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I need a new mainsheet (the rope that controls the boom on the main sail) for my sunfish.

Many of the online sailboat stores, are selling 8mm FSE Orion for the job. My local Hardware store does not offer this type of rope, but does have Blue Hawk 0.375-in Braided Polypropylene Rope (By-the-Foot), 7/16-in x 100-ft Braided Nylon Rope & 0.375-in x 25-ft Braided Polypropylene Rope All of these braided ropes seem fairly comparable to the rope at the online boat stores. 

MM      inch      inch
8 mm    0.3150    5/16 
9 mm    0.3543    23/64 
10 mm   0.3937    25/64

conversion source

Can I use any braided rope I find at the hardware store, or should make sure to find a specific type of rope?

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    I don't know your hardware stores, but I would not buy a sheet at a hardware store. Isn't there a sailboat shop near where you sail?
    – njzk2
    Oct 20, 2016 at 23:23
  • @njzk2 I agree that a better replacement could be found online but I still think it is a valid question.
    – Erik
    Oct 20, 2016 at 23:59
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    I count nine uses of the word rope in this question, and zero of line. Sigh. Landlubbers anyway.
    – cobaltduck
    Oct 21, 2016 at 14:50
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    @cobaltduck I actually struggled over that while writing the question. With mainsheet, line is implied. Everything else is before it becomes a boat part, so rope seemed more appropriate than line. Much like cement is concrete that has not set, rope is line that does not belong to a boat (or so I was thinking!)
    – James Jenkins
    Oct 21, 2016 at 14:58
  • 1
    Related When does a Rope become a Line?
    – James Jenkins
    Oct 21, 2016 at 15:05

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You most definitely want to be selective when selecting rope for a sailboat's running rigging. I'll group sailboats into two categories: racing and non-racing. Racing sailboat is simply any boat where it participates in a race, not any particular construction.

For racing sailboats, you will want to stick as close to the manufacturer's specified lines, and your class rules likely also require it. The details of that are beyond the scope of this answer.

For all other sailboats, what is most important is the rope material. For size, within a millimeter or so is usually fine, though you'll want to err on too small rather than too large so that the rope doesn't bind in through any blocks or winches. For dinghies and small boats, control lines are usually sized large to be easy to grip and hold on to, and as a result don't come anywhere close to their yield strength, so don't worry if you're a tiny bit smaller than the stock line.

For material, your hardware store options were nylon and polypropylene. Nylon is a no-go for running rigging. It is stretchy and loses strength when wet.

Polypropylene is acceptable but isn't the best. It's not very durable and will have to get replaced more often and you'll have to inspect it more regularly.

The materials that remain are basically polyester and a bunch of newish high-tech fibers. Unless you're in a world-class race, there really is no point in getting the other fibers unless you want to throw more money into the water than you would be normally.

Polyester is what you'll want. Virtually all sailboats not owned by millionaires are equipped with polyester running rigging. For a sunfish, pretty much any double-braided polyester line in the right size will do the job just fine.

This page goes into additional detail. (West marine is a boating retailer, but I have no association with them)

So in short: go to a specialty boating retailer.

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