Hi Plugcheck,
Thanks for the response, really appreciate it. I still question if this is just Gel fracture, if you look at the picks after using the grinder there are still deeper cracks into the glass, and they do seem to have been filled previously, if you zoom into the pics you will see what I mean, and that to me is worrying, either way I will do as suggested and keep an eye on things, I'll probably remove the larger battery and put the original one back where it was as currently I have 2 heavy battery's within the area of the splits worst area..
I know the transom was enforced a few years ago when this motor was put on the boat and since I have had it I've all about replaced everything on the engine, except the lower end, pistons, rings, heads and associated parts, oh and I have rebuilt the carbs not replaced and the engine is sweet, still using the oil injection rather than premix. She goes like a rocket, up on a plane within 3 seconds of hitting the throttle. I've only ever managed to hit 62MPH on a clear day when the water is calm, don't forget mine is used only at Sea. This was on around 3/4 throttle at around 3200 rpm if I remember correctly and even then I needed to back off as I could see the horizon moving from side to side by approx. 1 meter each side of the bow and to be honest I nearly flooded the boat from inside!! You only need to hit a wave of take a gust of wind at that speed with that much power behind you and you up and over, and this was when I used to have one of the widow maker steering arms before they improved the strength....