Sentinel Experiences by Ed Seling
In my years of cruising sentinels have become a bit of a hobby of mine (along with Primus burners.. but that's another story). They have several terrific uses none of which have to do with making an anchor hold better.
I was once blown out of an anchorage on the coast on Nova Scotia into the middle of a thankfully weakening hurricane because my boat became tide rode and the anchor line was severed by the aft end of my keel. In wind against the current conditions a vessel frequently sheers about and can wind up with her anchor rode fouled around the keel or rudder leading (in this case) to unpleasant events.This was an unnerving experience for a New England sailor who was not used to strong currents in anchorages and I was happy to discover that an anchor sentinel is of immense value in preventing fouling one's own anchor rode.
The idea is that when the tide and current oppose each other nearly equally and the boat wanders about, the sentinel will take any slack out of the system if it is positioned to hang a few feet under the keel. When the current or wind forces gain the upper hand the rode will be pulled taut and the sentinel will be "out of the system" until it is needed again. In north Florida, Georgia, and South Carolina where currents in anchorages are the norm this can be a very valuable technique.
The second use for a sentinel is a bit more worthwhile. Today's anchorages are frequently a bit crowded and it is often difficult to set decent scope in case a wind comes up. Boaters usually set scope for the worst (5-7:1) but the conditions are frequently such that little is needed. As a consequence the boats drift all around on their too long rodes bumping into each other in the night causing all sorts of social tensions when people run up on deck in the buff! (My wife and I once entertained an entire cruise schooner this way :-))
The sentinel provides a solution. I deploy 5:1 scope (or more) towards the most likely strong wind direction and then lower my 15 lb sentinel to the bottom (allowing a bit more for any tide). The boat will normally lay to the sentinel which frequently buries itself nicely in mud. This keeps the boat in one spot, away from others, on less than 2:1 scope. In the event the wind comes up in the night my long scope is ready to go. You still have to give a bit of thought about where you anchor but we use this trick all the time up in Maine. When anchoring Bahamian style with rodes going fore and aft from the bow I will frequently run the sentinel down one rode then use a smooth carribeaner to clip the sentinel to the other rode too. This holds the slack rode away from the keel and rudder. The boat will still wind the rodes up but at least you avoid chafe on the hull, topsides, keel and rudder.
Quite frankly I don't use a sentinel to increase holding power. In winds of any consequence the rode lifts the sentinel off the bottom anyhow. Maybe in lighter winds it does something. A good friend, who designed and tested anchors for a major manufacturer, felt that in conditions of surge a sentinel might actually INCREASE shock loading by bouncing on the rode, but there is no hard data on this.
So that's my thoughts on sentinels. Mine is 3-4 diving weights on a ss eyebolt shackled to a block that comes apart easily to fit on my anchor rode. It lowers on a 1/4" line. I wouldn't sail without it!
Fair Winds Ed Seling