snip>> I want to replace the bench chair in my flybridge area with a pedestal helmchair. >> Jonas, You didn't mention what kind of boat you have. Some early MT's used a plywood deck covered with just a little glass. Other brands might have a cored construction. Either way, you need to consider how you will spread the forces so you don't destroy the deck or pull out the mounting bolts when you are pitching about in a bad seaway. The bench seats do that inherently. One option to consider is bolting a pedestal seat with a short pedestal directly to the box that the bench seat is mounted to. It ought to be easy to reinforce the underside in that case. But if you want to be rid of the bench and don't know about the deck construction, I'd make a heavy, over sized base in a hard wood and securely fasten that to the deck and arrange what ever is convenient for mounting the pedestal base. This method will allow through bolting (head side down). As Rudy said, you almost certainly have a crown to deal with. You might also be able to put a finished 'backing' block on top of the headliner and bolting right through the whole structure. Bob Chateau De Mer 1981 Hatteras 48MY