I'm just about to change my main sheet from a 6:1 to 7:1 and need a triple plus becket at the top block. Has anyone any idea ( or calculated ) of what loadings ie how many kilos are actually at the block as the specs differ widely between sheeve size and manufacturers.

Many manufacturers also quote for example a 650kg working load maximum on their single blocks and yet the same 650kg applies to their triple block, any clues as to how they work out the max loadings. <img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" />

Max working load is probably more a function of the bearings than the strength of the block as a whole. The balls would squish and jam up or break before the block explodes.

As for working out the loads, multiply how much you can physically lift with the rope by the purchase you intend to use and that will give you a reasonable estimate of the applicable loads that are possible.

eg. 30kg x 7 = 210kg, a 650kg working load has a factor of safety of 650/210 = 3.1, this should be enough for unexpected peak loads.

The breaking load is defined by the weakest link. Often this is a shaft or shackle and as such you don't get much variation with number of sheaves for a given size block. Max working load will be a percentage of this based upon a safety factor and as Ncik mentions expectations of non-destructive (elastic) deformation.

The only problem with Ncik's calc is it doesn't take into account the extra holding power provided by the ratchet and/or cleat. I.e. when you're in conditions where you couldn't pull the sheet on but because of technology you can hold it. Having said that.... you wont see anywhere near 650kg on your mainsheet, tornados don't get loads that high.

I seem to remember reports where the typical Tornado max leech tension is given as 450 kg.

As a result I don't see F16 loads go 350 kg.

Wouter