You need to get a longer halyard
as well. The 2:1 will require that you have a dead eye at the top of the mast
. You can attach a block to the old halyard
and run the new longer halyard over the block and snap one end on to the headboard and run the other to the deck
turning blocks etc. if you have them. You haul the block up first and the the new external halyard is now 2:1. I think that works. Nothing banging against the mast
with the sail down - one external line with it up.
Alternatively you can go aloft and install a robust dead eye at the halyard sheave. Attach the NEW longer halyard shackle up there and run it down to a new single
block at the sail head
board and up to the dead eye. Now you have two line external (one actually) banging against the mast when the sail is down and no external lines when it is up.
I think this works.