My largest tuna caught aboard a sailboat was just over 75 pounds and the boat had to heave to in order to land the fish. I don't believe you could land a tuna in excess of 50 pounds from a boat that remains underway. Most of the fish I've caught offshore were taken from the decks of a 21-foot center console power boat, which also provides the maneuverability to land fish of this size. In many instances you would have to back down on the fish in order to bring it under control. My best advice would be to spend at least one day offshore fishing with a good charter fishing captain and learn the proper techniques for hooking and landing big game fish, which is usually what you encounter in the offshore, coastal waters. It would be a good investment in time and money, and most of the captains are more than willing to part with some valuable information.