Backstay adjuster for a beneteau 311

Jackdaw · Oct 27, 2015

A boat that size will need at least 16:1 purchase to be effective. That means a 8:1 block set with a 2:1 ring to make the 16:1.

You boat has one hard point on the transom for a backstay. You COULD make the 2:1 work off the same point, or you could add two hard points wider on the transom, and use the middle for the lower 8:1 block. That would be better. In any case, you would shorten your existing stay, and add a stout block to the end. A loop of Dyneema up and over makes the 2:1. Them comes the 8:1. The image below shows a 16:1 made from 4:2:2:1, not 8:2:1. You could do it either way.

You only trick will be where to adjust it; our First 36.7 uses a remote adjuster. The cleat is not on the block, but by the mainsheet lead by a series of turning blocks. A bit of work to set up, but much nicer. For your needs a simple cleat on the block should work fine.

Jackdaw · Oct 27, 2015

CapitalistSailor said:
Jackdaw,

Thank you for your response. I figured a block and tackle would be the simplest and most direct. Unfortunately, the backstay is attached to base of the "sugar scoop" style swim platform, and its structure is not visible. It's a few inches thick. There's no way that I can see to get inside to attach nuts and backing plates. But maybe something was set up in advance during construction.

How did you determine that 16:1 would be sufficient purchase? The breaking strength of 3/16" wire is about 4000 pounds, so I probably won't want to put more than 800 pounds of tension on the backstay (20% of breaking strength), and maybe not more than 600). Not counting friction, a 16:1 purchase would mean I would have to pull 38 pounds to get 600 pounds of tension, which is probably as much as I would want to pull.

Maybe a set of blocks for a boom vang would work. These setups often have an offset as part of a single block and tackle arrangement.

Because 8:1 would be way too hard?? ;^)

Maybe 12:1, but I bet you and crew will be much happier with 16:1. Now with some care, you can make a cascaded system from one lower point. Choosing small hardware helps (like low friction rings in place of blocks), and you can also use a triangle plate at the bottom to create some separation.

Like the new Sunfast 3600 does.

Jackdaw · Oct 28, 2015

CapitalistSailor said:
Jackdaw,

The triangle plate is a good idea.

On my question of how much purchase is enough, did you suggest 16:1 from experience, or calculate it somehow? Is 16:1 enough, or maybe 24:1 (8:1 with 3:1)?

I emailed Harken, but I didn't get a very thoughtful answer. They suggest a hydraulic tensioner, which would be $1500, overkill for what I need.

Mostly from experience, with a quick calculation like the one you did as a sanity check. Too little or too much will work, but its either harder and you have to deal with too much line. Its also what you can make with with the hardware you choose or have available. 24:1 would work great, but probably as 6:2:2:1

Also factor how much you will adjust it. Skip the hydraulics.

Jackdaw · Oct 28, 2015

CapitalistSailor said:
Does anyone have a comment on the type of adjuster shown in the attached picture? It is a turnbuckle with handles that close to lock the turnbuckle, and open to allow hand adjustment. The vendor says it is the right size for this application.

The pins may not fit the swages I have. I'll have to check that, and maybe add a toggle.

No enough range. And way to slow/hard to adjust.

Unlike shrouds that get adjusted +/- 5 turns across the entire wind range, you pull the mast tip back several inches to effect trim. And on a fractional boat you do it a lot. You'd be turning that for hours even if the range was enough. On masthead boat you used to see a big wheel that was rotated on a long screw, long enough to have effect. You don't see those anymore, and never on fractional boats.

Rick D · Oct 28, 2015

Allow me to butt in here with a question. On previous boats with standard 90 degree spreaders, I added backstay adjusters. However, with my fractionally-rigged Hunter Legend 40.5 with swept back spreaders (but not a B&R), I was told by my rigger that it wasn't a good idea since adding tension would have the effect of loosening the shrouds. Thinking about the geometry, that made sense to me. However, a friend who raced his 35.5 very successfully with the same rig used one regularly. Comment?

Jackdaw · Oct 28, 2015

Rick D said:
Allow me to butt in here with a question. On previous boats with standard 90 degree spreaders, I added backstay adjusters. However, with my fractionally-rigged Hunter Legend 40.5 with swept back spreaders (but not a B&R), I was told by my rigger that it wasn't a good idea since adding tension would have the effect of loosening the shrouds. Thinking about the geometry, that made sense to me. However, a friend who raced his 35.5 very successfully with the same rig used one regularly. Comment?

You are correct, on a fractional boat tightening the backstay:
Tightens the forestay
Bends that mast
Loosens the shrouds

The amount that is loosened can be known and factored into any rigging setup. On our boats, when we know we will be sailing in big breeze, we tighten up the shrouds to the max; 24% of breaking strength with backstay slacked. Putting on max backstay will loosen them to 20%

For boats that use just one general setting, I'd suggest checking the shroud tension with backstay full off, and full on positions.

Backstay adjuster for a beneteau 311

.

.

.

.

.

.

.

Attachments

.

.

.

.