THE FINE ART OF ANCHORING
Some lessons learned from a head-to-head anchor test.
Based on an article in US Boat.
For as long as humans have been floating around on the water in vessels, they’ve also needed a way to hold those vessels in place when not underway. One suspects that, from the very beginning, they have also argued about the best way to do that. And for as long as humans have been doing anchor tests, you can bet they’ve been arguing about the results, as well as the methodology, the bottom, the conditions, and the validity of the whole darn enterprise.
Fortress Marine Anchors, from Fort Lauderdale, Florida, organized the event to test 11 different anchors to determine the holding power of each in a typical soft mud bottom. The huge amount of data gathered during the tests shed some light on how well different anchors work, but as good science tends to do, also produced many questions, along with some surprises. The testing confirmed what many of us already know: Anchoring tends to be as much art as science and — even in the most controlled of circumstances — can be unpredictable. Despite that, every anchor test adds to our knowledge, not just of holding power in specific bottoms and under specific circumstances, but also of the importance of technique and experience in anchoring success.
The anchors were tested on a typical soft mud bottom, the kind in which many boaters routinely anchor. The location at the mouth of the Patuxent River near Solomons Island, Maryland in the southern part of the Chesapeake Bay was chosen because of the relatively consistent mud bottom — and the proximity of a first-class testing platform. The r/v Rachel Carson, a 5-year-old, 81-foot aluminum vessel owned by the University of Maryland Center for Environmental Science, is used most often for scientific research in Chesapeake Bay. She was selected because of her ability to use her twin 1,200-hp diesel jet drives and bow thrusters, combined with a state-of-the-art dynamic positioning system, to “hover” motionless over one spot regardless of wind and current. She is equipped with an aft winch that is capable of reading how much line is paid out to the inch and precisely controlling the speed at which it is paid out or retrieved. These last two things were important because they allowed precise and repeatable testing for every anchor.
Each anchor was attached to 20 feet of 3/8-inch chain, which was in turn connected to a 5/16-inch wire rope. The anchors were lowered by hand to the bottom of the bay and then the wire rope winch paid out 200 feet of wire over the stern while the boat moved forward at the same speed. The depth was pretty consistent at around 27 feet, and the aft deck of the vessel where the testing was done was four feet above the water, giving an initial scope of about seven to one. Once the wire was paid out, the vessel stopped, the captain actuated the dynamic positioning system, and the winch began pulling the cable at 10 feet per minute for 10 minutes until 100 feet had been pulled in. This had the effect of decreasing the scope gradually to about four to one and giving each anchor 10 minutes to engage the bottom.
The pull was recorded by a custom-built strain gauge that measures the force in pounds while sophisticated software recorded a real-time graph of each anchor’s holding power as it was hauled along the bottom. Each anchor was selected at random to be tested a total of five times over the course of three days. The vessel moved to a slightly different location between sets so that each anchor was pulled along a different trajectory, ensuring no anchor fell into the furrow of another.
We will cut to the chase and tell you that the Danforth and Fortress anchors, with their long, wide flukes, held in the mud with the most tenacity.
It’s not a huge surprise to those who have anchored often that a lot of the anchors didn’t set right away. When anchoring in the real world, most boaters know to let the anchor settle before they begin to back down on it. It took between 10 and 20 feet for most anchors to reach 300 pounds of holding power, a bare minimum to consider for an anchor of this size. During this period, it would often look as if the anchor was starting to set as the tension increased, only to have it break free again and the tension drop back down. A boater could easily be fooled into thinking that the anchor was set, even though it was just barely hanging on. Putting the boat in full reverse and powering back against the anchor for more than a minute after it appears to be set would ensure that the anchor had really found the bottom.
What was most striking was the lack of consistency in the results for the same anchor. Most had one good set that far exceeded the rest, and almost all had one trial where the anchor did not seem to engage the bottom at all. During the first test done with the Fortress 37, with its ideal 45-degree mud-bottom fluke angle, it failed to set during the entire 100-foot drag. Chagrinned, the Fortress representatives nervously joked that at least it was obvious the test wasn’t rigged in their favor. Subsequent tests demonstrated the Fortress’ mighty holding power, but the failure on the first set proves that no anchor will set perfectly every time, and if you want to be certain of staying put, you need to check the set and reset if necessary.
No anchor test will ever manage to be complete, and despite all the caveats, the process was a commendable attempt to conduct consistent and comparable straight-line holding power tests in a specific bottom. While the testing wasn’t perfect and won’t begin to satisfy everyone, it succeeded in adding to the limited body of knowledge that exists about anchoring and to confirm that anchoring remains as much fine art as hard science