pago tonga trench

8/14/2019 Pago Tonga Trench

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Hail to El Nino, the Supertrades are back!!!

Just got a letter from some good friends sitting in Pago Pago and thinking

about heading west soon..

It highlighted the weather situation in that region and the need to paycareful attention; their letter was dated 1st Sept.

A boat here in American Samoa threw caution away, and left for

Tonga during this past week - they reported a brutal passage on their

catamaran. The winds for the last week have hovered around 25-30 knots,and won't ease until next Monday

I checked my files and dug out Passage Weather issued on 19thAug going

through to the 27th Aug. Clearly the forecast indicated fresh to strongwinds from forward of the beam on both 26th and 27th. 15 and 20Kt areas

are indicated on the map so locally 20 to 25kt winds, with higher gusts would

have been quite likely.

I dont know about you but we wouldnt think of trying to get to windward in

those conditions and having a go in anything but a heavy duty, deep keel boat

would be mighty uncomfortable to say the least!


2/10

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We have returned to El Nino and this is a return to the more settled

conditions of yore Hence the Supertrades are back and they are

particularly relevant between 15S and 22S, from 160W all the way to the

coast of Australia.

Statistically they will tell you that the West Pac area will receive lighter

conditions during the El Nino period when compared with La Nina, well

sailing statistically can get you into trouble here, for whilst the general

winds may be down there will be long periods where the isobars are

compressed in the area where most of us are cruising.

This is not unusual and after a while you just get used to it; last time in the

Pacific I had a deep keel and a racing boat that loved crashing to windward

since then twenty years have passed, I have mellowed and Ive changed to a

more practical cruising boat. So, like most people on the circuit, getting

weather to suit our new situation in life is very important.


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Rule 1: WE DO

NOT CRASH TOWINDWARD.

Playing the weather game was far more difficult in the past, now we have so

much information that it is coming out of our ears so Im just going to

simplify it here.

In this region, the trade winds honk. La Nina or El Nino, they still honk when

they get going; more so mid season (June, July and August)! They honk even

more when there is a Big Fat High Pressure down to the south (somewhere

over NZ) and somewhat less when that area is between highs (or relatively

low).

Unfortunately, as you will notice, there is usually a nice big frontal trough on

a NNW / SSE line passing between the high pressure areas, originating from

some nasty LOW PRESSURE thats kicking the teeth out of New Zealand.

As the front / trough (or whatever they are calling it) moves in your

direction, from the west; the trades slacken off and move to the NE. You

then have a sloppy period where many things can happen before it

re-materializes, building quickly from the SSE.

The thing to remember here is that the wind may be predominantly from theEast but the systems that generate it are moving towards the east (coming

from the west). So, as the winter High Pressure areas drift eastwardsacross New Zealand and sometimes just north of New Zealand the isobars to

the north, in the trade wind zone, become compressed and the trade wind

honks.


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In this case you have about 48hrs of lighter (generally) Nly breezesbefore the SEly returns

Above you can see Ive amended the Passage Weather plate to show the

Highs, Lows and where Id expect a frontal line to be. As the line moves

East towards you the wind swings around anticlockwise and in the region of20S there is quite an area of lighter winds shown. Whatever, there may still

be a strong shearline / troughline between the systems (not shown on

these charts and not shown on Gribs), this we have to watch for!

What happens in, and the length of, the sloppy period is determined by

many factors, none of which are easy to predict

Normally, things dont get nasty until after the wind has passed west of

north, then you may get some fresh NWlys; you may sail through a violent

front with heavy rain OR you may get light and variables for the duration. Ifthat is the case, you need to be able to motor as there will be quite a lumpy,

left-over sea and without power youll sit around bashing your gear about.

In the example Ive laid out below, if youd cleared out (Tuesday) and left

Pago Pago for Tonga on the Wednesday, early morning; there would have

been a window of about 48hrs to reach Tonga On the Wednesday morning,


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the wind would be dropping quickly but the sea would take a little longer. The

afternoon honk would be ripping down each side of American Samoa but as

you move offshore it dies a little.

By Thursday, early morning, you should still be creaming along with a good

breeze just abaft the beam but that breeze now lightens up fast. As itlightens enough for you to do it comfortably, youd best be trying to get left

of track. This also brings you up a bit increasing relative wind and keeping

you moving.

How far left of track depends on what you expect to happen next but in this

case Id say five to ten miles.. Mr John should do 135/day in these

conditions so if you are faster stay closer to the line because you are going

to cover the 320 miles in the 48hr window.

BEFORE moving left of track. Check that you have the time to make it toVavau before the wind freshens up from the SSE; if you have any doubt,

now is the time to alter course for Niuatoputapu.Great place and a goodanchorage to wait till the next window (about a week).


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Thursday midday (as shown below): The Front is now approaching, during the

morning the wind goes light, more variable but remains in the north; you may

end up motoring and the sails wont do a lot of good so its best to get a reef

in and take the bag out of the mainsail. As the day progresses you have to

watch in case the front is active in your location.

HOWEVER, you do have to keep making progress, hopefully you get some

fresh NWlys with the front because you need to push south and you have to

make distance.

These windows are never big enough and you are in a race to get under the

lee of Vavau for sunrise on Friday because by then the wind is likely to be

filling in from the SSE.

Thursday Midnight and the situation still looks OK but close. With luck we

would be seeing the light on Vavau in the early hours and with luck there

would have been enough wind for us to motor-sail these last few hours. Itsa bit tight for a small yacht however, if that wind fills in a little early then

things could get real tough and all this motoring could easily come to nought!

Perhaps a deviation to Newpotatoes would have been the safest play.

Perhaps it would have been better to just head there in the first place?????


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Friday Midday (lets worry about the day change when we are safely at

anchor!)..The picture is clear, the frontal boundary has gone well passed

and the SSE is filling in fast at 15kts.

You are either arrived in Tonga or you are in Newpotatoes, waiting foranother window whilst enjoying yet another great pacific anchorage. Take

heed however, as every year someone ploughs into the reef here eithertrusting the GPS / Nav plotter or trying to get in with poor visibility!

The northern coast of Vavau has also seen a fair amount of wrecked yachts,

so its best to lay-off if there is any doubt.


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There are some rules which, when applied, will make life easier..

1) If you get the chance to go south of track, do it because that

freshening SSE trade may come back early.

2) Dont hang about in calms.motor like hell (Im no purist!)

3) Reef early, reef deep.especially if you have any lightning visible, hearthunder or see dark clouds to the NW.

4) If you get an adverse blow, Heave- to and let it all pass by whilst you

catch up on a novel. Do not run off to the east, SSE may be OK (if you

have to) but not East!


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5) Listen to what other boats around you are getting, especially those to

the west of you.

6) If you can shorten the distance, do so. Consider Niuatoputapu as

widows are not that long and this is a convenient stopover.

7) Never approach any island or reef in this area without GOOD

visibility, the islands here tend to move around a bit when you least

expect it!

8) If you cant get to Double Reefed mainsail and a storm jib sized

headsail in less than three minuets ..you really need to practice!

9) Keep a couple of hatch-boards in and your cockpit drains free.

10) Dont push it, if your window closes that SSE will only increase so look

to your alternative.

NOTE: More often than not, even with NO weather planning, this passage is

no problem for the average cruising boat; you really do have to be unlucky to

get your head blown off. However, it can be a really nice passage when you

just pay a little attention to those High Pressure Areas to the south and

time your passage accordingly.

You will note how we shift through the sequence from Wednesday to

Saturday and how that nasty Low Pressure forms in the disturbed air to the

NE of North Cape..Its just a reminder of why only the tough mess about

round New Zealand during their winter period!!!

Thats about it except to mention PATIENCE, which is where most of us go

wrong, if you want to have a thrashing just throw caution away. Im working

on patience but I have to admitI do have some problems!


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Mr John IV with the cockpit partly covered due to large amount of heads

getting aboard on the Samoa to Tonga run; beam reaching in Supertrades!

Note that the windward weather cloth got blown out, it was a trifle wet!

Have Fun and enjoy the sailing

John Wolstenholme 2nd Sept 09

yachtmrjohn.blogspot.com

[email protected]
mailto:[email protected]:[email protected]

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