Wave height and period: what's "comfortable"?

We're in Providenciales, TCI, getting ready for out next move, to San Juan, PR. We expect it to take 56 hours at 7 kts. We're looking for a nice 3 day weather window, but the REALLY nice windows are few and far between this time of year. You know - the ones with wind under 10kts, 2-3 ft waves, and a 10 second wave period.

So we're trying to figure out some "cut off", below which it is likely to be fairly comfortable, and above which, we just won't go. But we don't have that much "big water" experience - back and forth from FL to Bahamas several times, but those are 8 hour crossings, and we're looking at 56 hours.

We use PredictWind and Windy to get forecasts. If you want to play along at home, you can look at Windy for free (windy.com). Pick a point midway between Provo and San Juan, and look at the Waves forecast. You'll see, this coming Saturday, Sunday, and Monday a stretch in which the waves are forecast to be (at worst) 5 ft. with a 7.6 second "swell period", and (most common) 4 ft. with a 7.8 - 8.2 second "swell period". Waves are from the East that whole time. Wind during that period is from SE then E, up to 14 kts with gusts as high as 20 kts.

PredictWind shows 3 different model forecasts: one mostly in agreement with Windy, the other two are as bad as 6 ft. waves with a 7 second period.

Go, or no-go? (We are stabilized, but with the waves from that direction, won't be doing much.)

I don't know your and your crews tolerance for boat motion, nor the characteristics of your boat, so have no idea what's "comfortable" for you. Personally, those are no go conditions for me given my tolerances and boats I have run, such as 49' Grand Banks and a stabilized Hatteras 56MY, especially at night.

When we kept our boat on the west coast and often made the trip to Catalina Island, a forecast of 5-6' seas was our limit. This was on a 34' Mainship Trawler with no stabilizers. I would think you would be fine up to 7'.


YMMV of course.


David

It's real complicated. For example, at 7 knots you'll cover 11 ft/second. For my boat at that speed, I'll cover a boat length every 5 seconds. If the period of the waves is 6 seconds, but the waves are traveling at 3.5 knots (common), which direction they are traveling plus wave height determines the comfort of the ride. If they are nose on, I'm straddling them and pounding. If they are moving away, I'm riding up and down and the motion is just fine. In the first case, waves over 3 ft might be an issue. In the latter they could be 10' and be no issue.

Knowing boat speed, length, wave height, wave speed and direction, you can figure it out for a particular crossing easily enough.

Delfin said:

It's real complicated. For example, at 7 knots you'll cover 11 ft/second. For my boat at that speed, I'll cover a boat length every 5 seconds. If the period of the waves is 6 seconds, but the waves are traveling at 3.5 knots (common), which direction they are traveling plus wave height determines the comfort of the ride. If they are nose on, I'm straddling them and pounding. If they are moving away, I'm riding up and down and the motion is just fine. In the first case, waves over 3 ft might be an issue. In the latter they could be 10' and be no issue.

Knowing boat speed, length, wave height, wave speed and direction, you can figure it out for a particular crossing easily enough.

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Wow - that seems like a great way to evaluate this! Thanks!

djmarchand said:

When we kept our boat on the west coast and often made the trip to Catalina Island, a forecast of 5-6' seas was our limit. This was on a 34' Mainship Trawler with no stabilizers. I would think you would be fine up to 7'.


YMMV of course.


David

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That's with pretty gentle long period swells. Out these parts that's a Small Craft Advisory.

BrianSmith said:

Wow - that seems like a great way to evaluate this! Thanks!

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The route you mentioned, the direction from TCI to PR, wave height and period, makes this look like kind of an uncomfortable ride.

This could get into a rather lengthy and somewhat technical conversation.

As a general rule (I use that term rather loosely) if the duration of the Swell is 2 times the height, I.E. 5 ft at 10 seconds and the wind is not a factor, its a go. Now, when does the wind become a factor? Dont remember if the wind and swell have ever come from the same direction, but if the winds are 10-20 knots and low swells its a fairly comfortable ride. Get over 20 knots and things can change rather quickly, cuz now the wind waves are effecting things.
Since we are not stabilized, an ideal day for traveling south or north is 3ft swell from the west at 12 seconds with 5 knots of wind from just about any direction. Oh, and the sun is out with no clouds and 75 degrees, and the moon is gonna be a full one and bright.
Nice dream right?

Cheers