Looking at the math to verify course and check headings can get a little complex when you get obsessed with math. Good thing is that there are some simple calculations that can be done in Excel to help make the process a little easier when crossing an area with a current pushing the boat off course. Here is what I use to, mainly for looking at crossing the gulf stream. I know that it should not be much of an issue for a motor yacht, but I like to understand the calculations enough to check headings or plot course if the electronics fail. Of course I prefer to use Excel for my cheat sheets and make the process a faster.
Using a chart to plot the course, assuming you already know how to do that, you need a to make a few notes:
- Rhumb Line Course (RLC)
- Rhumb Line Distance (RLD)
- Speed of Vessel
- Average Current of Gulf Stream (2.5 Knots)
Let’s say we are leaving the Miami area heading to Bimini. Using our imaginary chart we will be heading on a RLC of 68 (cell A2) degrees straight to Bimini. This is a distance of 56 Nautical Miles (cell B2). We will be maintaining a constant speed of 30.7 knots (cell C2). The Gulf Current average is calculated using the current readings on the map which range from 1.8 to 3.3 knots (cell E2).
So lets make a spreadsheet:
|1||RLC||RLD||Knots||Time||Current||Adjusted RLC||Return RLC|
The first formula is for cell D2 to determine the amount of time the trip is going to take:
Next we will need to adjust the RLC in cell F2 to have a correct heading to allow for the drift of the current:
So now we need to get back so we need to make sure we know the return RLC in cell G2. For this we will use the MOD function in Excel that allows for a calculation to return the remainder when a number is divided by the divisor.
Not perfect but it gets you close without having to break out the trigonometry books.