James Stevens answers your Questions of Seamanship. This issue - when his GPS plotter packs up, Pete has to rely on the time old strategy of aim to miss

Question:

Pete is beginning to
 regret an ambitious trip across the North Sea 
in his newly purchased
 Rival 34, Levanter.

He and two friends are returning from 
Cuxhaven in Germany to their home port
of Blyth, Northumberland.

Levanter, over 
40 years old, came with very little kit and Pete, already out of pocket, is budget yachting.

There is basic safety equipment and a depth sounder and log – but the GPS plotter has packed up, so Pete is using an iPad to navigate.

The trip is about 370 miles long and has been pretty hard work with variable winds, shipping lanes, gas platforms and worse, patches of poor visibility. About two thirds
 of the way across, his patience is tried even more when the iPad runs out of charge and the screen goes blank.

Continues below…

The boat’s electrical system has never been very reliable and refuses to charge up the iPad, and there
 is no other GPS on board.

There is a paper chart of the North Sea, but Pete has kept 
no record of his position or his logged distance. His best guess is that they are more or less on the rhumb line between Cuxhaven and Blyth, and they are on the Dogger Bank.

The forecast is for more variable winds and poor visibility, but they have just enough fuel to motor if necessary. How are they going to get home?

Answer

Pete is going to have to use 
a strategy which was part 
of a day’s work for navigators before GPS: aim to miss.

He knows that he will eventually reach the English shore but needs to be certain which side of Blyth 
he will be so he can turn towards rather
 than away from home.

The first problem is to try and establish his position as best he can. He’s paying heavily for not keeping a record of his position either on the paper chart or the log book, so he’ll have to make an educated guess.

James Stevens

James Stevens, author of the Yachtmaster Handbook, spent 10 of his 23 years at the RYA as Training Manager and Yachtmaster Chief Examiner

He knows the time he started and the course he took, and has some idea of his speed. He also knows from the Almanac what the tidal stream has been doing, so he can get at least an approximate EP given that his last look at the iPad showed he was more or less on the rhumb line.

He needs to keep a close eye on the depth to pick up when he sails off the Dogger Bank which will give an approximate position line.

Assuming he has a circle of uncertainty about 10 miles across, it’s then 
a case of setting a course accounting for
 the tidal stream to a point about 10 miles south of Blyth.

He can then head north 
when the depth drops to about 15m and follow the contour.

If he’s lucky, the vis might lift and he’ll be able to identify where he is as he closes the land.

If not, he’ll have to feel his way in to Blyth with the echo sounder for a well-earned pint at the Royal Northumberland Yacht Club.