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Old 14-09-2010, 19:42   #31
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Originally Posted by jkleins View Post
i have heard many mention how it helps with sail trim and such. If you are watching SOG on the GPS instead of the output from the paddlewheel wouldn't you get the same exact change since the current would be constant. A trim change that increased boat speed .5 knot would still be good wouldn't it?
Sort of ....but not really.

Clearly if you tweak the sails and get an extra .5kts, then you've made an improvement - but is it enough? is it the optimum speed for the conditions? Well the answer is you don't know unless you have some targets.

You have a target boat speed for a given wind angle and wind speed and if you're racing seriously, for each sail configuration. These are your polars. The polars tell you what speed you should be doing in any given set of circumstances. If you're meeting your polars, you're doing OK, if not you need to do something.

You can't do that with SOG because you would need a set of polars for each and every possible current speed and direction.
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Old 14-09-2010, 19:45   #32
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Originally Posted by Ex-Calif View Post

Adding speed to the boat was very useful for sail trim as noted. Also makes you crazy wondering why you can get 5.6 knots on Starboard tack and only 5.3 on port - grrr...

I thought the difference was that the current created a
little more lift on one tack...creating a little more speed
or was it that I only got around to cleaning one side of
the boat!
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Old 14-09-2010, 20:55   #33
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Am I the only one who carefully puts a thin coat of anti-fouling on my paddle wheel? Takes a Q-tip, a few drops of leftover paint and a couple of minutes, lasts for a few months, and markedly reduces fouling.
My log has become the Holiday Inn for some sort of crab species. I don't get a "lot" of fouling but always get one or two little bugs that need to be chased out.

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Originally Posted by maxingout View Post
I look at the GPS for speed, but since I am not racing, I don't get too excited about the speed. Instead, I see how my boat is behaving in the seas, and I adjust my sail trim and course for the best ride rather than maximum speed.
This is not a racing thread but most classes don't actually allow gps navigation because of the obvious advantages of having VMG.

I am opposite. I don't have a gps on board most of the time.

So far we have explored eeking out the extra .5 knot at the top end.

I find the speed most useful when winds are low and I am fighting a 1.5kt current. This is when 2 vs. 2.5 knots boat speed makes a difference. Especially when racing. 2X the SOG compared to a competitor is a big deal...

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Originally Posted by hugosalt View Post
I thought the difference was that the current created a
little more lift on one tack...creating a little more speed
or was it that I only got around to cleaning one side of
the boat!
I also am suspicious that my assymetric mainsail might be cut wrong. Time to call the sailmaker and file a warranty claim.
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Old 15-09-2010, 00:57   #34
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Originally Posted by Unicorn Dreams View Post
I would rather know VMG which is what the boat is actually doing
VMG - velocity made good, or the speed at which your are closing on your destination waypoint is only "what the boat is actually doing" when you are heading direct to your destination. i.e VMG=SOG

Tacking into wind to reach your destination, your VMG will be considerably less. If the wind is say 50į off your destination you may get a VMG of almost 0 on one tack & nearly equal to SOG on the other.
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Old 15-09-2010, 04:10   #35
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True wind can be derived from GPS. Some instruments do refuse to calculate this, but the better instruments will do it.
A picture is worth a thousand words. My boat has no log.
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Old 15-09-2010, 19:15   #36
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OK, I'll bite - how do you get boat speed with no log?
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Old 15-09-2010, 20:00   #37
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Sort of ....but not really.

Clearly if you tweak the sails and get an extra .5kts, then you've made an improvement - but is it enough? is it the optimum speed for the conditions? Well the answer is you don't know unless you have some targets.

You have a target boat speed for a given wind angle and wind speed and if you're racing seriously, for each sail configuration. These are your polars. The polars tell you what speed you should be doing in any given set of circumstances. If you're meeting your polars, you're doing OK, if not you need to do something.

You can't do that with SOG because you would need a set of polars for each and every possible current speed and direction.
I wasn't addressing that. If you know about, and or care about, polars then you need the knot meter. I was only saying it was possible to trim your sails and see the results using either. The only thing I really want to know about "polars" is how to keep those things off my boat if I go to Alaska. So I am thinking I am probably a good candidate for not having one if my current one ever gives up the ghost.

Jim
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Old 15-09-2010, 21:01   #38
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OK, I'll bite - how do you get boat speed with no log?
Heh. Impossible. Think of it as 'water speed'. Can't know it without sensing the water herself.

GPS is lousy at boat speed and SOG. The computed position moves at .1 to .6 knots all the time, in random directions. Just now I just hit a burst of 0.5 knots tied to the dock. That small speed error matters if you are sailing slow.

You may not notice it if you have a great amount of position damping configured, but it's still there. And with excessive damping you are not looking at the real speed anyway.
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Old 15-09-2010, 21:14   #39
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Heh. Impossible. Think of it as 'water speed'. Can't know it without sensing the water herself.

GPS is lousy at boat speed and SOG. The computed position moves at .1 to .6 knots all the time, in random directions. Just now I just hit a burst of 0.5 knots tied to the dock. That small speed error matters if you are sailing slow.

You may not notice it if you have a great amount of position damping configured, but it's still there. And with excessive damping you are not looking at the real speed anyway.
Newbie Crew: "Hey Dan. Your boat speed is broken. It's reading 1 1/2 knots."
Me: "Why do you say it's broken?"
Newbie Crew: "We are still tied to the mooring ball."
Me: "Uh, yeah. I'll dive on the paddle wheel as soon as the tide stops rising. I don't want to get swept away by the 1 1/2 knot current."
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Old 15-09-2010, 21:32   #40
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. . .
GPS is lousy at boat speed and SOG. The computed position moves at .1 to .6 knots all the time, in random directions. Just now I just hit a burst of 0.5 knots tied to the dock. That small speed error matters if you are sailing slow.
. . .
You might need to upgrade to a DGPS or WAAS unit. They do not wander all off the place and are quite good at only indicating what you really are doing.
- - All the units get "spikes" now and then but they are very transient and don't last long enough to have much effect on SOG or COG. You can usually set the averaging function to dampen the wandering of the displayed SOG or COG. Just like your autopilot which can be set to respond instantly to an induced heading change or to average out for "sea state" and reduce excessive turn commands.
- - However, the SOG and COG are "after the fact" values as the unit needs to know where you were and then where you are to calculate those values.
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Old 16-09-2010, 04:00   #41
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GPS can be quite sensitive and not lagging necessarily, but it's not as precise as a calibrated speed log. And it can also produce errors and go down and years back they used to reduce the precision as well. And GPS is getting to be rather inexpensive so you can have several of them to provide data.

But the fact remains if you are trying to do serious sail trim you need to see instantaneous subtle changes in the boat speed through the water. I find that with the B&G precision to hundredths - .01 is much more precise than observing tell tales. For example, of you tension the leech or the luff you CAN find the optimum tension at these levels of precision which neither tell tales or GPS will reveal. Granted you need steady conditions to do the trim, but when you have them you can really optimize your performance.

Does picking up .05 knots matter? You decide. Does picking up .1 knots matter? Then you have the rounding function which can hide changes in speed and trim.

Getting all the lifting surface correct in a sailboat so that it moves at maximum speed through the water is one of the "pleasures" of sailing and to accomplish this we need relatively precise feedback and a calibrated speed log provides that in real time. Of course all this applies in relatively light air when trim is so critical and if you are prone to motor sail, all this matters not.
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Old 16-09-2010, 04:11   #42
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You might need to upgrade to a DGPS or WAAS unit.
Got it, but...

WAAS is a waste of transistors for a cruiser because it covers few interesting places.

DGPS coverage is a little better. Hey! There's one in Kuching! Drat, too far away because DGPS is only reliable out to 200nm. if that.
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Old 16-09-2010, 06:23   #43
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Actually WAAS is the USA name for this particular version of Differential GPS or DGPS. In Europe the service is named EGNOS and INWAAS for India and SBAS elsewhere in the world. But the various GPS manufacturers generally only mention the term "WAAS enabled" in their descriptive literature. Normally your GPS will display a message whether it is receiving the necessary signals to provide the more accurate level of service.

- - More importantly, GPS and Knotmeters/logs are two entirely different animals that provide different mutually exclusive information.
- - Knotmeters/logs only know what your boat is doing in respect to its movement through the water. This is very important information for "sailors" - those folks who want to perfect their sailboat's performance.
- - However, cruisers will find GPS and its Speed Over the Ground and VMG is more important because we want to know when we will get to our next idyllic destination. GPS has no idea what water is and could care less. It only knows how you are moving across its theoretical surface of the earth.
- - So whether you want to invest the hundreds of dollars for a new or replacement Knotmeter/log is strictly a matter of your personal preference as to knowing your boat's performance through the water. The rest of us are lazy, laid back and our favorite phrase is "wake me when we get there."
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Old 16-09-2010, 07:51   #44
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Osiris,

Good points but the two are not mutually exclusive and in fact a knot meter used in service of sail trim will enable you to get there faster.

In the extreme example, you lazy sailor roll out the genny and are moving at 4 knots over the ground or through the water. Your GPS will tell you when you will arrive at the waypoint and whether you need to fire up the engine to make it before the sun sets, for example.

Or you might want to hoist the main and even trim the sails to get the max VMG and though the GPS will show this, it is too slow and imprecise to help you with sail trim. But then again you can simply turn on the engine and run at the speed so that you arrive precisely at the time of your choosing.
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Old 16-09-2010, 08:19   #45
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Got it, but...

WAAS is a waste of transistors for a cruiser because it covers few interesting places.

DGPS coverage is a little better. Hey! There's one in Kuching! Drat, too far away because DGPS is only reliable out to 200nm. if that.
Now EGNOS is working there are lots of uninteresting places to visit.
Please stay away from this one its very boring.
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