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Old 21-08-2018, 12:10   #1
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How to estimate roughness of sea?

The marine weather reports I read don't seem to have enough information to estimate how rough the sea will be. For example, I want to take a 30-mile trip that has the following forecast:

Quote:
NW winds 5 to 10 kt. Seas around 2 ft.
Normally I go out in sub 10-kt winds no problem in my 32-foot boot with a V-hull, but if there are 2 foot waves spaced 5 seconds apart it will be a rough ride. The spacing of the waves is the difference between just rolling swells and slamming.

How can I estimate how rough it will be?
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Old 21-08-2018, 12:15   #2
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Re: How to estimate roughness of sea?

Quote:
Originally Posted by jsc7 View Post
The marine weather reports I read don't seem to have enough information to estimate how rough the sea will be. For example, I want to take a 30-mile trip that has the following forecast:



Normally I go out in sub 10-kt winds no problem in my 32-foot boot with a V-hull, but if there are 2 foot waves spaced 5 seconds apart it will be a rough ride. The spacing of the waves is the difference between just rolling swells and slamming.

How can I estimate how rough it will be?
you need the full report the report you posted said wind speed and wave height but nothing about preiodicity
A 10 ft wave pattern at 15 second dominate period will be much smoother ride than a 2 ft wave pattern at 3 to 5 second period.
Mostly you need to get a better weather forecast/ report .
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Old 21-08-2018, 12:19   #3
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Re: How to estimate roughness of sea?

Try the NDBC

https://www.ndbc.noaa.gov/station_pa...?station=44013

Both the existing conditions and the forecast.
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Old 21-08-2018, 12:25   #4
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Re: How to estimate roughness of sea?

What detailed report? I have used both the NDBC and NWS marine forecast and neither one seems to mention periodicity in the forecast.
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Old 21-08-2018, 13:15   #5
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Re: How to estimate roughness of sea?

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What detailed report? I have used both the NDBC and NWS marine forecast and neither one seems to mention periodicity in the forecast.
and it won't for inland waters look at the forecast for open ocean.

Inland forecasts are wind wave heights and are normally 2 ft are on 5 second 4 ft are usually about 7.5 second and so forth and in a smaller power boat I wouldn't want to be in 4 ft wind waves but in a sailboat not an issue till they reach 6 or so feet.on 10 second periods approx
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Old 21-08-2018, 13:37   #6
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How to estimate roughness of sea?

It also helps to know how the weather is changing. If thereís been a big shift in direction expect it to be lumpier. If itís been 5-10 for awhile in the same direction youíre better off.

Windy.com can be a good tool for visualizing sea state development if your target date is a few days out.
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Old 21-08-2018, 13:43   #7
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Re: How to estimate roughness of sea?

Take a look at the Wind Alert app. There are sure to be stations near your sailing area and they predict wave height and period as well as wind speed, direction and gusts.
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Old 22-08-2018, 08:36   #8
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Re: How to estimate roughness of sea?

You can also try Windy
Periodicity available for both waves and swell.

Good winds and FOLLOWING SEAS!
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Old 22-08-2018, 09:06   #9
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Re: How to estimate roughness of sea?

You can also try Windy
Periodicity available for both waves and swell.

Good winds and FOLLOWING SEAS!
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Old 22-08-2018, 09:14   #10
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Re: How to estimate roughness of sea?

Find the reports for off shore bouys for your area that are to windward of your course. They mostly report wave period as well as hight. Probably an overestimate for coastal wathers but you soon get an idea of the relationship.
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Old 22-08-2018, 09:59   #11
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Re: How to estimate roughness of sea?

Sea state is also affected by tidal stream, and in places with high flows the wind against tide effect can result in seriously unpleasant sea. Also knowing how your vessel handles following and quarter seas may be relevant depending on oneís preferred direction. Other factors affect wave patterns too, headlands, islands, swell waves etc. So knowing your area, planning your trips ahead, after working out tidal streams, looking at a variety of weather information are important skipper responsibilities.
Boats arenít like cars, and you usually canít just think Ďoh itís a nice day today letís go to ...'
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Old 22-08-2018, 10:40   #12
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Re: How to estimate roughness of sea?

I assume that was just a cut and paste with the 2' waves. A 32' v-hull that doesn't ride well in 2' waves...something is wrong with the boat.

But to the question: if you can find out the period of the wave that will tell you a lot but even if you get a report on the wave period, if you are in coastal areas the conditions can change a lot depending on the coastal shape and bottom conditions.
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Old 22-08-2018, 11:50   #13
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Re: How to estimate roughness of sea?

I agree with Tayana - the Wind Alert App is quite good and has changed it's name to "Sail Flow". Another superb app is "Windy" and it allows you to choose the forecast model most suitable to you.
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Old 22-08-2018, 12:37   #14
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Re: How to estimate roughness of sea?

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Sea state is also affected by tidal stream, and in places with high flows the wind against tide effect can result in seriously unpleasant sea. Also knowing how your vessel handles following and quarter seas may be relevant depending on oneís preferred direction. Other factors affect wave patterns too, headlands, islands, swell waves etc. So knowing your area, planning your trips ahead, after working out tidal streams, looking at a variety of weather information are important skipper responsibilities.
Boats arenít like cars, and you usually canít just think Ďoh itís a nice day today letís go to ...'
Yes the effect of tidal streams and currents on sea state are localised, often down to a few dozen yards, and changeable in a few minutes (say, as a tide turns). We have a local area (called "the rip") where I've seen the seastate change from a 3-4 foot windwave, over 25 yards, a 10-15 foot standing breaking wave for half mile.

I've never seen a forecast with fetch or periodicity (nor have I looked, actually)
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Old 22-08-2018, 14:38   #15
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Re: How to estimate roughness of sea?

The wind strength has differing effects on the sea surface determined by the length of uninterrupted flow, which when it is hundreds of kilometres and the wind has been constant for some days can build a huge swell.

Generally speaking though, the stronger the wind, the rougher the sea. Tides against the wind make that sea a steeper one, tides with the wind make the sea waves travel a little faster, coastal influence can produce HUGE standing waves if one gets too close to a lee rocky shore with nice cliffs and lots of jagged boulders many times the size of your vessel--

I vebntured forth in my trimaran with forecasts of 20 km per hour winds developing towards the afternoon. By about one o'clock I had winds abeam of 75 km per hour recording on my instruments, that same aforementioned rocky lee shore, and when in fifty metres of water, could see clearly the bottom in the troughs of the standing wave system that developed about two hundred metres from the rocks

Then the engine quit--bubbles in the fuel sloshing around.

Somehow, we edged our way out to sea.

NEVER TRUST WEATHER REPORTS, no matter how "Reliable" the source. They are only right about ninety percent of the time.
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