Sprit Booms


Sprit Booms
by Chris Wentz
(Excerpted from "Small Boat Journal" - May, 1984)

Basically undesirable things, booms spent some time in the sailing public's eye before that public adopted them for general use. No doubt even more time will pass before we remove them from applications that do not need them. And in spite of what history has taught us, we do not entirely understand booms.

A boom is supposed to be a great help when you sail a boat off the wind. To some extent, it is, but unless you have a means of controlling the way the boom hikes up when the boat runs before a breeze, it is not so helpful. When the foot of the sail lifts, the entire sail twists. Those nervous rolls to windward that keep your heart in your mouth as you sail downwind in a breeze are the result.

click to enlarge image
The correct way to lace the luff is shown. Don't combine this lace line with the head or tack lashing.

This type of snotter attachment to the mast combines a dead end and a fairlead. It's tapered so the lace line will pass over when the sail is hoisted or lowered.

A jaw in the forward end of the sprit is simple & good for smaller sails, say up to 100 sq. ft. above that blocks rather than dead eyes will help keep things easy.

A thumb cleat is good for the tack lashing. Two half hitches keeps things secure. The snotter can pass thru a bulls eye and go on to a "V" jam cleat. No moving parts herel

Twist, in this context, simply means that the top and bottom of the sail are trimmed to a very different extent because you have little choice but to trim the major part, or bottom, of the sail and let the top twist off. This overeased section causes rolling. To test this out, next time your boat sets up a rhythmic roll or rolls upon acceleration in a puff, rapidly trim the mainsheet and she'll settle right down. She'll also slow down because most of the sail area is over-trimmed.

A boom vang — or saltier and more correctly, a boom jack or kicking strap — is one solution to the problem of twist. The vang simply pulls down on the boom when it is eased too far outboard for the sheet to perform this function. Pulling downward sets the leech of the sail and allows you to choose the amount of twist that you want.

If she's a sloop, the jib leech must clear the sprit. The mainsheet should lead more or less straight down. If there is a conflict with the tiller, a rope 'horse' can be used. First reef: strike the jib. Second reef: shorten the main

I'm not suggesting that you put a boom vang on your traditional boat. It might look out of place, and traditionally proportioned spars are not shaped to withstand a bending strain in the middle. There is, however, one terrific way to get the advantages of a boom and avoid the pitfalls — equip your boat with a sprit boom

Scarce and generally misunderstood, the sprit boom is the closest thing to a panacea for a boomed rig. It requires no gooseneck, no outhaul to control the shape of the sail, and because it is only under compression, it can be much lighter than a conventional boom. The sprit boom is both boom and vang; the portion of the sail below the sprit prevents the sail from hiking up when you ease the sheet. The result is an untwisted sail on the wind or off. Forcing the sprit aft not only flattens the sail, it tensions the foot and the leech. Therefore, the biggest job of the mainsheet, that of tensioning the leech, is removed. The sheet, then, has only to trim the boom athwartships. Relieving the burden allows you to play the mainsheet in a breeze without help from a multi-part purchase.

Finish the clew with a becket. A strong seizing closes the loop. Any reef should be finished the same way.

The end of the sprit should be whittled down so there is a 1/4" shoulder
This type of sprit to sail attachment is simple, reliable, and causes very little distortion in the sail.

The modern variant of the sprit boom is the wishbone boom. It performs the same tasks as does the sprit boom, but it interferes with the sail a lot less. It might be the ultimate boom if it didn't take up so much space in the boat when the rig is struck. You must place the wishbone before you step the mast, and it's very much in the way when you go forward to raise your stick.

If the clew of your sail is so far aft that sheeting is a problem, a sprit boom may be the answer. It needs no horse (traveler) and may be the only arrangement for a double-ender. A block right on the center line is all you need. You can place it on the rudderhead.

I can't claim to be open-minded about sprit booms; I'm sold on them.