Seamanship_Secrets_185_Tips_-_Techniques_for_Better_Navigation-_Cruise_Planning-_and_Boat_Handling_Under_Power_or_Sail_(Re)_e..

easy calculations and adjustments 53

the cost of inadvisably high RPMs, fuel consumption, and engine and hull

stress. (Note: to fi nd the theoretical hull speed of a displacement boat, use the

formula 1.34 × the square root of the boat’s waterline length. A displacement

boat with a 36-foot waterline thus has a hull speed of 8.04 knots.)

2. Make the speed run. Time your run from the fi rst mark to the second mark
   at a steady RPM. Divide the distance by the time (converted to parts of an
   hour) to fi nd speed. For instance, if you run from A to B in 12 minutes,
   convert that to hours using the formula 12 ÷ 60 = 0.2 hour. If A and B are
   1.2 miles apart, your speed for the run was 1.2 ÷ 0.2 = 6 knots. Now time
   the run from the second mark back to the fi rst at the same RPM. Calculate
   the speed for this leg in the same manner.


3. Plot data onto the graph. Add the two speeds together and divide by two. Th is
   gives you the average speed for that RPM setting. Repeat at 200 to 400 RPM
   intervals. Remember to calculate the speed for each half of a round trip. Plot
   your speeds onto the graph and join the dots with a smooth curve. Post your
   graph at your console, into your logbook, and onto your sketchboard. Try this
   example.

You will run a 1-mile distance between Marker A and Marker B. Your first
run will be at 800 RPM.

1. Bring the boat up to speed at 800 RPM. Make sure the boat is at speed
   before you start the run.


2. Start the stopwatch the moment you are abeam of Marker A.


3. Read the stopwatch the moment you are abeam of Marker B. Time
   reads 12 minutes.


4. Make a wide turn and pass close abeam of Marker B. Start the watch
   when abeam.


5. Read the stopwatch the moment you are abeam of Marker A. Time
   reads 10 minutes.

Compute the speed on the first leg (A to B): 1 mile ÷ 0.2 hour = 5 knots

Compute the speed on the second leg (B to A): 1 mile ÷ 0.167 hour = 6 knots

Take the average: 5 + 6 = 11 ÷ 2 = 5.5 knots.

Plot 5.5 knots on your graph next to RPM 800.

Repeat this method for any other RPMs desired, all the way to maximum
speed. Now you have a gold mine of information for use with navigation,
cruise, and voyage planning. (See also Appendix I for Speed over a Measured
Mile.)