A boom vang is a useful control for your mainsail, especially if the mainsail is older and acting its age. That is, if it’s getting baggy and is difficult to flatten, particularly when you’re pointed off the wind. For better performance and safety, you need to be able to pull excess twist out of the mainsail and flatten the leech. The best way to do that is with a boom vang. It has the added benefit of preventing the end of the boom from raising so high during gybes that it can snag the backstay, a potentially dangerous situation if the wind is strong enough.

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How a boom vang improves mainsail shape

A boom vang is a tackle system attached at a 45° angle between the mast and the boom. Its purpose is to pull the aft end of the boom down when the mainsail is fuller and more twisted than desired. It can be used on any point of sail but is most useful on a reach. For this reason, the lower end of the vang is typically attached near the mast tabernacle so that the vang can rotate with the boom and keep equal tension on any point of sail.

Tightening the vang flattens the mainsail and provides more balance so that the headsail can be trimmed to make more forward power. Easing the vang allows the top of the mainsail to twist and spill air to avoid overpowering the rig. This technique is called vang sheeting and is an effective way to control heeling without adjusting the main sheet.

DIY materials list

You can spend hundreds of dollars on a pre-assembled kit or you purchase standard parts à la carte and save a lot.

What you’ll need:

• Two fiddle blocks, one (upper) with a becket and cam cleat. The other (lower) can be standard.
• Jaw snap shackles to attach to the ends of both fiddle blocks. These make the vang quick to attach and detach.
• Garhauer BT-2 boom tang or equivalent screwed to the boom
• Garhauer BB-2 boom bail or equivalent held by the mast step bolt. If your sailboat has a mast step plate with a vang loop built in, you can use that instead.
• 16′ x 5/16″ New England Ropes Sta Set, tied or eye spliced to the fiddle block becket, 30′ if you want to lead the line through a deck organizer with other lines from the mast.

Installation instructions

To install a boom vang on a C-22 or similar sailboat:

1. The vang should be installed at a 45 degree angle between the mast and boom. Measure the height of the top of the boom above the cabin roof with the mainsail at full hoist. For a Catalina 22, it should be 39″.

2. Measure the same distance along the bottom side of the boom from the aft side of the mast and make a pencil mark. This is where you will attach the boom tang.

3. Attach the boom tang so that it is angled toward the base of the mast as shown below.  If the boom tang has straight legs, carefully bend them to fit the combination of the boom curvature and the tang angle. When the fit is good, drill 5/32″ pilot holes and use #10 x 3/4″ pan head stainless steel tapping screws. Leave enough space between the tang and the boom to connect a snap shackle as shown in the following picture.

4. If your sailboat does not have a mast step plate with a vang loop built in, remove the mast step bolt, align the boom bail holes with the mast holes, and replace the mast step bolt as shown in the following picture.

5. Attach the snap shackles to the fiddle blocks as shown in the preceding pictures.

6. Tie or splice one end of the line to the becket on the upper fiddle block.

7. Reave the line through the fiddle blocks and the cam cleat.

8. Use the snap shackles to connect the lower fiddle block to the bail and the upper fiddle block to the boom tang.

9. Test for smooth operation of the vang and that the boom rotates freely.

You can rig this boom vang in one of two ways. In its simplest form, the fiddle block with the cam cleat and the standing end of the line can hang over the companionway hatch and doesn’t need to be led farther aft. You or your crew can just reach up and trim it as needed.

You can also rig it with the line long enough to lead it aft with other running rigging. For that, lead the standing end outside of the cam cleat, down through a turning block at the mast base, over the cabin roof, through a deck organizer, then aft to a cleat over the bulkhead. The advantage of this setup is that it’s easier to trim on a reach when the boom is swung forward. It’s also easier if you have a bimini that is in the way of reaching the cam cleat.

The longer line also makes the vang more versatile as a utility tackle system. With its 4:1 mechanical advantage and quick release shackles, you can easily detach it and move it to wherever you need some heavy lifting or pulling power such as hauling up a crew member that’s fallen overboard.

To get full control of your mainsail, also see Flatten Your Mainsail Foot with an Outhaul and Control Mainsail Draft with a Boom Downhaul.  All three controls work great in combination with each other to flatten the mainsail in medium to heavy winds.

For the complete collection of rigging projects like this one, purchase my ebook Do-It-Yourself Small Sailboat Rigging.

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