Relaxed riding on a sea anchor


Hal Sutphen’s recent article on sea anchors (“Anchor survey,” Issue No. 100, September/October 1999) has prompted me to ask some questions about sea anchor deployment on my own boat.

I have sailed on many vessels, including performance yachts, full-keeled voyagers, and commercial ships, and have seen the ocean get rough on all of them. I have set a sea anchor while voyaging on both a full-keeled sailboat and a fin and spade performance racer/cruiser. On the full-keeled boat we saw the rough weather coming and set the anchor early. We had no difficulty and sat through the entire storm in relative comfort, reading books and staying dry.

We didn’t use a trip line but waited until the wind had subsided enough to pull the boat up to the anchor and retrieve it. To prevent chafe trouble I used a multiple-layer approach: a piece of leather against the line itself, followed by a split piece of plastic tubing, followed by another, stiffer piece of plastic tubing. All of this was held on by some duct tape and line. I do not see how it could have been done at the last minute under extreme load on a wet, pitching deck, though.

The second time we set a sea anchor was not as successful. We were in a performance-oriented racer/cruiser and we avoided deploying the sea anchor until the last moment. Though I was able to get the anchor deployed with only the amount of difficulty that one would expect given the conditions, it did not serve us at all. As soon as it was deployed and had tension on it the boat began to sail back and forth at anchor. It tends to do this in a regular anchorage as well, but not to the same extent. The boat would “hunt” from side to side and go almost fully beam to the seas before it would begin the cycle and “hunt” back in the other direction. We attempted to use the method recommended by Lin and Larry Pardey: a bridle arrangement with a line to the bow and the stern. But we could not get this to work. When the tension was enough to get the boat to stop tacking by itself, the line held us so close to a beam-to-the-seas position that we were doing a death roll. When the line to the stern quarter was not as tight it would get pulled under the boat as the boat fought its way up to a head-to-wind position and then tacked itself across.

We ended up abandoning the sea anchor and going back to a sail configuration that allowed us to heave to. I much preferred the successful anchoring experience with the full-keeled boat, but I enjoy the sailing performance of the fin and spade underbody. Do you know what I was missing with my technique on the performance boat? Was it just that particular boat, and most fin and spade boats can be balanced at a sea anchor?

Scott Rhoads Clearwater, Fla.Answer:

A deep-fin keel, light-displacement racing boat will tend to sail around much more on a sea anchor or in a tidal flow than a full-keel, heavy-displacement voyaging boat. This is due to the high aspect ratio characteristics of these racing foils, which generate a much higher proportion of lift for the same profile area.Without getting into a lengthy dissertation on hydrodynamics and 3-D foil theory, it can be said that the aspect ratio (AR) of a foil has a profound effect on its efficiency and ability to generate lift and reduce drag. The AR of a keel is defined as the span squared divided by its lateral area. The resultant lift or side force (Fs) for a given foil section and angle of attack will be dependent on the side force coefficient (Cs), which is in turn dependent on AR. For example: Boat 1 and 2, at left, have the same lateral area and use the same foil section. Boat 1 has a keel with an AR of 0.5 and a Cs of 0.09, which generates an Fs of 90 pounds. Boat 2 has an AR of 5.0 and a Cs of 0.3, which generates an Fs of 290 pounds (ref. Sailing Theory and Practice, by C.A. Marcaj). The hydrodynamic benefits become obvious, and that’s why a more modern racing-type yacht will perform better to windward. The downside, as you have observed, is that they will override a sea anchor or a mooring in a tidal flow.

Next time you’re in this situation on this type of boat, try rigging a small flat riding sail off the backstay and sheet it down hard to somewhere on deck (assuming you have the sea anchor set from the bow). The aerodynamic drag of this sail will generate enough moment to counter the hydrodynamic lift produced by the keel and keep the boat tracking into the wind and waves. The wheel or tiller should be on center.

By Ocean Navigator