Control Mainsail Draft with a Boom Downhaul

A boom downhaul is one of the three possible control lines for the three sides of the mainsail. The other two are the boom vang (controls leech tension) and the mainsail outhaul (controls foot tension). Catalina 22 and similar sailboats have a  short length of line attached to the bottom of the gooseneck car on the boom that ties off to a cleat at the base of the mast as shown below.  It simply keeps the boom from sliding up past a certain point on the mast after you hoist the mainsail. It gets the job done but it isn’t easy to adjust, especially while under sail.

Some people call this simple arrangement a boom downhaul, which I suppose is technically correct, but I call it a tie-down since it isn’t as easy to trim as what I’m going to describe and it would be confusing to call them both the same thing.

Most cruising sailboat owners don’t adjust their tie-down often (or ever) because it’s too much trouble to go forward and adjust for the relatively small improvement it makes. Most racers adjust it whenever wind conditions change.

BEFORE – A humble boom tie-down between the gooseneck car and a mast mounted cleat

This project is a true boom downhaul that makes it so easy to trim from the cockpit, even while under sail, that it can become a regular part of trimming the mainsail.

A boom downhaul works like a Cunningham by tightening the luff of the mainsail to flatten it and to move the belly of the mainsail forward, which improves its aerodynamics and reduces heeling. It helps most in moderate to heavy winds and can make a significant difference in the mainsail shape.

A Cunningham works by connecting a hook on one end of a block and tackle to a grommet above the tack of the mainsail. Then when you harden the Cunningham, it pulls the luff flat while at the same time creating a wrinkle in the foot the mainsail. The boom stays stationary on top of a stop in the mast slot. A boom downhaul is simpler than a Cunningham

It may help to think of a boom downhaul as simply a boom tie-down line led aft to the cockpit. The advantages of a downhaul over a tie-down is that you can always trim it regardless of how high the mainsail is hoisted and it doesn’t make a wrinkle in the foot of the mainsail that disturbs air flow.

A boom downhaul is easy to set up if you have an empty turning block at your mast step, an open sheave in a deck organizer, and an unused cleat at the bulkhead. Just replace your existing boom tie-down line with enough line to lead it through the turning block and deck organizer to the cockpit like shown below.

AFTER – Simple 1:1 boom downhaul

I prefer to use 1/4″ New England Ropes Sta Set with a spliced eye in the dead end to receive a halyard shackle. Instead of tying the line to the gooseneck car with a bowline knot, the shackle is faster and easier to detach. This way, you can leave the downhaul line in place and just disconnect it from the gooseneck car if you need to remove the boom, such as to unstep the mast for trailering.

Rigged this way, the boom downhaul doesn’t offer any mechanical advantage over a boom tie-down. In fact, it has the disadvantage of the friction added by the turning block and the deck organizer. But it usually doesn’t take a lot of force to tighten it fully anyway.

To make it even easier to adjust, you can rig it with a block attached to the gooseneck car instead. Start the line at a becket on the turning block at the mast step or attach it to the mast step plate as in the picture at left. Reave the line up through the gooseneck car block, down through the turning block, and then lead it aft to a bulkhead cleat. That gives the downhaul a 2:1 mechanical advantage. If necessary for heavy wind sailing, you could increase the mechanical advantage even more by using double blocks in both positions.

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8 Comments Add yours

  1. Greg says:

    I’m looking to rig mine from the halyard plate, to the block attached to the gooseneck car, to the cockpit. Any idea the length of line needed? Also, did you use a simple shackle to attach it to the halyard plate?

    1. Hi, Greg

      I used 12′ of 5/16″ New England Ropes Sta Set. You can find the lengths necessary to lead all of the running rigging lines to the cockpit listed in Lead All Lines to the Cockpit for Safer Sailing.

      It’s hard to see in the picture but yes, I use a regular shackle to attach the dead end to one of the holes in the halyard plate. If you have a bail attached to the base of your mast for a boom vang like you can see in this post, another option for attaching the downhaul turning block is to shackle it to the bail also. That frees up a hole in the halyard plate for something else. You can see that in the third picture in Lead All Lines to the Cockpit for Safer Sailing.

  2. This cat has all sorts of really useful sail rigging ideas! I am well pleased and actually impressed.

    1. Thanks for the compliment.

  3. Ed in Gig Harbor says:

    On the last option, could you tie off the line to the freed up cleat, then up to the block and on to the cockpit? I seem to be missing the last image showing the 2:1 setup…

    Thanks for the explanation of how the three control lines impact the sail shape…now I just need to try them out (along with all the other great rigging suggestions on your site)!

    1. Hi, Ed

      Yes, you could start it there but it might interfere with a boom stop if you have one in between the cleat and the boom. That’s why I led it over the opposite side of my mast step plate. The 2:1 picture is the main picture of the post on the left. The top and bottom are cut off a bit, sorry.

  4. Hi Stingy,
    Your boom downhaul is a nice simple way to adjust luff tension.
    I’m curious if the boom downhaul offers any sail shaping advantages over a fixed boom and using the main halyard tension to adjust luff tension? Perhaps the friction of the sail slides or bolt rope gives an advantage to tensioning the luff separately from the head and tack.

    Thanks in advance for your reply. Really enjoy your site!

    1. Hi, Joe

      They both basically achieve the same thing but a downhaul is a lot easier to adjust, especially if you’re single-handed or short-handed. On my boat, to get it any harder than when I originally set it through a cam cleat, I would have to leave the helm and “sweat” the line from the cabin roof or lead the halyard to an unused primary winch and crank it tight. But if I had fully hoisted the mainsail to begin with, there wouldn’t be any more height to be gained anyway and I could easily jam the end of the halyard in the masthead.
      For that reason, if you stick with a boom tie-down, set it so that it stops your mainsail head board a couple inches below maximum hoist. Then you will at least have the space to harden it if you need to.

      With a 2:1 downhaul, I just reach over to the bulkhead and haul the line a couple inches and voila. Likewise, if I then turn downwind and want to ease the luff, I can do it just as easily.

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