You're reaching along, the boat's as steady as the proverbial church. You stretch up on your toes to peer ahead at the next channel marker just as a rude powerboat screeches by from astern. As your boat rolls to windward, the boom appears in your peripheral vision, and the next thing you know you're lying in the cockpit with a dreadful headache.
Or you're screaming along on a hard run when the wind shifts, the steerer can't keep up with it, and you jibe accidentally.
Or the wind dies leaving you rolling uncomfortably in the leftover sea and the boom waving like Leonard Bernstein's baton.
A moving boom is a wicked thing, second only to an overused liquor locker in the damage it can inflict on a crew. Last month I said a word about the solution to the problem; here I'd like to go into more detail about preventers. Unfortunately, preventers (like safety harnesses) are widely considered to be essential gear only for boats heading offshore. But as my list of examples (and, I bet, your own experience) demonstrates, the boom can fly around at any time on any boat, and with potentially terrible consequences. If you're cut, someone can patch or sew up the wound. But once you've been hit hard on the head, on-board first-aid treatment will be useless. On a Marion-Bermuda Race several years ago, a boat full of doctors, including a neurosurgeon, jibed accidentally, the boom smashed the skull of the helmsman, and none of his medical practitioner shipmates (including the neurosurgeon) was able to help except to hail a nearby Russian freighter to pick up their friend's body.
A preventer is an adjustable line that holds the boom out. Preventers should be rigged permanently on both sides of the boat using line whose diameter is at least as large as your mainsheet's, either Dacron or nylon (some sailors like nylon's stretch for absorbing shock loadings that might break the boom). There are two kinds of preventers: the combination preventer-vang and the offshore preventer. (Because the setup for each is simple and varies with the boat, I'm not including any illustrations.)
The most common and easiest to use is the combination preventer-vang. Tie or shackle two long lines to the boom one-third to one-half of the distance back from the gooseneck, preferably to a reinforced location like the boom vang or mainsheet fitting.
Lead each line to a side deck, one to port and the other to starboard, through a sturdy block on the rail near the chainplates at the base of the shrouds, and aft to the cockpit to a winch or cleat. The blocks should be slightly forward of the after chainplate on both sides so there's a forward pull on the boom when the mainsail is eased all the way. If the pull is only down, the boom will whip around as the boat rolls. Full-length battens accelerate the swing because their weight aloft adds to the sail's (and therefore the boom's) momentum.
When tensioned, the combination preventer holds the boom both out (for safety) and down (for better sail shape on a reach and run and to keep the mainsail from chafing on the leeward shrouds). Even if you have a permanently rigged centerline vang, the combination preventer is the best choice for normal cruising because it's very easy to set up and adjust on both sides.
The other preventer is the traditional offshore type. It provides no downward pull but is led all the way from the end of the boom to a block on the bow and then aft to the cockpit. It allows the boom to rise to keep it from dragging in a large ocean swell.
Because the line is much longer and the boom-end fitting is not easily accessible, this preventer takes more time and agility to set up than the combination type. It's often carried on boats making long trade-wind passages.
Port and starboard preventers should be rigged so that when the boat changes tacks, as one preventer is eased the other can be quickly and easily tensioned. A preventer isn't any good if it can't be adjusted without sending someone on deck, in harm's way of the boom. Keep the system simple with a single part (instead of a block and tackle) led to a winch, a rope clutch, or an easily operated cleat in the cockpit as near as possible to the mainsheet's cleat. When you ease the sheet, you tighten the preventer.
An alternative to the preventer is special gear that, instead of holding the boom out, slows its swing. One is the Boom Brake, a block that hangs under the boom and through which passes a line from the two rails. The block can be adjusted to increase or decrease friction on the line, slowing or speeding the boom's swing. I've never sailed with a Boom Brake, but people who have say it offers good protection against an accidental jibe.
Books on heavy weather sailing
Anybody who's serious about cruising should have a consult with a library of books on heavy weather and storm sailing. These volumes tend to fall into two categories, instructional manuals and collections of narratives.
In the former category are Lin and Larry Pardey's Storm Tactics Handbook and sections in general sailing manuals like my Annapolis Book of Seamanship.
Many advisories laid out there are drawn in part from books in the second category, with their many detailed, instructive stories of boats in storms. Look, for example, at Adlard Coles' and Peter Bruce's Heavy Weather Sailing, Tony Farrington's Rescue in the Pacific and my Fastnet, Force 10. Rob Mundle's forthcoming book on the recent Sydney-Hobart race disaster will surely add to this library.
Another valuable book that many readers may not know about, because it's privately published, is Victor Shane's Drag Device Data Book, now in its fourth edition, published last summer. A collection of 120 case histories of monohulls and multihulls in hard chances and other rough conditions, it covers a broad range of problems and solutions and offers a long list of practical lessons-learned about boat and crew preparation, storm strategy, and storm tactics for a wide variety of boats and equipment.
Shane is particularly interested in the use of drag devices. These are sea anchors and drogues. They either hold the boat's bow into the wind (the former) and slow her when running before a blow (the latter). Not only do most the accounts here involve one or the other, but there are three chapters describing and evaluating the types of drag devices available. Although Shane manufactures a line of sea anchors, he is not the sort of person (seen all too often in the safety business) who warns that you face certain death if you use a competitor's gear.
Drag Device Data Book is available for $36.95, plus shipping, from Para-Anchors International, P.O. Box 19, Summerland, CA 93067; www.dddb.com, (800) 350-7070
Boom Brake for a 40-footer costs about $300. It's available from Dutchman/MVB Inc., 160 Water St., South Norwalk, CT 06854; (203) 838-0375.