When it comes to speed over water vs Speed over ground, many of us get confused. May be, it is a confusing topic. What do you feel ?
And the confusion is not only what is the difference but also where and when these are used ?Why we need to have input of speed over water in radar ? Why do we have doppler log on board ? And many more questions like these.
So today i want to put all these doubt and confusions to rest once for all. But before i start, I need you to ask yourself this question. Do you know the answers of questions I asked above ?
Let’s test it. Can you tell what is the speed over ground and speed over water in these three conditions ?
Condition A:No wind, no current, absolute idealconditions. The GPS speed of the ship is 15 Knots.
Condition B:All other conditions being same but now we have 2 knots current from ahead. What will be speed over water and over ground in this case ?
Condition C:All other conditions being same but now we have 2 knots current from astern.What will be speed over water and over ground in this case ?
Write down your answers. Tick tick….Tick tick.
Ok, I hope you have your answers.If your answers for speed over water for all the three conditions is same, most likely you have got it. But if you have different speed of water for all three conditions, you must read on.
Speed.. What it is ??
There is nothing absolute in this world. Everything is relative to something. Speed too is measured with respect to something adjacent. While travelling on a train you might feel increase in speed when another train passes on opposite direction. Or Sometime on railway station,we suddenly feel our train moving even when it is other train adjacent to ours which has just started moving.
So what is the speed of your train in this case. You may say zero but I may disagree. Depends on with respect to what we are talking about ?
Speed with respect to station will be zero, but with respect to other train will be equal to the speed ofthat train.
In the same way, the ship’s speedis either measured with respect to water or ground.
Speed Through water & Speed over ground
Speed over water is the distance traveled in one hour with respectto water. Similarly speed over ground is the distance traveled in one hour with respect to ground. Now what does that mean ?
Let usunderstand this and look for the answers of our three conditions.Imagine your ship is moving from an island and there is a floating boat near to the island. Sea conditions are perfect withno wind andcurrent and your ship’s GPS is showing speed of 15 knots.
In one hour you would have covered 15 NMs with respect to island as well as floating boat. This is because the boat would have maintained its position as there is no wind/current.
So in Condition A, speed over ground and speed over water will be 15 knots.
Now as in Condition B, rest of the things being same, we have 2 knots current from ahead. In this case ship would only cover 13 NM in one hour because of adverse current. So when measured from island, vessel has only moved 13 NM. So the speed over ground will be 13 Knots.
Because of 2 Knots current, the boat would move 2 NM away from the ship. The distance covered by ship with respect to floating boat will be 15 knots.So the speed over water will still be 15 knots.
Now finally let’s assume that there is 2 Knots current from astern as in condition C.In this case ship would cover 17NM in one hour because of favorable current. So when measured from island, vessel hasmoved 17NM. So the speed over ground will be 17Knots.
But because of 2 Knots current, the boat would move 2 NM in the direction of theship. The distance covered by ship with respect to floating boat will still be 15 knots.So the speed over water will still be 15 knots.
What were your answers ??
Interpretation of Speed Through water
As you would have noticed, current has nothing to do with speed over water. Irrespective of how much current you have, the speed over water will remain same.
How can we use this information ? Let’s see.
Speed through water for collision avoidance
If you have been sailing on tankers you would already know how much emphasis vetting inspections put on this fact. They want us to use speed over water in radars for collision avoidance. Do you know why ?
To understand this, let me give you a situation. I know many of us are fed up with ROR situations during competency exams but this one would be interesting.
Your vessel is moving on a true course of 000 Deg. You have another vessel right ahead on a course of 180 Deg. There is strong easterly current and because of that your vessel is making good a course of 040 Deg. Other Vessel is making good a course of 140 Deg.
Your radar screen would look something like this.
If there is risk of collision, what action you will take and under which rule ?
If you think you would take action as per crossing situation, you would be wrong. Collision avoidance rules and situations are based on how we see the ship and not on how they are moving.
For example, as per ROR, Head on situation is when you see a ship right ahead ornearly right ahead. That is when you can see both her Mast headlights in line or nearly in line and both of her sidelights.
Now Just visualise the situation I described. Will the defination of head on situation fitin this situation ? Yes it would.
But the problem is that if you follow just the radar, It will give you a false sign that the situation is a crossing situation. This is because the radar would be showing the course made good of both the ships.
This problem is tackled by using the speed over water in Radar. And that is the reasonvetting companies require use of speed over water for collision avoidance.
When we use speed over water the same situation would look as a head on situation. That is because when we use speed over water, it does not consider the effect of current on the vessel.
But in both the cases, CPA and TCPA would not change. That is because CPA and TCPA are calculated with range and bearing of the target between two or moreintervals. Remember Radar plotting techniques ??
Is the GPS speed required in RADAR ?
There is no debate that GPS feed is required in Radars for position. But if we have to use speed over water in radar, do we require GPS speed then ?
Yes, we do. Infact i have come across some vetting inspector who ask to put the radar on standby. They then check what source ofspeedit is showing. If it shows GPS speed, they have an observation such as..
“Vessel did not use speed over water while approaching the present port as was evidenced from the radar screen in standby mode.”
In my view, this is really incorrect observation. Navigators need to use both speed over water as well as speed over ground judiciously. Speed over water is only required to be used for collision avoidance and not necessarily for navigation.
By using speed over ground, a navigator is more aware of the situation than otherwise. For example, if the ship is drifting towards a danger, navigator will know it better if he has speed over ground in radar. This is because, with speed over ground radar will show a vector of course made good. (Vector need to be selected as true in this case).
As you can see in above, with speed over water navigator may get an impression that ship is moving clear of the danger. In actual it may be drifting towards the danger.
So Navigators should use both the speed to their benefit and as a tool to better navigation.
Log speed as Engine Speed
As we have seen, current has no effect on the log speed. For this reason, in normal wind force, log speed is very close to the engine speed.
Engine speed is calculated by Propeller distance divided by the time. Propeller distance is obtained by multiplying total revolution to a constant. This constant is proportional to the pitch of the propeller.
So
Engine speed= (total revs x constant) / Time
In normal wind conditions, this engine speed will be equal to the speed over water (or speed obtained from log).
If it is usually not same on you ship, there are few important conclusions that can be drawn.
1. Log is erratic
This can be a good indication of you log giving erratic readings. The conclusion however need to be verified with other means too. These include comparison with the GPS speed in calm weather with no current.
2. Foul Hull
There is another conclusion that can drawn from the difference in actual engine speed and log speed. There are chances of hull having considerable amount of marine growth. This will increase the resistance to the ship and ship’s speed over water will reduce to that with the clean hull.
This can give considerable difference in engine speed and speed over water. There is another way to confirm this finding. If the hull is fouled, ship may not be able to run on NCR rpm because of excessive torque. In run on NCR with foul hull, it is very likely that you will get torque limit alarm on the engine.
Conclusion
Speed over water and speed over ground are two distinct tool. One does not replace other. Navigator should make full use of these speeds where these best fit.
For example, navigator should use speed over water for collision avoidance. Speed over ground should be used for navigation.
Using speed over water, a navigator can be sure of his assessment of colreg situation. Using speed over ground can give an early signs if the ship is drifting towards a danger.
FAQs
Speed over Ground (SOG) is the vessel's speed in one hour concerning the land or any other fixed object such as buoys. Speed through Water (STW) is the vessel's speed in one hour concerning the water or anything floating on water.
Why speed over water is used for collision avoidance? ›
It is shown that speed through the water is the appropriate reference for avoiding collision on the high seas since it will produce a correct measure of the target-aspect information that is essential in determining legal collision avoidance manoeuvres.
Why do we use speed through water? ›
True speed through water is essential for looking at your speed fuel curve with new accuracy,” he explains. Accurate speed through water measurements help to identify the true effect other performance optimisation tools or techniques have on a vessel's fuel consumption.
How is speed through the water measured? ›
Since it was originally measured as the number of knots per unit time, speed over water traditionally uses the unit of “knots” to indicate speed. One knot (kt) = 1 nautical mile per hour = 1.15 mph = 1.85 kph.
What is SOG and COG? ›
SOG = Speed over Ground - includes current forecast. COG = Course over Ground to the next waypoint.
Which type of speed log is more accurate and commonly used nowadays? ›
Most commonly used however (especially on merchant ships) is the electromagnetic log, more reliable as there is no mechanism subject to breakdown.
Which speed is used for collision avoidance? ›
Collision avoidance by braking is appropriate at low vehicle speeds (e.g. below 50 km/h (31 mph)), while collision avoidance by steering may be more appropriate at higher vehicle speeds if lanes are clear.
How do you calculate speed over ground? ›
Speed over Ground (SOG) & Speed over water (SOW)/STW Explained
What is the difference between course over ground and heading? ›
Course Over Ground (COG) is the actual direction of motion (the intended direction of travel). While heading is the direction in which a vehicle/vessel is pointing at any given moment (https://www.applanix.com/news/blog-course-heading-bearing/).
What does SOG stand for on a depth finder? ›
SOG is speed over ground.
Speed of sound in water at temperatures ranging 32 - 212oF (0 - 100oC) - Imperial and SI units.
| Temperature - t - (oC) | Speed of Sound - c - (m/s) |
|---|
| Water | Seawater |
|---|
| 30 | 1507 | 1546 |
| 40 | 1526 | 1563 |
| 50 | 1541 | |
9 more rows1. Speedboats – 70mph. Speedboats are the fastest boats, with top speeds reaching over 170mph. These are commercial speeds, however, as custom speed boats can reach much higher speeds.
How do you measure the speed of a boat? ›
The speed of a boat having an engine to deliver P horsepower and displacing D pounds is written as: S = Speed = sqrt(P / D) * C , where S is the boat speed and C is the Crouch constant. The above equation is known as Crouch's formula.
What are the modern methods of measuring speed? ›
Police use RADAR and LIDAR to measure traffic speed. Cars have speedometers for travel speed and tachometers for engine speed. Accelerometers are also used with car speed measurements. Meteorologists use anemometers and radar to create their weather forecasts.
Why is speed on water measured in knots? ›
Mariners would lower the wood piece into the water and allow it to float freely behind the ship for a specific amount of time (often measured with an hourglass). When the time was up, they would count the knots between the ship and the piece of wood, and that number estimated their speed.
How do you know if a course is over ground? ›
With your straight edge, draw a line from the last fix to the end of the set/drift line. Measure the angle of this line against the compass rose to get the Course Over Ground. The Speed over Ground is also the length of the line, if the measured time was an hour.
What is difference between course over ground and course made good? ›
Course Made Good (CMG) - Is the course actually steered which includes the effect of current set (direction) & drift (speed), wind and leeway and helmsman errrors. Note we distinguish COG (course over ground) from cmg (course made good), as one being present dynamic value, and the other being past.
What is the course over ground? ›
Course Over Ground is the actual direction of progress of a vessel, between two points, with respect to the surface of the earth. The vessel's Heading may differ from the Course over ground due to the effects of wind, Tide and currents.
What is safe speed? ›
A safe speed is a speed less than the maximum at which the operator can take proper and effective action to avoid collision and stop within a distance appropriate to the prevailing circ*mstances and conditions.
What are the four types of speed log? ›
Types of speed logs:
- Electromagnetic log.
- Doppler log.
- Impeller log.
- Pito meter log.
- Acoustic Co-relation Log.
Pitometer logs (also known as pit logs) are devices used to measure a ship's speed relative to the water. They are used on both surface ships and submarines. Data from the pitometer log is usually fed directly into the ship's navigation system.
What will you use in collision avoidance? ›
Collision avoidance systems use a variety of technologies and sensors, such as radar, lasers, cameras, GPS, and artificial intelligence. Not all collision avoidance systems are created equally—some warn or alert, while others override the driver to assist them in avoiding collisions and mitigating risk.
How can collision be avoided? ›
To avoid colliding with a vehicle in front, look well ahead for hazards, brake lights, and turn signals; and always maintain sufficient distance between you and the vehicle in front so that you have enough room to stop safely.
How will you differentiate the sea and ground stabilization? ›
To be sea-stabilized, your radar needs two key inputs: speed through the water, usually taken from your boat's paddlewheel log, and boat heading, usually taken from a fluxgate or satellite compass. By contrast, if your radar uses GPS speed over ground and course over ground, it's ground-stabilized.
What does SOG stand for in boating? ›
SOG. Speed Over Ground. This differs from boat speed- so for example if your boat is stationary in the water, it may still be moving in relation to the land due to current and wind etc.
What is GPS speed on ships? ›
The GPS speed of the ship is 15 Knots.
What does cog mean in boating? ›
COG, Course Over Ground, indicates the direction of the boat's heading relative to the land. Over ground means "in relation to the earth", see also the term SOG. Therefore, COG means the true direction free from the effects of sea currents.
What is speed over ground? ›
Speed Over the Ground (SOG) is the speed of the vessel relative to the surface of the earth. Speed Through Water (STW) is the speed of the vessel relative to the water. It is possible for the STW to be zero while the SOG is 5 knots, for example if the boat was drifting in a 5 knot current.
Do you fly heading or course? ›
Heading is the direction the airplane is pointed, whereas track is the actual direction of the airplane tracking across the ground. Bearing is the angle between any two points, whereas course is your intended path of travel to your destination.
Are heading and bearing the same? ›
Heading is the direction the aircraft is pointing. The aircraft may be drifting a little or a lot due to a crosswind. Bearing is the angle in degrees (clockwise) between North and the direction to the destination or nav aid.
SOG is Speed Over Ground. It is measured in MPH or Knots or Km/Hr depending depending on what you have selected.
What direction does a fish finder read? ›
If you know how to read a fish finder, you know how powerful these tools are to us anglers. Down imaging fish finders send sonar down vertical from the boat, and side imaging sends sonar horizontally away from the boat.
How do you identify fish on sonar? ›
Sonar Basics: How To Find Fish - YouTube
What's the fastest speed on water? ›
The current unlimited record is 511.11 km/h (317.59 mph), achieved by Australian Ken Warby in the Spirit of Australia in 1978.
How do you calculate the speed of a deep water wave? ›
The speed the wave crests travel at. (seconds). ✤ Speed (m/s) = 1.56 * wave period (seconds). ✤ Longer waves travel faster.
Does sound travel better in water or air? ›
Sound travels faster in water compared with air because water particles are packed in more densely. Thus, the energy the sound waves carry is transported faster. This should make the sound appear louder.
What is a good boat speed? ›
Do boats go fast? With some popular boats average speed values
| Type of boat | Average speed | Top speed |
|---|
| Deck boats | 30 mph – 50 mph | 60 mph |
| Cruiser | 16 mph – 30 mph | 50 mph |
| Cigarette boats | 100 mph – 150 mph | 150 mph |
| Racing boats | 150 mph – 200 mph | 200 mph |
6 more rowsMost recreational boats have an open helm, so you can feel every bit of 50 mph (the boat-speed gold standard) on the water, while maintaining that speed in a car will get you honked at in the slow lane.
How fast does a 80 hp boat go? ›
The boat is powered by 80 hp, and makes a speed 30 knots.
How do you calculate the speed of a boat in still water? ›
Boat's speed in still water = ½ (Upstream speed + downstream speed) = ½ (40 + 60) = 50 miles/ hr.
Find the Speed of a Boat with a Current - YouTube
What makes a boat fast? ›
A boat's ability to go fast is a balancing act between its length and the power driving it, whether supplied by wind or engine… A boat's ability to go fast is a balancing act between its length and the power driving it, whether supplied by wind or engine, says Jerry Milgram, William I.
What type of measurement is used in measuring speed? ›
Speed is the rate at which an object's position changes, measured in meters per second. The equation for speed is simple: distance divided by time. You take the distance traveled (for example 3 meters), and divide it by the time (three seconds) to get the speed (one meter per second).
Which methods are used by the police to measure average speed? ›
The police use three different methods to monitor driving speeds as accurately as possible to ensure that speeding drivers are penalised. Radar guns and mobile radar units in police cars send out radio waves. The radio waves are reflected from the moving vehicle.
What unit do we use for speed? ›
Speed is a measure of how fast something is travelling. The most common units of speed are metres per second (m/s), kilometres per hour (km/h) and miles per hour (mph).
Why do boats use knots instead of mph? ›
In modern times, a knot is a unit of speed that ties directly into the global latitude and longitude coordinate system. Therefore, in the aviation and nautical worlds, knots are oftentimes used in place of MPH and KPH since they are easier to navigate with.
Why is speed called knots? ›
The term knot derives from its former use as a length measure on ships' log lines, which were used to measure the speed of a ship through the water. Such a line was marked off at intervals by knots tied in the rope.
Why knots are used for measurement? ›
Ancient mariners used to gauge how fast their ship was moving by throwing a piece of wood or other floatable object over the vessel's bow then counting the amount of time that elapsed before its stern passed the object. This method was known as a Dutchman's log.
How do you calculate speed over ground? ›
Speed over Ground (SOG) & Speed over water (SOW)/STW Explained
What is the difference between course over ground and heading? ›
Course Over Ground (COG) is the actual direction of motion (the intended direction of travel). While heading is the direction in which a vehicle/vessel is pointing at any given moment (https://www.applanix.com/news/blog-course-heading-bearing/).
SOG. Speed Over Ground. This differs from boat speed- so for example if your boat is stationary in the water, it may still be moving in relation to the land due to current and wind etc.
What is BCR and BCT? ›
Bow Crossing Range of target (BCR) Bow Crossing Time of target (BCT)
Which speed is used for collision avoidance? ›
Collision avoidance by braking is appropriate at low vehicle speeds (e.g. below 50 km/h (31 mph)), while collision avoidance by steering may be more appropriate at higher vehicle speeds if lanes are clear.
What is course over ground in GPS? ›
The course is the intended direction of travel. Ideally (but rarely) it is the same as heading. On a GPS receiver, the actual direction of motion is called course over ground (COG) or, on some units, track.
What does cog mean in boating? ›
COG, Course Over Ground, indicates the direction of the boat's heading relative to the land. Over ground means "in relation to the earth", see also the term SOG. Therefore, COG means the true direction free from the effects of sea currents.
What is speed over ground? ›
Speed Over the Ground (SOG) is the speed of the vessel relative to the surface of the earth. Speed Through Water (STW) is the speed of the vessel relative to the water. It is possible for the STW to be zero while the SOG is 5 knots, for example if the boat was drifting in a 5 knot current.
Do you fly heading or course? ›
Heading is the direction the airplane is pointed, whereas track is the actual direction of the airplane tracking across the ground. Bearing is the angle between any two points, whereas course is your intended path of travel to your destination.
Are heading and bearing the same? ›
Heading is the direction the aircraft is pointing. The aircraft may be drifting a little or a lot due to a crosswind. Bearing is the angle in degrees (clockwise) between North and the direction to the destination or nav aid.
What is speed over water? ›
Speed over water is the distance traveled in one hour with respect to water. Similarly speed over ground is the distance traveled in one hour with respect to ground.
Is SOG the same as mph? ›
SOG is Speed Over Ground. It is measured in MPH or Knots or Km/Hr depending depending on what you have selected.
1. Speedboats – 70mph. Speedboats are the fastest boats, with top speeds reaching over 170mph. These are commercial speeds, however, as custom speed boats can reach much higher speeds.
Why do ships have 2 radars? ›
Where two radars are used, one radar can be kept on a longer range scale to obtain advance warning of the approach of other vessels, changes in traffic density, or proximity to the coastline. The other radar can use a short range scale, which helps to detect smaller targets easily.
What is BCR in radar plotting? ›
Bow Crossing Range (BCR)
The range at which the other vessel will pass directly. ahead (or astern). Found by measuring the distance from where the Line of Approach crosses the heading line to the centre of the plotting sheet.
What is CPA and TCPA in radar? ›
CPA: Closest Point of Approach. This is the closest that you will come to the vessel in question should you both maintain your current course and speed. TCPA: Time to Closest Point of Approach. This states when the closest pass will occur.
