Has GPS made Speed Through the Water (STW) irrelevant?

[Graham W]

Can anyone put their finger on a killing argument for knowing (as small boat sailors) Speed Through the Water (STW) as opposed to Speed Over Ground (SOG)?  You can easily measure SOG with the GPS in any mobile device or fixed chart plotter, to a high degree of precision.  STW on the other hand requires the installation of a paddle wheel somewhere under the boat, possibly through a hole in the hull.  The mechanism can be vulnerable to damage from grounding or trailer operations and can gum up if the boat is left in the water for any length of time.

So I’m thinking, what’s the point of knowing STW when measuring it can cause difficulties and additional expense and may even be unreliable?  I can understand if you’ve got a motor boat and need to equate STW with fuel (or electricity) consumption to calculate range.  But on a sailing boat, surely SOG, together with related Course Over Ground (COG) tells you as much as you might usefully need to know for navigation purposes?  SOG and COG measure progress towards your objective.  STW measures how close you are to hull speed but as far as I can see, that’s about it.  And you can get a pretty good idea about hull speed by glancing at your wake.

One of the reasons I’m asking is because my particular electronics system (Tacktick) refuses to measure progress to windward (VMG) unless it is provided with STW as well as wind measurements.  I haven’t tried it yet but I suspect that tablet and phone apps are quite happy to use SOG and wind measurements.  The resulting synthetic VMG seems more relevant to how quickly you’re going to get home than the ‘old school’ way of calculating it.

[AndyB]

Hi Graham,

The main proponent of STW is Raymarine and they do seem to have a point. 

Read this article:
https://raymarine.custhelp.com/app/answers/detail/a_id/3794/~/apparent-wind%2C-true-wind-and-ground-wind%2C-and-data-required-to-calculate-them or https://www.simrad-yachting.com/en-gb/world-of-simrad/technology/sog-versus-stw/

Your instruments should be able to display AWS; AWA but strictly speaking not TWS and TWA. You could display GWS and GWA ( G - ground) and maybe your instruments could display those.

You can get sensors that do not use a paddle ( ultra sonic - see Airmar as an example)  but you still have to make a hole in the boat. Others allow you to pull out the sensor  clean it an put it back in without taking the boat out of the water.

Some instruments receive SOG and just change the sentence and send it out as STW and some have an option to accept SOG if STW is not there.

I am lucky in that I calculate my own TWS/TWA so I use SOG and just remember that tide will play a part not shown in the answer.  For me the impact is not great enough to warrant spend upwards £500 on a non-paddle sensor.

VMG is effected by tidal stream flows as well as wind speed. So you would get a different target boat speed for different tidal streams/wind speed. I have asked ( sometime ago) if there is a polar chart for the B26 so I could see the target boat speed for each wind strength/wind angle however Swallow Yachts have not responded and not having STW means I can't create one.  There is an interesting article on VMG - https://sailzing.com/velocity-made-good-vmg/ which is worth a read as a reminder....

I have not calculated VMG on my instruments but may do for close hauled sailing just to try it out when I know tidal stream is low. It is a simple trig formula - VMG=STW∗ cos(TWA) - android calculator would be able to let you do it in the cockpit.

This topic seems to have created many discussions in the sailing community. When I was researching for building my instruments I was amazed at the different thoughts and approaches. It seems to me that whether you use STW or SOG depends on the tidal streams in your area. If they are small then SOG is OK. If they are great then STW is better. I think that is why a lot of instruments give you that choice.

Lastly I have not seem many tablet/phone apps calculate VMG/TWS/TWA. They expect these to come through the network as sentences and be calculated elsewhere. OpenCPN (KIP is particularly good) and ORCA seem notable exceptions. There are specific apps that calculate these and then display.

I am currently teaching myself Andriod development of apps to try and get this information on a smart watch though I sometimes wonder why as it is clearly visible in the cockpit 

[Sea Simon]

As Andy says (I think), and imho, STW is perhaps best viewed as more performance-orientated data?
Integrated via a system like tacktick, or BnGs performance sailing instruments (Nemesis, Triton etc) and with accurate boat polars, and with experienced crew/tacticians STW is very useful indeed. Another of those racing "arms race" type "must haves" if your class allows it.

On a more practical,  and in my case more realistic note (I flirted with BnG Triton, but realised that regardless of cost, I dont have the crew to use it effectively! Of course, I have no polars either)....the Raymarine/Airmar transponder can do 3 main functions, speed TW (paddle wheel), depth, and fishfinder type sonar...and temperature  too! Downside is that sensor alone is £250 ish. Wiring easy enough, but fitting a new unit thru hull much harder. Luckily Airmar sensors seem to be widely available, and not as subject to  "planned obsolescence " as the head units?
It gets clogged with SW beasties and crud after about 10 days afloat, and therefore needs cleaning weekly. The optional blanking plug, to fit in hull while cleaning the sensor was £40! I'm too wary to rely on the very flimsy integrated "flap valve" to keep me afloat!

I abandoned my integrated plotter/sonar/depth/wind instrumentation when it needed a new mast head wind sender £500, and while considering this the main processor/display unit (non serviceable item sir) at £1500 also failed. Within months, the tridata S/T/D/sonar sensor also failed....luckily...I had not decided to replace/repair.
Hence no more integration, but i still have STW and SOG fwiw....

My main use of STW is now limited to assessing sail trim changes, it seems faster than GPS and perhaps more accurate over a short time frame?
I don't think I'd bother to cut a hole thru the hull just for this....

[Philip L]

Matt - I hope you read this!
Having not long completed my Yachtmaster theory when commissioning our BC23, I insisted that Matt fit a triducer with paddle wheel.  I looked at the ultrasonic triducers but they were more expensive again.  Matt pointed out the weaknesses of the wheel being vulnerable to damage when winching out onto the trailer and how often they clog up. He didn’t recommend them and he was correct.
Although I have become very efficient at preventing a lot of water entering the bilge when I fit the blank, it adds another level of faff when hauling out/launching. Also as Graham says, the level of GPS accuracy now gives a reliable SOG and even on Ullswater the STW seems to disagree wildly with SOG.  The discrepancy improved with calibration according to heel but still doesn’t seem consistent enough to inspire confidence if I needed to know how much I was being effected by tide or to give a reliable log reading for dead reckoning.
I have multiple back ups on board in the space of tablets, iPhone and an old walking gps and if electronics on board fail then I lose the STW anyhow. 
Lastly, the week before I hauled out in October, the STW stopped working. When I swapped out the transducer, I found it had become home to a population of Ullswater freshwater crustaceans. If that can happen in freshwater then I expect it would be a pretty constant issue when coastal sailing.  At least United Utilities mustn’t have destroyed all aquatic life in the Lake District!
So my advice to anyone specifying a boat is to listen to Matt’s advice!

[AndyB]

Hi Simon,

For me STW is not really worth the pain of the maintenance and problems, especially for trailer sailors,  it seems to bring so you read my thoughts correctly. I guess Philip unfortunately you are evidence to this although it is not surprising to get growth in fresh water that is quite normal. Where I am moored we have a river joining the sea and we get very little growth.

VMG is very useful for planning  when going up wind to see how many tacks you need ( if you don't use a down wind sail then of course it is harder to know what angle when going down wind). I am still building up how close to wind can you go before it is best not to. EG only  50deg @3 knots VMG is 1.93; 60 deg @4 knots VMG 2 would say 60 deg is the closest but it does depend on wind speed.

You don't really need STW for this comparison but the idealists would say that you do.

On the  other hand if you have purchased very expensive chart plotters with all those extra features then maybe you do need STW.

[Graham W]

I have read some apoplectic reviews about non-paddlewheel speed sensors, along the lines of “we paid all that money for this useless piece of junk?”

My multiplexer can be told to substitute SOG for the missing STW, which will then force my instruments to show a synthetic version of VMG.  Also “true” wind speed and angle.  Not one for the purists but it will keep me happy, regardless of what the tides are doing.  I really can’t be bothered to spend money on a vulnerable and potentially unreliable and inaccurate paddlewheel.  You can be pretty certain that measurements of SOG are accurate, especially when averaged out.  But STW?  Not so much.