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Old 07-24-2007
KeelHaulin
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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-

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