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

Shaun Van Poecke wrote:

When sailing in shallow waters, what do you allow for swell/waves?

Half the wave is down and half the wave is up so, if you are cutting it real
close and just worried about bumping the keel, you subtract half the height.
That's a little nuts though because bigger waves come along, soundings are
scattered, old mooring blocks and other stuff gets left on the bottom, etc.

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.

--
Roger Long

* 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!

"Roger Long" wrote in message
...

Half the wave is down and half the wave is up Roger Long
This seems to be a good time to raise this issue. The Annapolis Book of
Seamanship, and others, agree with your statement, Roger.

But I am sure I have read somewhere that wave height, like the amplitude of
a sound or radio wave, is measured from the average to the extreme. This
would result in a value of one half what is commonly stated. This causes me
to pause when someone asks "how high are the waves ?".

Has anyone else heard of wave height measured this way ?

On Mar 2, 7:11 pm, "Garland Gray II" wrote:
"Roger Long" wrote in message

...

Half the wave is down and half the wave is up Roger Long

This seems to be a good time to raise this issue. The Annapolis Book of
Seamanship, and others, agree with your statement, Roger.

But I am sure I have read somewhere that wave height, like the amplitude of
a sound or radio wave, is measured from the average to the extreme. This
would result in a value of one half what is commonly stated. This causes me
to pause when someone asks "how high are the waves ?".

Has anyone else heard of wave height measured this way ?


Crest to trough = wave heigth
but the stinker is when we hear or read the "marine forcast" and it
mentions wave heigth. I that measurment is a mean or average
calculation of what?

* Bob wrote, On 3/3/2007 12:38 AM:
....
But I am sure I have read somewhere that wave height, like the amplitude of
a sound or radio wave, is measured from the average to the extreme. This
would result in a value of one half what is commonly stated. This causes me
to pause when someone asks "how high are the waves ?".

Has anyone else heard of wave height measured this way ?


Crest to trough = wave heigth
but the stinker is when we hear or read the "marine forcast" and it
mentions wave heigth. I that measurment is a mean or average
calculation of what?

This is why I posted the definition directly from the NOAA web site:

"the average height (trough to crest) of the one-third highest waves
valid for the indicated period."

For a full explanation of the statistics behind this read
"Oceanography and Seamanship" by William G. Van Dorn.

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

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!

On Fri, 02 Mar 2007 08:05:53 -0500, Roger Long wrote:

Shaun Van Poecke wrote:

When sailing in shallow waters, what do you allow for swell/waves?

Half the wave is down and half the wave is up so, if you are cutting it real
close and just worried about bumping the keel, you subtract half the height.
That's a little nuts though because bigger waves come along, soundings are
scattered, old mooring blocks and other stuff gets left on the bottom, etc.

Exactly. I never cut it that close.

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.

Matt O.

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.