Originally Posted by Zanshin
Typically any swell that size while at sea isn't going to impact you much at all, the issue with the north swell is in an anchorage where the waves begin to "feel" the bottom. Mathematically, this happens at depths of about half the wavelength (the actual formula has a TAN function, but for rough approximations this number is about right). This is the depth where the waveform changes. Then there is the wave period, a long wave period of >10s will be quite uncomfortable in an anchorage such as Cane Garden Bay even with just a 1m swell.

In any case a north swell will cause your boat to roll and depending upon the frequency this might turn into harmonic rolling. I've been in a north swell of just 1m but each wave got my boat rocking just a little bit more (kind of like being on a swing which is being pushed just a bit each time around) until I could barely crawl without being swept from side to side.

Whenever I see a north swell forecast in the BVI I head to either the North Sound (anywhere in there is protected) or to Diamond Cay on Jost.

I like to use https://www.windguru.cz/58 for wave predictions. A 12s north swell on Sunday of 1.3m will have me scurrying towards the North Sound. But I'm just chicken. I think that the Bight on Norman should be OK but I'd certainly stay out of Cooper or Cane Garden Bay.


We too have sailed the BVI many times and I have never really got a grip on the threshold at which the northern swells effect the anchorages. You say a 12 sec 1.3m swell would send you scurrying or the north sound. I’m interested in the threshold at which you would not head for the north sound. For example, would north swells of 1m see you heading for cover. How about .75m. We are bringing our kids and grand kids to the BVI for their first trip and, wishing to avoid mutiny, I am keen that everyone sleeps well! We are on a 46 foot mono hull.

Thanks
Tim

Last edited by TimS; 02/23/2020 10:19 AM.