A passage of 11M at 6kn will take about 2 hours.A passage of 6M at 5kn will take about 1 hour.
When we work out the approximate time of passage we are only looking to see if it will be 1 hour or more so that we know how much tidal information to look up. During the course you will always be told the boat's speed. With practice you will learn to guess what the speed will be in your boat on any heading. When motoring the speed is easy to deduce but if you are sailing it is not so easy. The next step is to measure the distance from the start to the destination, and by comparing this to the speed you expect to make decide roughly how long the passage will take. When you do this, you must draw the line past the point where you intend to stop, even if the line goes over the land! If you do not do this, sometimes you will be unable to complete the diagram correctly. The first step is to draw in the ground track to the destination.
Not only will this approach take longer, but also you may be swept into a angerous area or in a sailing boat, you may end up head to wind and unable to sail. This will take longer than doing some quick calculations first. If there is a tide running across the route and instead of allowing for it, we just point the boat to the destination, we will end up sailing in an arc. Before we make a passage across a current we must consider if we need to steer a course to allow for the tide.
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