If your new main is not of the loose foot variety (like the standard Elvestrom) then you may have a problem. To allow the bolt rope on the foot of the new main to fit the boom groove you’ll have to replace the Packaway bag with one designed to fit around the bolt rope, so that the bag and bolt rope fit into the groove together ( the packaway cover fits snugly around the bolt rope and they both then slide into the groove). Roger’s suggestion of setting up the new sail ‘loose footed’ may well work; worth a try before going to the expense of a new packaway cover.
Good luck…..
Ian Culley
Another Fantasy
