Log Speed vs GPS [SOG] Speed?

[noelex 77]

As I understand it true wind derived from the GPS and SOG is going to more accurate answer questions like:

What is the wind strength and direction going to be when I tie up to the fuel dock, pier or marina birth?
Has the wind increased strength and direction changed since this morning?
Does the wind strength and direction match the forecast?
What is the likely wind strenth and direction at my chosen anchorage?

If I want to know the direction and strength of wind that is influenced by current, boat speed I have apparent wind direction and speed display.
As a crusing sailor this is type of information I want from my wind instuments.
As this can be accieved with an instrument that does not require any calibration and cannot be affected by weed and growth ,therefore will not give inacurate readings is a major plus.
Even better I dont have drill a hole in boat or purchase another instrument.

[theonecalledtom]

Originally Posted by Sandyh

However, a caveat ! On my setup, there is a fore & aft alignment notch in the hull fitting which must be correct or the log/plug won't seal. You need to know if this is the case before you start. Of course, the compartment isn't lit and there's no third hand for the torch to see the little black-on-black arrow on both plug & log, so I've marked the arrow in white and put a marking pen arrow on the hull to show the alignment. This makes the switch easy with minimal water ingress.

In part, it's this transfer process which started the debate and prompted the original question.

Cheers

Thanks for the feedback, I'll pluck up the courage one of these days and do it, with a cold beer standing by as reward.

Luckily mine is in a well lit and accessible area (I got lucky this time) so I should be able to see the notch.

-Tom

[s/v Jedi]

Originally Posted by noelex 77

What is the wind strength and direction going to be when I tie up to the fuel dock, pier or marina birth?

Indeed, you need wind over ground for that, but it's not something the boat cares about when under sail, and the instruments are sailing-instruments... that is their primary function.

Has the wind increased strength and direction changed since this morning?

While under sail you need this info relative to the water, not the ground.

Does the wind strength and direction match the forecast?

That sounds like a good one. However, I never saw a forecast that accurately matched the real conditions during the forecast period. Also, sail trim doesn't care about the forecast as it only deals with the real conditions. You are already underway so the decision to leave was already made.

What is the likely wind strenth and direction at my chosen anchorage?

For a "likely" indication the normal instruments give enough information. Much of the real conditions will be influenced by shore features which are not accounted for in either method of calculating wind speed and direction.

If I want to know the direction and strength of wind that is influenced by current, boat speed I have apparent wind direction and speed display.

For upwind I agree. But for light downwind conditions apparent wind direction won't do. Also, many want to know much more for which wind over water is needed, like course on opposite tack, time to reach layline/tack, VMG etc.

As a crusing sailor this is type of information I want from my wind instuments.

I think we have different ideas about the definition of a cruising sailor. Also, remember that when tied to a dock or anchor, your apparant windinstruments are 100% accurate for wind over ground. We are talking about them while underway, not when static.

ciao!
Nick.

[noelex 77]

Nick, fistly I love the Avatar, anyone who can set and control that much sail has got my respect, but I will have to disagree with you about the information I want from true wind.
For example if conditions are OK I will sail tomorrow for a lovely little anchorage where the water is so clear you can see the individual grains of sand 12m down and the color of the water is an incredible Jade. (I would tell you where it was but everyone would want to go there) Sounds perfect and it is, but its not well protected and is only viable in a narrow range of wind directions and strength.
While sailing there I would like to look at my true wind display to check it still viable. If not I can divert, as early as possible, to a more protected anchorage.
You may not find this information useful and that's OK, but as I said it is the kind of information I want from true wind.

[s/v Jedi]

Noelex,

Avatar: that's not us, we can't afford hiring a chopper for pictures ;-) It's Steve Dashew on Beowulf. But we would look like that from above I hope...

Okay, lets take your example. Your TWD is based on STW so not relative to ground. Now, the difference between the two must be corrected for drift (as result of leeway and current). You are familiar in the area otherwise you wouldn't know which narrow range of wind direction allows you to anchor there. I am sure you can do without a wind-over-ground instrument.

Now us: we look out at the sea (not even an instrument) and think that anchoring in that spot is likely/possible so we sail there and anchor if we were right. Easy enough, right? ;-)

ciao!
Nick.

[goboatingnow]

I have no desire to engage in a back and forth banter

Lets look at the True Wind issue first

No, true wind speed is defined as wind over water.

Nope, nada, no. boats have computed this using STW becuase they lack SOG , true wind is the wind that is really there, with respect to ground, ie a stationary observer.

For external refernces have a look at Apparent wind - Wikipedia, the free encyclopedia or True and Apparent Wind Calculator

In both cases you will see that true wind is the wind relative to the ground, ie it the wind that the forecasters are trying to tell you is there. True Wind is NOT realtive to water, it is relative to the Ground. If I am in a sationary boat drifting in a 8 knot current, I am not experiencing true wind. Take for example. There is no wind.9 IE TRUE WIND =0) I am in a current , I experience a 8knot wind due to a 8 knot currrent, Thats is an apparant wind ( again see definitions) Whether or not I agree with the definition is irrelevant , its is the excepted definition and the calulations shown in the reference also agree, that SOG is used to compute true wind.

Again, true wind is of realtively little use to sailers, and its why instruments that were developed before widespread introduction of networked GPS used what was available , ie STW from a log paddle wheel. now that SOG is available on most boats, true wind should be properly calculated using that measure.

The sail boat doesn't care about it's SOG at all, it can't see the ground, doesn't feel it and the ground does not influence it in any way

I'm sorry thats a silly statement. SOG and its associated vector COG are actually what a sailboat feels in respect of apparent wind. A sailboat does effectivly feel the ground is this respect( ie apparant wind) as again its SOG ( and COG) that generate apparent wind and that is what a sailboat feels. A sailboats SOG and COG are effectively taking into accout set and drift

SOG is way more useful to know than STW. Firstly it allows the determine of EP's rather the DR's, its allows for determination of set and drift ( even by mere inspection of boat progress through the water and SOG. It and COG provide more accurate Navigation information.

STW can be usful in sailtrim, but as any seasoned sailor will tell you , you "feel" sailtrim more then what instruments tell you.

When you say that SOG as a basis is just as good as STW for sail trim, you know that this is a flawed statement; you mean that you don't care about the small difference.

...

However, we normally don't have that much current so you don't care.

Its not flawed, most of the time sailing SOG and STW are often very similar numbers, secondly in sailtrim you usually looking at deltas, rather then absolute, Triming and getting a SOG up by half a knot is as good as using STW. Most of the time sailing theres ( well what I do is bluewater deliveries)little current effects and even when theres is most tidal drifts a in the 0.5 to 1.5 knot range, so outside of significant river currents, drift has little effect on STW versus SOG. Also in sailtrim, one does not set ones sails by reference to true wind, once always uses apparent wind since thats whats you have to deal with. In practice one can trim expertly without either SOG or STW ( witness the many dinghy sailors in action).

I am trying to address the OP here, who said that SOG was possibly more useful to have then fitting a paddle wheel and getting STW.

Autopilots need STW of course.


Right lets get back to ARPA, ( I have worked on RADAR and I am a RADAR instructor).

firstly you are agreeing with me

it also has no clue at all about the targets sensor readings.

I said this isn as many words in my first post.

Remember all I was making a point about ARPA was that ground referenced ARPA should not be used in close quarter situations ( this has been the cause of several collisons and teh IMO have released bulletins about this).

Okay, compass course is needed for ARPA operation to correct for not steering a straight course all the time. The operator will see a relative vector, CPA and TCPA only in this setup.

Sure I agree, in fact ARPA doesnt need compass inputs either, remember your basic radar plotting. ( for CPA) However what I was referring to was not simple CPA and TCPA cals, I was referring to what ARPA shows and ARPA shows the targets vessels true vector ( ie as opposed to realtive speed, true = targets STW in this case)and on the bridge of ships and often more and more on small boats this is what people look at as it instinctly easy to understand, ie the target is doing 20 knotson a course of 220 dir. All I said was that you cannot compute this vector using SOG. Thats all I said, the rest is just english

But hey, I see the target's speed and course too!!! This is because you connect a GPS to the radar. The radar uses SOG and COG to calculate true speed, bearing and COG of the target. It uses the position info from the GPS to calculate the target position.

What ARPA shows you ( normaly) is the targets STW and CTW so that you can compare it to your STW and CTW. and that the vectors are from the same base.

[mudbug]

And just picture it . . . in the "old days" people simply peeked at the pennant atop the mast and licked their finger and held it up to the breeze.

Things sure have improved . . . I think . . .

Peace, love and transducers,

Buddy

[Hud3]

Heh! Buddy, you're right on the money!

My windspeed instrument failed on me, and I went a couple of years before getting it fixed. I put maybe 8,000 nm on the boat, both offshore and in the islands during that time. You really can tell a lot by just looking at the sails and the water, feeling how the boat responds, and listening to the sound of wind. Even at night.

p.s really enjoyed your Rio D pieces!

[Chief Engineer]

The pointy end goes first, wind is free, watch your telltales.....we'll get there
eventually.

[s/v Jedi]

goboatingnow:

Nope, nada, no. boats have computed this using STW becuase they lack SOG , true wind is the wind that is really there, with respect to ground, ie a stationary observer.

SOG has been available for a long time and instruments that can use it still default to STW for good reason imo. For a shore based instrument, true wind is related to ground and for boats we obviously disagree. I can find many web references that agree with me but you would argue that these are from the pre-GPS era and use flawed sensors because SOG wasn't available, so I won't even list them.

But you surely agree that at sea, there is no such thing as a stationary observer. Even if there was, what would you care about him?

You obviously take the word "true" in "true wind" in the strict sense of the word. I take "true wind" as the expression used by sailors for wind over water. I mean, you don't tie bed-linnen to your jib just because they are called sheets, right? Also, if you take the meaning of "true" strict, than you should take it into it's 3rd dimension and include vertical components (which are relevant for sailing).

I would not want to sail without SOG and COG information now that it is available. I agree these are important, but I never use them when I choose sail combinations or trim sails. I use them for navigation purposes.

I'll let the definition of true go as it doesn't really bother me that we have a different opinion on it's meaning. But what I can't let go is that you write that the boat experiences wind over ground and that this value matters. While underway, the boat is in the water, not attached to the ground and it needs no ground related wind information. You did not comment on my example of the downwind sail with favorable current. I invite you to do that and explain me how a wind over ground indication would work better or even as well as wind over water. I ask that because your reply has a lot of correct information that is, however, not relevant to sailing the boat.

I actually checked my radar guide: ARPA uses COG & SOG, not STW. So the info displayed for a target includes their COG and SOG and their compass bearing, which I can display in deg. True or Magnetic. Check your manual as well, I am sure you will find that your set uses ground-related information too as ARPA is standarized.

Ah, I also checked my B&G manual... it states that true wind is wind over water and specifically states that this is not wind over ground (sorry, no need to respond to this, I know what you would write ;-)

ciao!
Nick.

[goboatingnow]

I suppose its all to do with definitions and what you expect from the instrument.

true wind over the water, can be used in some sailtirm applications, but most people say with a Raymarine wind instrument, push teh "tru wind" button to get an idea of whats really out there , which is ground wind, ie they refer it to there ground wind frame of mind ( like the beaufort scale etc). In this case the instrument lies.

True wind over water removes the set and drift and gives you a symetrical "true" wind angle on each tack..sure and B&G gives you a few ideas on how to trim using it etc.

I still maintain that outside of sailing performance nuts, everyone assumes true wind as wind over the ground.

Lets leave it at that, at the very least the instrument should offer the choice.

ARPA and SOG, you are incorrect, MAPRA ie mini-ARPA is an abomination that has so many faults that I wont even start. ( your radar is MAPRA based) Most installations produce vectors that are competely wrong unstable or both. I have to turn the damm thing off when I use them

Furuno as befits a proper company does offer both , IMO mandates water reference, Raymarine which produces kids toys, uses SOG. ( go figure).

But what I can't let go is that you write that the boat experiences wind over ground and that this value matters. While underway, the boat is in the water, not attached to the ground and it needs no ground related wind information.

What a boat experiences is apparent wind nothing else, Apparant wind is a combination of boat speed and direction and the speed and direction of the wind. Since the wind is not generated by a big fan floating on the water, but by a medium not connected to the water, then the boat feels the true wind over the ground , and the boat speed over the ground. Your wind annemoter accounts for boat speed over the ground ( effectivly) as mine continues to spin , in no wind , but being pushed by a current.

I accept that we talk about a boat feeling the wind in the context of being an observer on the boat. But thats really a relative perspective

PS please stop quoting manufacturers manuals at me, these are businesses that produce teh stuff thats wrong in the first place..

[noelex 77]

Originally Posted by s/v Jedi

Noelex,


Okay, lets take your example. Your TWD is based on STW so not relative to ground. Now, the difference between the two must be corrected for drift (as result of leeway and current). You are familiar in the area otherwise you wouldn't know which narrow range of wind direction allows you to anchor there. I am sure you can do without a wind-over-ground instrument.
ciao!
Nick.

No my true wind data is calculated from SOG not STW. So it is relative to ground.
Yes I could sail without a wind over ground instrument. I could sail without radar. I could even sail without GPS as I did for several years, but these instruments add information which improves my safety reduces workload.
Without any instruments when out of visual range from land features "True wind speed and direction over ground" is one of the more difficult estimates to make as it requires some estimation of a multitude of factors such as STW, apparent wind speed and direction current strength and direction and compass heading.
My instruments can calculate these variables much better than I can . Not essential, but certainly nice to have.

[s/v Jedi]

goboatingnow: sorry but my radar is ARPA and Furuno, not Raymarine. And it gives COG and SOG of it's ARPA targets. I must admit I'm new to this unit, but I really can't find a setting to change this to water based vectors..

Oh I agree that many cruisers don't have a clue what their instruments try to tell them. When I talk about VMG I often get blank looks...

Noelex: Radar, GPS, that's all for navigation, not for sail trim. I'll keep at it: you don't need wind over ground for sail trim.

I keep hearing that apparent wind is enough but it isn't (not from noelex I think but in this thread). Consider down wind sailing, you need wind-over-water direction because apparent wind can show from the beam. When you don't have wind over water instruments you have to look at the water to see if you are close to gybing or not, which can be difficult.

Also, I think that posters are too quick to call something "racing" or "speed freaks" etc. Good seamanship on a sailboat means that you can trim your sails optimal. Only when you do that during a regatta, you racing and a speed freak. When you do that on a cruising passage, it's called good boat handling. Also, you don't have to set more sail for more speed to trim sails right. Optimal trim normally means more comfort, longer lasting sails & rigging, less exposure to weather etc. It's probably because I am Dutch but I hate it when I see a boat with luffing or even slamming sails or worse: motoring in good sailing conditions.

ciao!
Nick.