For a boat up to about 24ft and about 4000lbs or less, like the V20, you'll want to use 3/8 NYLON, 1/2 is too thick. The secret to good, safe anchorage is rope STRETCH to take the shock of wave action off the achor itself (and giving the anchor enough scope, at least 1:5, preferably 1:7). If anything 3/8 is a little too heavy for the weight of the V20, but most "Experts" say it's too hard to grip anything smaller than 3/8. The problem with the 1/2 inch is the V20 isn't heavy enough to get is to stretch out the nylon, so the anchor just get's the shock... and shock after shock CAN drag your anchor rather than have it hold fast.
Use NYLON, not Polypropylene(sp?). Aside from the fact it floats (BAD for anchors, if you anchor in 50+ feet of water the poly can actually float your anchor), it has NO stretch, so a good hard shock like a big swell can SNAP your anchor line.
Don't forget an anchor chain too.... it'll help the anchor in 2 ways:
1) the weight of the chain helps stabilize the anchor so the wave action is absorbed in the rope stretch.
2) prevents chaffing and cutting your anchor rope on obstructions (coral, rock, wrecks) on the bottom.
Here's one good article, but there are plenty of others online:
http://www.boatus.com/boattech/anchorin.htm
I'm a diver, so I take anchoring SERIOUSLY. It's one thing to set your anchor then fish, it's another to anchor then jump off your boat and leave it hoping it'll be there when you come back up.