“ROGUE” WAVES & ENVOY NEARLY READY TO LAUNCH

We are both well here in Marmaris, Southern Turkey - the weather is mostly fine and sunny in the low 20 degC, and progress getting Envoy ready to launch is on schedule..
ROGUE WAVES
This paragraph about the sea from a Clive Cussler novel appealed to me:
“Most of the time, the oceans appear to be at rest. Un-ending waves no higher than the head of a German shepherd give the image of a sleeping giant, the surface of his chest slowly rising and falling with each breath. It is an illusion that beguiles the unwary. Sailors can fall asleep in their berths with clear skies and calm seas and wake up to a frenzied sea that quickly sweeps over thousands of square miles engulfing every vessel in its path”
We’ve recently seen the term “rogue wave” used very loosely, particularly by the media. The use of this term could imply that such waves are unexpected or unpredictable, whereas this is not the case at all, and waves of about double the average wave height are entirely predictable.
Because our minds mostly register only the larger waves we see, and not the smaller ones, wave height is usually calculated not as the “average height”, but as the “significant wave height” – that is the average height of the highest third of all the waves (i.e. the ones our minds register).
To give an example, in a sea state where the true average wave height is 4m, the significant wave height (the height the mind registers) is 6.6m.
As the wave trains roll across the ocean, the peak of one wave eventually synchronises with the peak of another, and the resulting wave can be much larger than either of the two waves that coincided. The UK’s National Institute of Oceanography determined that:
- About one wave in 23 is twice the average height – that means in the above case 8m
- About one wave in 1,175 is three times the average height – that means in the above case 12m
- About one wave in 300,000 is four times the average height – that means in the above case 16m.
In practical terms, if we are cruising along in waves of 6.6m or about 21ft significant wave height (which I believe could be quite challenging), then the true average wave height is 4m or about 13 ft, but we could encounter occasional much larger waves up to 12m or about 39ft. If we are unlucky enough to strike that one wave in 300,000, it could be 16m or about 52ft.
I understand that the largest wave ever scientifically recorded at sea was one of 34m (112ft) in 1933 by the USS Ramapo in the north Pacific.
However it’s not all bad news – if you take a wave period of 10 seconds, which is typical in rough seas, that means 6 waves per minute, 360 waves per hour and 14,400 waves per day. So the chances of meeting that one wave in 300,000 are very remote, particularly as these encounters of the wave trains are transient, occurring in one place for a short time, and the resulting giant waves don’t travel much distance.
It’s also worth noting that according to the book “Perfect Storm”, the energy generated by waves does not increase linearly, and waves from a 40 knot wind are likely to be 17 times “as violent” as those from a 20 knot wind. The energy created by waves is huge, with impact pressures typically 250 to 1,150 lbs per square foot, while up to 6,000 lbs is possible from very large breaking waves.
The incidence of very large waves is quite limited statistically, and one study reported that for the open oceans in general, 80% of the time wave heights (not sure if they meant “average” or “significant”) are under 3.7m (12ft), 90% of the time they are under 6m (20ft), and that to encounter waves above 12m (40ft) is extremely rare. This partially explains how many cruisers travel thousands of ocean miles over many years and don’t ever encounter dangerous rough seas. Another part of the explanation is planning their cruise according to weather forecasts.
All of the above relates to waves generated by wind (force) blowing for a period of time (duration) across hundreds of miles of open sea (fetch).
Another entirely different source of large waves is seismic activity or landslides into the sea. These events have historically caused waves much larger than the largest waves caused by weather.
LIVING IT HARD
We’ve been back aboard Envoy over a week now up on the hardstand. We’re fortunate here, as many marinas don’t allow you to live aboard while the boat is out of the water, but it’s an unusual way of living. Firstly everything has to come aboard up a ladder. We have to use the marina toilets and showers, we do dishes in a bowl in the sink, empty the bowl into a large bucket which is emptied twice a day, and use a “chamber pot” during the night which is emptied into the marina toilet every morning. Diane still thinks it’s a huge joke when she passes the pot down the ladder to me, promising not to spill it over my head! Cars drive around us and the travel lifts are constantly moving boats.
During the day the boat is a bit chaotic with different contractors coming and going, but progress is steady with no unexpected issues so far. The most time-consuming job is getting Envoy’s hull and topsides cut and polished. This is a job we choose to have professionally done annually to maintain and protect Envoy in top condition. There’s a lot of gelcoat surface area, and it took two guys six days including during Easter, which is not celebrated in Muslim Turkey. Envoy’s gelcoat is gleaming, looking more like a 5 year-old than 22 year-old vessel. At a cost of NZ$1,240 (US$1,020) we think it’s money well invested.
Tomorrow Envoy goes back in the water.
If you don’t use a car for several months you just jump in it, start, and drive away. Not so with a boat because you normally combine annual maintenance with your winter haul-out. Although I mentioned in the last blog this is all “routine”, it’s still comprehensive, and on the next blog I’ll go into that in some detail.