Control Your Mainsail Shape Better With a Boom Vang

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.

Before I continue, a bit of legal housekeeping. This post contains affiliate links. That means I receive a small commission if you make a purchase using those links. Those commissions help to pay the costs associated with running this site so that it stays free for everyone to enjoy. For a complete explanation of why I’m telling you this and how you can support this blog without paying more, please read my full disclosure.

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.

Upper fiddle block attached to the boom tang
Upper fiddle block attached to the boom tang

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.

Lower fiddle block attached to the mast bail
Lower fiddle block attached to the mast bail

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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16 Comments Add yours

  1. Orion says:

    You may find it more convienent to reverse the way you have your vang mounted in the pictures. By placing the cam at the mast base you can adjust vang tension at any boom position. Also, this would eliminate the need for a turning block if you do decide to run the line aft.

    1. kbilling says:

      That’s a good point that I forgot about when I wrote this one. Thanks for noticing!

  2. John says:

    I’ve only read a couple of your solutions so fa, but I plan on reading them all! I’m already wondering where you find parts so inexpensive though. You reportedly completed this whole project for the price I would have to pay for a single snap shackle.

    1. Ah, that’s what separates the ordinary sailboat owners from stingy sailors! If I had to pay normal retail prices, I couldn’t afford to own a boat. Check out The 6 best sources for sailboat parts and supplies for where I shop.

      1. John says:

        Fantastic! I will definitely be including ebay in my search from now on. Thanks so much!

  3. Garhauer says:

    Why or what advantage to using a tang on the boom instead of another bail? (Great article by the way.)

    1. Good question!

      You could theoretically use another bail but the attachment to the boom would be a challenge. The holes in the arms of the bail accept a 5/16″ fastener so that would either be oversize sheet metal screws going into the boom or if you used a through-bolt, you’d want to prevent compressing the boom cross-section. Either that or let the bail swing freely on the bolt, which is unnecessary.

      The holes in the tang accept much smaller sheet metal screws that are more appropriate for the thin wall of the boom extrusion and because there are four holes, the tang can be solidly attached without allowing any movement.

      The bail works ideal for the tabernacle connection since it lets the vang snap shackle slide side-to-side in an arc with the boom movement and without affecting vang tension. It fits perfectly over the tabernacle as you can se, and the mast bolt fits the bail as well. It’s also nice to be able to remove the bail for stepping the mast or to fold it out of the way.

  4. Roy Thomas says:

    Have you ever done a piece on how to string holiday lights? I looked on your site and couldn’t find one.

    1. Hello, Roy

      I haven’t written one yet, but I did string LED lights all over Summer Dance for the night-time boat parade this past 4th of July. Since she’s out of the water until spring, I won’t be doing it again for Christmas. I do need to raise the mast for another project coming up so maybe I’ll write this one too.

      Stay tuned.

    2. CaptainBob Roberts says:

      Over the years, I have participated in numerous lighted boat parades. The first one taught me not to put white “icicle” lights on the lifelines. Looking forward, I couldn’t see outside the lights. Had to position a crew member on the bowsprit for visual relays. Hang them from your tow rails along the hull.

  5. Steve Fisher says:

    Great article! Thanks!

  6. Jeff millet says:

    I learned something I will DEFINITELY incorporate in my sailing. I have an ’86 C22, and will be utilizing your advice for several projects!

  7. Stingy, Thanks again for all the great articles!! What diameter mast step bolt do you have? I wanted to make sure I ordered the right Boom Bail and would like to configure like yours using the mast step bolt.

    1. Hi, Kevin

      The C-22 mast and mast step accept a 5/16″ diameter bolt but I prefer a 1/4″. It’s easier to insert through the 6 holes of the bail, step, and mast while stepping the mast. It only acts as a pivot pin and isn’t subjected to much force at all so the reduced size isn’t a strength concern. But a 5/16″ bolt would work too.

      1. Thank you and see how 1/4″ would make things easier to insert. Does the BB-2 accept a 5/16?

      2. Yes, so you can use either size.

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