NorthStar Navigation Multi-Function Unit M84 - Manual (Page 51)

Northstar M121/M84 Installation and Operation Manual 50
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
Current 4
knots at 45º
Gives a ground speed of about 13 knots
Water speed 10 knots
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.
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.