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Old 09-08-2003
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The Perfect Anchor Rode


More than we often realize, sailors are dependent upon something as mundane as an anchor rode.
Like many things in our cruising life, our anchor rodes have evolved slowly over the past 30 years. In fact, our concepts of the perfect way to attach the anchor to the boat have come nearly full circle.

Our first few cruising boats were under 30 feet, and like many of our youthful peers, a few feet of chain shackled onto a Nylon line was more than adequate. These easily managed rodes were perfectly suited to our infrequent weekend outings and lightweight boats without a windlass.

When we moved up to boats in the 31-foot to 37-foot range, and the limits of our cruising horizons expanded to include foreign locales, we succumbed to the literature of the day which claimed that a serious cruiser worth their salt only carries chain. In fact, our second "big" boat had primary and secondary anchors each rigged with 200 feet of 3/8-inch chain—a total of some 600 pounds not including the weight of the anchors themselves.

After living on this particular boat for six years, we were in the process of cleaning her up for sale when we discovered that the bitter end of both these rodes had welded themselves into a ball of rust in the bottom of the chain locker. We literally had to chip the links apart with a hammer and cold chisel. What was obvious was that we had never used the last 50 or 60 feet of these rodes in all of those years aboard.

It is impossible to conduct an enlightened discussion about the proper length of a rode without first talking about "scope"—an elusive concept at best. To arrive at a scope, you must know three things. First, the true depth of the water from the surface at the location of the anchor. Note that this is not what your depthfinder reads unless you have taken the time to set the keel offset function correctly—if not, the extra depth must be mentally added. Note also that it is not the depth where the boat lays, which can be much greater or less than where the anchor resides.

 
Most anchoring situations are easily handled with one hook.
 
Secondly, you must add the height to the chock or roller where the rode lies above the surface of the water. This is often forgotten.

Thirdly, you must add the highest rise of the tide. If you anchor at high tide in seven feet of water where there is a 10-foot rise and fall, you will surely be aground in about four hours. Conversely, anchored in seven feet of water at low tide, with a bow five feet off the water, you would put out 60 feet of rode to reach a 5:1 scope (7+5 = 12; 12x5 = 60). But when that 10-foot tide comes in, you'll find yourself with a paltry scope of 2.7:1 (7+5+10 = 22; 60/22 = 2.7). Expect to have the anchor pull out at about mid-tide.

If you're a stickler for detail, which you should be when it comes to anchoring, you should also add one-half of the wave height to your calculations. Ten feet of nominal water with a four-foot swell becomes 12 feet on the crests and eight feet in the troughs. This point is rather moot, however, as an anchorage with that kind of wave action would have most sailors at storm scope anyway. Better yet, find a more suitable spot to put the hook down.

But I diverge—back to the evolution. Our next boat weighed in at nearly 40,000 pounds of displacement and drew 6.5 feet. One thing became obvious to us as we prepared to order new ground tackle for this behemoth. The many rules of thumb, such as using one boat length of chain, are mostly inadequate. The dimension that is important is the boat's draft—not her length. A vessel with four feet under her will feel comfortable anchoring in six feet of water while a boat with eight feet of draft may not want to venture into much less than twice that much depth. The deeper draft vessel automatically requires an extra 30 feet of rode to maintain the same 5:1 scope as her shoal-footed cousin—even more for storm conditions. Even with the increased draft of our new home, we decided to cut the amount of chain on the primary anchor from the 200 carried on his predecessor down to 150 feet. For the secondary anchor we went back to a rope/chain rode with 50 feet of chain.

 
While necessary, swivels create a weak link in all-chain rodes.
 
There is a footnote here that is important. With all-chain rodes, it is critical that a rope snubber be employed to provide the stretch that the chain lacks. Snubbers not only insert a shock absorbing action that protects the deck gear from sudden overload, but they provide extra catenary, or weight induced curvature, to the chain rode. Again, like most of our peers, we started with a chain hook spliced to 20 or 25 feet of three-strand Nylon line. We quickly found these inadequate.

If we desired a finished rode length of 60 feet, letting 50 feet of chain go over the roller, clapping on the chain hook, and letting out another 10 feet of both chain and snubber was easy enough. With a few feet of Nylon used to cleat off, however, we only had about eight feet of snubber left. When a sudden night-time squall would come up, we would have to crank in the last 10 feet of chain, disengage the chain hook, let out the desired new amount of chain, re-insert the chain hook, and then let out the extra scope. All of this is usually performed in the dark on a pitching foredeck—both inefficient and potentially dangerous. These short snubbers were summarily replaced with lengths of 50 feet, which allowed us to easily veer out up to 38 feet more chain and snubber in an emergency.

With our new draft of 6.5 feet, we reasoned that we would be anchoring in deeper water, and accordingly increased the length of our snubber to 75 feet of Nylon. Note that we now only required one snubber as the secondary anchor had a rope/chain rode—we carried a second snubber as a spare.

All of this worked well, but I kept noticing that even with the increased draft, we continued to use about 100 feet of chain on a regular basis. The other 50 feet was used occasionally, but we found that we would rather work our way into 10 to 12 feet of water before anchoring, making 100 feet of rode acceptable for anything short of a survival storm. Now, however, I would turn the chain end-for-end each year to prevent the ball-of-rust-in-the-locker syndrome.

 
One anchor, and preferably two, should be ready to roll at a moment's notice.
 

When we sold our deep draft monster and acquired Sojourner, our present water-borne home, we cut our draft in half. Having less than 3.5 feet in the briny allows us to anchor right up on the beach with the catamarans. But it also cut our need for rode length a substantial amount. Anchoring in 10 feet of water (bow height of 5 feet) on a 5:1 scope only requires 75 feet of total rode. As we were pricing out the new ground tackle, the whole process suddenly became clear—why not just splice the extra-long snubber onto the chain and be done with it. So instead of dropping the chain length again, and building an even longer snubber, we spliced 100 feet of Nylon onto 75 feet of chain. So that's how we evolved full circle back to a rope/chain rode.

We realize that our cruising grounds are shallow by some cruisers' standards. Our half-chain/half-rope rode would allow us to anchor in moderate conditions in water up to 22 feet deep, which is more than adequate for the US East Coast, Bahamas, and most of the Caribbean. We have left provision to quickly add another length of Nylon to the bitter ends of our rodes should an emergency arise. Those in the deep waters of the Pacific, however, would have to recalculate the rode lengths to suit their own cruising grounds.

We recently finished an 800-mile shakedown cruise with the new system, anchoring Sojourner a dozen times along the way. We found it to be simplicity itself. Easing out rode with the windlass until rope went over the roller, we knew that we had exactly 80 feet out, 75 feet of chain with a 5-foot snubber. This gave a 5:1 scope in 11 feet of water depth and 7:1 scope in 6-and-a-half feet of water. For deeper water, or for more scope, all we had to do was to let out some more line, just as we would have done with our previous all-chain rode and long snubber system.

The only drawback seems to be with other conscientious sailors. Conning the anchorage, a few noted that we were lacking the telltale chain and snubber. Assuming that we were anchored on the more traditional boat length of chain, they then proceeded to anchor too close to us with a mostly Nylon rode. Boats on the two types of rodes behave very differently, especially in a change of tidal stream or in light breezes. So we began thinking of having some lightweight plastic faux-chain to drape over Sojourner's bow in order to ward off these close encounters.

Scope Tables

Sojourner's Scope for Various Conditions
 All Chain/ Snubber 1/2Rope/ 1/2ChainShort Chain
Short day stop—light weather345
Overnight—light weather456
Moderate weather678
Heavy weather7810

Sojourner's Scope Table
     Water Depth plus Bow Height at High Tide 
1015202530
S
c
o
p
e
33045607590
4406080100120
55075100125150
66090120150180
770105140175210
880120200250240
990135180225270
10100150200250300
*Do not reduce scope on either rode if anchored with two or more.




Last edited by administrator; 02-07-2008 at 10:54 AM.
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