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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation > Steep/Breaking Waves
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Topic Review (Newest First)
07-24-2007 11:44 PM
sailingdog TsingTao-

Might also be worth noting that if the wind is coming off the shoreline, then the fetch will be insufficient to build significant wave height... the fetch has to be fairly large for significant wave height to build as a general rule, and if you're just limiting yourself based on wind speed, but not accounting for wind direction, you're going to miss out on a lot of good sailing.

For instance, on Buzzards Bay, if the wind is out of the Northwest, there isn't significant fetch distance for most of the bay, since the bay runs SW-to-NE. SW winds will have significant fetch and can have some serious wave height.
07-24-2007 11:22 PM
bestfriend While this may not answer your questions specifically, may I suggest reading this book. I found a few other interesting books about the dynamics of open ocean waves too. Here and here.

By the way, enjoying the thread. I can sink my teeth into this, all that solar electrical stuff makes my head hurt.
07-24-2007 11:15 PM
chris_gee Sorry Keelhaulin you are miscalculating the wavelength. Although the figure is correct it is in metres not feet so that alters the slopes.
07-24-2007 10:49 PM
tsingtao Chris Gee,
Think you've finally helped me get things sorted out. Just read the web site you suggested (Australian Gov, Bur of Meteor Researh Cntr) and Table 4.1 seemed to make the most sense about what I've been trying to figure out.
20 hours of 10 kt winds produce 3 foot waves rated Moderate
25 hours of 20 kt winds produce 6 foot waves rated Very Rough
30 hours of 30 kt winds produce up to 24 foot waves rated High
Based on everyone's input, I think I'll limit my coastal passages to forecasts of sustained winds of 20 knots or less until I get more experience and the practical knowledge that many of you have. Thanks

Plenty time mon. If it takes a year, so be it. I have found out that the weather WILL dictate when Ya get there.
07-24-2007 10:45 PM
KeelHaulin Let's work with the 8' wave every 8s a little. The calculation gives a wavelength of 125'; so that makes the wave crest to bottom of trough distance ~60 feet. From the bottom of the trough to the middle of the next wave face is about 30 feet; so if you are in a 30' boat your bow will be digging into the middle of the next wave when the stern is at the bottom of the trough. You also have to consider the fact that you are moving forward into these waves (not sitting still) and the time that you will be going from crest top to crest top will be much faster than every 8 seconds. The sailing would probably be rough and uncomfortable, possibly dangerous depending on wind conditions. I'm not saying that every combination of H=T will produce a dangerous situation; but the "rule-of-thumb" that I read was if T is equal or less than H it can be bad.

I can't comment on your sail up the coast; we have not had the need or desire to go on a long passage yet. The California coast can be treacherous; I'm sure you are aware of that. With winds regularly 35 kts in the slot and steep chop in the gate we get enough white water to keep me content with weekend trips.

Plan your trip around weather windows and you should be fine tsingtao; take your time traveling up if the weather is not permitting; people tend to get into trouble when they are rushing to get to their destination. Some say that the winds are lighter and more consistent farter offshore; but then you are farther from a safe harbor if it gets bad or the fog comes in.

JMHO...
07-24-2007 09:44 PM
tsingtao Chris Gee,
I certainly appreciate your input. If one simplified the "general" go/no go decision making process for a port to port passage to a forecast of less than 25kts and period equal to, or more than, wave height would that constitute a relatively conservative approach on the Central California, Oregon, Washington coasts? From your figures and analysis that "appears" to be the case. I use the word "general" because I realize you have to factor in what has been happening, what is forcast for later, bars, harbor approaches etc. etc.
PS Best Friend, give me an email. Wouldn't mind sharing a cold one and your local knowledge.
07-24-2007 08:54 PM
chris_gee Theory and practice are not divorced.
The original question was about breaking waves. All waves break in shallow water ie at the shore.
In deep water the wave characteristics depend on fetch ie over what distance the wind has been blowing, duration and wind speed. As these increase so do wave height period and wavelength.
For a wave to break the relationship in deep water of 1/7 holds.
For practical coastal purposes a F8 fresh gale blowing for 30 hours over 400 miles gives a significant (average of the highest third) wave height of 8m and a period of 10 seconds with a wavelength of 150 m. I suspect the average coastal sailor would avoid going out in that forecast let alone knowing a gale has been blowing for 30 hours. Nevertheless he should be able to handle F8 and the waves will not be breaking.
However against a strong current or in shallow water the wave period will shorten and the waves steepen, and may well break, say on a bar. Hence if it is bad it is better to head offshore rather than through shallow water and against a current.
i don't agree that a 12' swell every 12 seconds is bad news, the wavelength is 216 m and you would hardly notice it.
I haven't heard of the waveheight in feet = period rule of thumb but it doesn't sound right. An 8' wave at 4 seconds would be 24 metres apart and at 8 sec 128m. The former you may wish to avoid but the latter is hardly a problem.
Period and wave height inmetres may be equal but only with strong gales over a long time and fetch see http://www.bom.gov.au/bmrc/pubs/tcguide/ch4/ch4_3.htm.
In shallow water you can get a nasty chop particularly with wind against tide but the wave height will be quite low and the period very short. That may be unpleasant but not dangerous in a reasonable sized boat.
A rule of thumb is to stay home if the forecast windspeed is greater than the boat length in feet. As mentioned a 32 ' boat should be able to handle 30 knots adequately reefed, because intended or not it is likely to encounter such winds. However the seastate depends on fetch current depth and wind duration which are local conditions.
A 5m ocean swell should not be a concern, assuming a reasonable depth because to reach that height requires time fetch and quite a long period unless you are talking hurricanes.
I daresay you can get water over the bow in 8' but the period is likely to be less than 5 seconds and the wavelength short.
07-24-2007 01:56 PM
tsingtao KeelHaulin,
Thanks for the info. Sounds like theory and practical are two entirely different things. With all the talk about "breaking waves" was trying to find out how you could anticipate/predict them. Looks like you are in deep doo doo long before the theoriretical point, at least the way it's calculated. You're right about the bouy averages--Pt Conception and Pt Arguello taught me that. Bouy data said 6' at 9". Know there were MUCH larger sets. Real question is still, looking at forecasts and bouy reports, wind aside, what sea conditions trigger the response "I,m gonna sit under a tree with a cold one" and wait for things to calm down? All this assuming your intent is a coastal passage from one safe harbor to the next. After analyzing available data, would like to be able to visualize what I'm going to encounter. Is the "square wave" a real life indicator? What is the "community" standard for sensible operations? Have the Capital Yachts site on my favorites and just found the Newport forum. Thanks.
07-24-2007 02:34 AM
KeelHaulin
Quote:
Originally Posted by chris_gee View Post
.14 is 1:7.
So ten seconds is 150 and the critical height is 21.4 m roughly 70 feet. That depends on the water being on the water being more than half the wave height deep.
If it were 5 seconds it would have a critical wave height of about 5.5 m.
Of more importance to you is that in shallower water waves slow down so decrease in wave length and increase in height and break at a depth of 1.3 height generally that will be approaching shore. So if the waves were 3 m they would break in 3.9 m a wave height and depth you would want to avoid, but usually evident by breaking waves.
A 5 m ocean swell is not a concern because the wavelength is long so the slope is more gradual.
Whaat??!!

I don't think I would really want to be in 10ft seas every 10s; LET ALONE 70ft SEAS EVERY 10s! Something is wrong with that calculation; please don't use it as rule-of-thumb. I'm not sure you could get a wave much taller than 20ft on a 10s period without it breaking, or the top being blown off and washing down the face.

I would not at all say that 5m (16') swell is not a concern; it really depends on the wave period; not just how big the lumps are. We've been in 8' swell that was more like 8' chop and it was a coaster ride with the entire bow pulpit in green water each time the boat crested and dove down into the next wave face.

One rule-of-thumb that I have heard is that you don't want a period in seconds equal or less than the height of the wave in feet. So a 12 ft swell every 12s is bad news. A 12ft wave every 20 seconds would be more of a 'lump'. Also remember that bouy data are averages, not maximum. If the seas are confused you can get stacked and/or rouge waves in addition to the "average" waves.

For sailing out the SF Gate, the wind waves at the gate and out to Pt Bonita are mostly due to opposing currents in the Summer. You want to catch the ebb when you go out (otherwise you probably won't get out) and that is in opposition to the wind direction on most days. So you want to go on days when the ebb is early in the day before the wind has built up to it's afternoon howl and when the ebb is light (below 2 kts) if possible. We have tried going out on days when the current was moderate ebb but heavy wind, and west of Pt Diablo it was like a washing machine. I'm sure you could sail through it but it is a tough beat to get out into the open ocean. Also be aware that north and south of the gate are shoals that produce nasty wind/current induced waves also.

tsingtao-

If you have not already found the owner's website for your Gulf 32; here it is:

www.capitalyachts.info

In addition you might want to join the Newport owners email list:

http://list.sailnet.net/read/?forum=newport
07-24-2007 01:22 AM
bestfriend Kudos to you. Can I bring you a 12 ounce welcoming gift, or perhaps something in a red?
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