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Old 07-19-2018, 07:50 AM   #1
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Wave height vs period

Didn’t see a thread on this in the archives, so:

I understand that, as a rule of thumb, wave period should be about two times the height for “comfort” in open water. And I know that comfort is a relative term.

Just wondering what sea conditions most you in 40-ish sized boats would consider comfortable?
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Old 07-19-2018, 08:27 AM   #2
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When we lived in SoCal and often cruised to Catalina Island for the weekend, my personal rule of thumb was a forecast of 5' seas or less. Once or twice we got into 6'+ and regretted it. It wasn't unsafe but uncomfortable at that level.


Can't say what the period was at 6' but I would guess less than 12 seconds. Would 15 seconds have been more comfortable. Maybe but not much.


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Old 07-19-2018, 08:42 AM   #3
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When I crossed over from the Bahamas we were in 5-6’ (I think, I never know for sure) on the stern quarter, and it pretty much sucked. Had the sails out but we were pretty much running with the wind and motorsailing so I think the apparent wind was very low so the sails didn’t help with stability.

So, 5-6’ may be my limit, unless it’s a long period, which it never seems to be.
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Old 07-19-2018, 09:08 AM   #4
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Lots of valuables in determining comfort.

Direction plays a big roll. Going head on into seas and quartering is very different from traveling in beams seas or taking them on your stern, depending on your speed. As previously mentioned, period has a great deal to do with comfort. Duration of exposure is also a big factor. Have been in 7'+ seas a couple of times, but only for a few hours before moving into the lee of the land.

3 to 4' is my general go. Above 4', period, duration of exposure and importance of moving today, become factors. I try my best to keep stuff from sliding around. While stuff is generally stowed well, I'm not interested in cruising with a seatbelt on as my coffee cup and cellphone slide back and forth at the helm. We're out here to have fun aren't we?

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Old 07-19-2018, 09:11 AM   #5
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Quote:
Originally Posted by angus99 View Post
Didn’t see a thread on this in the archives, so:

I understand that, as a rule of thumb, wave period should be about two times the height for “comfort” in open water. And I know that comfort is a relative term.

Just wondering what sea conditions most you in 40-ish sized boats would consider comfortable?
Not a bad rule of thumb for starters. However your route vs the waves is an extremely important factor as well. Also in a lot of areas, wind waves on top of inherent swell come into play. Likewise, wave direction vs any current that may be present.
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Old 07-19-2018, 09:54 AM   #6
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We look at the period just as much as the height. We also look at the direction.

5’ x 7 Seconds is not an issue as long as they are not straight abeam, but even then they are doable, we just hit the throttle to dampen roll.

5’ X 10 seconds is no problem at all

8’ X 7 seconds is OK but “wet” as a head sea, but we don’t generally go out in those conditions. Abeam they can be extremely unplesant, requiring a quartering angle of attack.

Here is a video of us in 5’ x 7-8 second seas a couple weeks ago. These were bouy measured.

https://youtu.be/lX36WQJuEUE
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Old 07-19-2018, 10:09 AM   #7
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sorry, unable to view, for some unknown reason

Quote:
Originally Posted by ksanders View Post
We look at the period just as much as the height. We also look at the direction.

5’ x 7 Seconds is not an issue as long as they are not straight abeam, but even then they are doable, we just hit the throttle to dampen roll.

5’ X 10 seconds is no problem at all

8’ X 7 seconds is OK but “wet” as a head sea, but we don’t generally go out in those conditions. Abeam they can be extremely unplesant, requiring a quartering angle of attack.

Here is a video of us in 5’ x 7-8 second seas a couple weeks ago. These were bouy measured.
Nothing comes up but voice over, but no pics.
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Old 07-19-2018, 10:20 AM   #8
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Keep in mind that boating on the Pacific is different than the east coast or the Great Lakes. We have been in 18’ waves on the Pacific and the period was long enough so that it was exciting but we were not in fear for our lives. 18’ on the Great Lakes I would definitely be in fear for my life. Lake Erie in particular is so shallow that the waves are steeper and the period is usually short. We crossed Lake Ontario a couple of years ago in 6’ quartering waves and it was uncomfortable but not dangerous. The auto pilot worked very hard trying to keep us on course. I believe that a lot of it depends on where you are and what your comfort level is.
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Old 07-19-2018, 11:01 AM   #9
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On our top heavy 40' Nova, we can do 5' swells with long intervals all day, provided they're not on the beam, which is sometimes the case during a Predicted Log contest.
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Old 07-19-2018, 11:40 AM   #10
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Quote:
Originally Posted by Gordon J View Post
Nothing comes up but voice over, but no pics.
Here is a link to the youtube video

https://youtu.be/lX36WQJuEUE
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Old 07-19-2018, 12:07 PM   #11
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We just had a bout with really short period 4 footers on Delaware bay. It was very unpleasant. Not dangerous but we'd rather avoid it when they're that close together.
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Old 07-19-2018, 12:14 PM   #12
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For our typical cruising (here in the NE), 3-4 is about the max I "want" to cruise in. Above that, it becomes a matter of what direction, for how long and how badly do we want to make the next port today? Then there's also the very real matter of forecast vs reality. I find its always better (and often closer to reality) to add at least 1 extra foot to whatever is forecast.

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Old 07-19-2018, 12:16 PM   #13
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So many variables.

I've been out in a 10+ foot swell where with a 20 second interval and it was a pleasure to boat in. The water was almost glassy with no wind.
I've also experienced ugly 4 foot conditions with mixed up swell swell against current that was horrid.

The comfort limit also depends on the crew. I don't mind rough water, but often I don't go out if I suspect the others aboard would not enjoy themselves. My wife is more adventurous than most, but still prefers not to have to hold on to stay seated. There's a couple fishing buddies that are willing to tackle anything for a big snapper.

As far as boat handling characteristics, my boat probably likes the swell on the stern more than most. It rolls like a sausage in a beam sea with out the sails; and hobby horses into a head sea like any 30 footer. Usually there's an angle thats fairly comfortable, but it doesn't always lead to where I want to go.
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Old 07-19-2018, 12:18 PM   #14
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I don't think of it as height vs period, but height and period. It's also important to recognize what makes up the "wave." Is it wind or swell or both or multiple swells.

The Pacific is much different than the Atlantic because of the normal prevalence of swells which may be very high but very long periods.

The period double height rule of thumb is mainly for low wind waves. 2'@4 seconds, 3'@6 seconds. I think more in terms of period at least 2 greater than height and preferably 3 greater. 4' wave at 6 seconds is fine and at 7 seconds better.

There's also the separate issue of bars or inlets. You might have 3'@6 seconds outside as you cruise but find you have very large, confused waves at the inlet as they break over the bar, so that's always a separate consideration.

Waves I'll cruise on depend on so many other factors in addition to the wave itself. What boat? Experience? Guests? How long? For instance, I might be very comfortable with an hour and half run to Bimini with just my wife and myself and maybe another experienced boater in rough conditions that I would never do an 8 hour cruise off shore in with inexperienced boaters aboard.
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Old 07-19-2018, 12:34 PM   #15
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After my first round of steep 6 footers in a 3 second period, I'll be doing my best to avoid that battle next time! Ouch!

Holy crap!! What was that?
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Old 07-19-2018, 04:01 PM   #16
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I don’t get swell in the waters that I boat in except when crossing the Strait of Juan de Fuca. Most of the time we deal with wind waves. Height vs period are key. Most of the time we don’t get good forecasts on wind waves. Also, because of the variable currents, the conditions can vary wildly over a small area.

The trawler doesn’t handle the rough water nearly as well as the sailboats. Or maybe I should say that the boat handles it fine but the passengers don’t handle it as well in my trawler vs my past sailboats.
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Old 07-19-2018, 07:27 PM   #17
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Gulf Coast off Al and Fl panhandle.... a 5 second period is rare. Mostly choppy disorganized and steep.
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Old 07-19-2018, 07:48 PM   #18
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Gulf Coast off Al and Fl panhandle.... a 5 second period is rare. Mostly choppy disorganized and steep.
You mean something like this?

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Old 07-19-2018, 08:15 PM   #19
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Yeah, that's it.
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Old 07-19-2018, 08:27 PM   #20
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I call that "washing machine chop"
Remember, the only thing worse than being at the dock wishing to be out on the water is to be out on the water wishing to be back at the dock!
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