How bendy is your mast?

[bitsailor]

SO419, commissioned 2018, we have done over 3,000 miles with her.

If we are sailing in 20+ Kts, I notice that during big gusts or crashing waves, the mast bows significantly backwards and forwards. This appears to be due to the fractional rig, as gusts hit the foresail, it puts pressure on the forestay which pulls the upper part of the mast forwards, this in turn puts pressure on the back stay which pulls the top of the mast backwards. 

I am fairly sure that this is the expected behaviour, and I know the mast is designed to be very flexible, but at points in strong winds it appears almost floppy and i am surprised by the amount of oscillating and bowing that the mast does. Note that it is always bending the top backwards, not inverting which I know would be a concern on a sweaped rig.

I am considering having a rigger come down to tune the rig, but wondering if this is just the norm with modern rigs?

Note that we have in mast furling, so the mast is not swepped back when the sails are not up.

[Charlie-Bravo]

Masts are bendy, which is why we all have standing rigging , they don't stand up long without support ..... just stating the obvious .

It would seem your rig is not set quite right, from reading your post, the fact that you can see the flexing of the mast is a concern, rigs needing a little bit of a re tune don't normally result in visibly wobbly masts, just a slightly slack leeward shroud perhaps, and very rare to see fore and aft flexing.

A good read is as ever, the seldon hints and tips section of their web site , even if you have another brand of mast, it explains how a rig should be tuned. Not everyone is keen on diy rig tuning, and employing a competent rigger would seem wise, but having an understanding of what is amiss will keep you informed of the state of tune of your rig for years to come, so worth a read, and if you are a bit of a spanner twiddler , worth a go yourself.

A lot of folk get their boats lifted each year for maintenance, but don't relax the rig prior to the lift, and checking the rig tension is often ignored after launch as well. The rig tension also will change with time as the shrouds stretch and in some cases the hull settles to the rig tension, it's a dynamic thing, so needs a bit of a tune from time to time.
The final tune of a rig should be done under sail in a reasonable wind to get it perfect, a rigger in a marina will get it close, but not spot on.

I have learnt that a well tuned rig improves the performance of the sailing to a degree that is unexpected, higher pointing, and more drive forward for a given wind ..... and I am but a casual cruiser , no racer.

Hope you get some further rigging advice perhaps specific to your model.
CB

[zaphod]

What you are describing is called "pumping", and the mast should not be doing that in a significant way.

Definitely have a rigger inspect and tune your mast.

[bitsailor]

Thanks for the input. I’ll arrange to have a rigger come round and report back what they say.

[rene460]

Hi Bitsailor,

That loosening of the rigging during the early sailing of a new boat is quite expected, but as others have already said, it needs to be attended to.

The stranded wire settles in a bit under load, and as it does so, it becomes a little longer, hence looser. This is what allows the mast to move as you are observing and is part of the normal settling in of a new rig. Every time the mast comes against the tension, it experiences an impact load which is considerably higher than the load when mast rigging is tight. These alternating loose and impact cycles also encourage failure of fittings, shackle pins etc. and hence the need to re-tension.

I think it’s a good decision to get a rigger to get it right at this stage as it’s a complex job if you don’t have a good understanding of the process. Also if you don’t fancy hanging around in a bosuns chair at the top of the mast for the time it will take. But it is worth having some idea of what the rigger will be trying to achieve.

The mast is held vertical by two points, the base whether cabin top or keel stepped, and one of the points where the shrouds or spreaders meet the mast. “Vertical” means side to side as well as fore and aft, including any rake required. Then the mast is further restrained where other shrouds and spreaders attach, as well as the forestay and backstay. By adjusting the balance between these extra points and the selected fixed points, any curvature can be put in the mast if required. As always, the Selden guide is an excellent guide to the process, but putting it into practice is definitely tricky.

It might help your understanding of the rig to remember that the stays can only act in tension, while the spreaders effectively only act in compression, in both cases in a straight line between the end fixings. For example, the lower spreaders generally meet the mast at the lower shroud attachment point. In plan view, the spreaders push the mast forward, while the lower shrouds are pulling it aft and stabilising sideways. The sideways force on the upper shrouds pushes the end of the spreader so it in turn pushes forwards on the mast to balance all the forces.

When it’s all done, the mast should be straight when viewed from the bow or stern, and there should be minimal loosening of the Lee shrouds, even in stronger winds, but no noticeable movement of the mast.  From the side, you will see the mast bend if any. For mast head rigs, probably no bend, for 3/4 or bendy rigs, the mast bends forward in the middle, never back. By the way, Selden specify the max bend equal to the width of the mast for the mast section on my 7/8 rig, but probably none needed as the main sail can be adjusted to fit at the cars on the end of the battens. For the common mast slides which are not adjustable, the sail maker will provide the appropriate shape to the luff to properly control the draft.

When the job is done, it is worth getting your mechanic in to check the motor alignment. Boats will commonly bend a little under the high loads of a well tensioned rig, this will change the shaft alignment and shorten the life of the cutlas bearing. Then it should all be good for quite a while with just reasonable observation to pick up any signs of wear and tear. Some forum members who are keen racing sailors have a Loos gauge to check rigging tension and make adjustments for differing weather conditions. This almost certainly is significant in the racing environment, however I am not sure how much help it is to cruising sailors, even though they still want to go fast especially to get to a safe harbour before bad weather arrives.

I hope this helps,

Rene460

[NZL50505]

Apr 6, 2022 19:52:29 GMT bitsailor said:

Thanks for the input. I’ll arrange to have a rigger come round and report back what they say.

Yup. And ideally take the rigger for a sail especially if your rig is a far out of tune as it seems to be.