NorthStar Multi-Function Unit M121 Installation And Operation Manual page 51

13-5-2 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.
When the current is from ahead, ground speed is less than water speed
Water speed 10 knots
Gives a ground speed of 6 knots
For this example:
If the boat travels for one hour, uses 3 gallons of fuel per hour and has 50 gallons of
fuel left:
Using water speed:
Using ground speed: 6 knots
When the current is from behind, ground speed is more than water speed
Water speed 10 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:
Using water speed:
Using ground speed: 14 knots
50
Current 4 knots
Speed Log
10 knots 10 nm
6 nm
Current
4 knots
Speed Log
10 knots 10 nm
14 nm
Northstar M121/M84 Installation and Operation Manual
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. To select a speed sensor,
see section 13-5-1.
Water speed 10 knots
Gives a ground speed
of about 8 knots
Economy Range
3.3 nm / gal 165 nm
2.0 nm / gal 100 nm
Water speed 10 knots
Gives a ground speed of about 13 knots
Economy Range
3.3 nm / gal 165 nm
4.7 nm / gal 235 nm
Current 4 knots at 45º
Current 4
knots at 45º