Hello all
So if i am in a 24' trailer sailer with my head hight aprox 5' off the
water and all i can see is sky and the side of the wave and a 80.5 meter
light house 1.5nm away is out of sight in the troughs (worked by
sounding and bearing) LOCATION:
Latitude 32° 0.5' S, Longitude 115° 30.1' E
i estimated 8' to 12' breeze was forcast 30/35 kn i was knocked down
twice and had a ball.
http://www.lighthouse.net.au/lights/....htm#Operation
shaun

Ryk wrote:
On Mon, 20 Dec 2004 02:56:36 GMT, "Roger Long"
wrote:


The method for getting accurate wave height data from a boat is to know the
height above the waterline of some points at different heights. You then
stand on one and stretch or crouch until you find yourself looking across
the tops of the waves when the boat is in the trough. It usually takes a
few waves to get an average but it's quite striking when you get the right
position. You then measure from eye to feet and add it to the height known.
That's the average wave height.


I'm reassured. That's the approach I take when trying to make
estimates and I'm usually fairly consistent with what the Coast Guard
is broadcasting. I was wondering if you had some other secret trick.


I've often been amused to have even fairly experienced sailors say that the
waves must be eight feet. I usually don't point out that our eyes are
perhaps six feet above the surface and we can still see all the tops when we
are down in the trough.


Losing all the shoreside lights in the troughs can be quite dramatic,
and they are definitely higher off the water than the rest of the wave
crests.

Ryk

That sounds like fun. However, the fact that the lighthouse is out of
sight is meaningless - that could occur in a modest swell. 80 meters
at 1.5 miles is roughly 3 parts in a hundred, or a 2 degree elevation.
Even is you have a 5 foot eye height from the water line (doubtful
unless you're standing) if you're in the trough of a 10 foot swell the
light would not be visible if the wavelength is under about 200 feet.
If the wave height is only one foot above eye level the view of the
light is cut if the half wavelength is under 30 feet.


shaun wrote:
Hello all
So if i am in a 24' trailer sailer with my head hight aprox 5' off the
water and all i can see is sky and the side of the wave and a 80.5 meter
light house 1.5nm away is out of sight in the troughs (worked by
sounding and bearing) LOCATION:
Latitude 32° 0.5' S, Longitude 115° 30.1' E
i estimated 8' to 12' breeze was forcast 30/35 kn i was knocked down
twice and had a ball.
http://www.lighthouse.net.au/lights/....htm#Operation

shaun

Ryk wrote:

On Mon, 20 Dec 2004 02:56:36 GMT, "Roger Long"
wrote:


The method for getting accurate wave height data from a boat is to
know the height above the waterline of some points at different
heights. You then stand on one and stretch or crouch until you find
yourself looking across the tops of the waves when the boat is in the
trough. It usually takes a few waves to get an average but it's
quite striking when you get the right position. You then measure
from eye to feet and add it to the height known. That's the average
wave height.



I'm reassured. That's the approach I take when trying to make
estimates and I'm usually fairly consistent with what the Coast Guard
is broadcasting. I was wondering if you had some other secret trick.

I've often been amused to have even fairly experienced sailors say
that the waves must be eight feet. I usually don't point out that
our eyes are perhaps six feet above the surface and we can still see
all the tops when we are down in the trough.



Losing all the shoreside lights in the troughs can be quite dramatic,
and they are definitely higher off the water than the rest of the wave
crests.

Ryk

On Sun, 19 Dec 2004 17:56:54 GMT, "Roger Long"
wrote:
Twelve footers would
considered pretty big however, even on the ocean.

==============================================

Roger, if you can believe the NOAA weather buoys, 12 footers on the
open ocean are almost routine anytime the wing is blowing 25+, and
that happens with a great deal of frequency.

12 average or 12 max? What I'm talking about here is 12 foot, wind driven
waves, average height so that wave after wave is in the twelve foot range.
This means that the occasional big sea will be 18 to 20 feet. These will be
intimidating conditions to the average coastal sailor. They are not
uncommon overall but not frequently encountered by people who listen to
weather forecasts and have a choice about going out.

The distinction between swell and waves is significant. A 12 foot, long
period swell would hardly hamper a 30 foot boat at all whereas wind driven 8
footers could give it a real beating.

There is much more to it than the measurement from trough to crest which is
what got me asking about Lake Erie in the first place.

--

Roger Long



"Wayne.B" wrote in message
...
On Sun, 19 Dec 2004 17:56:54 GMT, "Roger Long"
wrote:
Twelve footers would
considered pretty big however, even on the ocean.

==============================================

Roger, if you can believe the NOAA weather buoys, 12 footers on the
open ocean are almost routine anytime the wing is blowing 25+, and
that happens with a great deal of frequency.

A National Weather Service table shows probable wave heights of 12 feet
being produced by 27 to 28 knot winds. These heights are for fully
developed seas and it takes several hours for them to build up.

Another common over estimation is wind speed. I was quite surprised when I
started carrying a pocket wind gauge with me while sailing. We tend to
perceive wind force rather than speed; especially when observing the
response of a sailboat. Adding a bit less than half to the wind speed
doubles its pressure. If a breeze feels twice as strong as one we know to
be 15 knots, most people would call it 30 knots whereas it would actually
only be 21.

The formula is Velocity squared x .0041.

--

Roger Long

On Tue, 21 Dec 2004 11:31:58 GMT, "Roger Long"
wrote:
If a breeze feels twice as strong as one we know to
be 15 knots, most people would call it 30 knots whereas it would actually
only be 21.
=============================

Somewhere between 30 and 35 knots the wind begins to rip off the wave
tops and send them through the air like the stream from a fire hose.
I've found it to be an infallible guide to near gale force conditions.
Also, the rigging begins to howl like a banshee in that wind range and
above.

the wind starts blowing the tops off waves in streaks of foam about 24 knots.

Somewhere between 30 and 35 knots the wind begins to rip off the wave
tops and send them through the air like the stream from a fire hose.
I've found it to be an infallible guide to near gale force conditions.
Also, the rigging begins to howl like a banshee in that wind range and
above.

On Tue, 21 Dec 2004 10:00:32 -0500, Wayne.B
wrote:

On Tue, 21 Dec 2004 11:31:58 GMT, "Roger Long"
wrote:
If a breeze feels twice as strong as one we know to
be 15 knots, most people would call it 30 knots whereas it would actually
only be 21.
=============================

Somewhere between 30 and 35 knots the wind begins to rip off the wave
tops and send them through the air like the stream from a fire hose.
I've found it to be an infallible guide to near gale force conditions.
Also, the rigging begins to howl like a banshee in that wind range and
above.

That's a good rule of thumb. By sitting at dock on squally days, you
can compare the "note" of howling rigging to anenometer readings.
That's how I can tell over 30 knots...it gets pretty damned noisy and
stuff not secured begins to leave the boat.

If you are close to shore, however, on a beam reach relative to shore
and coming from there, you get lots of wind sound and a lot less water
sound, because you can have 40 knots and 2 feet of waves. That's why
lake sailing is fun because you can train your ear in less
"uncontrolled" conditions.

R.

Roger Long wrote:

Another common over estimation is wind speed. I was quite surprised
when I
started carrying a pocket wind gauge with me while sailing. . .

Part of your surprise may be due to the velocity gradient. The wind
blows harder the higher you are above sea level, with the gradient
being stronger or weaker depending on how stable condtions are. It is
caused by friction with the sea; at the sea surface windspeed is
effectively zero, rapidly increasing in the first few feet, but is
still several knots different from deck level to mast top, where
sailboat wind sensors are.

I've seen conditions (building wind from a calm) where it's blowing 10
knots 55 feet up, and only 5 knots on deck, measured by acccurate
sensors. It's one of the reasons the big boys with the ninety foot
sticks seem to be able to move in drifting conditions while the punters
are becalmed.

On 24 Dec 2004 22:16:50 -0800, "Mark" wrote:

Roger Long wrote:

Another common over estimation is wind speed. I was quite surprised
when I
started carrying a pocket wind gauge with me while sailing. . .

Part of your surprise may be due to the velocity gradient. The wind
blows harder the higher you are above sea level, with the gradient
being stronger or weaker depending on how stable condtions are. It is
caused by friction with the sea; at the sea surface windspeed is
effectively zero, rapidly increasing in the first few feet, but is
still several knots different from deck level to mast top, where
sailboat wind sensors are.

I've seen conditions (building wind from a calm) where it's blowing 10
knots 55 feet up, and only 5 knots on deck, measured by acccurate
sensors. It's one of the reasons the big boys with the ninety foot
sticks seem to be able to move in drifting conditions while the punters
are becalmed.

That would probably explain why so many more trees were blown down in
my neighborhood in Pensacola when hurricane Ivan came ashore. We lost
8 of 10 trees in my front yard. That was normal in our neighborhood
on the NE side of Pensacola where we are at 115' above sea level. The
neighborhoods at only 10' above sea level did not lose nearly as many
trees.

mike
--

Mike Hendrix
Pensacola
http://travellogs.us/