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

"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.

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.

"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.- Hide quoted text -

Oh, Id let you talk to a friend of mine who took a 36' somthing in to
a snug little harbor on the south Oregon coast. But he is dead. Oh,
thats right................ while entering the channel between the two
jetties he hit bottom, next wave hit an pivoted his boat on the keel,
kicked him beam-to, the next one rolled him over a few times. Cause of
both fatalities: shallow water with big rollers make for dead sailors.
Ups, guess they should have measured water depth, wave height, and
draft.

"BrianH" wrote in message
...
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.

This sounds like a case where deep-water swells ran onto a shallow bar or
entrance. In this case, when the swells start to feel the bottom they slow
down, getting closer together and steeper, and taller -- not a good place to
be, of course. On the other hand, when you are surrounded by shallow water,
any large deep-water waves will quickly lose their energy (often by
breaking) and become smaller. And as Roger said, wind-driven waves can not
build beyond a certain height in shallow water.

-Paul

Paul wrote:
"BrianH" wrote in message
...
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.

This sounds like a case where deep-water swells ran onto a shallow bar or
entrance. In this case, when the swells start to feel the bottom they slow
down, getting closer together and steeper, and taller -- not a good place to
be, of course. On the other hand, when you are surrounded by shallow water,
any large deep-water waves will quickly lose their energy (often by
breaking) and become smaller. And as Roger said, wind-driven waves can not
build beyond a certain height in shallow water.

This whole area is shallow, around 3-6 metres out to at
least 2nm off-shore. The channel is dredged to about 5
metres, on average. The fishing boat was slightly out of the
channel, converging on it and well off from the entrance, in
about 3 metres depth. The wind was fresh and directly on-shore.
It sure shocked us sitting in harbour wondering if we should
be going out.