January 2014 Issue
Table of Contents
Where Credit Is Due
Mailport: January 2014
I very much appreciate the service that Practical Sailor provides. I read the recent article on snubbers (PS, November 2013), and once again, you probably kicked an ant hill. I offer a few comments on the topic; note that throughout this letter, I use made-up figures.
You may find the following website to be valuable. It provides several graphs for calculating parameters that relate to dynamic loading of anchoring systems for boats: http://alain.fraysse.free.fr/sail/rode/rode.htm
Comparing Snubber Systems: I can’t think of a practical way to accurately calculate the amount of energy a snubber system will absorb. I think the only way to gain meaningful information is to perform tests and collect data. Below is a proposal.
If the last link of an anchor chain is picked up at 270 pounds of tension (100 feet of 3/8-inch chain at 5:1), and the tension on the anchor rode continues to increase, at some point (say, at 2,000 pounds), the maximum safe working load of the anchoring tackle will be reached. The amount of energy absorbed by the decreasing catenary of the anchor rode can be calculated with satisfactory accuracy by summing the data readings that were taken during the tensioning process: (0*275) + (1*303) + (2*333) + (3*366) + (…) + (21*2035). The values of 303, 333, 366, … 2035 are the tension readings at 1-inch increments.
In this scenario, about 276,000-inch-pounds of energy is required to tension the anchor rode from 275 pounds to 2,035 pounds. If a snubber is added to the anchoring system, then the system will have more elasticity, and hopefully, it will absorb more energy than a chain-only anchor rode. That said, sometimes snubbers will carry a smaller safe working load than the chain.
The point is that this analysis lends itself to comparing various snubber systems that many people will find useful. This analysis requires some assumptions and simplifications, but they may not significantly dilute the results.
Initial Tension: As you mentioned in the November 2013 article, one advantage of an all-chain rode is that some energy is required to lift portions of it off of the bottom before the last link is lifted. This contrasts to an all-nylon rode which requires less energy before it is nearly straight (more on this later).
I don’t think that the energy required to lift portions of the chain off of the bottom should be ignored. I suggest that initial starting point for comparison tests of snubbers should be the average tension on the anchor rode in a given situation. If the “last link” tension is used for comparison, an all-nylon rode will always absorb more energy than a chain rode or a combination rope/chain rode.
Final Tension: I suggest that the safe working load of the weakest link in the anchoring system should be used as the upper limit of the test.
This may be impractical to perform at your test setup due to structural limitations, but the test could be done in two phases. The first phase would mimic the profile of an anchored boat. After sufficient tension is applied to lift the last link off the ground, a second test setup would be used to replicate the depth of the catenary; ignore the water depth.
Combination Chain/Rope Rodes: On my former boat, a 36-foot C&C 110, I had an anchor rode with about 60 feet of chain and the rest nylon. This allowed me to anchor in 20 feet of water, with a 4:1 scope, and automatically have a 20-foot snubber. Perfect. If the wind kicked up, I let out more rope and got more scope and a longer snubber. Perfect again.
I recommend using braided rope in a chain/rope anchor rode rather than three-strand because three-strand can hockle and be hard on anchor-chain gypsies.
Rope Rodes and Yawing: I have been in some blows where the boat is yawing back and forth with significant force, and I have observed that the rope rode forms a fairly large horizontal catenary as the bow hunts. I guess that the depth of the catenary is about half the length of the boat. As the bow falls to leeward and tugs on the rode, the rode vibrates as it surges through the water. In practice, nylon rodes are not straight when boat is yawing.
Yawing and Pitching Loads: In addition to “yawing loads,” large waves and ocean swells can impose a large pitching motion on the boat. When the anchor rode is tight and a large wave lifts the bow high, the resulting load on the anchor line—and snubber—can be tremendous.
Bridle Snubber: The article did not mention a bridle snubber system. I currently use such a system on my power boat. When the boat swings to one side, only one leg of the bridle takes the tension. So, in effect, the snubber system receives half the wear of a single line system, and if one snubber line fails, the remaining one should still be functional. I wonder if a bridle system has slightly less yawing, even for a monohull. If the attachment points are well spaced from the centerline of the hull, the leeward snubber will come into play a couple of feet before a single snubber would, if it were attached on centerline.
Navigator, Kadey Krogen 48