Pokey,
Your design looks real fine (and you diagram is really helpful to visualize it), but after looking at it for a couple of days now, I have a question about it. In one contiguous system like your mainsheet, why would you use a 40 mm Block (max working load 970) and then use a 75 mm block (mwl 2426)? It seems like the 75 is so oversize for the system. The 40 will fail so much sooner that it isn't even close. I can understand that you want a bigger line for the gross control which then necessitates a somewhat larger block, but that difference in size seems extreme to me. Any comments on that disparity in size & strength?
Kieth,
The size of the blocks relate to their position in the cascade system.
Note that the 75 mm block at the boom takes 100% the load when the boom lifts. The load at the fiddle is slightly less (75%) because the double block at the top of the fine trim takes 25%. That 25% is further divided in quarters by the three blocks on the cockpit sole. I think the center block holds 50% of the 25% and the other two blocks each hold 25% of the 25%.
If my math is correct (a highly suspect supposition), that's how you get to the 16:1 purchase at the fine trim.
That's not to say that the 57 mm blocks are rated at 75% of the 75 mm blocks and that the 40 mms rate 50% of the 57 mms. But I think they’re close enough.
By the way, I use twelve strand 3/8” Samson Apex for the gross and ¼” New England Ropes T100 for the fine trim. I love the Apex. This is the third boat I’ve rigged with an Apex mainsheet. Unfortunately, it’s not widely available because it’s been replaced in the product line by Control-DPX, which I’ve never seen.
