Courtney:

Well, I can give you an example of what is too small.

Sailing wing on wing with one reef in the main and the genoa rolled up a bit
in 25-30 kts of wind with 8' seas of the stern in a Saga 43, the preventer
was challenged by a big sea and let go. It was 5/15" dacron reeved with two
parts from the aft of the boom to the toe rail. The block on the boom was a
light weight component, probably good for only 3/8" line or smaller and it
let go and allowed an uncontrolled jibe.

We then rigged a preventer from the aft of the boom direct to the bow
fitting with one part of 5/16" dacron. Another heavy sea started a jibe that
stretched the dacron by several feet and allowed a somewhat controlled-
uncontrolled jibe. We subsequently saw that the stresses had destroyed the
roller wheel on the bow chock.

So, the point is that preventers need to be stout, but maybe not so stout.
At some point of wind and sail you are going to break the boom, particularly
if the preventer is only rigged to one point.

Probably the best scheme would be to rig the preventer from the toe rail to
the boom in multiple parts at two points- midboom and at the end. With this
rig you have several parts so you can use reasonable sized line and blocks
and the stress on the boom is spread out along its length.

Any others have thoughts?

David