26
FUEL 3100 / DIESEL 3200
Installation and Operation Manual
NAVMAN
Different instruments measure different boat
speeds. A paddlewheel sensor measures
boat Speed Through the Water (STW). A GPS
measures boat Speed Over the Ground, ie
over the bottom of the water (SOG). If there
is a current, these speeds will be different, as
explained below. Therefore, the readings for
speed, log, trip log, economy and range will
depend on whether the speed input is STW or
SOG and if the current
is constant.
Appendix C - Speed through water and over ground
When the current is from ahead, speed over ground is less than speed through water
When the current is from behind, speed over ground is more than speed through water
STW is 10 knots
Current
4 knots
Gives an SOG
of 6 knots
STW is 10 knots
Current
4 knots at 45°
Gives an SOG of
about 8 knots
STW is 10 knots
Gives an SOG of 14 knots
Current
4 knots
STW is 10 knots
Current
4 knots
at 45°
Gives an SOG of about 13 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 STW:
10 knots
10 nm
3.3 nm / gal
165 nm
Using SOG:
6 knots
6 nm
2.0 nm / gal
100 nm
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 STW:
10 knots
10 nm
3.3 nm / gal
165 nm
Using SOG:
14 knots
14 nm
4.7 nm / gal
235 nm
