Navigatie is een combinatie van wetenschap en kunst. En voor wie daar meer over wilt weten schrijft Albert De Nijs, instructeur bij de De Zeezeilers van Marken wekelijks een rubriek met tips&trics van de Royal Yachting Association.
When sailing in restricted visibility it can be a valuable option to proceed outside the fairway into shallow waters. Safely out of the way of the bigger ships and following depth contours you can proceed to a safe place to drop anchor and wait for improved conditions.
When training for this situation we choose a curving depth contour. We need to know the offset of the depth sounder, from keel, transducer or waterline. Knowing the rise of tide, we can choose a contour to follow (following the 2m contour and rise of tide 1.7m you are looking for 3.7m on the indicator when the depth sounder is set to indicate from the waterline).
The navigator sits downstairs with a detailed chart, chart protactor and depth indication. He gives steering orders to the helmsman. When the depth decreases he steers towards deeper water and vice versa. If the SOG is 5 kts, the yacht covers 0.5nm per 6 minutes, and 0.25nm per 3 minutes. This way the navigator can assess the progress along the route. Starting at a known position (buoy) on a contour he can sail the yacht to safe waters and anchor over there (in this example into the Amsteldiep).
If you are crossing the fairway on your way to the Amsteldiep, aim to arrive offset of the exact position. Otherwise you will not know whether to turn to port or starboard if you did not arrive at the exact spot and finding the entrance would be a challenge. Better to be offset and know which way to turn and follow the contour!
Albert de Nijs, Dutch Offshore Sailing Academy