I have a 30-footer that is about 10,400 pounds and I plan to anchor in approximately 20 feet of water. What size and lenth of rope should I carry? At present I have 20 feet of 1/4 chain with 150 feet of 7/16 three-strand line. Is this adequate for the boat?
Thanks for your question. We get asked this on a regular basis, so you're not alone in your query. Regarding the dimensions of your anchoring gear, for a boat the size of yours, John Rousmaniere recommends (in his useful tome The Annapolis Book of Seamanship) that you have about 16 feet of quarter-inch chain for your lightest anchor (he says a 12-pound Danforth styled anchor would be a minimum) and 46 feet of quarter-inch chain for your heaviest anchor (he recommends a 35-pound yachtsman). For the same size boat he recommends nothing smaller than 7/16-inch line, so it looks like the gear you have is appropriately sized.
Now, regarding the amount of rode you need for 20 feet of depth, Rousmaniere recommends 7:1 scope for a rope rode and 5:1 for an all-chain rode until the hook is securely dug in, and then a little less only in light to moderate conditions. He says that a lot more rode should be veered out as conditions get rougher.
Rousmaniere cautions that the length of the rode may vary with rode type and anchor type, and of course with how well the anchorage is protected and how crowded it is in a shifting wind. He also reminds us that the most important factor in what type and rode and what kind of scope to use is windage, which is a function mainly of beam. "A lightweight multihull may pull harder than a heavy monohull."
So with your 20 feet of chain and 150 feet of line, you've got plenty of rode to use relative to Mr. Rousmaniere's recommendations. Here's hoping that this helps to answer your question. By the way, you might want to consider getting a copy of Rousmaniere's book. It can be very useful. Best of luck on the water this summer.