When your mainsail is lowered, the task of holding your boom up usually falls to the topping lift. This piece of running rigging can support the vertical load of your boom, but it offers little to keep the boom from swaying laterally. Tightening the mainsheet will help reduce the swing from the boom, but it will still jostle back and forth.
if you are leaving your boat in a marina for the week, a little noise from the traveler and mainsheet is inconsequential. If you are passagemaking and the off-watch crew is trying to sleep, the rattling of the boom could keep them awake.
How do you hold the boom steady when it is lowered? The old answer was to rest the boom in its gallow. This was a support that would hold the boom in its place when the mainsail was lowered in lieu of a topping lift. Boom gallows can be readily found on historical yachts but are less commonplace on modern production yachts.
If you don't have a boom gallow, like me, and don't have the time to spend fabricating and installing a boom gallow, there is an alternative: your Bimini.
A well made Bimini will be made of stainless steel pipes that can easily support a lot of extra weight. Think about it, the Bimini and dodger need to withstand a boarding wave crashing hundreds of pounds onto your boat. A boom (on a boat below 50 feet in length) is not going to weigh that much, and it can safely be supported for the night while your crew sleeps.
Another great advantage to resting the boom on the Bimini is there is no movement, and therefore, no chafe! The sound of a mainsail sliding back and forth on the Bimini is not only annoying, but it is the sound of damage being done to both parties! Minimizing chafe usually entails raising the boom and tying the sail up nice and tight. This is effective, but also takes time, effort, and a lot of work. The alternative is to lower the topping lift to rest the boom on the Bimini and call it a night!
If you don't have a gallow and need to silence a noisy boom for the night, try resting the boom on your Bimini by easing the topping lift until the boom rests securely in place.