Nav in a nutshell: Deploy the thinking man’s passage plan

Brain power is still best for calculating weather, wind and tidal information, says Dick Everitt

TAGS: boating Nav in a Nutshell navigation sailing

Navigation instruments are only tools. They can measure, but they can’t think! In order to plan ahead we’ve got to use all the information available to us and then distil it down in our brains.

Top racing navigators have very sophisticated instruments and computers, but still make subtle adjustments to get into a favourable position to take advantage of predicted wind shifts.

As far as I know there’s no onboard computer that can integrate weather forecasts, tidal information, wind instruments and GPS to give you the definitive course to get home before the pubs shut!

Instruments can give us all the information – but it’s the skipper or navigator who makes the ultimate call

We mentioned VMG (Velocity Made Good) in a previous Nav in a Nutshell article and showed how, by bearing away in choppy conditions, we’d get a more comfortable ride, less leeway and arrive roughly at the same time as a bumpier heading closer to the wind.

Wind instruments linked to a log will give VMG in relationship to the true wind angle, but VMG from a GPS is better as it ignores the wind and tide and shows how fast we are moving directly towards a waypoint.

In the diagram we’re heading home from A to B and not sure what the wind will be doing later on in the day, as it’s very variable in high-pressure conditions.

So, we start off on the red heading and check our VMG. Nevertheless, if we crack off onto the green heading we can get the cruising chute up, which greatly improves our boat speed and VMG.

This will get us quickly towards the waypoint – while the wind holds – but we’ll be set much further downtide. If we get there fast, however, the tide will be quite weak when we arrive and shouldn’t be too much of a problem.

Also, in these weather conditions, the wind tends to disappear in the evening, so we’ll be motoring the last bit of the trip anyway. This will be easier for the crew as they’ll be tired and looking forward to getting home.

On the other hand, if we were heading into a stronger tidal stream area we’d have to modify our course at some stage, to get the boat much further uptide.

This Nav in a Nutshell was published in the Summer 2012 issue of PBO. For more useful archive articles explore the PBO Copy Service.