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.

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.

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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