Re: Howto drill center of a rail.
Is the rail already on a boat, or is it just tubing in a workshop?
This is easy to do in a workshop by laying down the tube on a flat surface, then drawing an old machinists square along it. You will get a reference line down the highest point on the tube. Use it as the reference for your holes.
If the tube is already on the boat then I'd use a good sturdy and flat piece of plywood to make a temporary surface then do the same trick. That piece of plywood can be mounted to the nearest vertical members of the railing.
Don't forget to center punch the holes before drilling (to keep the bit from wandering) and to use sharp drill bits, lots of coolant, and a slow and steady speed that always keeps the bit cutting fresh metal. If the bit makes a pass without taking a cut it will work harden the stainless and dull the bit.
boats that I sail on regularly: Pearson 28-2, Rondar 505, Yankee 30, Blanchard Junior