Reduce Boat Roll with Flopper-Stoppers
Seamanship: When the ocean in motion becomes untenable, try flopper-stoppers or paravanes to settle things down.
Many factors contribute to a successful flopper-stopper installation. Several commercial designs on the market provide excellent value because they’ve been fully engineered and the companies that build them offer good technical support. If fabricating them yourself, be sure to have a solid understanding of the forces involved. The length of the poles makes a huge difference. The amount of load placed on the rig is governed by the surface area of the stopper and the length of the pole. A relatively small change in the pole length can have a huge effect on the loads imposed on the rig.
|Flying a “Bird”
Unlike flopper-stoppers, paravanes, or “birds,” are lowered to reduce the roll on a vessel that’s under way. When properly deployed, they “fly” through the water at a downward angle.
On the other hand, many fishing vessels, trawler yachts, and the occasional motorsailer use paravanes to ease their motion when under power. Paravanes are triangular metal wings with small vertical fins on top, to which a lift/towline is attached, and weighted bottoms, to help keep the wings horizontal as they move through the water. They’re lowered from and towed by structures that extend out from each side of the boat. While they can somewhat reduce rolling when the vessel is stationary, they’re very effective when the boat is under way and they “fly” through the water with a slight down angle that continuously resists upward motion.
To use flopper-stoppers on Bonnie Lynn, the 57-foot offshore schooner that my wife, Bonnie, and I launched in 1998, we installed 18-foot aluminum poles that we had specially fabricated for the purpose. We experimented with several types of stoppers, finally settling on a pair of stainless-steel trays that utilize opening and closing baffles. Because the poles are attached at either side of the rig of Bonnie Lynn, which has a beam of 15 feet 6 inches, when the floppers are deployed, they’re almost 52 feet apart. If we used shorter poles, we’d need larger floppers. We’ve used our floppers often in the eastern Caribbean, and they work great.
Since we had the poles and the rig in place, we decided to experiment further and designed and fabricated a pair of aluminum paravanes that were smaller and lighter—12 pounds apiece—than the standard size usually deployed on power vessels. For the bottom weight, we used lead in a streamlined tube. They came in handy on several windless days while we motored through swells. We also tried them while sailing, although rolling usually isn’t a problem unless you’re sailing downwind. When deployed at 7 knots, our speed was reduced to 6.5 knots: not a bad trade-off for the comfort.
Especially when the poles are long, using flopper-stoppers in a crowded anchorage can be problematic. At night, we always illuminate ours with small LED lights so that passing boats can see both the pole and the lines. The proper depth and pole angle will vary by boat and conditions. The units should never be allowed to touch bottom, but they must be set deep enough so they aren’t affected by waves or turbulence from beneath the boat.
While there are many good references on the subject, most focus on flopper-stoppers for powerboats, although the principles are the same. An enormous help to us was Captain Robert Beebe’s Voyaging Under Power; originally published in 1975, it’s been recently updated and is available online.
If rolling anchorages have been a problem, give flopper-stoppers a try. You won’t be disappointed.
Earl MacKenzie holds a U.S. Coast Guard 500-ton ocean master license and skippers the charter schooner Bonnie Lynn (www.bonnielynn.com).
For More Information
The following companies manufacture commercial-grade flopper-stoppers (also known as roll stabilizers) and/or paravanes: Forespar, Kolstrand, Magma Products and Prime Fabrication, Inc.
This article first appeared in the May 2013 issue of Cruising World.