NAVMAN
8120/8084
Fuel, AIS, & Radar Addendum
4
Water speed and ground speed
A paddlewheel sensor and a pitot sensor
measure water speed, the boat speed
through the water. A GPS measures ground
speed, the boat speed over the bottom of
the water. If there is a current, then these
speeds will be different, and the log, trip
log, economy and range will be different, as
shown below.
Water speed is better for measuring the
boat’s potential performance, Ground speed
is better for going to a destination because it
takes currents into account.
Current 4
knots at 45º
Gives a ground speed of about 13 knots
Water speed 10 knots
Current 4 knots
Gives a ground speed of 6 knots
When the current is from ahead, ground speed is less than water speed
For this example:
If the boat travels for one hour, uses 3 gallons of fuel per hour and has 50
gallons of fuel left:
Speed
Log
Economy
Range
Using water speed: 10 knots 10 nm
3.3 nm / gal 165 nm
Using ground speed: 6 knots
6 nm
2.0 nm / gal 100 nm
Water speed 10 knots
Gives a ground speed
of about 8 knots Current 4 knots at 45º
When the current is from behind, ground speed is more than water speed
Water speed 10 knots
Current
4 knots
Gives a ground speed of 14 knots
For this example:
If the boat travels for one hour, uses 3 gallons of fuel and has 50 gallons of fuel left:
Speed
Log
Economy
Range
Using water speed: 10 knots 10 nm
3.3 nm / gal 165 nm
Using ground speed: 14 knots 14 nm
4.7 nm / gal 235 nm
Water speed 10 knots
