Understanding Wave Period

Hello. Our first time chartering the USVI will be next week. From our research we see that a north swell can make things pretty rough in certain anchorages/moorings. We have been trying to familiarize ourselves with the info on windguru but I’m a bit lost on “wave period” and what that might mean for us.

Bottom line is we would love to know what places to steer clear of in STT and STJ when a north swell is happening and also know how to recognize when this swell event in coming (wave period?)

Thank you in advance.

You want to look at swell rather then wave. Swell is larger ocean effect. Waves are the more acute wind influenced effect. Waves are more wind based and aren’t really bothersome at most anchorages. Swell can be very bothersome when is rocking your boat all around.

In a big north swell avoid Honeymoon, Caneel, etc on St John. Maho and Francis are good. In south swell avoid most southern anchorages. Everything feels worse on a mono vs cat.

Instead of windguru, check windy.com

(Just my opinions and observations)

Wave period refers to the amount of time it takes for the peak of one wave to pass a given point before the peak of the following wave peak reaches the same point. Used to look at the swell/wave period back when we surfed more. Think of it like this - you’re out in the ocean on your board and a wave just passes over you, you have 15 seconds until the next wave peaks at your same spot and it’s a smooth, calm wait. Likewise if you only had 5-8 seconds it’s going to seem much more ‘choppy’ and the waves are likely more disorganized vs the smooth swell of the 20 second period.

In a sense the shorter the wave period the conditions. Longer wave period should seem smoother (although the chances of the waves being larger when breaking on the shore is greater). Something to consider when planning on taking the dingy into the beach.

I use this buoy report to gauge the N. Swell in real time

https://www.ndbc.noaa.gov/show_plot...uom=E&time_diff=0&time_label=GMT

Ok awesome. Windy.com is very helpful. Very easy to see the direction of the swell and I see it’s shifting literally every day. My follow up question is how big of a swell should start to concern me for any given anchorage/mooring?

In a big north swell Honeymoon and Caneel would be a better anchorage than all the bays from Hawksnest to Francis Bay.
Water lemon Cay on the east side will be calm along with all the anchorages on the south shore.

This is interesting because I find the exact opposite. Maho good in a moderate north swell. Caneel is terrible. None are good in a big swell.

Intriguing. Great discussion.

How about Magens in a big north swell?

Anyone else with input, observations?

Can someone describe what a “big” north swell might look like in swell height? When is it too small to even worry about from any direction?

It’s relative sailor to sailor. I don’t like caneel in any swell over 1 meter. That’s fine in maho. In my mind in the USVI “Big” swell is 1.3-2 meters. This is daily stuff, not talking storms nearby, etc. The swell isn’t as bad if the wind stays up overnight but when the wind settles to only a few knots the swell just rolls the boat all over. Most of the time swell is under 1m.

I base much of this knowing that you can just plan your trip differently knowing the conditions. If I’m sitting in caneel knowing i could be comfortable 40 minutes away in maho I would much prefer to be there.

With a north swell Caneel/Honeymoon is protected somewhat by the cays to to the north whereas hawksnest to Francis Bay are pretty much wide open.
If you have a 1 meter swell your boat will be rocking and rolling all night long.

A big N. Swell, -storm offshore cause - makes Magens Bay very rolly -- not common but does happen.
USE *****
https://www.ndbc.noaa.gov/show_plot...uom=E&time_diff=0&time_label=GMT