Winter Boat Project: Boom Re-rig

We had a number of boat projects on the “to-do” list this winter as we hunkered down at the dock, and I’m going to try and put together some blog posts to cover them as we often get asked for details on these things from other sailors. Some of these were inside the boat and were ones we could take care of regardless of the weather, and some outside once the weather started to dry out and warm up.

A high priority in the category of outside jobs was a re-rigging of our boom. Three lines run inside the boom on our boat: the outhaul, and two reef lines. I also wanted to modify the way the boomvang was rigged.

For non-sailors reading the blog, the outhaul is a line which pulls the bottom corner of the sail (called the “clew”) towards the end of the boom. The reef lines are used to reduce the size of the mainsail when it’s very windy. And the boomvang is a setup which runs diagonally down from the middle of the boom to the base of the mast. The boomvang (sometimes also called a “kicker”) is used to hold the boom down because the wind pressure on the triangular mainsail would otherwise lift it up. On our boat the boomvang also has a spring-loaded strut which holds the boom up when the mainsail is down.

One of the things we noticed last summer sailing was that there was a lot more friction in the outhaul and reefing lines than there should be. Nothing outside the boom seemed wrong, so the first part of the job was to take the boom off the mast, remove the end caps and have a look inside where the lines run.

Taking off the end caps revealed two things. 1) the outhaul was rigged with a pair of blocks in the boom that provided 4:1 leverage, and 2) the remains of a decent-sized bird’s nest gumming up the works. That explained some of the friction issues! Cleaning out the boom was easy enough.


The blocks we discovered in the boom for the outhaul weren’t great quality, and with two cars for the reefing system already needing to run in side the boom (see below), it struck me as a bad idea to keep the outhaul blocks in the boom. Not only the concern about tangling with the reef cars, but also the fact things inside the boom are not very serviceable. It was easy enough to recreate the 4:1 leverage outside the boom… 2:1 at the end of the boom, and another 2:1 at the mast. I also replaced the outhaul line running inside the boom with a piece of HMPE (aka dyneema) line… strong and slippery to reduce friction.

Reefing Lines

Our boat has the ISOMAT single-line reefing system. It’s a bit complicated to explain, but looks like this:

Two cars inside the boom (5) enable a single line (“Reef 1” or “Reef 2”) to pull down on the sail at the end of the boom and at the mast, at the same time. Properly rigged, it actually works fairly well and makes it reasonably easy for a single person to take in or shake out a reef in a controlled way. But there are a lot of turns in the system, so reducing friction is important. We pulled out the two cars and gave them a good cleaning an lube. Instead of running the reef line through the cringles on the leech of the sail (the rings on the back edge), we added low-friction rings, and swapped that part of the reef lines for new HMPE.

New reef lines and low friction rings on leech grommets


The original rigging for the boomvang had a block and tackle to provide leverage. At the base of the block and tack (at the mast), the bottom block was connected with a snap shackle, the idea being that this could be unclipped and the block and tackle base moved, clipping into a padeye on the toe rail to act as a preventer. Mid-boom preventers aren’t ideal in a big blow (we’d rig something from the boom end instead), but it is a convenient way to quickly hold the boom in place at other times (for example: sailing in light winds and bigger swell, or when in a bumpy anchorage).

The problem with the original rigging, however, was that this preventer setup pulled sideways on the plastic sheave (pulley) built into the boomvang strut near the boom. At some point in the past on our boat, this had resulted in damage to that sheave. I’d purchased a replacement, but also decided to redesign the rigging to avoid this sideways pull and prevent the damage from happening again. Reworking the system with some spliced HMPE and low friction rings, I eliminated the snap shackle and moved the detachable end up to a block (on a swivel) at the boom. Now when we want to use this as a preventer we just undo the soft shackle on the boom and lead it down to the padeye on the toe rail.

Updated boomvang rigging. Soft shackle on the boom (top left in image) can be undone and relocated to a padeye on the bulwark for use as a basic preventer.