin as described above and laid up glass cloth and epoxy to the thickness of the core and skin. The lay up was about 1” and far better than a ply core

 

[William_H]

Hello Swens Dad. My boat is called a Castle 650. Not much help to you being 10000 miles away and certainly none of my kind of boat has ever got to UK. However to explain the cabin top does slope downwards where the mast base is. So it has a wedge under the SS base over the cabin top skin. Underneath is a stout wooden post about 80mm square which also is the front of the centre board (lifting keel) case with a similar post at the back of the cb case. On later versions of my type the cb case extends right up to under the cabin top. I presume the top of the post on mine matches the under side of the cabin top. That area is hidden by 2 stout ali angle pieces from one post to the other to take the loads of lifting the keel. Photo of whole boat on my details. ol'will

 

[lw395]

If you use an angle grinder with a super skinny disc you can cut a nice neat square under the mast base. Lift out the skin and you will have a good look at the core. If all is good then epoxy the skin back on to the core and nothing is lost. No harder to repair than a few 8mm holes and no damage to the core if you are careful.
I have replaced the ply core under a mast base on my old boat and had a new mast base made. The core was rotten. I cut out the glass skin as described above and laid up glass cloth and epoxy to the thickness of the core and skin. The lay up was about 1” and far better than a ply core

One might say that the integrity of the top skin has been lost. I'd prefer to see some overlap between the new glass and the old top skin. But in the case of a mast step, the main loads are not going to stress that. It's hard to theorise and generalise at a distance, you have to see the job and do what's needed. But in general I think in terms of removing the minimum of sound original material. If one does have to remove an area of GRP, I like to replace with more than I've removed, with plenty of overlap to sound bits of the original. Depending of course on how it will be used.
Sandwich is not generally much about the core, the strength is in the skins. The point load of a mast is an exception to that.

At some point I need to repair the tip of my Laser centreboard, I will just glop on some resin with a bit of glass in it and sand it to shape.....

 

[geem]

Sandwich is not generally much about the core, the strength is in the skins. The point load of a mast is an exception to that.
QUOTE
But we are talking mast compression loadings here. The core is everything as its taking the compessive load and the skin thickness is irrelevant in this application

 

[anoccasionalyachtsman]

Sandwich is not generally much about the core, the strength is in the skins. The point load of a mast is an exception to that.
QUOTE
But we are talking mast compression loadings here. The core is everything as its taking the compessive load and the skin thickness is irrelevant in this application

If there's a core.

 

[lw395]

Sandwich is not generally much about the core, the strength is in the skins. The point load of a mast is an exception to that.
QUOTE
But we are talking mast compression loadings here. The core is everything as its taking the compessive load and the skin thickness is irrelevant in this application

Mostly, yes.
But we still have to pay some regard to sheer loadings from the the kicker and halliards led back etc.
Also the deck loadings from some crew stood next to the mast.
It's probably going to be fine so long as it all ends up stuck together. When you actually do the job, you get to see the thicknesses and everything else and make a judgement. Not being able to see it and prod it, my vague feeling is not to encourage people to cut away any more of the original than is necessary.
Not remotely suggesting what you did was wrong when you could see it in front of you.

 

[lw395]

If there's a core.

Fair comment, I'd be surprised if there wasn't at least a block of something under the mast step, with another layer of glass under that, but life is full of surprises!