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Rig tension
Posted by: BCCTakayna (IP Logged)
Date: June 20, 2018 06:03AM
Category: Spars and rigging

Hi all,

Does anyone have table of sensible tension for the shrouds and stays on a BCC? I picked up my wire for re-rigging this morning and after hearing yet another horror story of local riggers over tensioning rigs I'm keen to find out what I should be aiming for.

Thanks

Jonathan

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Re: Rig tension
Posted by: John Cole (IP Logged)
Date: June 20, 2018 06:16AM
Category: Spars and rigging

I don't think there are tables for this.

The optimum rig tension is where there is no "slop" in the rigging on either tack, or on the forestay, without putting excessive tension on the chainplates.

A well tuned rig gives you better performance upwind.

There in an anomaly in the BCC hull mold which will defeat any tension gauges!

John Cole
[www.coledata.com]

Re: Rig tension
Posted by: Bil (IP Logged)
Date: June 20, 2018 12:17PM
Category: Spars and rigging


> 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


Attachments: rig tension.gif (11kB)  
Re: Rig tension
Posted by: BCCTakayna (IP Logged)
Date: June 21, 2018 03:09AM
Category: Spars and rigging

Thank you Bil,

That was exactly the information I was hoping for. Just one question. Is there a way to calculate what the tension should be if the wire is one size up? My boat is 9/32 wire throughout and I am replacing with 7mm so for example my whisker stays will be one size larger than the ones in your table. Does that mean it needs more or less tension than in your table?

Thanks again

Jonathan

Re: Rig tension
Posted by: Bil (IP Logged)
Date: June 21, 2018 08:05AM
Category: Spars and rigging

I'm no mechanical engineer or professional rigger, so I'll do everything except answer directly. If you want a different approach, one that might garner a response from a mechanical engineer, post a question to cruisersforum or sailinganarchy.

You can find big mobs of published advice from professional riggers, including Brion Toss and even from Selden Mast. Selden has a neat Hints and Advice document available freely from:

[www.seldenmast.com]

Pages 42-4 directly address the tuning of masthead cutter rigs. It also has the standard message (which I will restate and supplement in my own words):

1. the prime directive is to get the mast straight (no sideways bends, no forward bend) and then to pretension the rigging so the mast is in compression.

2. the function of pretension in the rigging is to avoid buckling the mast in a seaway, when pitching and rolling forces could whip an unstayed mast around.

3. the best test of acceptable rig tune is sailing on the wind in relatively smooth water conditions that generate heel of 20 degrees: the rig is acceptably tuned if the mast remains in column and the leeward cap shrouds are not slack (the test being that when plucked, the shroud responds with something like a musical note rather than a rattle) until at least that 20 degree of heel has been reached. The Selden document also suggests reefing and again checking that the mast is in column and any mast bend is smooth and positive (no forward bend).

4. the idea of the table of standard tensions, and carrying a tension gauge aboard, is that so you can (a) get back to standard tension quickly if you have temporarily disconnected a stay or shroud; and (b) check the tension in new rigging over time. So I recommend almost everyone buys an inexpensive tension gauge (such as a Loos gauge) or becomes familiar with the folding rule technique and marks the stays and shrouds for easy reference.

5. tension in the intermediate and lower shrouds maintains the mast in column (laterally straight) when heeled to wind pressure and also when reefed. So the standard tensions in the table are just a guide for repeatability, not a recipe for getting to correct tension.

Page 32 has an important note at item 8, about the permanent deformation excess rig tension can create on a hull that is not stiff enough. The BCC hull was designed by Lyle C. Hess who also designed the rig. Brion Toss, in his The Complete Rigger's Apprentice, spends time analysing a Lyle C. Hess rig that is close to identical to that of a BCC. So a BCC hull should be stiff enough.

If you're using the equivalent of 9/32 wire all around, the big differences to the wire used by Tom Harrer and I are in the cap shrouds, intermediate shrouds, and fore & aft lower shrouds. That's where Tom and I had/have 1/4" diameter wire rope.

Looking at the specs for 316 1x19 wire, the difference between 1/4 and 9/32 is tiny. Standard breaking strength figures are (as in that table) 6900 lbs for 1/4 wire and 8700 lbs for 9/32.

"Tiny" is relative of course. Quite a few BCCs in N America were rigged with 302 wire, because their owners intended them only to sail in cool temperate water, not tropical or subtropical water where the increased corrosion resistance of 316 is an advantage. The standard breaking strengths of 302 wire are 8200 lb for 1/4 and 10300 lb for 9/32. So when I write 'tiny' I mean that the difference between 316 1/4 wire and 9/32 wire is smaller than the difference between 316 wire and 302 wire, to the point that the standard breaking strength of 302 1/4 is close to that of 316 9/32 wire.

Back to that Selden Mast document (because I did not write an essay plan for this answer), note:

* p. 31 has a table on 1x19 wire in mm; and

* p. 32 explains the folding rule technique for measuring rig tension, a technique you can use to get reliably and repeatably to tension in terms of % of breaking strength without a tension gauge.





BCC 116 Zygote,
Scarborough Marina, Moreton Bay, Queensland, Australia



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