AusCan
Guru
- Joined
- Jul 15, 2012
- Messages
- 3,219
- Location
- Australia
- Vessel Name
- Kokanee
- Vessel Make
- Cuddles 30 Pilot House Motor Sailer
I'm convinced that it is not only the wave height which makes it sometimes nasty for us. Even smaller waves will force our boat into serious motions if they hit us with a frequency matching the rolling Eigen-frequency of the hull. We have been in (long) waves as high as 9' without any inconvenience, but experienced very nasty rolling angles up to 35-40 degrees in 4-5' (short and steep) seas on other trips.
Rolling Eigen-period of our hull is in the range of 3.5 seconds. Based on the forecasted seas (heights, direction and period) we try to predict the period with which the waves will hit us depending on our intended course. Sometimes we can avoid resonance by adjusting the speed and / or course, sometimes those measures won't give us enough safety margin. That are the days where we nowadays stay in port even if the forecast is below 5'.
And then there are those days where the forecast fails and you see the equipment flying through the pilot house ...
Ahhhah! A new word for me. I always just called it my boat's natural roll cycle, but it has a name - Eigen period.
Yes - it does get ugly when the swell period and the Eigen period match up.
Thanks Waddencruiser.