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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #21  
Old 07-24-2007
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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.

Last edited by tsingtao; 07-24-2007 at 08:57 PM.
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  #22  
Old 07-24-2007
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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...
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Old 07-24-2007
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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.

Last edited by tsingtao; 07-24-2007 at 09:52 PM.
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Old 07-24-2007
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Sorry Keelhaulin you are miscalculating the wavelength. Although the figure is correct it is in metres not feet so that alters the slopes.
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Old 07-24-2007
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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.
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Last edited by bestfriend; 07-24-2007 at 10:31 PM.
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Old 07-24-2007
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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.
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