Does anyone have table of sensible tension for the
shrouds and stays on a BCC?
If all goes to plan, you’ll find attached a screenshot of a table from my first version of an operating manual for Zygote. The late first owner of BCC White Wings III, Tom Harrer, and I worked together to devise a guide to standard rig tension for the more recent Sam L. Morse Co. boats.
Tom and I used Loos rig tension gauges and the rig tension number on those gauges. So if you’re using a Loos gauge, the table in this screenshot will give you a guide.
I’ll talk you through the table:
The rightmost column, column 7, is Tom’s preferred tensions. Column 6 was my preferred tension (Tom and I ended up agreeing on tension).
Column 2 is wire diameter in inches. Column 3 is approximate pin to pin length for the more recent Sam L. Morse Co. boats. Column 4 is the published breaking strength of the 1x19 wire that SLM Co. used.
I still use those figures as my tension guide today. Too much tension is of course a bad thing - within its elastic limit, wire rope will return to its original length after tension is released. But GRP does not. So if you overtension your rig, you’ll bend your boat. Of course, too little tension is not a good thing either.
If you’re not using a Loos gauge, it is possible to work from Loos gauge numbers back to pounds.force.
In the BCC Construction Manual, I think you’ll find Roger Olson’s description of how to rig and tension for the first time. From memory, Roger used qualitative descriptions (e.g. bar tight).
Back to the attached table: White Wings III and Zygote both had roller furling on the head stay, so the head stay was tensioned by the backstay. And there’s no tension figures for the bobstay or the two boomkin stays because the deal was to tension the backstay and then tension the bobstay so the bowsprit was straight. Likewise for the boomkin stays: each leg of the boomkin should be straight and not pulling up and away from the deck.
Note the whisker stay (or whisker shroud, should you prefer) tension. Tom and I agreed on that figure to guarantee stability on the bowsprit. You need to hold the tip of the bowsprit fairly steady. Apart from collision, bowsprits fail by buckling or by flexing so the glue lines between the laminae are stressed. If the glue lines get stressed, rainwater can enter and cause rot. Tension in the headstay, bobstay, and whisker shrouds puts the bowsprit in compression and holds it stable - and that’s one solution to buckling and flexing.
rig tension.gif, about 12 KB
