Northerly Swells forecast interpretation

We are chartering a Sunsail 38 monohull down in Tortola in a couple of weeks and are having a bit of trouble correlating the marine forecast swell conditions to what folks are referring to as "bad" northerly swells. It's our 5th trip, but first in the high season, so it looks like swells affect where we are going to have comfortable anchorages. In the past we've really just planned our itinerary on wind direction to avoid beating to wind on vacation (we do plenty of that at home at the PHRF races!).

That being said, I do remember folks surfing stand up boards at the entrance of the channel of Cane Garden Bay! I suppose that's what is meant by "wrap around" swells?

Is it mostly wave height, or a combination of height and period? One meter (3ft) doesn't sound too bad if spaced far enough apart, but looking for some salty advice here based on the vulnerable anchorages. Also high season means dropping the hook more often, so would want less stress on the anchorage watch.

Sites with swell forecasts: (feel free to suggest others, or good phone apps)

http://www.myweather2.com/Marine/Global-Marinas/British-Virgin-Islands/Moorings-Marina.aspx
https://www.windy.com/18.425/-64.662/waves?waves,2020-02-23-21,18.397,-64.669,13,m:ebUaenj

Thanks!

How about this site...

Note swell on Sunday

https://marine.weather.gov/MapClick.php?zoneid=amz715#.XlE7sShKg2x

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.

Well said. Great Harbor and Little Harbor on Jost are well protected. Also, the Bight and Trellis Bay.

This site will give actual wave height North of the Virgin Islands and is good for prediction interpretation of graph

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

Great asset, thanks.

Today there is no swell..we are getting our beach back here at Apple Bay, there were no cruise hips in and it felt like June on a perfect CGB day..just saying. Late breakfast at Rhymers, beach walk and dip, meeting old friends ,a couple of cold ones..perfect. Soup at The Sugar Mill tonight for dinner.

Sounds like heaven!

Had lunch at Sebastian’s a few days ago. Always great to see the surfers, and nice to see the beach returning.

Tomorrow feels like more of the same, Mal. Let me guess... Brunch at Tremonti, beach walk at CGB, cold ones at CRC, dinner at the Elm? See you at a few of those places!

We are always in 38’ monohull and swell affects us much more then the cats. Swell with no wind is even worse. 1 meter most places is ok if it’s deep.

Thanks for the detailed reply with explanation. Looks like we'll be on the lookout for swells over 2 ft from the North and adjust our evenings appropriately. Thanks for the link as well.

We'll be on the lookout for deeper moorings if there is a choice.

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

Are the readings at that buoy (170NM from PR) an indication of swell conditions in the BVIs one or two days out? Or nearly real time conditions?

Very close to real time - if the graph indicates a building swell, IT is already there and building.

The longer the period of the north swell the more energy in the swell. When cruising the BVI I usually have 3 weeks of time (what I get in my passport when I clear in) so I am not under any pressure to visit a number of sites in a week; and since I prefer calm to rolly when given a choice I will seek sheltered anchorages when there's just a chance of getting a north swell roll. So a period of over 10 seconds gives the waves a lot of energy, but in 20 feet of water they won't be affected by the bottom and a swell of 1m won't have me scurry to the North Sound; I'd probably stay in Cane Garden Bay if it were a Sunday and I could see the Elmtones play live.

I got caught in Cane Garden Bay by surprise once, as I'd not checked the weather. Shortly before sunrise the swell set in, I believe it was 1.5 to 2m with a very long period. I left the anchorage at sunrise and was worried about bottoming out while departing the CGB channel with surfers coming right by me surfing the break.

https://www.windy.com/18.456/-64.680/waves?swell1,18.296,-64.605,10

Windy is also a nice site with swell forecast information. They also have a nice app for download.Windy

Interesting -- got any good links for this?

Thanks,
Dan

Here's a link: https://earthscience.stackexchange....t-mean-for-waves-to-feel-the-bottom/2601

The N swell in the BVI is a zero issue for being out on the water sailing . It’s a big issues for many dive sites and anchorages.

Swell picked up overnight....right now waves are big and lumpy in Apple Bay.

Waves crashing on the reef off Scrub Island, which happens in a north swell.

was there in January, a couple of days of N swells & sustained 20+ kt winds; great for going to Anegada cos lots of folk don't like the crossing in winds, the mooring area was pretty calm & the North shore beaches were fine to bathe & snorkel off; came back down to CGB, the surf was up :), great fun if you like that sort of thing but even though it wasn't disastrously role on the ball the waves were crashing into the jetty so it was out to hope to tender there for dinner or anything. Great Harbour was fine and protected; little Jost was ok to moor but waves put The B line out of tender commission, tying up outside Foxy's was fine; White harbour was rough on the West side but East side looked fine & a few boats had anchored there.

Excellent, thanks!

Dan

Thanks for the data points. Interest about the different conditions on each side on Nany Cay. That is one of our favorite spots.

Do you remember the swell forecast details were with these conditions?

Nanny Key will not be affected by a North Swell.

Ah, I misread that. Wind action waves, not wrap around swell prevented making a B line to B Line. 😀

Just curious - is the North East swell a problem, or just the North

Thanks
Tim

Generally as you get away from N the effects are less noticeable. ENE will still be uncomfortable but NE swell will be a lot less. And then there's size and period of the swell to consider.