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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > Why Waves Look Bigger From a Boat
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Topic Review (Newest First)
11-06-2012 10:05 PM
Roger Long
Re: Why Waves Look Bigger From a Boat

Quote:
Originally Posted by Hartley18 View Post
Maybe what we need is a reference table of Actual Wave Height vs Perceived Wave Height.
If you are looking at the waves, just ask yourself how high they look and then divide by 2. You'll be close enough for such a fuzzy number.

If anyone says they were out in XX foot waves and you know they are reporting a visual estimate, just divide by 2. If they are in a bar, divide by 4.

Oh yes, divide all the wave heights in the book "Perfect Storm" by two as well. I was at first disappointed that the author didn't research and get this right. I then realized that the book is about the experiences of the people. They experienced waves that were twice as high as a proper measurement would show so I think the book is find. The movie then took those waves and doubled them again but that's Hollywood.

BTW, my former boss did the last stability test and analysis on the Andrea Gail. I asked him after the book came out what he thought happened and he said, "They never saw the storm. That piece of **** would have rolled over long before the real weather got there."
11-06-2012 09:29 PM
Classic30
Re: Why Waves Look Bigger From a Boat

Quote:
Originally Posted by Barquito View Post
A related phenomenon is why waves look so flat in photographs. I presume this has to do with a frame reference problem, in that, you don't know where the horizon is in a photo.
Maybe what we need is a reference table of Actual Wave Height vs Perceived Wave Height.

If Roger's original post is correct, it should be possible to work this out empirically.. Roger?
11-06-2012 12:08 PM
JulieMor
Re: Why Waves Look Bigger From a Boat

Quote:
Originally Posted by Minnewaska View Post
Why he would say it is probably at the root of this entire calamity.
Sadly, I've thought that too...
11-06-2012 11:53 AM
Barquito
Re: Why Waves Look Bigger From a Boat

A related phenomenon is why waves look so flat in photographs. I presume this has to do with a frame reference problem, in that, you don't know where the horizon is in a photo.
11-06-2012 11:51 AM
Barquito
Re: Why Waves Look Bigger From a Boat

Quote:
In those sea conditions, that boat should have reefed by now.
Not on the s/v BFS
11-06-2012 08:04 AM
Roger Long
Re: Why Waves Look Bigger From a Boat

Quote:
Originally Posted by JulieMor View Post
I'm not sure if Walbridge was referring to continuous wave heights when he mentioned 70 feet or if he was referring to a rogue wave.
I am and I think you will be too if you watch the video:

HMS Bounty - YouTube

I've seen two rogues, one of which put a 105 foot schooner so far under water that just the two masts were sticking out of the ocean. They are very dramatic. You look out at the horizon and all the waves are going up and down but one isn't. It just hangs against the skyline like an island until you are suddenly looking into the huge hole in the ocean ahead of it. It is this deep trough that does as much damage as the crest in most cases. I remember looking over the bow of the schooner and seeing the keel dry back to the foremast before she basically sailed dropped and dove into the crest.

Rogue waves are such dramatic and infrequent events that I'm sure he would have described it as such instead of going on about how easily the ship was riding in wave after wave.

Waves are officially reported and forecast by "Significant Wave Height" which is the mean of the largest 1/3 of waves. This graph shows the distribution:



Note that wave height runs along the horizontal axis. Most waves are smaller than the nominal height but a few can be up to twice as large. I could believe the captain of the Bounty if he said, "We saw A 70 foot wave.", but not when he talks many 70 foot waves easily. I have no doubt however that the waves appeared to be 70 feet high to him and that he sincerely believed they were.
11-06-2012 07:44 AM
Minnewaska
Re: Why Waves Look Bigger From a Boat

Quote:
Originally Posted by JulieMor View Post
....so Walbridge may have experienced a wave 70' high in 98 knot winds.
In the same testimony, he claimed to ride those 70 ft waves like he was standing on the ship at the dock. Even if you assume they were so far apart that you might gently roll over them (still ridiculously unlikely), the wind necessary to create them would try to take you off your feet.

Bottom line is, what he said just could not be true.

Why he would say it is probably at the root of this entire calamity.
11-05-2012 11:52 PM
JulieMor
Re: Why Waves Look Bigger From a Boat

I'm not sure if Walbridge was referring to continuous wave heights when he mentioned 70 feet or if he was referring to a rogue wave. Rogue waves used to be considered a freak of nature, occurring something like 100 years apart. Now we know through satellite imaging and storm buoys that rogues are a rather common occurrence. During the "Perfect Storm" of 1991, a buoy off the coast of Nova Scotia reported a wave height of 100.7 feet, so Walbridge may have experienced a wave 70' high in 98 knot winds.
11-05-2012 07:46 PM
Omatako
Re: Why Waves Look Bigger From a Boat

Roger

Firstly I agree that 70 foot waves are improbable/unlikely. Many times I watch videos of rescues where the seas are reported as 60 or 70 ft and when you do simple comparisons with other elements (boats, aircraft, etc.) you can see that it is not true. It is also no coincidence that the graph posted stops at 55 feet. Anything above that is seriously abnormal.

The graph that you posted leaves out one vital element (IMHO) and that is duration of the wind. We were caught in a strong blow in the South Pacific, a condition known as a squash zone. Winds of 75 knots gusting 85 but the storm system moved very fast by comparison to normal tropical depressions (35 knots compared to some storms that travel at under 10) so the duration of "our" blow was just 24 hours compared to sustained blows of 4 days or more.

After 18 hours of 75 -85 wind speed we estimated the wave heights at 35 to 40 feet so considerably smaller than the 55+ feet suggested by the graph. I'm not contesting the graph, I think it is probably correct for sustained high-wind conditions.

We gauged the height of the waves by observing how much wave as behind and ahead of our boat when going up and over the swells and figured that the surface of the wave was probably about 60 feet but that is not its height. It is the hypoteneuse of the triangle if you will, much longer than the vertical.

The problem with 35 foot waves is when "just the top" breaks and becomes a foamy roller, it can be up around 10ft of white water which on your local beach is a really scary-sized wave and it hits the boat with astonishing force. I know, we got hit twice.

Thanks for the geometry lesson, most interesting.
11-05-2012 05:53 PM
CaptainForce
Re: Why Waves Look Bigger From a Boat

There's another perception factor that is related to the size of the boat. Have you ever noticed how much faster your speed appears in a low sports car compared to sitting in an SUV at the same speed? In the same manner, the waves appear larger on a smaller boat. It's a parallax factor.
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