Mast top flexing

[Surfer81]

Dear sailors,

during upwind sailing I ve noticed that the top of the mast flexes for approx. 5-10cm. This happens when the boat (Bene First 42) hit a small wave. The boat has a masthead rig.

I am wandering If is a normal situation or the rig should be completely tight.

I tried to trim with backstay but there is not much difference. When I tight the runners there is less flexion but the mast is bent more forward all the time.

Thank you for all your suggestions!
Andrej

[Jim Cate]

Which direction is it moving?

Jim

[danstanford]

Originally Posted by Jim Cate

Which direction is it moving?

Jim

My first question also Jim followed by these:

-are any of the shrouds going slack (wobbly) at this same time?
-is this flexing happening above the diagonals?

[Surfer81]

Thank you for your quick feedback.

The top of the mast (approx 3 meters) is flexing in forward/backward direction. I did not notice any shroud movement (slack). The upper shroud is attached approx 60cm below the top of the mast. In flat water there is not FLEX.

Thanks!

[chrisr]

yes, as jim asks : the direction of movement is rather vital. top of mast for a masthead rig should not move much at all in fore & aft plane (that's what forestay and backstay are for) but might fall of to leeward slightly depending upon where cap shrouds are fixed

not that familiar with the rig set up on this boat - can you tell us exactly what is the whole configuration. i'm guessing double in-line spreaders ? in such a rig the runners are really only there to stop the middle of the mast 'pumping' in a seaway

what control do you have on the backstay - hydraulic ?

is it just the masthead moving or does the whole rig give a shake when hitting a wave ?

feel you are doing well to pick 5cm of movement by eye in the masthead of a 42'er !

subject to more info, i'd be looking for more backstay and less runner but this is only a guess until we know more

cheers,

[chrisr]

hang on

you mention boat has a 'masthead rig'...then you say 'The top of the mast (approx 3 meters) is flexing in forward/backward direction.'

just so we have the terminology correct, a masthead rig has the forestay and backstay both attached to the masthead

is this the case on your boat ?

cheers,

[Surfer81]

You can see some boat pic on sail4fun.si.

In line spreaders, hydraulic backstay control, masthead rig. I d say there is very little leeward movement.

Regards from Slovenia.

[chrisr]

thanks...nice boat.

you mention "did not notice any shroud movement (slack)" this is in side stays (V1) ? leeward side should be just firm when going to windward in 15k

if so, and mast is straight in column, then i would just be a bit more vigorous with backstay tension. forestay sag should be abt 15cm or so

cheers,

[MicHughV]

It's also possible you may have some hull flexing depending on point of sail and wind strength, this could affect the various shrouds and stays.
You could also have some compression issues with the mast compression post or bulkhead that supports the mast, especially so with an over-tensioned rig.
Hard to say, without being there.
5 cm is about 2". You must have some kind of eagle eye to spot that on a mast that is likely 50' plus above the waterline.
Stainless steel wire can also stretch under load.

My advice would be to get a good rigger out to your boat to inspect and tune your rig. We are all guessing here.

[skipperpete]

Maybe longitudinal hull flex.... it can happen[ATTACH]265705

[Rucksta]

Originally Posted by Surfer81

I tried to trim with backstay but there is not much difference. When I tight the runners there is less flexion but the mast is bent more forward all the time
Andrej

Foward deflection of the masthead is known as inversion.
The mast should not be considered "in column" while it exists.

Baby stay and/or forward lower shrouds (D1) are typical control measures.
An effective vang helps.

[MicHughV]

I don't know if you can open this....but can you some helpful pointers

https://support.seldenmast.com/files/595-540-E.pdf

[SurlySailor]

Does the mast have a babystay and checkstays? Mast head rigs and racing rigs of this era were built to be highly tuneable but they need proper support. If you have a ton of flex foreword and backwards you need to potentially add some prebend into the mast at the partners and affix your babystay and checkstays to stabilize the rig. You’re likely seeing the flex as a result of the middle of the mast moving.

The prior owner may have removed the babystay and checks because they are annoying. On our boat they stay removed in light air, and flat water sailing. Anytime we get any chop upwind you need to add them back to the equation to support the middle of the mast section.

[Surfer81]

Thank you for all your suggestions and help!

Regards.
Andrej

[rhubstuff]

You can check for hull flexing at the dock or under weigh. Run a wire or some light line between the bow and stern pulpits; make it very tight. Slacken the backstay right off and mark the position of the line at the mast. Then tighten the backstay to sailing tension. If the line at the mast moves upward, your hull is flexing. If you go sailing with the line attached, you may see the line rise and descend at the mast as the hull flexes in the seas. The C&C 43 I raced on had almost a centimeter of hull flex at the dock.