Learning from Failure: The Case of the Perilous Paravane
Published July 15, 2022
By: Christopher D. Barry, PE
A marine disaster movie that has an important lesson for mariners is the 2000 film, The Perfect Storm. Though no one knows what happened aboard the FV Andrea Gail after her last call to the Coast Guard, the film depicts a harrowing fight to cut loose a paravane when the other one was lost. In the film, it is implied that the requirement to release the paravane was because it was thrashing around and bashing stuff on deck, but the producers of the film did get the fundamental need to release the paravane correct.
WHAT IS A PARAVANE AND WHERE ARE THEY USED?
A paravane is a small hydrofoil, generally sort of a steel delta wing, connected to an outrigger via a wire. It is ballasted nose down, so it applies “lift” forces downwards due to being lifted by the outrigger when the boat rolls away from it. (The other paravane on the down roll side just sinks when its wire goes slack.) It exerts a very strong roll damping moment because it is well offset from the roll center of the vessel. This means that rolling the vessel away from the paravane lifts it upwards rapidly (the speed increases with the distance off the roll center) so that the dynamic force increases greatly. The roll moment generated from the dynamic force is the force times the distance off the roll center as well, so a relatively small paravane can provide a lot of stabilizing, “roll damping”, effects.
Paravanes are commonly used in many fisheries, on oceanographic research vessels, and very frequently on “passage maker” type large motor yachts. They are generally regarded as very effective and are relatively inexpensive both in terms of actual cost and in terms of any fuel required to make up the drag they cause, which is small.
HOW DOES A PARAVANE CAPSIZE?
The problem might be considered reasonably foreseeable, but in a 1998 paper, D. W. Bass, of the Memorial University in St. John’s Newfoundland, did an extensive study including model and full-scale tests and mathematical analysis that detailed the mechanism by which a paravane could cause a capsize. Since this study was inspired by the loss of the FV Straits Pride II, it is a reasonable conclusion that a single paravane, especially to weather, can be very dangerous, and a vessel losing one paravane should recover or release the other immediately. It is interesting that the FV Straits Pride II initially developed a relatively small angle of lift, which gradually increased until water taken aboard became critical, but other types of sudden motions are possible as well. There have been other casualties involving paravanes, including an injury to a crewmember when a boat took a sudden roll, having lost one paravane.
FV Straits Pride II was unable to recover or release the single paravane due to heavy weather, (the Perfect Storm involved climbing out on the outrigger). It is important that designers and builders design paravane installations to ensure that release or recovery of a paravane is easy, fast, and safe. One way would be to rig the paravane so that it is made fast, only while aboard, in an accessible place, with a line that can be cut as well as be mechanically released (with a fitting that cannot foul on its way overboard). Crews also need to be made aware of the issue. Not much has been published about this problem in either the fishing industry or in the yachting press, though it would seem a simple message: Paravanes are often very useful and effective in roll control, but there is one key precaution for users; recover or release.
This paper also proposes the use of anti-roll tanks in fishing vessels and studies their effectiveness. These devices basically absorb and damp much of a vessel’s roll energy in a special sloshing tank. They have become less common, but should be considered more often, at least for vessels that can stand a weight increase of a few percent for the tank, because they don’t cause any drag or absorb power, and unlike the more common anti-roll fins work well at anchor.
This also invokes another question: How can technical people and societies like SNAME bring wider public attention to this kind of hazard?
USING SERIES DROGUES
Another issue, fortunately with good news rather than bad, provides another example. After the Fastnet disaster, SNAME, the Coast Guard, and the USYRU, studied sailing yacht capsizes and published a series of papers on the subject. Donald Jordan and Carol Hervey developed a simple “series drogue” that seems to be very effective in preventing capsizes of sailing yachts and passage maker type motor yachts and life rafts. Though series drogues are becoming more common, even though there is no patent for them, and the design drawings are provided in the report, they are still not as common as they could be, again, because they are probably not as well publicized as they deserve and provide another example of something that a technical society might well take on.
This also goes to the far wider issue of the role of technical societies in public outreach, especially as regards supporting STEM education, but that is an issue for another day.
ABOUT THE AUTHOR
Christopher D. Barry, PE, s an experienced naval engineer who has, over the years, gained extensive experience in the maintenance, overhaul, acquisition, design and construction of commercial and military ships and boats, offshore oil platforms and other floating equipment. He is well versed in the areas of hydrostatic, structural, hydrodynamic and mechanical engineering analyses of resistance, motion RAOs in waves, mooring systems and more. See Christopher’s full CV here.
D. W. Bass, “Roll Stabilization for Small Fishing Vessels Using Paravanes and Anti-Roll Tanks” SNAME, Marine Technology, Vol. 36, No. 2, Apr. 1998
Jordan, D.J., Investigation of the Use of Drogues to Improve the Safety of Sailing Yachts and Life Rafts, U.S. Coast Guard Report CG-37-84, December 1984
Hervey, C. and Jordan, D., “Investigation of the Use of Drogues to Improve the Safety of Sailing Yachts”, U.S. Coast Guard Report CG-20-87, May 1987