Basic question from a learner.
Is there any reason that I cant use one block and tackle to serve as both a vang and preventer and just move the bottom block from the mast to the rail when needed? (I have mid boom sheeting) The preventer would need more line which is messy but I dont lead them back to cockpit. Similar enough ratios? Why not?
Scosch, The short answer is "yes you can" use your vang as a preventer. This is not the ideal arrangement -- better to have a separate preventer that you can deploy when necessary -- but it is better than running without a preventer at all. Many many sailors do exactly as you describe.
In my experience, the block system that goes from the boom to the base of the mast (a lot of modern boats use a block system in tandem with a sprung telescope or hydraulic rams) to control tension of the leech is called a kicker and is not the same thing as a vang.Andre
Omatako, Not wishing to get into an etymological debate here, but I think this may be a question of regional differences in terminology and usage. In North America, the distinction between a vang and kicker is more-or-less as SailingDog describes in his post above. "Preventer" is a more general term used to describe any device that is deployed to prevent the boom from jibing unintentionally when sailing off the wind. They can resemble the same block and tackle system often employed for the vang or can be rigged in other ways such as Billyruffn described above.
My new end-of-boom-to-bow preventer uses more line and is a bit more work to set up and change over when we do jibe, but I think it works better. The end-of-boom set up allows me to get the preventer much tighter (less movement when we do jibe unintentionally).
Billyruffn, Your end-of-boom preventer is more often seen on boats sailing off-shore or making longer coastal passages. It seems to be a preferred arrangement for many experienced off-shore sailors. Personally, I have never been comfortable with this approach.
One important consideration when rigging a preventer is that while it may succeed at preventing a sudden accidental jibe, it may not be able to prevent a jibe altogether. In certain heavy wind and sea-state conditions, the sail may become backwinded and overpower the vessel's ability to correct its course. With an end-of-boom-to-bow preventer, the crew is then left with two options: (1) allow the boat to continue spinning up into the wind before releasing the preventer line (which would probably entail a very dramatic broach); or (2) if sea-state or limits on navigation do not permit option (1), the preventer must be eased and the boom allowed to jibe across.
The problem with option (2) is that following this course of action will require easing the preventer line until it bears up hard against the shrouds, placing enormous lateral loads on them. And this will occur at about the same moment that the angle of the line from the bow to the boom-end will lose most of its purchase (i.e. mechanical advantage), so the preventer will no longer be able to overcome the tremendous leverage being exerted on the boom via the backwinded sail. At that point the only option will be to release the preventer entirely and let the boom jibe violently. In addition to risking damage to the boom or the leeward rigging with the jibe, there is risk that the windward rigging, probably the spreader, will be damaged if the preventer line is permitted to bear up against the shroud prior to release.
For these reasons, I prefer the mid-boom preventer that is secured to the reinforced portion of the boom where the vang attaches, and led outboard to the point of maximum beam just aft of the shrouds. To prevent shock-loading the boom, the preventer line can be stretchy and can be secured to the boom via a shockles or similar high-strength elastic material.
As SailingDog mentioned, the best all-around solution may be to rig a sturdy break that simply permits a slow controlled jibe rather than preventing it altogether.