View Full Version : log: SOG V Water Speed

07-03-02, 10:55
What are the advantages if any of having a log showing speed through the
water as against GPS log showing speed over the ground?

Is the feedback from one "better" than the other to allow adjustments
to increase boat speed?

I have a Stowe trailing log and a GPS which shows SOG.

I'm not inclined to use the Stowe log and am thinking of fitting an inhull
log but then again don't want to put another hole in the boat ( sailing).


<P ID="edit"><FONT SIZE=-1>Edited by duke on Thu Mar 7 11:02:49 2002 (server time).</FONT></P>

07-03-02, 11:29
Speed through the water is the most important log measurement of all! A good measure of speed through the water is vital for racing or cruising.

It is, as you surmise, the best measure of boat performance. It is also what harbour speed limits are measured in. e.g. your speed over ground may read 6 knots in Harbour but you may well be going much faster through the water. thus causing more wash and annoying everyone - and you may well get fined.

Your speed over ground is a combination of your speed through the water, tidal effects and any leeway you make.

I understand the issue of putting a hole in your boat but I honestly think it worth it - especially if you race.


07-03-02, 11:39
If you have both you can see the real affect of the tide which is useful for confirming any calculations you've made that include tidal data.

I've just fitted a GPS repeater next to the log so I can see both.

The SOG of course give you a much better idea of when you're going to be back in the pub!

Roger Holden

07-03-02, 11:41
This is not the complete answer but from my experience I would make the following comments;
Log showing speed thro water will be quicker to show the affects of ''tweaks and adjustments'' than GPS SOG.
SOG will show changes of ''tactics'' in using tidal streams and VMG read-out especially on different tacks is very useful in ensuring you are on the best approach to a destination(this will also incude wind shift effects)
For traditional paper chart navigation using distance sailed(thro water)/tidal vectors/course steered/leeway you need the info from your log not GPS.
I'm sure theres lots Ive omitted and others have a different view

07-03-02, 12:39
Don't worry about the hole in the boat - especially if you are going to fill it with a transducer!
I think both are good, particularly if you are sailing in very tidal waters such as Scottish West Coast where feedback from the log on sail adjustments can help you monitor whether the boat is going faster or the current is gaining on you. Really useful for longish slogs such as Islay Sound when you've just left things a little late!
John S

07-03-02, 13:01
Out of the two and bearing in mind that you need always to be able to fall back on trad navigation, some method of calculating speed through the water is essential.

The though hull transducer will be more convenient.

07-03-02, 13:21
Hang on to that Stowe log. maybe inconvenient, but you've got it, it works, and it doesn't require a hole in the hull. If you do decide to ditch it, let me know. I'd like a spare whirligig jobby for mine!

07-03-02, 13:43

I have the same set up stowe trailing log, GPS and handheld backup GPS (Garmin 12). What I am interrested in is what is my speed over the ground which I get from the GPS. I look up the tide flow rate before I leave harbour and plan my passage to take advantage of the tide. The trailing log has not been connected up for years, I also fear forgetting about it and reversing over the cable.
Before we had GPS & Decca to get speed over the ground we either had to add or subtract LOG speed from the tide flow rate obtained from the almanac. The only advantage I can see with the LOG is it is immediate which is essential for racing types.
As for another hole in the boat I must confess I am anti more holes in boat, also I have been on many boats to be told the logs not working due to it being fouled by our barnicle friends. My depth sounder works without any problems it is in a tube attached inside the hull without any hole in the boat.
So my feeling is unless you are racing don't bother.
Your boat your hole!
Good sailing

07-03-02, 14:24
How does one navigate without a log? I think I better go back to school oh and what happens when the batteries fail?

07-03-02, 14:35
Don't trust the GPS for 100% unless you have a DGPS. I'd experienced that it showed SOG at 6 knots at one moment and then only 1 knot at the other second.

Why? Normal GPS gives you a random fix within 100 meters dia. You'll have a faster speed when the 2nd fix is at the front edge of the dia, or a slower speed when the 2nd fix is close / backward to the 1st fix.

Or you can set the speed damping to the smallest scale to improve the accruacy of the GPS reading.

07-03-02, 15:01
The modern GPS sets give very accurate SOG seadouts, unlike some of the older ones. Unless you are racing and need tuning feedback, forget the hole in the hull and use GPS. BUT, always have a backup, and your trailing log will make an excellent backup fro when you need to go back to basics. If you do fit a paddlewheel log and ignore it until you need it a backup, the chances are that the paddlewheel will be fouled just when you need it most.

07-03-02, 20:07
If the holder of the radar gun is ashore, how does he measure your speed through the water?

If he is on a boat drifting with the ebb or flow, how does he correct for his drift if your speed over ground is to be measured?

Are speed limits over ground or through the water?

5 knot ebb tide and 4 knot speed limit over ground would mean leaving a harbour entrance or navigating down a river becoming an interesting problem.

Advice please (main point is to keep wash to a minimum but could you still be pinched?)

Can anybody settle this one, preferably quoting chapter and verse of some regulation?.

Whichever applies, over ground or through the water could easily be challenged successfully in court because of the above parameters (or could it?).

07-03-02, 20:28
GPS tells us where we are now and, from the chart, whether we are safe. However, the most important thing is to know if we will continue to be in safe water in future. To know that, we need to predict future COG and SOG. The combination of log and SOG from GPS will help to validate predictions of tidal stream, but this changes over time and place. Without a log, it is necessary to rely on the predicted tidal stream for the current position to reverse engineer speed through the water and then calculate Estimated Positions ahead. Simply projecting the SOG (and COG) from GPS ahead is a recipe for disaster.
GPS is very useful, but even when it is working as well as advertised, I would still rather do without it than log and compass.

Nick F

07-03-02, 21:01
You still have to rely on the predictions for tide/current direction, for your vector diagrams.

07-03-02, 21:51

Do you think that a 30 mph speed limit only applies on the flat and not downhill due to the effects of gravity. Officer my foot was at the same throttle setting it was gravities fault I was doing 50 mph. Don't hold much hope for you getting off, the same applies on the river 8 Knots is 8 Knots your speed is relative to a fixed object of zero speed such as a buoy. If the police launch is heading towards you at 8 Knots and your apparent speed is 16 Knots on the gun, then you are actually doing 8 Knots. Fortunatly we do not hear of many convictions for doing just over the limit most convictions are for speeds well in excess of the limit which is clear for all to see, with or without a radar gun.
Out of interest does anybody know of a speed limit which is less than the spring tidal flow?
Clearly if you want to know if you are exceeding the speed limit the GPS is your guide and not the LOG.
If your GPS reads 1 Knot and then 5 Knots something is wrong with the damping in the GPS data it should be stable +- 0.2 Knots.


07-03-02, 23:07
so without GPS you could defend yourself if you were "speeding" ? - until last weekend my GPS was at the chart table only so I couldn't really nip down there to check when passing through the busy stretch of water outside Cowes for example.

Interesting issues.

Roger Holden

08-03-02, 11:54
The harbour launch keeps pace with you. Also if you are making too much of a wash it is his sign to come and pace you.

08-03-02, 14:37
Not being unkind roger but you will hardly be done for speeding in your boat. It is us Stinkies that have to watch it.


08-03-02, 18:01

I remember trying this when caught doing 43 mph in a 30 limit my mitigating circumstance were "I was busy negotiating past a guy in the road painting a white line at the edge of the road, also a pensioner in the road which distracted me from observing the speedometer" result £30 fine.

Nice try!

08-03-02, 18:21

Evidently Roger can do 6 Knots so with a tail wind and tide he could probably just break the eight knot limit. Against the wind and tide is another story see his posting regarding lack of umph!
As you point out stink pots are the main speed targets, lets hope they don't bring out exhaust emission limits, which I suspect could be a bit of a problem for some.

Us sailing types do it with no noise or pollution well most of the time!

All the best

08-03-02, 20:05
I agree, of course, but having speed through the water and SOG avaialable allows you to compare the actual affect of set and drift with predictions. You can then take that into account in constructing the EP therefore achieving a more accurate prediction.

09-03-02, 09:35
As the master of yor vessel you are clearly responsible for complying with regulations - if you only have a speed through water log then you will obviously have tidal atlas almanac etc and a calculator (slide rule?) from which to calculate your SOG as any navigator......but, as said earlier in this post and many times before, you will normally stick out like a sore thumb to both your self and the authorities before anyone goes pointing a radar gun at you.

Above all we are all supposed to be enjoying this sport!

09-03-02, 13:02
Thanks for the replies - all very interesting and as usual on this forum
thought provoking.



09-03-02, 13:59
Absolutely right. You need both.