How to Read the Period of Waves

[Alecadi]

Sure I understand the meaning of the period of waves, but I would like to benefit from more experienced sailor and be able to visualize what it means in the reality when, for example I read: "Dominant period 5 seconds"

[pressuredrop]

period would be determined by timing the distance between wave crests, (with the boat hove to, or drifting, clock the amount of the between peak, do this a few times and youll have a good sense of the wave period)

[daddle]

Watch some flotsam in the water. Count the seconds between crests. In confused seas you'll get confused answers. You can watch the horizon from a moving boat, too, but the boat speed affects the time.

[hellosailor]

The period between wave crests is rarely perfectly uniform. Often there are two or more wave patterns coming in, and the "dominant" period is the prevailing one. You may see seven crests, five seconds apart, and then the next "set" of seven doesn't arrive until 3 or 8 seconds later.

[Alecadi]

I probably didn't express myself correctly.
I said I understand what it is, I know the definition .
What I would like to get is how to interpret it in real life.
When I read a weather forecast indicating the period of waves what should I expect to encounter at sea.

[daddle]

We call it 'square' when it's getting bad. That's when the period in seconds gets short enough to nearly equal the height in feet.

20 feet at 40 seconds, good. 10 feet at 12 seconds, unbearable.

[boatman61]

Originally Posted by daddle

We call it 'square' when it's getting bad. That's when the period in seconds gets short enough to nearly equal the height in feet.

20 feet at 40 seconds, good. 10 feet at 12 seconds, unbearable.

Thats not square.... thats $hit....

[Capt Phil]

My experience taught me that the closer the interval, the rougher the ride and usually the steeper the sea. LWL and tonnage also affects the ride.
With a newbie crew member on a delivery making the turn south from Cape Flattery one night, I had timed the intervals at around 5 sec's give or take. The wave height built fairly consistently through seven waves then the wave height receded to begin the cycle all over again. As a commercial fisherman working the Swiftsure Bank just northwest of Flattery years before, I recalled the fairly reliable repetition of wave sets unless a storm sea had set up.
So what does this mean? I demonstrated that once we had made our westing, wait for the seventh and steepest wave to pass, slam the starboard throttle wide open and spin the wheel to port. Voila, a safe, easy course change in a fairly heavy sea and into a much more comfortable motion, throttle back and enjoy the ride. Under sail in similar conditions, prepare for the turn by sliding off the back of the last wave and depending on wind direction, sheet out both main (which should be reefed at 1 or 2), headsail(s) and be careful not to jybe. You can handle that maneuver after you get set on your course with the wind aft and pole out any headsail you need to.
The wave performance in a particular area is influenced by wind, fetch, bottom depth and sources far outside the immediate cruising area. and varies somewhat depending on where you are cruising as hellosailor pointed out.
I've forgotten all the scientific explainations and base these comments soley on my personal observations and experience and recollections borne pretty much out of boredom and having nothing else to do but catch fish and count waves and seabirds... Capt Phil

[Sunspot Baby]

My experience matches Daddle's. The period alone means nothing. It is the combination of height and period. The rule of thumb is that when height in feet approaches the period in seconds, it is not comfortable unless you are on a much larger vessel.

George

[hellosailor]

"What I would like to get is how to interpret it in real life."
OK, when the wave period is ten seconds, you're going to get thrown in the air, knocked on the ground, and doused with cold water once every ten seconds. When the wave period is five seconds, you're going to get pummelled twice as often.

Have you ever been to an ocean beach? Hit by the waves?
Up! Down! Cough! (repeat). That's the period.

[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

[Alecadi]

THANK YOU ALL GUYS!!
As always I have learn a lot here...

:-)

Alec

[svcambria]

Originally Posted by Alecadi

I probably didn't express myself correctly.
I said I understand what it is, I know the definition .
What I would like to get is how to interpret it in real life.
When I read a weather forecast indicating the period of waves what should I expect to encounter at sea.

Yup, generally the greater the height of a wave for a given period, the more uncomfortable to dangerous it becomes, same with the shorter the period for a given height. Longer periods are generally from far off storms and are - generally - smooth until you get into shallow water. More confusing is the prediction for wave height:

http://www.mxak.org/weather/pdfs/waves.pdf

You get a "spectrum" of heights, so for a particular period you will get a spectrum of steepness and so a spectrum of discomfort or danger. They may come from a variety of directions as well causing some crests to add to crests and some troughs to other troughs and cause "confused" seas (like they cause confusion in people who are trying to understand them...)

Michael

[boasun]

It is the height of the swells in relation to the wave period that makes the difference in comfort.
I've found that swells with the height of 4 ft and a period of 7 seconds to be good. But again waves with the height of 14 ft at 7 seconds to be rough. So going by the wave period alone isn't enough. You really need the height of the seas out there or around you to determine your comfort zone.
So know your boat and when she becomes very cranky in what kind of seas.
A young lady and her S&S 34 took the southern route around the world and was only knocked down around seven or so times... But then she has that boat buttoned up tight and only the items that weren't secured properly where thrown about.

[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.