Slightly off the original topic but still worth discussing is the length of chain that you use. Many of the posters have suggested that more chain will increase the holding power of your anchor but I don't agree with this in most practical circumstances. Having an all chain rode is ideal if your boat can take the weight because of chain's resistance to chafe but it won't help in ultimate holding power.
The vast majority of the time, the ultimate holding power of your setup is not an issue, the force on the rode goes up as approximately the square of the windspeed. Taking a design load of 1500 lbs for the OP's boat, that likely equates to 60 knot steady winds or even more depending on the sea state. During a breezy day, the load is more likely to be on the order of 2-300 lbs max. On these days, the weight of the chain will improve catenary which helps holding power and provide a bit of shock absorption. However, on the days where the ultimate holding power of the anchor is tested, the chain will be essentially a straight line. Another benefit on the calmer days is a reduced swing circle provided that it matches the other boats around you.
There is only 1 way for the chain to increase the ultimate holding power of the anchor and that is by changing the angle of pull on the anchor. In calmer conditions, chain achieves this through catenary. However, in extreme conditions, the chain is straight and it provides the same angle of pull as a piece of line. In these conditions, the only way to change the angle of pull is to increase scope. A good measure of whether there is sufficient scope is whether there is always at least 1 link of chain laying on the seabed. Since chain can only transmit force in tension and not bending, if even only 1 link is lying on the bottom, then the angle of pull is as low as it can be. In truly extreme situations, even this is not possible.
There are also 2 ways that a rode can lower the loads on the anchor for given conditions making it less likely that you will drag. The first way is by damping out dynamic loads that occur as the boat moves around. Chain does a very poor job of this in extreme situations because it is essentially already stretched to its maximum by being in a straight line and has no more give. Nylon line is an excellent shock absorber whether it is employed in a mixed rode or as a snubber. The other way to decrease the pull on the anchor is by the rode acting as an anchor itself and having resistance in the bottom. While it sounds like this should be a big benefit, it has been shown that there is almost no increase in holding power. Recently, I dragged 50' of 3/4" chain across a gravel parking lot and through the mud by hand and I am certainly no Hulk Hogan and this was big chain. Additionally, it must be remembered that in extreme conditions, the chain is not in contact with the bottom anyways.
You do need to keep enough chain in the rode to help the anchor set initially. Anchors set at relatively low loads so it does not take that much chain to keep the last few links on the bottom. In the OP's case 20' would probably be fine for all situations.
I hope that I have convinced you that during conditions where ultimate holding power matters, chain will not significantly increase it. The only way to increase holding power is by going to a bigger or different style anchor. To keep the shock loads down on your ground tackle, you should either use a mixed rode or a snubber. There are very good reasons to use more chain or all chain such as chafe resistance. Personally, I feel that mixed rodes are the best option for smaller boats that don't anchor in coral and all chain rodes are best for larger cruising boats. Like everything in boating, it is a balancing act but the weight should be put into the anchor not the rode.
Agreed. I LOVE chain, but I love it for being able to have less scope out and thus fit into a crowded anchorage. I also think it has benefit in helping an anchor reset
as wind and current change. I believe it does this by keeping the pull angle very low as the boat swings around its anchor.
I totally agree though that it doesn't help much with ultimate holding power. More so I agree that too much emphasis is put on ultimate holding power when ease/quickness of setting and ability to reset have more real-world value.
I'm a big fan of the Fortress for example and used it as my primary for 100+ nights in a long cruise up the inside passage. It takes more skill to set and be sure it's set than a Bruce or other types though. It also doesn't set quickly where you drop it (as my Bruce does). This can be problematic in a tight spot such as when you stern-tie, or are trying to fit into a crowded anchorage. When stern tying it doesn't do you any good to have the anchor set 2 boat lengths closer to shore than you wanted. Part of my problem though, may have been that I used 55' of chain, when you're supposed to used 6' only on the fortress and initial setting on short scope is also recommended, which I didn't do.