I use a boom brake AND a tradional boom-end preventer.
A preventer is great, but you really don't want it to be the only thing restraining the boom when the poo hits the proverbial...
A preventer's principal role is to keep the boom (and mainsail) from flogging when running in moderate conditions, especially when the main is set by the lee. Under these conditions, the rolling of the boat can make the boom swing back far enough to backwind the main and start off an accidental gybe.
Regardless of how you set it up, during a planned gybe the preventer has to be let off and re-set on the lee side. SO, this is NOT a part of the running rigging that you can (or should) rely on when the going gets rough or you need to do things quickly. This was graphically illustrated to me recently when crewing on a fifty footer in the Sydney - Gold Coast race: with wind gusting to 45 knots we had to gybe, and opted for a "granny gybe" (tacking the bow through the wind) to reduce the strain on the rig. The crew responsible for the preventer was a bit slow throwing it off, resulting in a pad-eye ripped off the bow and a broken preventer line which proceeded to wrap itself around the prop... Thank gawd we didn't attempt a gybe!
Also, if you really get the boat out of shape and the main is backwinded, you need to release the preventer pretty smartly, and if you can't, it needs to be fail-safe - i.e the line should break at a lowish force rather than a high one (this is where the boom brake comes in). Imagine the 2 tonnes of force in a 1/2in preventer being released instantaneously when it breaks! I use 8mm polyester braid (stretchy) for my preventer and it runs all the way to the stem, through a snatchblock and back to a jammer on the cabin top where I can reach it from the cockpit and put it on a winch to set and (most importantly) ease off gradually when necessary.
A boom brake has a number advantages:
- The forces on the boom are controlled by friction, so you can never over-stress the boom with a boom brake.
- Depending on how you set it up, it can assist your boom vang by pulling down on the boom, or can even be an emergency main-sheet.
My boom brake is very simple - a figure 8 "rescue descender" used for rock climbing which i got off EBay for $45 and 20ft of 1/2in polyester braid. The line is shackled to the starboard toe-rail and runs through the figure 8 to the other toerail, through a block shackled to the toe rail and then back up to the cabin top jammers via a free sheave in the line organiser. I use a winch to grind it on. Because of the set-up, it effectively has a 2:1 purchase.
There's a photo of the boom brake on my blog at svsunnyspels.com.
The boom brake does create a bit of an obstacle on the side decks, but you get used to it pretty quickly. I take the point re. the jacklines. I run my jacklines OVER the boombrake lines, which also keeps the tether hooks off the deck, at least over that area. However, having an intermediate "catch point" in your jacklines is not such a bad thing - if you get washed along the deck you'll get stopped midway rather than dangling over the transom!