Speed Through GPS Versus Old Fashioned Paddle Log

[OceanSeaSpray]

Originally Posted by El Pinguino

Thank you for the advice ( doffs cap ).

However I was actually seeking enlightenment regarding Dockhead's 'Shining Path' but I guess I will have to ride the Humbolt up to Peru to gain enlightenment in that regard.

The Humbolt isn't really an issue on that passage ... what you do have to watch out for is the South East Pacific High which can glass you out well south of 40*S and a long way offshore as you approach Chile.
The rhumb line is a definite non starter unless you want calms and easterlies and a 60 day passage.
The GC isn't real flash either ... best is a composite GC and a crossing in about 45*/48*S.

So... once again Dockhead's Shining Path has been found impractical in real life. There is a lot more to a successful passage than the shortest distance through the water...... consider Valpo to Auckland if you will.

I am afraid that the only thing impractical here for a while has been overcoming the pride of being ignorant and staying so - for a very select few - and the inability to admit that his initial statement that "STW is not useful" was plain wrong for one other.
I am amazed at the patience exhibited by Dockhead and a couple of others in repeating the same over and over again and this thread could probably be closed by now, as everything and much more has been said and explained...

[Moonos]

Originally Posted by Robin3

Also how about where speed limits are involved, they surely are for boatspeed not groundspeed so in a strong current you might think you are breaking the postedspeed limit and slow right down but in fact are way under it or you might be well over the set limit because you are battling a foul current...

I always thought that speed limits were based on SOG not STW. Am I wrong? Seems silly to use STW.

[El Pinguino]

Originally Posted by Moonos

I always thought that speed limits were based on SOG not STW. Am I wrong? Seems silly to use STW.

I think that was adressed back at about #15 ( the OP got his answers pre #30 ) long before the thread wandered off course - which is surprising as it was on a constant heading.

Most speed limits on the water are to do with 'wash' - or 'wake' if you prefer - issues ....'wash' is directly related to speed through the water not over the ground.

Not sure if this is addressed anywhere in port regulations , however I suspect that you would have a good case if someone ashore measured your speed by doppler and didn't have a satisfactory means of correcting your speed for the rate of the tidal stream ie didn't know what the rate was at the moment you were 'booked' in the bit of water you were sailing in..unless of course the limit was 5 knots and you were doing 25.

[Robin3]

Originally Posted by El Pinguino

I think that was adressed back at about #15 ( the OP got his answers pre #30 ) long before the thread wandered off course - which is surprising as it was on a constant heading.

Most speed limits on the water are to do with 'wash' - or 'wake' if you prefer - issues ....'wash' is directly related to speed through the water not over the ground.

Not sure if this is addressed anywhere in port regulations , however I suspect that you would have a good case if someone ashore measured your speed by doppler and didn't have a satisfactory means of correcting your speed for the rate of the tidal stream ie didn't know what the rate was at the moment you were 'booked' in the bit of water you were sailing in..unless of course the limit was 5 knots and you were doing 25.

That ( in bold) is the norm I believe, plus the ability to avoid physical collision perhaps with fixed objects, like maybe bridge supports, other vessels and maybe swimmers or protected marine life like manatees.

[valhalla360]

Originally Posted by Moonos

I always thought that speed limits were based on SOG not STW. Am I wrong? Seems silly to use STW.

While I believe the intent is STW to account for wakes, I believe you will find most don't actually clarify that. The problem being how do you measure the STW...on a boat at a distance?

That's why most signs (at least in the USA) speak of no wake zones in marinas and other sensitive areas.

[valhalla360]

Originally Posted by OceanSeaSpray

I am afraid that the only thing impractical here for a while has been overcoming the pride of being ignorant and staying so - for a very select few - and the inability to admit that his initial statement that "STW is not useful" was plain wrong for one other.
...

I believe that is a gross misrepresentation of the discussion.

Repeatedly, it has been stated for racers and other performance oriented folks, it is useful. If you have one hand on the tiller and another on the sheet, constantly trimming the boat second by second to get that extra 0.1kt, it is definitely useful.

For the typical cruiser, who sets the autopilot and sits back and keeps an eye on things, it is of minimal use. The we wandered off into the world
of tidal currents and the like. All of which can be handled without a separate instrument quite satisfactorily for cruising.

[El Pinguino]

Originally Posted by valhalla360

While I believe the intent is STW to account for wakes, I believe you will find most don't actually clarify that. The problem being how do you measure the STW...on a boat at a distance?

That's why most signs (at least in the USA) speak of no wake zones in marinas and other sensitive areas.

I have found no clarification , probably down the fact that when most rules were written speed through water was all they had to go by... and then only just.
Where I used to sail in Oz you would find both speed and 'no wake' areas..... Water Polis would tie up to channel markers and use radar guns... that is when they were not out there revenue raising by checking boat driver's licences...

[Dockhead]

Originally Posted by El Pinguino

.. .

Most speed limits on the water are to do with 'wash' - or 'wake' if you prefer - issues ....'wash' is directly related to speed through the water not over the ground.

Not sure if this is addressed anywhere in port regulations , however I suspect that you would have a good case if someone ashore measured your speed by doppler and didn't have a satisfactory means of correcting your speed for the rate of the tidal stream ie didn't know what the rate was at the moment you were 'booked' in the bit of water you were sailing in..unless of course the limit was 5 knots and you were doing 25.

I don't know about other places, but in the Hamble, it's STW and for exactly these reasons you point out.

Some power boaters here don't have STW, and try to use SOG for this, which causes problems when they are running downstream with the tide, which can run 4 knots. The speed limit is 6, so occasionally you see them slewing downstream with no steerage at 2 knots STW, trying to stay under 6 knots SOG, a menace to navigation.


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[valhalla360]

Originally Posted by El Pinguino

I have found no clarification , probably down the fact that when most rules were written speed through water was all they had to go by... and then only just.
Where I used to sail in Oz you would find both speed and 'no wake' areas..... Water Polis would tie up to channel markers and use radar guns... that is when they were not out there revenue raising by checking boat driver's licences...

How exactly do you measure STW from a remote location in a harbor where currents are variable and swirling. I think that's why no wake is more common as that's the real issue.

Of course, no wake has it's issues. I used to have a tunnel hull inflatable that left about a 3" wake at 60mph.

[Ericson38]

Originally Posted by Dockhead

Well, I think that the distinction between Apparent, True, and Ground is very useful:

Attachment 99360

But your point about measurement inaccuracies is well taken. In many cases, STW measurement is too far off to give a reliable True Wind calculation. You're right about that.

But still -- you do sail in True Wind -- that's the sheer between air and water which is our source of power. Apparent Wind is only what you sail in on this point of sail -- because your speed vector is added. You need True Wind to know how you can tack, what DDW is, etc., etc, etc. -- and only True Wind. Ground Wind is completely irrelevant to actual sailing on this tide, and you will get in trouble if you use Ground Wind to decide what course you can make a safe number of degrees from DDW, for example -- the answer may be very different than if you correctly use water-referenced True Wind. Ground Wind is useful to compare to forecasts and to know what will happen when the tide turns, but not for figuring out changes to points of sail.

The difference between Ground Wind and True Wind is relevant mostly to people with strong currents. As I said, if like a lot of sailors you never experience currents of more than a knot -- then you won't care about the distinction. Especially since the sloppiness of True Wind calculations based on speed readings from dirty paddlewheel logs may make them completely incoherent.

As to the terminology: You're right, of course -- there was no such thing as water-referenced True Wind until we had electronics capable of calculating it. It is a term invented by the instrument makers. But I think they have done us a big service -- it's much more precise, and the precision is highly meaningful, if we are dealing with at least moderately strong currents.

"True", as someone above said, just means corrected from some kind of deviation or error -- it's a fairly generic term, and it does not cause me any problem that people on land use the term "true wind" differently from what our instruments say. For someone on land, naturally true wind is ground-referenced -- that's the only reference they have. For us, I submit, it's much more natural to reference "true wind" to the water, since that is what we sail in. And therefore, I personally am really glad that the terminology is now like this and that our instruments work that way. I think it's valuable progress that we now have True, Apparent, and Ground Wind -- which as you said our ancestors did not. Progress which was made possible only with modern electronics.

Different people will have different taste about this, but I think you're swimming against the tide (so to speak! pace Shakespeare) if you try to stick to the old ground-referenced True Wind, at sea. You won't even be able to get your instruments to display it (unless you have Furuno, which does have that option, unique among the instrument makers AFAIK).

Dockhead-

What is illustrative in that picture is-

(1) with STW, boat heading (true-use calibrated fluxgate), apparent wind speed and angle at the mast head, a true wind speed and angle can be determined, as an academic exercise, or with electronics.

Then (2) with GPS SOG, COG, STW, and boat heading (neglecting leeway), the water current vector (direction and magnitude) can be determined, electronically.

With (1) and (2), the ground wind vector (speed and direction) can be determined, again electronically.

I wouldn't want to compute this information on the fly manually or repeatedly with spreadsheet vector calculations, but since we are all running around with chartplotters and sailing instruments anyway, why not have it available as real-time information ?

Not perfect information in an absolute sense, but it is real-time, tailored to your immediate location, and with decent apriori calibrations performed on wind and water sensors, it will in a relative sense, change appropriately as conditions change around you, which will allow you to make the best tactical sailing decisions.

If at some point in the scene above, cruising along some coast for the first time, heading to a planned cove to anchor, if I lost power and were out of penlight batteries for handhelds, I would revert to the paddlewheel powered knotmeter, wristwatch, and mag compass and just dead recon from there.

In this discussion on the utility of the ubiquitous STW sensor, it is a rather hard position to take (I think) to say, STW...not having or needing one of those, thank you.

[Dockhead]

Originally Posted by Ericson38

Dockhead-

What is illustrative in that picture is-

(1) with STW, boat heading (true-use calibrated fluxgate), apparent wind speed and angle at the mast head, a true wind speed and angle can be determined, as an academic exercise, or with electronics.

Then (2) with GPS SOG, COG, STW, and boat heading (neglecting leeway), the water current vector (direction and magnitude) can be determined, electronically.

With (1) and (2), the ground wind vector (speed and direction) can be determined, again electronically.

I wouldn't want to compute this information on the fly manually or repeatedly with spreadsheet vector calculations, but since we are all running around with chartplotters and sailing instruments anyway, why not have it available as real-time information ?

Not perfect information in an absolute sense, but it is real-time, tailored to your immediate location, and with decent apriori calibrations performed on wind and water sensors, it will in a relative sense, change appropriately as conditions change around you, which will allow you to make the best tactical sailing decisions.

If at some point in the scene above, cruising along some coast for the first time, heading to a planned cove to anchor, if I lost power and were out of penlight batteries for handhelds, I would revert to the paddlewheel powered knotmeter, wristwatch, and mag compass and just dead recon from there.

In this discussion on the utility of the ubiquitous STW sensor, it is a rather hard position to take (I think) to say, STW...not having or needing one of those, thank you.

Very well reasoned and useful way to look at it

In other words, without STW, you are missing one key datum without which you cannot calculate a number of things which you want to know.

The only question I have is about paddlewheel logs -- do you really have one which works without power? I haven't seen one like that in decades. Mine, unfortunately, does not.

[OceanSeaSpray]

Originally Posted by valhalla360

I believe that is a gross misrepresentation of the discussion.

Repeatedly, it has been stated for racers and other performance oriented folks, it is useful. If you have one hand on the tiller and another on the sheet, constantly trimming the boat second by second to get that extra 0.1kt, it is definitely useful.

For the typical cruiser, who sets the autopilot and sits back and keeps an eye on things, it is of minimal use. The we wandered off into the world
of tidal currents and the like. All of which can be handled without a separate instrument quite satisfactorily for cruising.

I think you precisely and entirely missed the point and the core of the discussion went exactly against what you just stated.
I am not going to once again repeat it in the thread, because you can just go back several pages and read why it is plainly wrong at long lengths.

[El Pinguino]

Originally Posted by Dockhead

........

The only question I have is about paddlewheel logs -- do you really have one which works without power? I haven't seen one like that in decades. Mine, unfortunately, does not.

Chernikeefs ( sp) are long gone but I used to have a VDO analogue which ...I think.... was self powered. Lovely bit of kit, pity they don't still make them.... analogue display of speed or wind is always superior to digital in my opinion.

If you know your boat you should have a good idea what speed she is doing even if your log isn't functioning.

I'm surprised that many big stinkboats on the Hamble (and elsewhere) don't have pitots.

[El Pinguino]

Originally Posted by OceanSeaSpray

I am afraid that the only thing impractical here for a while has been overcoming the pride of being ignorant and staying so - for a very select few - and the inability to admit that his initial statement that "STW is not useful" was plain wrong for one other.

Any chance of getting that re-written in plain understandable english?

[Dockhead]

Originally Posted by El Pinguino

Chernikeefs ( sp) are long gone but I used to have a VDO analogue which ...I think.... was self powered. Lovely bit of kit, pity they don't still make them.... analogue display of speed or wind is always superior to digital in my opinion.

If you know your boat you should have a good idea what speed she is doing even if your log isn't functioning.

I'm surprised that many big stinkboats on the Hamble (and elsewhere) don't have pitots.

Pitots . . . LOL . . . you crack me up

Actually, there are very few power boats on the Hamble -- the ratio is about 20:1. About 4000 sailboats. And the powerboaters are mostly extremely courteous and skillful. The whole problem here is blithely assuming that COG will do just fine if you don't feel like keeping up the paddlewheel -- good enough for government work, as they say . . . why do we need STW anyway? But in some cases, it is glaringly not.