Moving the traveller to the cabintop won''t really do anything the boats performance. It will, however, introduce significant additional load to your boom because the sheet would then be pulling on the center of the boom rather than at the end as with your current configuration. This is why boats with mid-boom sheeting usually have three sets of blocks at different locations on the boom - to try to distribute the increased load better. You''ll also need a mainsheet tackle with a greater purchase than your current end-boom mainsheet, which means a longer mainsheet (or a mainsheet winch). Same thing goes for the traveller itself; the new cabintop traveler will need greater purchase and longer control lines.
How big is your boat? This could be a considerable engineering problem. You will need to be sure that the boom can handle mid-boom sheeting, and that the deck structure can accomodate the traveler loads. Either or both will likely need to be extensively modified.
Personally, I dislike cabintop travelers and would rather have it on the transom (as with my current boat) or bridgedeck. I like having the mainsheet accessible from the helm.
divad hit the nail on the head. I do think that a cabintop mounted traveler could impact performance a little. First of all you will probably end up with a heavier boom. This is slightly offset by not having hardware hung on the end of the boom but not completely. The bigger issue is controling twist. I have found it much more difficult to control twist in the mainsail of a boat that has a cabin top mounted traveller. This results in a powering up a little in heavier going when you''d rather blade out to control weatehr helm.
Good point about controlling mainsail twist Jeff... I hadn''t thought of that. I suppose that the further aft the traveler is, the closer to vertical the mainsheet remains when eased, which would more effectively keep the end of the boom down to flatten the sail when you drop the traveler to leeward. However, with mid-boom sheeting, you''ll have a greater range of angular horizontal adjustment than an equivalent length of traveler track mounted for end-boom sheeting. That might be a minor advantage of mid-boom sheeting, unless the end-boom configuration has a wide enough traveler to enable the same range of motion.
Whatever; I''ll take end-boom sheeting any day, as long as the traveller doesn''t end up atop a "radar arch"!