Choosing the right mainsheet can often be a decision that can make or break your day of racing. Factors to consider are length, thickness and material.
First off, length is important to consider. If you’re unsure, as when buying all lines, buy it a bit longer and cut it down to your taste after a few sails. It’s good to sail with a roll of electrical tape so you can mark where you think you’d want to cut it and try it out during practice. A mainsheet on the short end will mean that on a windy day, if you lose your sheet your sail won’t get too far away from you. However, in by the lee conditions, you may not be able to sheet out far enough. The ideal length seems to be between 42 and 46 feet.
The thickness of your mainsheet will affect both the grip you can have on it and the amount of friction as it goes through the blocks (a thicker line will have more friction). What this simply translates to is that in stronger winds, you’ll want a thicker mainsheet for better grip, and since the loads in the sail are higher the increased friction through the blocks are negligible. A common thin mainsheet is 6mm while a thicker, heavier air sheet will be 7mm.
Mainsheets come in different styles and materials and it’s important to get a strong and long lasting mainsheet. Constantly sheeting will put a lot of wear on the sheet, and you also need something that’s comfortable to grip and not slippery in your hands. A nice polyester cover is great, and having a strong core such as Dyneema can help prevent stretch and breakage.
Note that new mainsheets often come with a protective coating that is very slippery, so it’s good to break it in far ahead of an important event!