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

I have been accumulating blocks as I found them on eBay, Harken
closeouts and other sources and thought I had a full complement but
now I am wondering. .

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. That is 156% of the 20 knot load. The
sheet is a 2:1 "admiral's cup" layout with a single block on the
traveler, a double at the boom end lead to the mast and back to a
pair of 40's at the cockpit.

The single block on the traveler and the pair at the mast are 75mm
Airblocks rated for 5,000 lb SWL. More than I thought I needed but I
got them cheap. My problem is the double on the boom. The way I
figure it a 90 degree turn has a load factor of 141% so 3,877 lb SWL
should be enough. I have a Lewmar 80mm double with a 4,000 lb SWL
but the Harken calculator says I need a block with a 7,000 lb SWL
which costs $550!

Where do you say enough?

From personal experience it's usually the shackle that goes before the
block. Besides, shackles are cheaper! Always design a weak link into a
mainsheet set-up!