S.O.W. vs S.O.G.
- Thread starter butchrobbins
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butch, there is a difference. SOG is the speed that you are moving at over the surface of the Earth. SOW is the speed through the water. Water is a fluid and rarely stationary. On a small lake the two may be close to equal but in tidal areas and large bodies of water, water is anything but stationary. I have sailed in areas where the current of the water equals the speed of the boat. I can remember once sailing along a shoreline into a 5 knot current with a boat speed of 5 knots. We sat practically staionary with respect to the fishermen on the shore although the knotmeter read 5 knots.
There can be. SOG - speed over ground is what your GPS will show. SOW - speed over water is what your knot meter will show. Where I sail, Galveston Bay, there is little tide current so SOG and SOW are close to the same. However, if you sail where there is significant tide current there will be a difference. Assume you are going with a 3 knot current, SOW shows 6 knots, SOG should read 9 knots. Headding dead into the same current, SOG is 3 knots.
As I understand it, speed over water is the speed of the boat through the water, and speed over ground is the speed of the boat relative to the ground under the water. The two would be the same in a lake or pond with no current, but could be quite different in a river, or in an ocean current.
Example: River current = 4 knots; boat SOW = 5 knots. If the boatgoes upstream, its SOG is 1 knot; downstream its SOG is 9 knots.
If this is wrong, someone chime in and let all of us know! -Paul
Example: River current = 4 knots; boat SOW = 5 knots. If the boatgoes upstream, its SOG is 1 knot; downstream its SOG is 9 knots.
If this is wrong, someone chime in and let all of us know! -Paul
Here's an Example
Try the Golden Gate in San Francisco. Currents can run up to 6 knots or more. Oh, they also change direction a couple of times a day.
Knowing the difference between the two numbers can make a big difference in any trip.
As I mentioned in another recent post, it can also be helpful learning about where back eddies occur.
BTW, this is basic "current sailing" in Bowditch, Duttons, Chapmans and all the rest of the "authorized publications." It probably is also in "Sailing for Dummies" so it's not or should not be one of those "here's what I think" issues, it is a reality. It's important information to know.
Try the Golden Gate in San Francisco. Currents can run up to 6 knots or more. Oh, they also change direction a couple of times a day.
Knowing the difference between the two numbers can make a big difference in any trip.
As I mentioned in another recent post, it can also be helpful learning about where back eddies occur.
BTW, this is basic "current sailing" in Bowditch, Duttons, Chapmans and all the rest of the "authorized publications." It probably is also in "Sailing for Dummies" so it's not or should not be one of those "here's what I think" issues, it is a reality. It's important information to know.
This is also why a knotmeter, in addition to a GPS, is important as sailing instruments go. While SOG is useful, there are times where KNOWING BOTH STW (SOW) and SOG is vital.
thanks for the responses
and I guess I really want to know is of what relevance is SOW? Isn't all u really care about is how fast you are getting where you are going (SOG)? So what if my knotmeter is showing 4 when what I need to know is that I am really going 2? What good is the 4?
No, because having a current against you, especially if it isn't coming from dead ahead, can be setting you off course, and that means you may think you're sailing a safe course, but because you're now going slower and the current will have more time to set you off course, you could be headed for a reef or submerged hazard and not realize it. Seeing that there is a difference between STW (SOW) and SOG should alert you to check what is really going on.
BTW, the difference in a passage that having the current with you versus against you is huge. Say there is a two knot current that reverses. If you time your passage so the current is with you, that is a four knot difference in overall speed. This can be the difference between making your next port before nightfall or not.
BTW, the difference in a passage that having the current with you versus against you is huge. Say there is a two knot current that reverses. If you time your passage so the current is with you, that is a four knot difference in overall speed. This can be the difference between making your next port before nightfall or not.
It's also often not realized, especially among small plane pilots, that a current directly on your beam slows you down. This mistake will now be made more often in boats as people follow GPS track lines. Aircraft instrumentation, pre GPS, was designed to help stay on a track as opposed to just heading towards the destination. Aircraft run with smaller fuel reserves than boats so failure understand this often led to worse outcomes than on the water.
Roger has it exactly. A current (or force) from the side will push you off course. Sailing by a given heading may not get you to your destination. After calculating cross track error you can choose a new heading that will get you were you want to go. For example, if your destination is due north and you sail a course of 360 but have a current driving you east you will arrive too far east of your destination. You will need to steer a course lower than 360 to compensate for the current but arrive exactly at your destination.
SOG is fine for calculating ETA but as Ross points out, SOW is needed for proper sail trim. This number lets you know when your sail controls are optimal.
SOG is fine for calculating ETA but as Ross points out, SOW is needed for proper sail trim. This number lets you know when your sail controls are optimal.
Way back when we had to use the Eldridge when crossing the sound and figure the crab angle BUT i have had GPS so long i forget HOW 
Its surprising that with all the current stations info in my 5" GPS it cant do things we use to do with a pocket calculator
Its surprising that with all the current stations info in my 5" GPS it cant do things we use to do with a pocket calculator
It's easy and easier than usually described. How long will it take you to get there? How far will the current carry you in that time? Put your finger on the chart that distance from your objective at the angle the current is flowing in the opposite direction and then steer for the greasy spot on the chart. (Eating potato chips is an important part of paper chart navigation).
There you go.
Airplanes are the same way...SOG is ground speed, Airspeed is speed through the air. If you have 300 knots of airspeed with a 30 knot head wind...your ground speed is 270.
Can't teach a navigation course here on the forum but for lake sailors there is no big deal. Leave the lake and get into open water, current, and needing to know how much fuel you will have at any given point, where you are on a chart, how far you have gone in a length of time is a different storie all to gether.
If you really serious abt wanting to know theses basic navigation issues, i'd suggest take a power squadron or USCG auxilliary course.
Come to Mobile,AL. and I,ll teach you for a hamburger and glass of tea..
..
Seriously take a course and learn something usefull.They are usually FREE. !
Capt Charles Creel
200 ton master
Most things have been covered here (Swiftsure winners almost invariably win because they chose their course based on tide in the Juan de Fuca), but one little thing about current: it affects relative wind! Going upwind with a following tide (SoG > SoW), you will get more wind. Downwind, less wind. Sailing downwind and down-tide in light air is a real pain!
druid
druid
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