Glenn Ashmore wrote:
How much safety margin do you really need when choosing blocks?


For example Rutu is designed to take it first reef at about 20 knots so I
figured sheet loads at 25 knots. At 25 knots the main sheet load will be
about 2,750 lb.

Glenn-
As I understand it, the 'force' required to restrain the end of your
boom at 25kt is 2750#. You have a 3-part tackle between your boom and
the traveller, and each part will experience a force of 900# or
thereabouts.
The total force on each block (not sheave) shouldn't be much more than
3000# or so.
If you're worried about 'airblocks' blowing up, you can always put a
swaged cable through the 'hole' in the block as a backup (as the
Maxiboats seem to do).

Also, the sheet loads generated by the various formulas for a given
sail area tend to assume that the boat is upright. Unless you have a
huge ballast ratio or are sailing in concrete, the 40kt gust mentioned
will cause your boat to heel and spill air from the sails..The force on
the sheet will not necessarily increase as much as one might suppose.

Your sailmaker could be a useful resource for questions like this as
well. Of course, finding a similar-sized boat that's been offshore and
having a chat with the owner about gear choices can be educational.

Having said all this, bigger blocks are certainly preferred. Simce you
seem to have quite a few 'creature comforts' on your boat, you might
find that heavier,'non-racing' style blocks (eg Garhauer) might provide
the safety margins you'd like at less cost.

Cheers
John

sv Manali- (29-ft Joel White Double Ender)