NorthStar Navigation EXPLORER 657 Water speed and ground speed, Speed Log Economy Range

12-5-2 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 12-5-1.

When the current is from ahead, ground speed is less than water speed

	Water speed 10 knots			Water speed 10 knots
		Current 4 knots	Gives a ground speed
Gives a ground speed of 6 knots
			of about 8 knots		Current 4 knots at 45º

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
When the current is from behind, ground speed is more than water speed
Water speed 10 knots				Current 4

	Current		knots at 45º
		Water speed 10 knots
	4 knots
Gives a ground speed of 14 knots		Gives a ground speed 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 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

48 Northstar Explorer 657 Installation and Operation Manual