UseCalcPro
Home
MathFinanceHealthConstructionAutoPetsGardenCraftsFood & BrewingToolsSportsMarineEducationTravel
Blog
  1. Home
  2. Marine

Boat Speed Calculator

Calculate SOG, STW, VMG, and current set/drift

SOG

6.5 kn

VMG

4.6 kn

Current

0.9 kn

Direction of wind source or destination mark for VMG calculation

Speed Over Ground

6.5

knots

MPH

7.5

km/h

12.0

STW

5.8 kn

Current Set & Drift

Drift (Speed)

0.9

knots

Set (Direction)

80°

true

Calculated from the vector difference between SOG/COG and STW/HDG

Velocity Made Good (VMG)

4.6 kn

45° off target bearing

SOG6.5 kn
Angle off target45°
VMG efficiency71%

Speed Conversions

1 knot =1.15078 mph
1 knot =1.852 km/h
1 knot =1 NM/hour

What You'll Need

Star Brite Ultimate Aluminum Cleaner & Restorer 64oz

$18-$254.5
View on Amazon

3M Marine Adhesive Sealant 5200 Fast Cure White 3oz

$14-$204.5
View on Amazon

Shoreline Marine Bilge Pump 600 GPH 12V

$15-$254.3
View on Amazon

Garmin Striker 4 GPS Fish Finder with CHIRP Sonar

$100-$1504.4
View on Amazon

Standard Horizon HX210 Floating Handheld VHF Radio

$120-$1604.5
View on Amazon

Hondex Portable Digital Depth Sounder

$150-$2004.3
View on Amazon

Star Brite Ultimate Aluminum Cleaner & Restorer 64oz

$18-$254.5
View on Amazon

3M Marine Adhesive Sealant 5200 Fast Cure White 3oz

$14-$204.5
View on Amazon

Shoreline Marine Bilge Pump 600 GPH 12V

$15-$254.3
View on Amazon

Garmin Striker 4 GPS Fish Finder with CHIRP Sonar

$100-$1504.4
View on Amazon

Standard Horizon HX210 Floating Handheld VHF Radio

$120-$1604.5
View on Amazon

Hondex Portable Digital Depth Sounder

$150-$2004.3
View on Amazon

As an Amazon Associate, we earn from qualifying purchases.

Frequently Asked Questions

Q

What is the difference between SOG and STW?

Speed Over Ground (SOG) is your actual movement across the earth’s surface, measured by GPS. Speed Through Water (STW) is how fast you move relative to the surrounding water, measured by a paddlewheel or electromagnetic log. The difference between SOG and STW reveals the current affecting your vessel.

  • SOG includes the effect of current — a 5-knot boat in a 2-knot favorable current shows 7 kn SOG
  • STW is independent of current — the same boat still shows 5 kn STW
  • When SOG > STW, current is pushing you in your direction of travel
  • When SOG < STW, current is opposing you (or pushing you sideways)
  • The vector difference between SOG/COG and STW/HDG gives current set and drift
MetricSensorAffected by Current
SOG (Speed Over Ground)GPSYes — includes current
STW (Speed Through Water)Paddlewheel / EM logNo — relative to water
COG (Course Over Ground)GPSYes — actual track
HDG (Heading)Compass / gyroNo — where bow points
Q

What is VMG and why does it matter for sailing?

Velocity Made Good (VMG) is the component of your speed directly toward (or away from) a target bearing, typically the wind direction or a racing mark. VMG = SOG × cos(angle off target). A boat sailing at 6 knots, 45° off the wind, makes 4.24 knots VMG toward the wind.

  • VMG = SOG × cos(angle between course and target bearing)
  • At 0° off target, VMG equals SOG (100% efficiency)
  • At 45° off target, VMG = 70.7% of SOG
  • At 90° off target, VMG = 0 (no progress toward target)
  • Optimal VMG upwind is typically at 38°–45° true wind angle for most sailboats
Q

How do I calculate current set and drift?

Current set (direction) and drift (speed) are found by vector-subtracting STW/Heading from SOG/COG. Plot your GPS track (SOG/COG) and your water track (STW/Heading) as vectors; the difference is the current vector. Drift is the current speed in knots, and set is the direction the current flows toward.

  • Set = direction the current flows TOWARD (not from), in degrees true
  • Drift = current speed in knots
  • Vector math: Current_X = SOG×sin(COG) − STW×sin(HDG)
  • Vector math: Current_Y = SOG×cos(COG) − STW×cos(HDG)
  • Drift = √(Current_X² + Current_Y²); Set = atan2(Current_X, Current_Y)
Q

How do I convert knots to mph or km/h?

One knot equals 1 nautical mile per hour, which is 1.15078 statute miles per hour (mph) or 1.852 kilometers per hour (km/h). A nautical mile is 1,852 meters, based on one minute of arc of latitude.

  • 1 knot = 1.15078 mph — multiply knots by 1.15 for approximate mph
  • 1 knot = 1.852 km/h — multiply knots by 1.85 for approximate km/h
  • 1 knot = 1 nautical mile per hour = 1,852 meters per hour
  • 6 knots = 6.9 mph = 11.1 km/h (typical cruising speed for a sailboat)
  • 20 knots = 23.0 mph = 37.0 km/h (fast powerboat cruising speed)
KnotsMPHkm/hTypical Use
3 kn3.5 mph5.6 km/hDinghy sailing
6 kn6.9 mph11.1 km/hCruising sailboat
15 kn17.3 mph27.8 km/hDisplacement powerboat
30 kn34.5 mph55.6 km/hPlaning powerboat
Q

What is the hull speed formula?

Hull speed is the theoretical maximum speed for a displacement vessel: Hull Speed = 1.34 × √LWL, where LWL is waterline length in feet and the result is in knots. At hull speed, the bow wave and stern wave align, creating a single wave as long as the hull.

  • Formula: Hull Speed (kn) = 1.34 × √(LWL in feet)
  • 25 ft LWL: hull speed = 1.34 × 5.0 = 6.7 knots
  • 36 ft LWL: hull speed = 1.34 × 6.0 = 8.0 knots
  • Light boats (DLR < 200) can exceed hull speed by surfing or planing
  • The 1.34 constant comes from the relationship between wave speed and wavelength in deep water

Example Calculations

1Sailing Upwind with Current (6.5 kn SOG)

Inputs

SOG6.5 kn
COG045°
STW5.8 kn
Heading040°
Wind Bearing000°

Result

VMG to Wind4.60 knots
Current Drift0.88 kn
Current Set080°
SOG in MPH7.48
SOG in km/h12.04

Angle off wind = |45 − 0| = 45°. VMG = 6.5 × cos(45°) = 6.5 × 0.707 = 4.60 kn. Current X = 6.5×sin(45) − 5.8×sin(40) = 4.60 − 3.73 = 0.87. Current Y = 4.60 − 4.44 = 0.15. Drift = √(0.87²+0.15²) = 0.88 kn.

2Downwind Run (8.0 kn SOG)

Inputs

SOG8.0 kn
COG180°
STW6.5 kn
Heading180°
Wind Bearing000°

Result

VMG Downwind-8.00 knots
Current Drift1.50 kn
Current Set180°
SOG in MPH9.21
SOG in km/h14.82

Angle off wind = |180 − 0| = 180°. VMG = 8.0 × cos(180°) = −8.00 kn (negative means moving away from wind source — good downwind). Current: same heading, SOG > STW by 1.5 kn, so 1.5 kn favorable current from 000°.

Formulas Used

Velocity Made Good (VMG)

VMG = SOG × cos(Angle off Target)

Calculates the component of boat speed in the direction of a target bearing (wind or mark).

Where:

SOG= Speed over ground in knots (from GPS)
Angle off Target= Absolute angle between COG and wind/mark bearing in degrees

Current Drift

Drift = √((SOG×sin(COG) − STW×sin(HDG))² + (SOG×cos(COG) − STW×cos(HDG))²)

Calculates current speed from the vector difference between ground track and water track.

Where:

SOG= Speed over ground in knots
COG= Course over ground in degrees true
STW= Speed through water in knots
HDG= Vessel heading in degrees true

Speed Conversion

MPH = Knots × 1.15078; km/h = Knots × 1.852

Converts nautical speed (knots) to statute miles per hour or kilometers per hour.

Where:

Knots= Speed in nautical miles per hour
1.15078= Conversion factor from knots to mph
1.852= Conversion factor from knots to km/h

Understanding Boat Speed, VMG, and Current Effects

Boat speed involves more than just reading a number from a GPS or knotmeter. Speed over ground tells you how fast you are actually getting somewhere, while speed through water reflects your boat’s true performance through the fluid medium. The difference between these two measurements reveals the invisible hand of current affecting your passage.

For sailors, velocity made good (VMG) is often more important than raw speed. A boat that sails faster at a wider angle may actually make less progress toward the destination than a slower boat pointing closer to the wind. Optimizing VMG is the key to efficient upwind sailing and competitive racing.

Understanding current set and drift is critical for accurate navigation, especially in areas with strong tidal flows. By comparing your GPS track with your compass heading and log speed, you can determine exactly how the current is affecting your passage and adjust your course to compensate.

Related Calculators

Tidal Current Calculator

Rule of twelfths prediction

Compass Deviation Calculator

TVMDC heading corrections

Boat Displacement Calculator

Hull displacement & DLR

Water Depth Sonar Calculator

Calculate water depth from sonar ping time using speed of sound in water. Adjust for temperature and salinity effects on acoustic velocity for accurate depths.

Hull Speed Calculator

Calculate your boat's theoretical hull speed from waterline length. Find the speed-to-length ratio and understand wave resistance limits for displacement hulls.

Fishing Weight Calculator

Calculate the right sinker weight for your fishing conditions. Factor in current speed, water depth, and line diameter to keep your bait in the strike zone.

Related Resources

Tidal Current Calculator

Predict tidal current speed with the rule of twelfths

Compass Deviation Calculator

Convert between true, magnetic, and compass headings

Boat Displacement Calculator

Hull displacement from dimensions

More Marine Calculators

Explore all marine and nautical tools

View All Marine

Last Updated: Mar 25, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

UseCalcPro
FinanceHealthMath

© 2026 UseCalcPro