The Difference Between Speed Over Ground and Speed Through Water

What is Speed Over Ground (SOG)?

SOG is used to measure the speed of a boat relative to the Earth’s surface. This considers the effects of external factors – such as wind, currents, and tides – on the vessel’s motion.

This gives boaters a more precise understanding of how quickly their vessel travels from one place to another, across the land and water.

For modern vessels, GPS technology is employed to calculate SOG. GPS devices – integrated into chart plotters or standalone – receive signals from satellites orbiting the Earth.

The GPS can accurately determine its speed and direction over the ground using multiple satellite signals to triangulate the boat’s position.

Monitoring and understanding SOG is essential for various aspects of boating:

1. Navigation: Knowing your SOG helps plan courses and estimate destination arrival times. It also allows for adjustments to be made to compensate for external factors, such as adverse currents, helping ensure that navigation is more accurate.
2. Safety: Keeping tabs on SOG is crucial for avoiding collisions and maintaining other vessels, navigational hazards, safe distances, and restricted areas. Furthermore, it can help boaters adhere to speed limits in certain waterways or zones, thus reducing the risk of accidents and fines.
3. Performance: Comparing SOG to Speed Through Water (STW) reveals the influence of environmental factors on a boat’s performance. This information enables boaters to make informed decisions about when and how they should adjust their course or speed to remain efficient while remaining in control of their vessel.

What is Speed Through Water (STW)?

STW is the speed of a boat relative to the water it travels through. Unlike SOG, which evaluates speed relative to the Earth’s surface, STW only considers the vessel’s motion through the water itself.

This measurement helps judge propulsion efficiency and detect changes in water conditions.

STW is generally measured with a speed log or paddle wheel sensor. These devices are typically installed on the boat’s submerged hull.

As the vessel moves, water flow causes the paddle wheel to rotate and generate a speed signal – this signal is sent to a display unit and shows STW in real time.

Monitoring and understanding STW provides various advantages for boaters:

1. Fuel efficiency and optimal performance: Comparing STW to engine RPM or power output can help to identify the most efficient operating conditions for a boat. This information lets boaters reduce fuel consumption and engine wear while achieving desired speeds.
2. Monitoring a vessel’s response to changing water conditions: Checking STW offers insight into how the boat is affected by changes in water density, viscosity, and temperature, allowing adjustments to be made to course or speed to maintain control of the vessel and optimize performance under different circumstances.
3. Troubleshooting propulsion system issues: Abnormal changes in STW, especially when compared to engine RPM, may signal problems with the boat’s propulsion system. Keeping an eye on STW can assist in detecting such issues quickly, permitting corrective action to prevent damage or breakdowns.

Other Ways to Measure Boat Speed

Here are some other ways to measure speed while boating:

1. Pitot tube: A pitot tube measures a boat’s speed according to the water pressure created from its movement through the water. The tube is attached to the hull, and as speed increases, it registers a reading on an analog or digital display. It may not be as precise as GPS or speed logs, but it’s economical and low-maintenance.
2. Doppler speed log: This sophisticated approach uses the Doppler effect to ascertain a vessel’s speed with high accuracy. A transducer installed in the hull sends sound waves to the seabed and reflects them, so any frequency shift can be used to calculate speed accordingly. This option is especially popular in large vessels due to its precision, though it tends to be more expensive and complicated than other options.
3. Traditional dead reckoning: Before modern technology entered the scene, sailors relied on dead reckoning to estimate their speed – by timing how long it takes to cover a known distance – often using a “chip log” (a wooden quadrant tied to knotted rope). Despite being less accurate than today’s methods, understanding this skill is still essential for boaters in case tech fails.

Using Speed Over Ground and Speed Through Water Together

By using SOG and STW measurements in tandem, boaters can gain several critical advantages for their vessel:

• Get a comprehensive understanding of their vessel’s performance in various conditions.
• Plan efficient courses and navigate challenging environments.
• Enhance safety by monitoring environmental factors and making necessary adjustments.
• Detect potential issues quickly and reduce damage to the boat or costly repairs.
• Improve fuel efficiency and extend the life of the engine.

The following examples demonstrate how various factors can impact Speed Over Ground (SOG) and Speed Through Water (STW) measurements, highlighting the importance of understanding and monitoring speed metrics.

Strong currents:

Imagine sailing in an area with a strong current flowing in the same direction as your intended course. While your boat’s STW may indicate a speed of 6 knots, the current’s assistance might result in a SOG of 8 knots. In this scenario, the current positively impacts your SOG, allowing you to reach your destination faster than expected. Conversely, if you were sailing against the current, your SOG would be lower than your STW, potentially causing delays or necessitating course adjustments.

Wind influence:

Suppose you’re navigating a powerboat with an STW of 20 knots. If you encounter strong headwinds, your SOG might drop 18 knots as the wind slows your progress over the ground. Understanding this difference will help you adjust your course or speed to maintain control and optimize your boat’s performance in windy conditions.

Tidal effects:

Consider a scenario where you’re sailing in a coastal area with significant tidal movements. During an outgoing tide, your boat’s STW might be 5 knots, while the tide’s influence results in a SOG of only 3 knots. Knowing this discrepancy lets you plan your route and timing more accurately, avoiding potential navigational hazards and ensuring a safer journey.

Final Thoughts

In conclusion, understanding and effectively utilizing Speed Over Ground (SOG) and Speed Through Water (STW) measurements are essential aspects of boating.

These speed measurements provide valuable insights into a vessel’s performance, interaction with environmental factors, and overall progress.

By comparing SOG and STW, you can make informed decisions regarding course adjustments, speed optimization, and troubleshooting potential issues.

To ensure the best boating experience, it’s crucial to maintain and calibrate your speed measurement instruments, stay informed about changing environmental conditions, and continuously develop your boating skills.

By integrating SOG and STW with other navigation data and following the tips provided, you can maximize your vessel’s performance, maintain control in various conditions, and navigate safely and efficiently.