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#1
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posted to rec.boats.cruising
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This sounds like a case where deep-water swells ran onto a shallow bar or entrance. -Paul- Hide quoted text - Yes. Good call. Add to that missing the high tide and trying to cross with the tide running out. swells got steep, even steeper with outgoing tide, troughs really brought the depth down, bump thud Wut ?!......................... the rest is history. Excellent assumption on your part. |
#2
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posted to rec.boats.cruising
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On Mon, 05 Mar 2007 07:20:39 +0100, BrianH wrote:
Matt O'Toole wrote: On Fri, 02 Mar 2007 08:05:53 -0500, Roger Long wrote: Shaun Van Poecke wrote: If the waves are big enough to worry about this, the real issue is breaking waves. Any time you are in water less than about twice the depth of the average size waves going by, there is the possibility of one of the few largest waves of the day breaking on that spot. If you think a wave twice the size of the average breaking hard on your boat would create more excitement than you care to deal with, you should keep at least twice the average wave height under your keel and a little more when passing over isolated ledges. Shallow water may cause steep waves, but prevents them from building beyond a certain point. So you don't have to worry about the occasional huge wave in shallow water like you do in the open ocean. "You don't have to worry"??? Last summer in the entrance to Laguna di Marano, northern Italy, a returning professional fishing boat was slewed and rolled by an extra large breaking wave and dropped on the bottom while inverted, smashing in the wheelhouse and killing two men. A third, who was on deck, was thrown in the sea and survived. Well of course waves breaking as they hit shallow water, such as when they hit a bar, is a big problem! But then the energy dissipates, and large expanses of shallow water cannot carry big waves. Matt O. |
#3
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posted to rec.boats.cruising
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* Shaun Van Poecke wrote, On 3/2/2007 6:17 AM:
When sailing in shallow waters, what do you allow for swell/waves? oviously this would depend a bit on the bottom, the boat, and the type and size of waves/swell, but are there some good rules of thimb for this? ie. if you are sailing through waters 10' deep and there's a 3' swell, do you consider it as being 7' waters? No - the wave height usually used is "significant wave height" which is defined as "the average height (trough to crest) of the one-third highest waves valid for the indicated period." Solving the simple geometric problem leads to a depth of 8.5 feet. However, things can get complicated real quick. For starters, "significant wave height" allows for the probability of some waves being higher. In addition, if the ocean swells are deflected by an island, you can easily create interference patterns with occasional waves twice the height of the normal waves. Anyone who has sailed behind Block Island when there are ocean swells coming in has felt this. And then we get to the issue of surface waves interacting with the sea bottom, which is a whole study unto itself. You certainly don't want to be in a position where you're in breakers. Personally, I don't like being in real skinny water when there are any waves at all. Trying to infer the last few inches of clearance from roughly understood data is futile. If this stuff is of interest to you, read Van Dorn's text on Oceanography; I think I might pull it out today since its pouring rain here now! |
#4
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posted to rec.boats.cruising
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I think it is important to understand what the waveheight number means when
anounced in a weather report. They mean the "Significant Waveheight" and it is a mean average of the top 1/3 of highest recorded waves in the recorded period. Steve "Shaun Van Poecke" wrote in message ... When sailing in shallow waters, what do you allow for swell/waves? oviously this would depend a bit on the bottom, the boat, and the type and size of waves/swell, but are there some good rules of thimb for this? ie. if you are sailing through waters 10' deep and there's a 3' swell, do you consider it as being 7' waters? Shaun |
#5
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posted to rec.boats.cruising
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On Mar 3, 2:06 pm, "Steve Lusardi" wrote:
I think it is important to understand what the waveheight number means when anounced in a weather report. They mean the "Significant Waveheight" and it is a mean average of the top 1/3 of highest recorded waves in the recorded period. Steve So if I am sailing through the Keys with a NOAA report calling for 6'-8' waves I should expect..... and prepare for, lots of much larger waves? Do you mean NOAA EXPECTS mariners to be able to read and understand weather reports when preparing for a voyage? And NOAA assumes that mariners know how to use the reports as intended? Which includes the fact when a report calles for 6-8 I WILL incounter much larger waves regularly!!??! Darn, that really puts a lot of responsibility on me the sailor. I just wanted to follow my dream, drink wine in my cockpit, and cleverly Blog my way through the Keys............. Damn! Can't sombody do somthing about the weather! |