Selden Mast Vibrations and Rolling — Instructions
INSTRUCTIONS
Why a mast vibrates or makes the boat roll at the mooring, and what Selden recommends to damp it: sail strips, spiral strips and mid-mast staying.
DocumentSelden Mast Vibrations and Rolling Instructions
FormatPDF · 2 pages · 0.1 MB
LanguageEnglish
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Cause
- Appears at moderate wind speed, approximately 2-6 metres/second, almost always with the wind athwartships.
- Caused by alternate eddy shedding on the lee side of the mast ("Karman" effect); resonance occurs when the mast's natural frequency matches the eddy frequency.
- Noticeable from approximately 80 % of the resonance wind speed, fading again at approximately 1,5 times that speed.
- A thinner section (fore-and-aft) and more weight per meter reduce the amplitude.
Remedies
- Selden's recommended sail strip: the flexible strip randomises vortex shedding on the lee side and prevents resonance.
- Spiral ("scruton") strips on the middle fifth of the mast: projecting 0.1 x D, pitch approximately 5 x D, 1.7 x D between neighbouring strips (D = fore-and-aft mast dimension).
- Staying the middle of the mast fore and aft with stiff wire (angles not less than 20°) raises the resonance wind speed; ordinary rope has no staying effect.
- Increasing fore/backstay tension by 50% lowers the resonance wind speed by only 15% – marginal.
Rolling (595-585-E)
- Rhythmic heeling with the wind roughly fore-and-aft: seen on P sections with wind from the bow, and on D-sections 137/113 and 160/132 with wind from the stern.
- Remedy: mount strips on the upper part of the mast, from 30 cm below the masthead and 3-4 m downwards, to detach the airflow.
- Material: Selden aluminium strip (art no. 535-012-01) x 3-4 meter.
- A change of displacement can shift the boat's natural pendulum frequency enough to stop the rolling.
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