Bob Blog 15Aug

Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the
patterned world.
Compiled Sunday 15 August 2021
More about waves
Last week I mentioned that shorter period waves are steeper and less
manageable.
I also mentioned that a sharply inclined seabed encourages breaking waves.
You can feel this increased choppiness when approaching land and crossing
the continental shelf.
This requires further explanation,
1. As you approach land, the swell arriving from the open ocean
interacts with the reflections of waves that bounce (rather than break) on
the coast, and this produces a diffraction pattern and increased choppiness.

1) There is usually a difference in the height of sea level on either
side of a continental shelf. This is maintained by strong ocean surface
currents sometimes as a series of eddies. When these currents flow against
the wind the waves that are produced are made steeper. A knot of current
builds the wave height up as much as a 10 knot increase in wind. =
I had a few readers reply to lat week's blog with stories of their own
encounter with steepening seas.

Rory recalls being off the Portuguese coast on a winter yacht delivery in
the 90s. Sailing south into a southerly gale, we tried going into the coast
to get some relief. Nope, inshore conditions were worse, so we headed back
to the ocean, As we crossed the continental shelf we sailed over two of the
biggest waves I have seen in 40 years of sailing. they were two very
organised swells that stretches as far as could be seen in both directions.
The first was just a very steep and impressive swell. But the second was
breaking at various points along its length, big enough to roll us.
Fortunately, we were between the breaking sections.

Stan writes about a trip across the Tasman Sea in March 2019. Finally
approaching land after a five-day passage, on a moon-less night and with
seasick crew, and being on the helm for over 12 hours, we continued
(doggedly) directly westwards to get ashore. But increasing wind from the
north and a strong current from the south meant a steepening sea. In
daylight we realised the sea state and altered course to the south so that
the swell was behind us. The sea would suddenly settle down, and then build
again dramatically. In retrospect we realised this was due to our transiting
areas with less current. As the waves settled in those area, we turned
westward towards Sydney. It was during one of these periods going west that
I looked to starboard and saw a large wave approaching directly on our beam.
When the wave was about 50m from the boat it suddenly combined with a wave
from a slightly different direction and the manageable 5m wave became an 8m
wall of water, nearly vertical and ready to break. We didn't have time to
turn into this monster and it hit us directly on the right side. We listed
to the side as far as we've ever experienced and at the top, luckily, we
proceeded over to the back of the wave. The lesson: If our course is into an
area of increased risk, be prepared to change course early, especially when
wind speed is greater than 30 knots.
Full blog is at buffalonickelblog.com/2019/03/24/crossing-the-tasman-sea/

Finally, a note about wave height. This is given in marine weather
forecasts as "significant metres". The significant wave height Hs is
defined as the mean height of highest third of the waves. In practical
terms it is the average height of the waves you can see. Using the Rayleigh
distribution Hs gives us a handle from which we can work out the probability
of wave height.

So 1 wave in 7 is higher than Hs, or around 1 per minute The occasional
wave, 1.5*Hs is 1 in 100 waves, or around 1 in 15 minutes The maximum wave,
2*Hs is 1 in over 1000 waves around once a day.

And to experiment with interference and diffraction patterns play with a
ripple simulator at www.falstad.com/ripple/ (simulates sound , radio and
microwaves but not sea waves. Click and drag the mouse to make waves).

TROPICS
Haiti is about to be hit by Cyclone GRACE soon after a very damaging
earthquake see edition.cnn.com/2021/08/14/world/haiti-earthquake/index.html
The remnants of FRED is heading for Louisiana.
LINDA is a very active system travelling westwards n the NE tropical
Pacific.
And SIXTEEN is a depression heading westwards across the NW tropical Pacific
towards Philippines.

WEATHER ZONES
SPCZ=South Pacific Convergence zone.
The SPCZ is expected to build and stretch from Solomona to Tuvalu to Samoa.
Trough forming over Fiji and Tonga from mid-week and travelling east to
reach Southern Cooks by local Friday. A low may form in this trough near
30S.

HIGHS and LOWS
H1 is a High above 1025 north of NZ tonight and expected to travel southeast
to east of NZ maintaining good trade winds for much of this week.
T1 is a broad trough that is expected to travel over the Tasman Sea and NZ
on Monday and Tuesday and the further east triggering a trough in the
tropics.
H2 is a HIGH that is expected to enter the central Tasman Sea from Australia
by Thursday and build to 1030+ over NZ this weekend. It is good for getting
across the Tasman Sea. However, it may bring a squash zone to the tropics on
its northern side. The EC model is at present producing a LOW to southeast
of Fiji early next week, but this may change. Be aware.
T2 is a trough that is expected over Tasmania by Thursday and then travel
fast east across NZ on Friday and then further off to the East.
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If you would like more detail for your voyage, then check metbob.com to see
what I offer.
Or Facebook at /www.facebook.com/metbobnz/ Weathergram with graphics is at
metbob.wordpress.com (subscribe/unsubscribe at bottom).
Weathergram archive (with translator) is at weathergram.blogspot.co.nz.
Contact is bob@metbob.com or txt 64277762212
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