How to Read the Period of Waves

[Roy M]

Plus, the clearance between the waterline and the bridge deck will be greater in a bigger boat, and the bow wave wake will have to travel further aft before it hits the other hull of the longer boat. In a lot of ways, bigger can be better.

[MarkJ]

Originally Posted by Limpet

But I'm wondering if there is some rule of thumb to determine a relatively comfortable ride considering wave height, period and boat length.

There was a rule about when a sea gets dangerous... when its half the size of the boat so 20 feet high for a 40 foot boat.

8 seconds period is bumpy no matter the size wave...

[Hydra]

There is a relation between the period of waves, their length and their speed, provided that the depth is more than half the wavelength.

In metric units, wave length in metres=1.56 * period (squared) in seconds

In feet, wave legth = 4.5 * period squared

And the speed in knots= 3 * period in seconds

Waves with 10 seconds period are roughly 150m or 450 feet long and propagate at 30 kts.

If the depth is less, the length and the speed are reduced.

Alain

[Bashleys]

Susan Casey - "Wave". More than u will ever want to know!

[Hydra]

There is some detailed information in these *free* documents:
http://www.wmo.int/pages/prog/amp/mm...702/WMO702.pdf
CHL - CEM Distribution (Download Chapters) (see Part II)

Alain

[rtbates]

It relates to how many times your bow will be going up and down in any given time period. A 5sec period would raise and lower your bow 12 times every minute. Not comfortable unless the height is maybe 6".

[pkpdjh]

I have read through this post and followed a bunch of these links. I think I have an idea of what to expect for my trip this weekend. However, I would love feedback from someone who is experienced reading this wave information.

Below I have posted the expected conditions for a couple day sails off the coast of San Diego this weekend. The fetch for these waves will be very long (across the Pacific). The depth will also be very deep except perhaps right when I am entering the open ocean.

Could some experienced folks give me an idea of what this will "feel like?"

I want to make sure I am being safe and won't have sick passengers.

Friday Feb 3, 2017
Wind: W 5mph, steady.
Waves: 2 meters, 12 seconds apart
Boat: Catalina 250, fin keel (LWL = 21.5 ft)
Location: Mission Bay Buoy
https://www.windfinder.com/forecast/mission_bay_buoy

My guess: The waves will be big, but fairly consistent and not breaking. Still, 2m of wave on a boat with a 6.5m LWL is not that small. If a "monster" wave comes, it could be a wild ride. 12 seconds apart for 2-meter waves is not too choppy. Might be best to stay in the bay for day one of the trip.

Saturday Feb 4, 2017
Wind: W 5mph, steady.
Waves: 2 meters, 12 seconds apart
Boat: Beneteau 373 (37")
Location: Point Loma
https://www.windfinder.com/forecast/...oma_south_buoy

My guess: Again, the waves will be big, but fairly consistent and not breaking. 2m of wave on a boat with a 10m LWL is okay. 12 seconds apart means they will be somewhat steep, but not crazy. 2-meter waves will still test passengers' stomachs.

Thanks in advance,
Dave ...

[pkpdjh]

Originally Posted by Hydra

There is a relation between the period of waves, their length and their speed, provided that the depth is more than half the wavelength.

In metric units, wave length in metres=1.56 * period (squared) in seconds

In feet, wave legth = 4.5 * period squared

And the speed in knots= 3 * period in seconds

Waves with 10 seconds period are roughly 150m or 450 feet long and propagate at 30 kts.

If the depth is less, the length and the speed are reduced.

Alain

Thanks for this. So:

The forecast says 6-foot tall waves, 12 seconds apart in deep coastal water.

So, the length should be 4.5 * 144 = 648ft.

That means the length:height of the wave is 100:1. So is that not too choppy?

[Hydra]

pkpdjh,
In fact, what your forecast describes is a swell, not wind-generated waves. In deep water, the crests will be rounded and not breaking.

If the Point Loma south buoy is the safe water buoy I see in Navionics webapp (Navionics Webapp) approximately 3 NM south of the point, the depth is about 70ft. This is not deep water (depth less than half the theoretical wavelength in deep water) but not so shallow as to cause this swell to break.

However, the wavelength will be reduced. In this page (Oceanography: waves), I have found a graph that gives the theoretical wavelength (in meters) with respect to the period (in seconds) and the depth (in meters). At this buoy, I would expect 180m / 600ft wavelength, still long by comparison with the height.

There are 2 other things to consider:

- The forecast gives the "significant height" of the waves and the swell. This is the average of the highest third of all waves. This means that you will encounter waves higher than the significant height. In fact, without even considering rogue waves, you must expect to encounter some waves twice the significant height.

- When the energy of the swell arrives from offshore and meets the continental slope, part of it is reflected and refracted over the continental shelf. This means that in deeper water, you will encounter higher crests.

Anyway, I would not consider this as choppy.

5mph is Beaufort 3, so no risk of having big wind-generated waves superimposed to the swell.

I hope this helps.

Alain

[pkpdjh]

Alan, thanks for all the great information. I will look again at the graph you referenced.

Also, I made a copy/paste mistake on my initial post. The wind for Point Loma on Saturday will build from 7-12 to 13-17 mph. I would expect that create more confused seas and more potential chop. At the same time, the current forecast is only showing 3ft swells.

Map Results | Wind, Forecast, Radar, Weather & More | SailFlow

Unfortunately, the outlook for my Friday at Mission bay has changed to 4mph wind and rain. I guess I will have to stay tuned.

Thanks again.

[JSLExplores]

Originally Posted by Limpet

How does the boat's length factor in? Certainly a large boat is less affected by the same wave height and period as compared to a small boat. But I'm wondering if there is some rule of thumb to determine a relatively comfortable ride considering wave height, period and boat length.
A 40' Cat would have a tough go of it in 8 foot waves with 6 second period, but a 60' foot cat may not find it all that rough as the boat length might bridge several waves.

Great question! Would love to see a graphic or chart that shows this.

[Stu Jackson]

pk,

To your original question: the way I learned what it meant was to ask as you have, then also do what you did of finding out what it was forecast to be, and then go out in it! Nothing beats personal experience. When I got back, I looked at the historical buoy reads on noaa's website.

Mixed in the replies: the noaa weather forecasts always give SWELL (x feet and y seconds) and WIND WAVES (only in feet) which are on top of the swell (literally and figuratively ). The windier it is locally, the bigger the wind waves.

Go out and see for yourself. Eventually you'll learn what they are, and be able to figure out what they are from what you see. Then go back check online.

[westsailwill]

Originally Posted by sailorboy1

if the period in seconds is 1.5 times the height in feet the ride is medium calm, if 2 times it is calmer, etc etc

but the problem is in trying to use this that all bets are off is the seas are confused

everyone of course will in time decide their own period/height rules and it will even change with the boat

[Don C L]

It’s an old thread, wonder what happened? I also wonder which app or forecast had wave heights in meters around here.
Edit, ok found it. I hadn’t been using windyty for wave height forecasts.