Bob Blog 25 June 2017



Compiled Sun 25 June 2017

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.

Perigean tides:Take extra care over the next few days as we had a new moon on Saturday and the moon’s perigee was on Friday (NZ dates). The close proximity of these exaggerate the tidal forces causing extreme low and high tides. Combine that with onshore swells and lower than normal air pressure, such as at Timaru and there is a chance that waves will wash higher inland at high tide.


Up to date graphs of the sea level are at, the yellow moon marks perigee and purple is for the new moon  SS = storm surge. IB= inverse barometric effect caused by air pressure.

Thanks to PJ at Perceivingacting channel for posting info to my Facebook site last week about the tip on how to edit the earth.nullschool url to change date.

And thanks to Captain Simon Catterson for showing me around the three-masted, square rigged barque Tenacious when it was in Auckland this week. They are providing valuable training around the world, see their web site at


These weathergrams have been going weekly for over 20 years, making this blog one of the longest surviving on the Internet. The original inspiration for my blog was to help by mate Mike Harris as he started the YOTREPS scheme. I was hoping this would be a way to get yacht weather observations into the view of those meteorologists who compile the marine weather warnings. It has now come to the stage that Mike Harris is seeking to hand the YOTREPS reporting scheme onto a new enthusiast/manager. I am willing to allow my weathergrams to be part of the package.

So here’s Mike’s request, and if you can see this in your future please get in touch with him straight away.


YOTREPS 20 years of passage Reporting 20/6/2017

Everyone likes to talk about the weather, but for cruising sailors it’s more than just a topic of casual conversation. It has a say on where you can go, when you can go and whether you will be comfortable or sick. It is the power in your sails.

Most long term, live-aboards will be familiar with the many amateur and commercial marine radio stations as a source of forecasts. For safety reasons, many also run traffic lists taking daily reports of positions and weather from boats on passage. The volume of data collected is quite considerable and I’d often wondered what happened to it after the net had closed down. On further investigation I found that often it was kept for a short time then dumped. This was not because it had no value, but because it was not prepared to an agreed standard and could not be delivered to forecasters quickly enough for them to make use of it.

For decades commercial vessels taking part on the VOS (Voluntary Observing Ship) scheme have contributed observations to forecast agencies so the idea had president. In some parts of the world, sailing boats may be the only visitors seen, so their reports can be especially useful, but on the other hand they are unlikely to have trained observers and the calibration of their instruments is not assured. For these reason their data is not used to prepare forecasts but as an independent ‘reality’ check on their accuracy.

In 1995 these were the ideas that in consultation with Bob McDavitt of New Zealand MetService, lead to the formation of YOTREPS. It provided radio net controllers with a program for logging reports and forwarding them to a server for checking, encoding, forwarding to met. Forecasters and plotting on a web page chart along with a text for news and comments. This has been shown to be an enormously popular feature and the reason that many use the service.

The first YOTREPS report was received in 1996 and now has several hundreds of thousands of reports. Many complete circumnavigations and expeditions have been tracked. The largest vessel was the Queen Elizabeth II. and smallest, I am not sure.

A New Future for YOTREPS

The original php script that I wrote for YOTREPS has, except for some brief breaks while upgrading, operated for some 20 years with very little need for attention. Within the past few weeks I have re-written the scripts to run under current versions of PHP, MySQL and Nginx and should be good for some time to come. However a deteriorating health condition has shown me that now would also be a good time to look for a new manager and place in cyber-space for YOTREPS.

Requirements for the new enthusiast are:

1. A deep and committed interest in all aspects of long distance sailing.

2. A good depth of technical experience in radio communications, and computer technology.

An alternative future could be with an organisation such as a yacht club or radio station. Here the management and technical details could shared and in all probability the computer components installed as an adjunct to an existing web site.

YOTREPS provides a position and weather reporting service. It’s free and open to anyone, anywhere in the world. You do not need to be a member of any particular club or have any special license to take part. If you think it could have a future with your club or cruising organisation or fit within your own cruising life style as it did with mine, I should very much like to hear your ideas*.

Mike Harris VK7AAA

19/6/2017 Please first be sure to be familiar with the YOTREPS pages on


One of the many achievements of the YOTREPS website is to give yachts that only have email access the tools to look at the bigger weather picture. In the South Pacific there is a weather feature unlike anything found anywhere else, called the south pacific convergence zone. It behaves like a wriggly dragon and contains squally showers but is almost invisible in the computer grib data, which tend to show it as an area of light variable winds. Cruising yachts need a weather map which shows exactly where this zone is located, and this is exactly what is given in the Fiji Met Service weather maps which are converted into FLEET code. The FLEET code dates back to WWII and was used to send weather maps to the whole fleet via morse code.

This is how the yachts get and view it:

FIRST (before departure) download and install PhysPlot.exe on your PC (no Mac version) from , part of the YOTREPS website.

Then request the Fiji Fleet code by sending an email to, no subject needed, with the message “SEND nadi-fleetcode”

(without the quotes).

An email should arrive back. To view the map, save the number-table in that email to your desktop (or somewhere you can find it) as , for example, fleet.txt then open this file with PhysPlot-


The various convergence zones that make up the SPCZ are shown on this map as brown lines. Black lines are isobars and red/blue lines are passing fronts.

The Tropics

Last Thursday (local) Tropical Cyclone CINDY made landfall on the Gulf coast bringing heavy rain from Louisiana to the Florida Panhandle.


The monsoon is making progress across Asia and into China, and recent heavy rain has triggered a massive landslide at Xinmo village in Sichuan province.

The last week’s rain map shows an enhancement in the rain over China, the Solomon Islands, the Gulf of Mexico (CINDY) and a decrease in activity around Samoa and the equatorial Atlantic.



Rain for the past fortnight from



Weather Zones (see text) as expected mid-week on Wednesday (GFS model) showing wind, isobars, current, lightning, Sig wave height purple lines, SPCZ and STR.

SPCZ=South Pacific Convergence zone.

The SPCZ stretches as an almost continuous band from Papua New Guinea/Solomons to around Tuvalu/Tokelau. It is expected to slowly drift south this week, reaching Samoa Islands around Thursday/Friday, and maybe reaching Fiji this weekend or early next week.

A passing trough is expected to cross the southern cooks by Thursday and then the Austral Islands around Friday/Saturday local.


Tropical accumulated rainfall for next week from

Subtropical ridge (STR)

HIGH that is over eastern Australia tonight is expected to spread into the Tasman Sea by mid-week and then onto central NZ around Thursday and further east this weekend.

Australia to New Caledonia:

Not for much of this week. As that High travels east across the Tasman Sea there is likely to be too much in the way of southeast winds. A low is expected to form in the Tasman SEA from Thursday, and that may allow a reasonable voyage eventually in its wake, too far away to tell just yet.

Departing northern NZ to the north for the tropics:

Looks ok with a departure on Monday or Tuesday with a High in the Tasman sea and a LOW to east of NZ. Only light winds on Wednesday, and after that the voyage is likely to encounter northerly winds ahead of the next incoming low, so not a good idea.

New Zealand to the east (Tahiti)

Maybe ok for departure in the SW winds early this week and then with the lighter winds of the high that is expected to travel east of NZ late this week. There are lows near 30 to 40S at present, and the voyage may be able to ride in the westerly winds north of these lows at around 25S this week.

French Polynesia to the west:

The recent squash zone of enhanced trade winds and over 3m swells is expected to ease off on Sunday/ Monday. There is a passing trough with variable winds and possible showers, reaching the southern cooks by Thursday local, and Society Islands around local Friday/Saturday, and then settled trade winds with average swell heights.

If you would like more detail for your voyage, then check to see what I offer.


See my website for more information

Feedback to or txt 6427 7762212

Weathergram text only (and translator) is at

Weathergram with graphics is at,

Click FOLLOW at bottom right to subscribe.

To unsubscribe from WordPress: click the “unsubscribe” link on the bottom of the email.

Or, if email wasn’t from WordPress then send a reply email saying LEAVE.



Published by metbob

Pattern and Chaos

Leave a comment

Please log in using one of these methods to post your comment: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.