The rudder lashings on my Tiki 30' have torn out of the stern post leaving a 4"x2" void. Would I be best to fill the void with thickened epoxy or a putty, then lay up glass and cloth over it, or plug the void with hardwood then glass it up. Then drill oversize holes, glass them and drill the correct size hole 6mm for the lashings
Would I be best to fill the void with thickened epoxy or a putty,
Not a good idea I think! Best to go with your replacement plan. 
Maybe even replace the whole post
Before you do anything it may be advisable to try and ascertain why your rudder lashing pulled a 4x2 chunk out of your stern post and then work on a plan to prevent it happening again.
I suspect that maybe the holes through which the lashings went weren't fully sealed against water ingress into the timber and if that is the case then maybe you need a better plan B.
My guess is that the holes weren't properly epoxied. You can't bog it up. Bog (filler) is not strong enough to do the job. You must either replace the timber (and epoxy all of the drilled holes) or do something new. I would replace the timber and then glass over it with at least two layers of 400gm double bias. This should stop it ripping out.
You could replace with some tufnol or some hardwood. Best probably to just do it well with well epoxied holes in good low oil hardwood - spotted gum perhaps. SG epoxies well.
cheers
Phil
just had a good look at the holes, did have the go pro on charge but left it at home so no photos. looks like it was a void cut into the wood and filled with epoxy that has pulled out so will beach the boat, clean it all up and fill with some epoxy and a strip of hardwood on the trailing edge, few layers of glass and cloth, re drill the holes and lash the rudders back on. Hopefully photos to follow
