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 Roger Long 11-05-2012 12:22 PM

Why Waves Look Bigger From a Boat

The recent discussions about the loss of the Bounty and statements by her captain that she had previously survived 70 foot waves make the subject of wave height perception timely.

Here is a graph showing the wave heights that can be expected in various conditions:

http://oceanservice.noaa.gov/educati...tion_fig01.jpg

You can see that 70 foot waves are rather improbable. We certainly would have heard about the storm that produced them if the Bounty had been in them. Because of their circular flow, hurricanes don’t actually produce extremely large waves but large and confused waves. Waves larger than 50 feet only occur in deep water in a couple parts of the globe where very strong winds blow in the same direction for days at a time.

Waves always look higher than they are and there are two reasons for this:

One:

A lot of people looking at waves are nervous and scared.

Two:

This one is rooted in physics and explains how even an experienced mariner can report something shown to be so improbable by the graph above.

We first have to look at a little known aspect of wave physics. Imagine a vessel which sits flat on the surface of the water and rolls little such as a raft or an inner tube. Mount a tripod on this float with a plumb bob and watch it go over a very smooth swell. The plumb bob will not move but will continue pointing to the same place on the raft as it would in flat water. There are even wave tank movies of a plumb bob pointing straight up inside the curl of a breaking wave when the float is carried to the top.

Take a cup of water and swing it around with your arm so that you can keep the water from sloshing out. Once you have learned to do this so that the water stays in the cup even when momentarily inverted, watch the path your hand takes and you will see that it almost perfectly represents the path of an object being carried up into the curl of a breaking wave.

So, the down that you perceive in waves tends to be towards the surface of the water under you and not towards the center of the earth. It is the confusion in the brain that this effect causes which is at the heart of seasickness.

In a large and perfect wave, your perception of “down” as a reference point for estimating the height of the wave will therefore be like this:

https://sphotos-a.xx.fbcdn.net/hphot...78671825_n.jpg

You can see the difference between the height that you would report and the height that a device such as a weather buoy would report.

Most vessels roll and this further scrambles up the “down” perception. The way that you got your sea legs was by your brain learning to sort out the changes in “down” caused by the rolling motion and filtering out those motion effects. However, it does this primarily using the water surface as a reference. Once it thinks it knows where “down” is, it’s happy. When the waves get large enough that people ask themselves how big they are, the effect shown in the diagram above begins to effect perceptions and waves generally appear to be about twice as high as the actual vertical distance from trough to crest.

 ctl411 11-05-2012 12:39 PM

Re: Why Waves Look Bigger From a Boat

Good to have you here Roger I don't get over there as much, kind of died off. The forums seem to rotate in and out of favor.

 ctl411 11-05-2012 12:46 PM

Re: Why Waves Look Bigger From a Boat

You also forgot the bar reason better story if the waves are bigger.

Re: Why Waves Look Bigger From a Boat

Very good explanation. I've always found it funny to get back from one of the New England islands and hear someone else, who also made the trip, describe the sea state we were just in. Almost always sounds bigger and meaner than I recalled, even when I thought it was tough. Maybe its just the fish tale in us all.

 Frogwatch 11-05-2012 01:08 PM

Re: Why Waves Look Bigger From a Boat

I cannot judge wave height because I always think I am chicken so I think that anything I am out in must be small. People ask me the wave height and I tell em am no judge of such.

When I sailed from West Palm to West End across the Gulf Stream on my 28' boat, the waves seemed "unreasonable" but I thought, "Oh, I'm just being a wimpy Gulf of Mexico Sailor" cuz the waves there are small. Two days later when I went back on the 525' long MV Discovery and the ship was rolling, I asked a crew member what he thought the wave height was, he said, "Not bad, it was worse two days ago......"

 Rudder Nonsense 11-05-2012 01:16 PM

Re: Why Waves Look Bigger From a Boat

That is a great description of why those waves look so huge sometimes. I never thought to apply geometry to it.
However there is one small problem with the diagram.
In those sea conditions, that boat should have reefed by now.
:D

 pdqaltair 11-05-2012 01:41 PM

Re: Why Waves Look Bigger From a Boat

Additionally, if your depth sounder has a relatively high sampling rate you can watch the depth go up and down. A fishfinder, for example, will give a virtual wave trace in terms of depth vs time.

As Roger eluded, you can't look at the nearest wave, you need to look in the distance. And everyone knows that small breaking waves can be far worse than big rollers.

 CaptainForce 11-05-2012 04:53 PM

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.

 Omatako 11-05-2012 06:46 PM

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.

 JulieMor 11-05-2012 10:52 PM

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.

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