If it's like my V-21 there is no core.
The old Macs were solid glass with no core. The cabin top is also 1/2" solid glass which I verified while working on the mast step after dropping my rig. Now that doesn't mean the ODay Mariner is the same but it's possible. If the glass weave is fractured like my transom was after my "First Sail" incidents, he'll need to do a proper repair. I had the shop do mine for about $300 and they did a great job. That $300 covered two deep fractures in the transom and one 2" hole in the bottom of the hull. Either get the job done by a professional or do the job right yourself.
The old Macs were solid glass with no core. The cabin top is also 1/2" solid glass which I verified while working on the mast step after dropping my rig. Now that doesn't mean the ODay Mariner is the same but it's possible. If the glass weave is fractured like my transom was after my "First Sail" incidents, he'll need to do a proper repair. I had the shop do mine for about $300 and they did a great job. That $300 covered two deep fractures in the transom and one 2" hole in the bottom of the hull. Either get the job done by a professional or do the job right yourself.
