Adjustable backstay help, please? - SailNet Community
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post #1 of 11 Old 01-25-2008 Thread Starter
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Adjustable backstay help, please?

Supposedly one of the best modifications you can make on a San Juan 21. I've more or less settled on this design:



My questions regard loading of these elements and sizing of components. I figure 5/16" line on the block-and-tackle, using Ronstan Series 30 double blocks (swl 880 lbs). But I'm unsure about the upper block (dark blue): should I plump for something really burly?

Finally, the cascade line, drawn in red: Is wire rope called for, or would you recommend hi-strength cordage, like Dyneema? What diameter? My preference is for cord, just cuz I don't like the way cable frays at fittings. But I'll defer to your advice.

Some numbers: Total working sail area on this boat is under 200 sqft; the spinnaker is another 250 sqft. The shrouds are not particularly swept back, so the backstay takes the load downwind. Not sure what kind of loads that translates into.

Thanks!

Buccaneer18, Grainnia
SJ21, Diarmuid
Albin Ballad 30, Fionn
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post #2 of 11 Old 01-25-2008
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Bob, use as much dyneema as possible. Its a good thing, and its light.

All my back stay is Dyneema, from the mast top to the bottom, and I am very happy, its light and almost strech free.

Now for the arrangment, I dont think thiose tiny little things from Ronstan would do. Besides, I would not use ronstan for that...I don't like them that much, they are ok for non critical stuff.

I think the load on that boat with thoise areas is around 1000 to 2000 lbs force. For real!!!!

For the blocks this is what I think:

For the main single block, and for those sail areas, I would use a Lewmar 60mm block or a harken 57mm. With a SWL of at least 2500lbs. With a breaking load of at least double that.

For the purchase, I would use a triple block or a beefier double with a SWl of at least 2000 lbs. A Lewmar 60mm or a Harken 57mm would do.

However for the cleat I would use a triple with becket and cleat, also with a SWL of at least 2000lb, instead of cleating the line on the boat structure. Several reasons:

Less holes,
more flexible
easier to release
easier to cleat
lighter
simpler
pretty

Give me some time I will tell you some references. By the way, prepare at leat $400 to do it.

Alex

Last edited by Giulietta; 01-25-2008 at 01:03 AM.
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post #3 of 11 Old 01-25-2008
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OK

Here is what I think. You may be OK with the 40mm racing series if the winds you sail are weak, or 60mm if you sail in up to 30 knots.

The Lewmar for the single line is reference for 60mm:

SINGLE FOR BACKSTAY LINE CLICK HERE

FOR THE UPPER TRIPLE CLICK HERE

AND FOR THE LOWER TRIPLE CLICK HERE

Lewmar is very good and cheaper than Harken, the only differnce is Harken uses for the same loads, sheaves that are 2 to 3 milimiters smaller.

I think that you would be OK if instead of using the 60mm blocks you used the 40mm, but depends on the winds you sail in.

If the winds are normally weak, the 40mm series is:

Single

Triple

block with cleat

and if you want to go even cheaper and the winds are not very strong:

40 mm doubles will do. also


In a boat I had a few years back I had a system like that but used fiddle blocks from Lewmar, also in the 40mm range.

This is my favorite arrangment bellow

They were:

this one for the top one

This one for the bottom one

Last edited by Giulietta; 01-25-2008 at 01:22 AM.
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post #4 of 11 Old 01-25-2008 Thread Starter
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Wow, that strong? I figured 1/8" rigging has a breaking strength of only 2100 lbs. But you're all about overkill, for good reason. I was planning to cleat off on the cockpit floor, via the existing swivel/fairlead/cleat presently used for the mainsheet (which will be moving to a traveler soon). The advantage is easy access from either side without tiller interference.

Many people get by with straight 4:1 vangs as backstay adjusters on these boats. Others rig bizarre 3-stage cascades of 16:1, which seems silly on a 23-ft mast.

Buccaneer18, Grainnia
SJ21, Diarmuid
Albin Ballad 30, Fionn
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post #5 of 11 Old 01-25-2008
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Nahh you don't need that.

please see the post above yours as I added another better solution for you.

Just use the fiddle blocks I showed you. Unfortunately the smaller is 60mm.

But if you find 40mm should be fine.
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post #6 of 11 Old 01-25-2008
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By the way, for your info:

at 30 knots a sail area of 200sq feet has a sail wind load of around 778 Lbs.

With the Spi on its 425, so the force is 1750lbs roughly.

At 20 knots:

200sq feet = 345lbs
425sq feet = 767lbs

at 10 knots

200sq feet = 88lbs
425sq feet = 185lbs

These numbers are roughly calculated.

So 2000lbs seems appropriate, what do you think?
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post #7 of 11 Old 01-25-2008
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Bob-

Before you do anything... make sure those padeyes that you're attaching the system to are well secured to the boat. Ideally, they should be chainplates that spread the load across a section of the hull and deck, not just padeyes.

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post #8 of 11 Old 01-25-2008
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Quote:
Originally Posted by bobmcgov View Post
Many people get by with straight 4:1 vangs as backstay adjusters on these boats. Others rig bizarre 3-stage cascades of 16:1, which seems silly on a 23-ft mast.
And so can you. Go to - http://garhauermarine.com/ for choices of 4:1 vangs from $95 to $175 depending on how much overkill you want to buy. Building something piece by piece usually almost doubles the cost.

Stan
'Christy Leigh'
NC 331
Wickford/Narragansett Bay RI
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post #9 of 11 Old 01-25-2008
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Assuming your boat currently has the split backstay in place, I personally would use a configuration like this http://mauriprosailing.com/Merchant2...de=HARBACKSYS3

My C&C 30 came with this backstay configuration - what I like about it is that you can release the backstay tension simply by fully releasing the tensioning line. My concern with an adjuster that's inline in the backstay is that if someone releases it too far, the backstay may cease to provide its intended function, and SURPRISE! you have a mess.
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post #10 of 11 Old 01-25-2008
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I have a San Juan 24 with the same setup as Sailingfool's link shows and it works great. Just remember the attachment points need to be strong enough. Mine are tangs that are bolted through the transom with large backing plates and 4 Ľ” bolts each
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