Mast shake and storage ashore with mast stepped

[syvictoria]

Hi. Here in Friesland, the norm appears to be to leave masts up when ashore. Our mast develops quite a shiver/shake in anything over about an F4 and vibrations can be felt through the boat when she gets going! Will she be okay left in a cradle with the mast up? TIA!

 

[michael_w]

What your mast suffers from is vortex shedding. The solution is to wrap a halyard round and round the mast to break up the airflow. My old boat's mast and furled jib suffered from this. The halyard wrap stopped it completely.

 

[syvictoria]

What your mast suffers from is vortex shedding. The solution is to wrap a halyard round and round the mast to break up the airflow. My old boat's mast and furled jib suffered from this. The halyard wrap stopped it completely.

Yes, we've looked into this a lot in the past. Unfortunately we've never found a solution. Wrapping ropes, hoisting fenders, rigger inspection, all make minimal difference...

 

[Ian_Edwards]

This is a very well known bit of physics, but often difficult to solve. Vortex Induced Vibration (VIB) happens when the frequency of the vortex shedding matches the natural frequency of the mast.
There are 3 ways of stopping it.
Stop the vortexes locking on. A vortex produces a low pressure area on one side of the mast. If all the vortexes occur on the same side of the mast at the same time, the low pressure will move the mast sideways. The movement of the mast disrupts the air flow and the vortexes stop, the mast move back, and vortexes form on the opposite side of the mast and the mast moves to the opposite side.
If this side to side movement locks onto a natural frequency, the amplitude can grow to a point where it can be destructive.
Wrapping a spiral around the mast is know solution it stops the simultaneous formation of vortexes along one side of the mast.
If that doesn't work then either the rope is to small, or the pitch of the spiral was to shallow.
You could try alerting the natural frequency of the mast, varying the rigging tension might work. Reducing the rigging tension will lower the natural frequency and increasing the tension will increase it. In a mast this tends the be a marginal effect.
Add damping, this takes energy out of the system and reduces the amplitude of vibration. A practical method may be to fly a very small sail part way up the mast. A small triangle, perhaps as small as 0.5m x 0.5m x 0.5m (I don't know the size of your boat, so this is just a guess) attached to the mast with 2 slides or a bolt rope, hoisted on the main halyard, with a down haul and a sheet to the stern of the boat. Set up bar taught.
You often see this arrangement on aluminium head sail foils.
My boat has a 10mm, D12 dyneema backstay, when I first installed it resonated with a very annoying hum. Slackening it of stopped it (changing the natural frequency), but this was a real pain.
I resorted to rapping a length of 3mm cord in a spiral around the dyneema with a pitch of about 150mm. That works most of the time, but if I've been using a lot of backstay tension and then forget to release it. It still hums but at a higher frequency.

 

[vyv_cox]

The mast on my Sadler 34 shakes heavily in winds of certain strength and direction. I have tried changing babystay tension and backstay tension but these have no obvious effect. The problem with spiral wrapping to overcome vortex shedding is that it can only be done usefully below the spreaders. I wrap my stackpack lines around when leaving the boat for winter but not certain that this overcomes the problem. However, after 40 years the rig is still intact.

 

[Blueboatman]

You might try simply playing with loading up the main sheet and vang tension, or windup the aft lowers a bit, that should mess with the mast enough to preload it just enough to stop this, if you’re lucky

 

[syvictoria]

Just wanted to thank everybody again for their comments and advice. I was inspired to play with wrapping ropes around the mast again, and by going outside the spreaders, I succeeded in getting one long turn above them which, together with seven below, seems to have made a difference, fingers crossed... We have consequently left the mast up. Hopefully all will be fine. We both had a tear as we left - how we become so attached to a great lump of plastic, I don't know!!!!

 

[Refueler]

Have never bothered about mast shake .... but what I really hate - and this is on water or shore ............ the lazy s*** that don't secure there lines better to stop that nerve jangling slapping on mast !