Backstay Adjustment

[Bora]

Having learnt all my sailing trade in dinghies the whole idea of a backstay for sail trim is a bit alien to me.  My 469 only has turnbuckles so not something to adjust on the fly but i'm looking to replace with a hydraulic Harken model.

Still gettting used to the loads on running rigging and noises that they make so the whole idea of bending such a big mast still kinda scares me....there is the pre-bend in the mast already and i haven't noticed any slack in the shrouds when sailing so i'm happy that the rig is set up correctly.

I understand what bending the mast does to sail shape/power etc but I guess my real question is how far can you really move the mast without breaking anything?  What sort of travel is "normal" from a backstay adjuster?

A friend that delivered a 509 talked about easing the backstay when not sailing, again how much is "eased"?

Is there such a thing as a tuning guide for these?  I've seen the Selden generic one and read some other sources but I was thinking more a table of measured pre-bend and mast rake etc.

Thanks  

[zaphod]

First of all you should consult with a rigger regarding how much benefit you will get out of a backstay adjuster. I though about one for my 39i, because it is a tool that I have always had on previous boats, however, after speaking to several riggers they convinced me it wasn't worthwhile on my mast.

On a racing rig where the spreaders are in line with the mast you can induce quite a lot of mast bend, and tension the forestay. My old boat also had a baby stay to help induce more bend, so I could get dramatic results with the Navtec hydraulic adjuster it was equipped with. My Jeanneau's spreaders are swept quite far aft, so it is the shrouds that are maintaining forestay tension. The mast section is quite substantial, and there is no babystay, so my ability to bend the mast is limited, and would take significant power.

Also remember that mast bend is only going to benefit a classic mainsail. If you have in-mast furling then bending the mast will have zero impact on the main because the luff is attached to the furler inside the mast and will remain straight.

[johannes]

Backstay tension helps two things: mast bend and forestay tension. With a masthead rig the mast bend is typically not very relevant. 

The shrouds don't provide enough tension on the forestay (the angles are too small) even with back-swept spreaders on most boat types. So you need a good tension on the backstay to get the forestay straight. Rule of thumb for swept spreader rigs is 20% of breaking load on the backstay. You can easily do this with the turnbuckles, but the problem is that the hull will deform over time. This can give problems with doors jamming in the interior and a misaligned propeller shaft. This is why most cruisers settle with less tension and a forestay that sags a bit. 

So the real advantage of using a backstay tensioner, in addition to being able to trim the main on a fractional rig, is that it allows you to sail with high tension on the forestay, and release the tension when you are in port. In principle you can take all tension off the backstay when not sailing, but it makes sense to keep a little tension so that the backstay does not rattle.

[Bora]

Brilliant, thanks.  Got a local rigger coming to have a check of the rigging in general so will pick his brains to see if it's worth it.

[zaphod]

Jun 4, 2019 12:55:34 GMT johannes said:

Backstay tension helps two things: mast bend and forestay tension. With a masthead rig the mast bend is typically not very relevant

Not true at all. My last boat was a masthead rig and the hydraulic backstay induced a huge amount of mast bend to flatten the main. Same on the Olson 30 I race on. And just about any IOR era masthead rig for that matter.

[johannes]

You're right, but you need a babystay or forward-directed lower shrouds. Most modern cruising rigs don't have those.

[MartyB]

My 85 Arcadia can bend a bit with the back stay adjuster. Can make a difference in how much or little weather helm I have. Then again, I do have a double perpendicular to centerline spreaders, Mini forestay, smaller mast section and thickness than many boats of that era. As a comparison, a First 285 that is inches and lbs of displacement etc different, as a single spreader, no mini stay, next size bigger thicker walled diam mast. Very possible a back stay adjuster will not effect that boat as it does mine.
With that said. I keep the shrouds around 20%, back stay in the 12% at rest, know in my case, how many turns of the adjuster I need to get to 20-22 max % of breaking strength.

Marty

[garyj]

Can anyone give me their thoughts on whether installing an adjustable backstay in a Sun Odyssey 469 or 479 is beneficial?  I can't see why it wouldn't allow me to induce some bend to flatten the main and tension the forestay.  I expect that the hull isn't racing boat stiff so too much tension might just flex the boat.  Thoughts? 

[fimacca]

I think you answered your own question.
It may make a little difference, but there are other upgrades that would make much more......