Boat speed / speed through the water / sog

[matt1]

My B&G manual doesn’t specify what boatspeed is even though it’s a distinct data field I can select to display. I get what sped through the water is and speed over the ground (Obviously) but does anybody know how B&G define “boat speed”?

 

[RichardS]

Do you have a paddlewheel? if so, it's probably speed through the water or can be set to that. If you have no paddlewheel then it's reading GPS so it's definitely speed over the ground.

Richard

 

[Boathook]

My raymarine says SOG when it is using gps for speed.

 

[MapisM]

My raymarine says SOG when it is using gps for speed.

Does also your Raymarine equipment say SPEED when it refers to STW instead?
Mine does, which seems to me a rather weird wording.

 

[Boathook]

I assume STW means speed to way point. Haven't looked at the type of function that I'm aware of so can't comment. I do use way points but SOG is adequate for me.

 

[MapisM]

Nope, actually STW stands for speed through water, and it's the one measured by the paddlewheel, hence unaffected by current.
What is imho a weird choice is calling SOG ...ermm, the SOG, and calling SPEED the STW.

 

[Refueler]

If the log is using Doppler - then it can be SOG based on either the seabed ........ or in deeper water - the thermal layer that exists ..

Just throwing a curve ball in there ................... doubt if OP's is Doppler though ...

 

[RJJ]

I think it makes sense to call "speed" your STW. Perhaps just because it's what I grew up with and also as a trimmer/helmsman what you are glued to when racing.

Either way, it's not hard to verify if there's any tide around. Park the boat up against a transit or hold station under power near a buoy. Your "speed" will either be the speed of the tide, or zero. If the latter, it's SOG.

 

[Catalina36]

I assume STW means speed to way point. Haven't looked at the type of function that I'm aware of so can't comment. I do use way points but SOG is adequate for me.

Surely STW is speed through water? There are only two options, water speed (log) or ground speed (GPS).

 

[johnalison]

It was all much simpler in ye olden days, when you had to divide the distance travelled on the log by time, or was it the other way round? Most of us have come round to accepting:
1. boat speed from the paddle wheel, ie through the water, displayed on my Raymarine as 'SPEED'
2. speed over the ground, shown as ' SOG'
3. VMG to the wind or, aaaah
4. VMG to waypoint. Both the latter appear at different points in my system. You just have to know which is which.

 

[RichardS]

It was all much simpler in ye olden days, when you had to divide the distance travelled on the log by time, or was it the other way round? Most of us have come round to accepting:
1. boat speed from the paddle wheel, ie through the water, displayed on my Raymarine as 'SPEED'

Although I'm one of the least of us who does not have a paddlewheel but still manages to display Raymarine "SPEED" and have a log which displays distance travelled.

Richard

 

[capnsensible]

Although I'm one of the least of us who does not have a paddlewheel but still manages to display Raymarine "SPEED" and have a log which displays distance travelled.

Richard

Probably even fewer of us with a Walker Log....

 

[cpedw]

It's clear from my Raymarine system (dating from circa 2000) that it was developed using speed through the water because that was available. Then they discovered GPS and some things were adapted to use it so we now have the "True WInd" and the "Ground Wind". I can't remember which way round they are but one is based on COG and SOG, the other on Heading and Speed through the Water (I think).

Derek

 

[johnalison]

Although I'm one of the least of us

Richard

Your modesty does you credit.

 

[Refueler]

Probably even fewer of us with a Walker Log....

I sold mine on ... never used it.

Wish I still had it now - I'd frame it and put in the wall !!

 

[Refueler]

Surely STW is speed through water? There are only two options, water speed (log) or ground speed (GPS).

Three if you count Doppler ... SOG

 

[capnsensible]

I sold mine on ... never used it.

Wish I still had it now - I'd frame it and put in the wall !!

I like trailing it behind my yacht to measure speed and distance whilst hoping a large fish passing by isn't hungry. ?

 

[Boathook]

Nope, actually STW stands for speed through water, and it's the one measured by the paddlewheel, hence unaffected by current.
What is imho a weird choice is calling SOG ...ermm, the SOG, and calling SPEED the STW.

My for speed through gives just the sp

Does also your Raymarine equipment say SPEED when it refers to STW instead?
Mine does, which seems to me a rather weird wording.

Says Boat kts and SOG kts. Also has VMG kts along with max and average.

 

[matt1]

Thanks everyone. I think it must be speed to waypoint, on the basis that it was displaying 0.0 today (with no waypoint being navigated to) whilst “boat speed” was displaying x.xkn. So I think the problem was me assuming “STW” stood for speed through the the water.

Would have be helpful to have an appendix with all the abbreviations in the instruction manual

 

[vyv_cox]

Would have be helpful to have an appendix with all the abbreviations in the instruction manual

The instruction manual for my Zeus2 bears all the hallmarks of having been written by a computer geek who has never seen a boat. Full of the highly aggravating 'you can do this, that or the other' without saying how. The plotter is superb but there are still functions that I cannot use and things still happen that I cannot explain. The manual needs to be at least twice as thick as it is, with proper explanations. Not helped by the fact that I live on my boat for six months and then leave it for another six, by which time I have forgotten much of what I remembered last time. Thanks to Covid it will be 18 months this time.