Yachts & Yachting – August 2019

the potential for much greater accuracy.

e European Centre for Medium-

range Weather Forecasting (ECMWF),

based in Reading, has long been seen as

having the most accurate global model.

However, a recent change to the licence

terms means other organisations are

no longer allowed redistribute its GRIB

data to third parties. A consequence of

this is that routeing using ECMWF data

can only be done on a remote server

and so can’t be used during a race.

Restricting the use of weather data in

this way seems like a step backwards on

the part of World Sailing. In addition, it

can be dicult for a user who’s not fully

tech savvy to realise whether routeing

calculations are being made on board

or remotely and I suspect a number of

boats inadvertently break the rules.

It’s always worth routeing with as

many dierent models as possible as

this gives a better understanding of what

might happen during a race. At the same

time it’s also important to recognise the

level of resolution that model output

represents. Typically a feature such as

an island – or a sea breeze – needs to

be ve times larger than the model grid

size to be modelled accurately. Localised,

ner-grained models are therefore

needed for this, but are only available

over a relatively short time frame.

e most popular model with a

small grid by far in home waters and

the north-western Mediterranean is

the French Arome model, which has

a grid size of approximately 1.3km.

Unfortunately, although the UK Met

Oce has both an excellent global

models and 1.5km inner domain

model, the outputs of these are not

available at an economic price for

our weather routeing purposes.

OCEANIC RACING

Once out of range of a 4G mobile phone

signal GRIB data must be downloaded

via a satellite link. For most boats below

50 this means a 2.4kbps connection via

Iridium. is is essentially a 20-year-old

system that’s incredibly slow: just ve per

cent of the speed of old-school dial-up

internet and 1,000 times slower than

a modest broadband connection. e

smallest possible le sizes are therefore

required – with an external antenna

it’s possible to download up to 100k (ie

0.1megabytes) although a le size of

around 70k is more reliable and is likely

to take around 15 minutes to download.

Although 4G coverage is very poor

around the coast of the UK compared

to many other countries, a race like

the Rolex Fastnet has a number

of key opportunities to download

weather data on a mobile phone: o

Portland, a little before Start Point,

o Salcombe, Plymouth, Falmouth

and Penzance, passing the Scilly

Islands and near the Fastnet Rock.

Even so, a quick update via satphone

of the French Arome model on the

return leg across the Celtic Sea in last

year’s 605-mile SORC solo Round

the Rock Race help cement my place

as rst British boat in that event.

GMDSS FORECASTS

e Inland Waters, Shipping and

High Seas forecasts oen seem very

old school and lacking in detail

compared to those that can be

found online or via GRIB les.

However, a key benet is they are

not just raw model data – they are

prepared with human interaction. is

ensures signicant features that might

otherwise be missed are highlighted.

As an example, in the outbound

leg of this year’s MailaSail AZAB

(Azores and Back) race the GMDSS

forecast highlighted the track just

ahead of us of the then-young storm

Miguel, that later went on to cause

the death of three French lifeboatmen

and dump unseasonal quantities of

rain on the UK. It provided sucient

information to allow us to route via

the lighter favourable winds behind

the system, rather than getting

the full blast of the depression.

LIMITATIONS OF ROUTEING

SOFTWARE

No algorithm can accurately predict

the future and in any case routeing is

not solely about the weather. Just as

it’s important to protect your position

against the eet in windward-leeward

races, even when racing across an

ocean the position and likely intentions

of your key competition are critical

to deciding exactly where to go.

In addition, routeing can only be

as accurate as the data the system has

about your boat. is means the polar

performance data must reect real-life

speeds as closely as possible. To achieve

this top navigators put considerable

eort into logging and analysing past

performance across as wide a range

of dierent conditions as possible.

It’s also worth remembering that

routeing soware can’t calculate an

accurate route without this data and

it’s clearly particularly important

for a race such as the Rolex Fastnet

that has a large coastal element. An

add-on is needed for this purpose.

Additional inputs include ocean and

tidal currents. e latter is critical for

most races in UK waters, including

the Rolex Fastnet. It’s also possible to

download GRIBs of predicted wave

heights, which can help route around

the most dangerous parts of a storm.

BUYER’S GUIDE WEATHER ROUTEING APPS

Left

A satphone

may be useful

but bandwidth

is limited so

download speeds

are often very slow

Don’t fret unduly if you don’t appear to get a signal from

an external Iridium antenna while in port – the density

of masts in most marinas is very e€ective at blocking

the signal. A quick trip outside a harbour is needed to

check the system works and to confirm that there are

not loads of emails backed up and waiting to download.

TOP TIP

I suspect a number of boats

inadvertently break the rules

76 Yachts & Yachting August 2019 yachtsandyachting.co.uk