Rudder internal foam

[Lady Campanula]

I'm considering ways of replacing some water-absorbing foam, or voids, inside a rudder and am exploring the merits of a limited-expansion PU foam such as the Soudal 'Flexifoam' product.

The current idea is to cut out some 15-18mm holes ( holesaw ), scoop out the damaged water-absorbing foam, dry the cavities, then apply this new foam - believed to be 'closed cell'. The circles of grp, cut-out by the holesaw, can then be re-inserted and the surfaces made good.

Any experience to offer?

 

[Quandary]

I rebuilt a Westerly foam cored rudder for a friend but it was already broken so splitting it was the obvious way, then I was able to laminate layers of rigid polyurethane to replace most of the core. I am pessimistic about both your chances of extracting all the old foam and of filling all the voids through the holes. I would be inclined to consider splitting it or at least taking a side off.
The alternative, which I recommend considering, is drilling a couple of holes in the bottom and leaving it for a few months to drain and then dry out. Then fix the crack that you will find on the top where the shaft goes in. The straight bit of the crack can be repaired with epoxy with sikaflex to allow for movement around where the shaft goes in. Do not worry about the internal voids, every rudder has some and even if you fix yours there will be water inside it in a year or two.