Most of us know how to calculate scope when it comes to anchoring, but is there a rule of thumb that equates a foot of chain to some length of nylon rode?
Mark Matthews responds:
Thanks for the question. To my knowledge, there isn't a set formula for equating the amount of line to chain in an anchor rode. Tom Wood, an anchoring expert, has written extensively about types of anchors, rodes, and anchoring techniques on this website. His counsel regarding how much chain you should have in a chain-and-rope anchor rode is not fixed because of the variety of factors involved (boat displacement, draft, surface conditions, etc.). And John Rousmaniere, the noted author of The Annapolis Book of Seamanship, recommends at least six to 10 feet of chain accompany every anchor rode (except of course those for dinghies and those that are all-chain), but that's a bare minimum for anything over 25 feet LOA. You could say that twice as much line as chain would keep you safe in almost any situation.
What most anchoring formulae tend to ignore, however, is that length of rode needed for the proper amount of scope should be calculated from the point at which the chain or rode leaves the bow, typically several feet above the water's surface. Tidal state is likewise important to factor, especially in areas of large tidal ranges. So when calculating the scope, consult the tide tables for your area and pay out accordingly. If you're just working off the sounded depth of the chart, keep in mind these are in mean lower, low water, so the water is likely to be getting deeper at some point during your stay. Use a depth sounder, or if you're lacking that, take soundings with a lead line.
Any anchoring system is only as strong as the sum of its parts, and as such care must be taken to ensure that all swivels, shackles, and other components are properly tightened, wire-tied, and in good working order. Snubbers are also recommended when using all-chain to keep shock loads to a minimum on the windlass, deck cleats, rollers, and fittings. Make sure you use nylon rode, which will stretch in a blow and leave life below a bit quieter when out on the hook—there's nothing worse than a grinding chain on a bow roller when you're trying to get to sleep. And if it really starts howling, consider using a sentinel, or a weight that can be paid down the length of rode or chain. This reduces the angle of the rode, increases the canterary, and gives a dramatic increase in holding power.
If you're using rode, you'll want to make sure any weight you send down the line won't give you chafing problems. I've had luck with a number of homegrown devices. We came across the head of a CQR that we attached to a retrieval line once, and sent it down the chain. Or you can check out something like the ABI Rode Rider.
For additional information, you can also check out a number of articles on the site concerning anchoring that we ran a while ago. Most were written by Tom Wood and Liza Copeland. Here's the link to the first one and you can take it from there: "Choosing Anchors, Rodes, and Windlasses".