[JChorlton]

Yes, it's another one of those backstay rigging questions. I am trying to decide what size blocks to select for re-rigging the mizzen running backstays on my 44' Spencer Ketch. Previously, the backstays had been moved by a previous owner and oddly attached to large davits as fixed backstays. The main and mizzen are independently stayed on this boat (except for the stay that everyone erroneously calls a "triatic" which connects the mastheads) and the mizzen backstays never seem to load up much even in high winds when I prefer to use the mizzen to balance the helm. Also, the mizzen sail area is only 177 sq-ft.

In order to re-rig the running backs I performed the following calculations:


Calculation of max backstay load due to righting moment

The waterline length is 32'6" and using this, I arrived at an estimate for RM30 of approximately 40,000 ft-lbs (from Skene's and Rigger's Apprentice).

Ultimate Righting Moment of Mizzen = 40,000 * 0.5 = 20,000 ft-lbs

RMult / (1/2 Beam) = 20,000 ft-lbs / 5.5 ft = 3,636 lbs

Using 1.5 to 2 for a factor of safety for mizzen loads and assuming that the backstay will take ~25% of the load:

Max working load on mizzen backstays = 3,636 * 2 * 0.25 = 1,818 lbs


Max load capacity of existing mizzen backstay pennants

The existing pennants are 1/4" 1x19 316 S.S. wire.

The highest manufacturer's tensile strength value that I have found for 316 wire is 7,481 lbs so I will assume that is the largest breaking load that the backstays will see.

Assuming that the safe working load is: 55-65% * BL (I think this was from Rigger's Apprentice)

SWL of 1/4" 316 = 0.65 * 7,481 lbs = 4,863 lbs


So ...my question is: what load should I size blocks to - the max calulated working load of 1,818 lbs, the max working load of the 1/4" wire of 4,863 lbs or the tensile strength of the wire 7,481 lbs?

Sizing the blocks to match the breaking strength of the wire just seems a bit excessive in this situation. If everything is matched in breaking strength then you still can't predict which component will fail first.

Harken 75mm Black Magic blocks would seem to fit the numbers and Garhauer has some blocks in their 70 series that have 5,000 lb working loads. Lewmar's Ocean blocks seem pretty huge and heavy to reach a 5,000 lb working load and I can't even tell if Schaefer's blocks are suitable because the Series 12 weigh about 4 lbs each! I do want a rugged and durable rig but that much weight swinging around on a backstay pennant just seems like a real brain basher.

By the way, I'm just contemplating a simple 4:1 or 6:1 tackle. Any and all thoughts on this would be appreciated.

Cheers,
John