TWL: Capsize Risk and Waves
Reference: Oceanography & Seamanship, Van Dorn, 1974, page 334.
Tables are not everything there is to know, but this table on page 334,
although a little hard to follow has the prime data about chances of
encountering a wave large enough to capsize, and of course the stuff that
comes along with it but is not quite large enough, can still do a lot of
damage.
Here is a summary distilled down for those with small craft.
Assuming a fully developed sea state.
At 30' of LWL a vessel should be safe up to about 27 knots.
At 37 knots, the chances of an encounter are about 1 every 6 minutes.
The 10 knots of difference may not seem like much, but you can see the
possibilities.
At 40' of LWL a vessel should be safe up to about 30 knots.
At 40 knots, the chances of an encounter are about 1 every six minutes.
At 60' of LWL a vessel should be safe up to about 37 knots.
At 47 knots, the chances of an encounter are about 1 every six minutes.
Notice that at each vessel length, the difference between safety and
catastrophic danger is only 10 knots.
From the safety line, which is estimated to be about 1 encounter in 100
hours, to the next level of once in ten hours requires only about 2 knots
increase in the wind speed. From 1 in ten hours to once in 1 hour it takes
about another 3 knots. And for the chances to increase to 10 per hour,
requires only about another 5 knots of wind.
So, here is another table.
30', 27, 29, 32, 37 knots.
40', 30, 32, 35, 40...
60, 37, 39, 42, 47...
Boat size, safe at once every 100 hours, every 10 hours, every 1 hour, 10
times an hour.
These number of hours before encounter, describe the number of hours before
a breaking wave will be encountered that can cause a capsize. Be warned
that these tables are derived from mathematical models and the chances in
any given storm may be somewhat different. Say, 3 waves every hour.
Now, I know what you are thinking. Lots of boats have encountered worse
than this and have had no serious trouble, at all. The answer is that they
never encountered a "fully developed sea state". If they had they would
have encountered about what the table shows.
The fully developed sea(FDS) state is another topic unto itself. See page
195 of the Van Dorn's book.
If the fetch(distance over which the wind can act), in effect the distance
upwind to the nearest land, or edge of storm, plus the wind speed and it's
duration are the components of a FDS. If every sea state made it's way up
to fully developed, there would be a lot fewer sailors. In practice very
few storms ever reach FDS.
When you are doing your own calculations about survivability, these tables
can be pretty sobering. Make sure you don't assume a sea state that is not
FDS, if there is any chance of there being enough fetch distance, enough
wind and enough hours to create one.
Mike
Capt. Mike Maurice
Wilsonville, Oregon (Portland).
Wow! Interesting tables! I'm glad I didn't know this when I came down the
New Jersey coast in 44 knot winds and 12' quartering seas! Instead of being
just scared, I would have been petrified!
Regards....
Phil Rosch
Old Harbor Consulting
M/V Curmudgeon MT-44TC
Currently moored in Captiva Pass, FL
Hello All,
OK, now I get to ask some of my seemingly endless dumb questions:
Do these wind speeds and resultant wave heights apply only to boats caught
broadside or do they apply to vessels headed bow on, quartering or running?
I know that the key phrases are "breaking waves" and "fully developed sea
states". Further, I understand that to mean waves that are actually curling
over, rather than having the tops blown off and sea states where the
constant wind has enough fetch (distance over sea) AND TIME to develop the
sea state to the potential indicated by the tables Mike referred to.
I've been out in the Pacific in huge rollers (in less than 20 kt winds) for
most of the daylight hours that never caused us any concern beyond the worry
of what caused them and if anything bad was coming from behind.
Fortunately, nothing bad ever appeared and I don't have any sea stories to
relate beyond enjoying the roller coaster ride on a 44' sailboat. I often
think that such conditions are improbable judging from what we read on the
list but there they were. I guessed that seas such as these were the
remnants of a storm or other event. Now I'm thinking that isn't a proper
explanation. Has anyone else experienced 15 or 20 foot rollers without very
high winds? This was about 700 miles out on the way to Hawaii.
I've never been out in "breaking" seas, just seas where the tops of the
waves were blowing off from the wind. These were some of my most fun days
sailing but weren't fun for the boat at all. I envision breaking waves as
the breakers we see in inlets and at the beach, actually breaking over the
bow and sending a wall of green water towards the windshield. Or, if
running, such a wave would actually break over the transom of the boat. Is
this the actual scenario or is my imagination running away with me?
In any event, I can't imagine how a certain sea state can "guarantee a
capsize", so to speak. No one actually said that in a post and I'm speaking
figuratively.
Mike Maurice offered up a fairly comprehensive set of conditions that are
cause for concern, and if experienced probably demand that the captain and
crew start thinking about survival strategies for the vessel and themselves.
That I'd agree with.
Now I'm at the point of wondering what strategies make the most sense for a
given set of conditions. Certainly, if my assumption is correct, us trawler
types in a 40 - 50' full displacement trawler can't outrun anything over 12'
in the fully developed sea state MM describes and I don't think we want to
be stern on to a breaking 15 footer that was overtaking our boat. What to
do besides don't get caught in the first place?? Now, this question may
contain a fatal error since I put 2 of Mikes posts together and assumed (bad
word, I know) that ANY 12 foot breaking wave will travel at about 9 kts as
he wrote in his Icy Straits post. This may be enough of a mistake to
invalidate the supposition that a 40 - 50 foot full displacement vessel
cannot outrun a 15' wave. If that's the case the question is meaningless
and I need to buy a couple books and start studying.
Knee Knocking Regards to All,
Frank & Claudette Weismantel
Elverta, CA
Boatless for a little while longer
-----Original Message-----
<SNIP>
"Mike Maurice" mikem@yachtsdelivered.com
At 01:52 PM 2/22/04 -0800, you wrote:
So, here is another table.
30', 27, 29, 32, 37 knots.
40', 30, 32, 35, 40...
60, 37, 39, 42, 47...
Boat size, safe at once every 100 hours, every 10 hours, every 1 hour, 10
times an hour.
These number of hours before encounter, describe the number of hours
before a breaking wave will be encountered that can cause a capsize
These figures are from Van Dorn's book. Please don't shoot the messenger,
as there is very little interpretation involved, since I have simply copied
numbers out of his graphs. These numbers imply severe hazard if the sea
state can become fully developed. Which rarely ever happens at least at the
higher wind speeds.
Here are some additional figures regarding fetch and duration to produce a
fully developed sea, these from "Wind Waves", page 393.
20 knots, 75 miles and 10 hours.
30 knots, 280 miles and 23 hours.
35 knots, 500?? miles, 33 hours??
40 knots, 710 miles and 42 hours.
45 knots, 1150?? miles, 57 hours??
50 knots, 1420 miles and 69 hours.
60 knots, 2000?? miles and 110?? hours.
70 knots, 2500?? miles and 125?? hours.
?? these are interpolated numbers.
Take as an example the 30 ft. boat. The danger lies at about 35 knots, in a
fully developed sea(FDS).
The fetch would have to be at least 500 miles and the wind would have to
blow STEADY for 33 hours to achieve a FDS. If you were in a 30 ft boat and
the wind had been blowing for greater than 33 hours but the fetch was less
than 500 miles, then a FDS could not have developed and you would be on the
border line of being safe(??). If either of these conditions, fetch
distance of duration were somewhat or well below the required levels to
produce a FDS then you could consider that you were reasonably safe (based
on the tables).
One note, and this is important. For each of the wind speeds above,
increasing the fetch distance or number of hours duration will NOT cause an
INCREASE in the sea state. There is an upper limit to the sea state
(roughness) that can be induced at any steady wind speed and at the limit,
increasing the numbers for fetch and duration will not increase the overall
wave heights or roughness. In other words, at 40 knots, a fetch of 2000
miles and a duration of 200 hours, will not increase the sea state, beyond
the FDS. That's why it is called the fully developed sea state. If there
were no upper limit, there would be a LOT fewer sailors. For instance, the
60' boat is virtually safe at a wind speed below 40 knots, regardless of
the fetch or duration, since the conditions will reach an upper level and
not continue to grow.
At 50 knots or above, it takes a fetch of such magnitude (length) that it
is a rare storm indeed that is large enough and is not restricted by
landmass that the wind can ever build a FDS. But, there are a couple of
places where this is possible.
- The seas around the Antarctic Landmass, where the fetch is virtually
unlimited and the duration can go on and on. - Areas of the No. Pacific and the So. Indian Ocean.
So, here is some kind of a summary.
30', 27, 29, 32, 37 knots.
40', 30, 32, 35, 40...
60, 37, 39, 42, 47...
The thirty foot boat should avoid a FDS that has been created by winds of
over about 32 knots.
Over 35 for the 40 footer and over 42 by the 60 footer.
To determine if the situation is tolerably safe, in other words, less than
the FDS and by how much, examine the fetch over which the wind is
operating. For this you need to know how large the storm is from front to
back and whatever you can about the distribution of winds and directions
within the storm. This is where things get complicated, so don't use what
follows as if it had been brought down off the mountain by Moses.
If I were doing this, I would take the wind information, extrapolate the
size of the area of highest winds and their speed and use that to calculate
a percentage of the FDS that could be generated from that size fetch and
wind speed. You can do this for the conditions as they exist at the present
moment or you can use the forecast, 12 or 24 hours or whatever as the
"snapshot". If you use a future forecast and you expect to be the object of
the storm's attention, then you will have to create a "projection", based
on the storm moving, you moving and the time that will elapse. That
projection will only apply for the wind speed and fetch area that you used.
Since there is most likely another larger area of winds, but of lower
speed, you should do the same for that wind speed area. And it make take
several of these projections using lower wind speeds to make a complete
picture.
From all this you can build a danger profile, by time, area and conditions
expected. I don't think you want to start learning this at sea, so try
practicing in your armchair. It is liable to take a while to learn to do
this with any speed or proficiency. Suggest Van Dorn's book, starting at
page 187 for further reading. There is a tendency to decide that all this
way too much trouble to ever be worth the effort to learn, since you can
always make your own guess estimate and with enough years experience your
guess may be pretty close. But, there is some satisfaction in being able to
put into practice that which is based on the testing done by some of the
best minds on the planet.
There are OTHER factors which enter into sea state condition which I have
not delved into, like current running against waves, gustiness, changes in
wind direction, and so on. The material presented is strictly limited to
the problem of fetch and duration.
By the way, I haven't got all this figured out either. So much to learn. So
little time.
Mike
Capt. Mike Maurice
Wilsonville, Oregon (Portland).
"Mike Maurice" mikem@yachtsdelivered.com
At 03:39 PM 2/23/04 -0800, you wrote:
One note, and this is important. For each of the wind speeds above,
increasing the fetch distance or number of hours duration will NOT cause
an INCREASE in the sea state. There is an upper limit to the sea state
(roughness) that can be induced at any steady wind speed and at the limit,
increasing the numbers for fetch and duration will not increase the
overall wave heights or roughness. In other words, at 40 knots, a fetch of
2000 miles and a duration of 200 hours, will not increase the sea state
I have just received an email from the fellow who has the Oceanography Book
online book, site.
The bad news is that the fully developed sea state, may not be as fool
proof as was thought when Van Dorn's book and others were printed. I will
see about this. In other words, the sea state can rise beyond what the FDS
tables in present books would show. This is not great news.
I know what someone out there is thinking... So, much for tables!
Perhaps, but it beats trying to learn all of this wave complexity, strictly
by field experiments, using your own body.
There is a remark in "Heavy Weather", something to the effect that since
the tank tests do not demonstrate that there is any proven way to prevent
all capsizing, it's pretty discouraging to know that the limits of human
strength, guile, stubbornness, preparation can not prevent catastrophy,
where small boats are concerned.
Regards,
Mike
Capt. Mike Maurice
Wilsonville, Oregon (Portland).