top of mast bending forward

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Darrel_Walters

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Hi All,
looking for a bit of advice

7/8 rig, in mast, swept back spreaders double.

Been playing around with the tuning of the mast as was out this weekend, where the wind max'd at 23 to 25 knots apparent, close hauled, looking up the mast for some hard points on the mast and noticed that the top of the mast was bending forward, I already have 20% strain/load on the backstay, in fear of over tensioning the rig, can I apply more tension, or would this bending forward be normal under the conditions. The boat was not over pressed on a full set of sails but another 5-7 know we would be looking to reef. The wind steadied down to 17-20 knots and the mast came true again, and as we got further into the harbor and the wind settled the mast straightened.

I have read large amounts about shape by altering cap shrouds d2 etc. I am fearful off applying more back stay tension as in harbor it is already at 20% (10mm)

Is this normal for a rig and worrying over nothing, or should I apply more backstay to straighten the mast

Any advice would be warmly appreciated
 
Amount of backstay the mast will take depends on the mast! As a basic premise, you're unlikely to damage the mast with any effort you can put on by hand - obvious exception if you've got a powerful hydraulic adjuster, but on those the travel is often quite limited. I'd whack on some more backstay and see what happens - but did you also have plenty of kicker on? That puts a force on the mast at gooseneck height - trying to move the lower mast forward can also help move the mast head back - bit like an archer's bow. Take a look at some pix of racing boats to see how much bend a mast can take.
 
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The diagonals might be too tight, they tend to straighten the mast so suggest you slacken them or tighten the caps . There's a rig tuning guide on the Selden site
 
Darrel_Walters said:
Hi All,
looking for a bit of advice

7/8 rig, in mast, swept back spreaders double.

Been playing around with the tuning of the mast as was out this weekend, where the wind max'd at 23 to 25 knots apparent, close hauled, looking up the mast for some hard points on the mast and noticed that the top of the mast was bending forward, I already have 20% strain/load on the backstay, in fear of over tensioning the rig, can I apply more tension, or would this bending forward be normal under the conditions. The boat was not over pressed on a full set of sails but another 5-7 know we would be looking to reef. The wind steadied down to 17-20 knots and the mast came true again, and as we got further into the harbor and the wind settled the mast straightened.

I have read large amounts about shape by altering cap shrouds d2 etc. I am fearful off applying more back stay tension as in harbor it is already at 20% (10mm)

Is this normal for a rig and worrying over nothing, or should I apply more backstay to straighten the mast

Any advice would be warmly appreciated
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A rig displaying a forward bend is not normal under any circumstances as far as I am aware.

Does the mast have visible pre-bend when at rest in harbor? Sounds like it may not have (as you refer to the mast straightening again with less wind). It should normally have pre bend, without (!) having to tension the backstay noticably.

Applying the backstay exerts a bending force both downwards as well as backwards. I think it is quite possible that you are increasing (or at least supporting) the forward bend by tightening the backstay (once the forward bend occurs that is).

I imagine that the forward bend under sail (especially as it appears to increase with wind force) is created by the pull of the fore stay on the mast top by the jib/genoa. Sounds like the middle section of the mast is not supported enough (allowing the forestay to pull in the forward bend, a reverse back stay as it were), or is tensioned the wrong way (essentially creating a forward facing pre-bend at rest, visible or not at rest)

Please remember that the backstay pulls against the the fore stay and the shrouds, and pushed the deck at the mast foot. Its a balancing act. If the shrouds and forestay are not set up properly, no amount of backstay tensioning will give you the backward bend your are looking for. It will simply lead to over stressing the rig and hull.

Basic rig re-tuning is not very difficult, but it can be time consuming (requiring all the rigging screws to be loosened) and difficult to understand the forces acting on each other for the first time tuner. Might be worth having it done once by a professional, ask lots of questions, and do it yourself in future.
 
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" and the wind settled the mast straightened."

I would normally expect some aft mast rake and aft prebend in a fractional rig at rest. Sounds to me like you resting setup is wrong. Either start again following the Selden Guide or get a rigger in.

As Twister Ken says a non-hydraulic backstay tension very unlikely to cause any damage when pulled on as tight as possible,. Certainly in high winds you need alot of backstay on and its the backstay that is your principle main flattener (with the clew outhaul, cunningham and flattener - if fitted)
 
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The aft wall of a mast is far weaker than the others so you want to avoid the mast bending forward at all costs (it's called inversion).

I presume it is not a racing rig with runners etc. A fractional cruising rig should not need the backstay to stop it bending forward.
 
Hi all

Many thanks for all your input so far. having in mast furling bending the mast aft would stop the furling system from working. so in harbour the mast has no pre bend. I am thinking that the caps are not tight enough but we would need to ease of the d2's first, has anyone got a view on this

Thanks
 
Darrel_Walters said:
Hi all

Many thanks for all your input so far. having in mast furling bending the mast aft would stop the furling system from working. so in harbour the mast has no pre bend. I am thinking that the caps are not tight enough but we would need to ease of the d2's first, has anyone got a view on this

Thanks
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D2s? Do you have discontinuous rigging? I would've thought you'd just have caps, intermediates & lowers all adjusted by bottlescrews at the chainplates. Having separate D1s & D2s is more a racing thing.

It might be an idea to see how much a rigger would charge to sort it out as it could be expensive if you ge it wrong.
 
Darrel_Walters said:
The set up is from seldon with discontinious caps on twin spreaders
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Ok. You're going to have to be hoisted up to adjust the diagonals. I'm guessing this would be your first time tuning a rig. The combination of discontinuous rigging and in-mast furling makes it a bit more complex. Maybe worth playing cautious and calling in a professional rigger.


However if you do do it yourself as long as everything is reasonably tight and there's no inversion it shouldn't come down. That however is absolutely not a guarantee from me.
 
I wouldn't worry too much about putting on lots of backstay when the main is set - but you'll probably need to ease it off before doing any furling or unfurling. I have a Selden fractional rig and it is very bendy and happily takes all the tension I can put on the backstay with a 16 part tackle.
 
W"hat is it that you are sailing where you reckon not to reef at all until 28 to 32kn apparent?

Anyway, the mast should never invert and bend forwards. Indeed your main sail will have been cut with a gentle forward curve in the luff ( in my case it is 5 inches) so if you have the mast bending forwards you will have a great bag in the centre of the mainsail luff.

The most likely cause is that you dont have enough pre bend and the mast is straight at rest and leaning stern wards. There will be official measurements for how much pre bend you should have and how much back slope but basically you set up the mast to give both the backward slope you need and by pulling on the babystay / forward lowers you put in the curve you need.

Normally if your backstay is 10mm you will find the forestay is 12mm so that the load %ages balance out allowing for the trigonometry of the rig and the aftwards pull of the shrouds.
 
Just noticed you've got in-mast furling. I'd certainly see a rigger about this one if I were you; you have a "higher" tech rig with a lower tech sail and a furling mechanism to get sorted - not an easy one to optimise......
 
Darrel_Walters said:
Hi all

Many thanks for all your input so far. having in mast furling bending the mast aft would stop the furling system from working. so in harbour the mast has no pre bend. I am thinking that the caps are not tight enough but we would need to ease of the d2's first, has anyone got a view on this

Thanks
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Page 22 (ie page 24 of the entire pdf) of the Selden manual linked earlier above by VicS, clearly states that an in-mast furling rig should also have pre-bend (max 1%, compared to 2% for non-in-mast furling).
-------------------
d = Depth of curvature
0.5% x FH
d must not exceed 2%
(1% for in-mast furling).
-------------------

I have a very similar rig to yours, and have no problems with the furling unit under pre-bend.
 
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