Control Mainsail Draft with a Boom Downhaul

A boom downhaul is the last step toward adding trimmable controls for all three sides of the mainsail. The other two are the boom vang (leech) and the mainsail outhaul (foot). A boom downhaul replaces or modifies the boom tie-down that is standard equipment on most C-22s. A boom tie-down is the short length of line attached to the bottom of the gooseneck car on the boom and tied off on the cleat at the base of the mast. It simply ties down or keeps the boom from sliding up the mast after you have hoisted the mainsail.

Some people may call this 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’ll be describing and it would be confusing to call them both the same thing. Most cruising owners probably 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. The goal of setting up the boom downhaul described here is to make it so easy to trim from the cockpit that it becomes a regular part of trimming the mainsail.

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

A boom downhaul works like a Cunningham in that it stretches the luff of the mainsail flat and moves the belly of the mainsail toward the mast, which improves its aerodynamics and reduces heel. It helps most in moderate to heavy winds and can make a significant difference in the sail shape. A Cunningham attaches by a hook to a grommet above the tack of the mainsail. It pulls the luff flat while at the same time creating a wrinkle in the foot the mainsail. The boom remains stationary on top of a stop in the mast slot.

You can do much the same thing as a Cunningham or boom downhaul by hardening your main halyard. This assumes that you haven’t hoisted it fully to the masthead sheave already. But if you have hoisted it fully already, the only alternative is to tighten the boom tie-down.

A boom downhaul is simply the 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. In its simplest form, it also has one less sheave to go through (the masthead) with its accompanying friction and far less line to stretch.

A boom downhaul couldn’t be easier to rig 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. I use 1/4″ New England Ropes Sta Set and spliced an eye in the dead end to receive a halyard shackle. Instead of tying it to the gooseneck car with a bowline knot, I connect the line to the car with the shackle. This way, I can leave the downhaul line in place and just disconnect it from the gooseneck car if I need to remove the boom, such as to unstep the mast for trailering.

AFTER – Simple 1:1 boom downhaul

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 pull and to provide finer adjustments, you can rig it with a block where attached to the gooseneck car and with a becket on the turning block or attach it to the mast step plate as in the first picture. That gives it a 2:1 advantage. If necessary for heavy wind sailing, you could increase the mechanical advantage even more using double blocks.

The Bottom Line

Suggested price: n/a
$tingy Sailor cost: $35.60
Savings: n/a

Do you have a trimmable boom downhaul or just use a tie-down and adjust your halyard?


4 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.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.