Panbo

Airmar CS4500 Part II (Reliable True Wind)

... written for Panbo by Dan Corcoran and posted on Oct 27, 2009
WindVectors2d5.jpgIn Part I of this series, Dan Corcoran, and many commenters, got into not just the value of accurate boat speed, but the true meaning of True Wind.  Click on on Dan's illustration above and check out Part II...

In Part I, I touched upon the no maintenance nature of the CS4500 vs. the venerable paddlewheel it replaces, and the reliable boat speed measurements that result.  For me, reliable means that every time I set sail, my boat speed indicator is always showing my boat speed through the water (not SOG), and the reading is consistent and not subject to how recently I performed maintenance on my speed sensor.  With the CS4500, I get one other measurement reliably, and that is the display of true wind angle (TWA) and true wind speed (TWS) on my wind display and chartplotter.

(If you're a power boat owner, please note that I am referring to True Wind Relative to Water, not True Wind Relative to Earth, which we generally call Ground Wind. In fact, you can just skip to the last paragraph to learn why this True Wind could matter to you, as most of what is written below is from the sailing point of view.)

True wind readings can be anywhere between fustrating and useless on a sailboat without BOTH reliable boat speed and reliable apparent wind measurements. If you do any sailing and don't appreciate true wind, it may simply be because the true wind angle displayed is near useless.

Your apparent wind angle measurement can have errors introduced by calibration inaccuracy and very large errors from excessive mast movement. Quoting 'bobc', an expert in the area of wind measurement from a May 5th 2009 Panbo comment, "Your wind vane (mechanical apparent wind sensor, vendor name excluded here) is sailing's equivalent of a random number generator and you could reasonably expect to beat it with a damp finger held up in the air and that alone would probably be more reliable over time."

On Breeze Pleeze, I have a really good apparent wind measurement (the Airmar ultrasonic PB200) to combine with the reliable boat speed measurement of the CS4500 to determine true wind angle and speed.  The reliable boat speed is really key.

* A boat speed of zero, such as from a stuck paddlewheel, prevents true wind measurement.

* A boat speed provided from your GPS (called speed over ground or SOG) will make your true wind value useless in the presence of even a � knot of current, and each time you tack your sailboat the inaccuracy will vary.

* A boat speed that varies by as little as � knot resulting from a somewhat dirty paddlewheel can introduce true wind angle errors of 10 degrees at the very low boat speeds a sailboat experiences.

Ten (10) degrees inaccuracy is a very big deal if I am trying to cross reference boat polars (documentation) that shows my fastest VMG (velocity made good) at a certain true wind speed is 45 degrees true wind. If due to inaccuracy I am steering either 35 or 55 degrees, my VMG will be cut by 50% which is saying that if my destination is upwind of my current location it will take me twice as much time to get there (that is if my crew doesn't first loose tolerance for sailing upwind and insist I start the engine).

Since it would be intolerable to take twice as long to a destination if the boat is steered 10 degrees off optimum, sailors have other methods to maximize their VMG without ever referencing true wind angle. For instance, we can look at that same documentation or just past experience, to understand what our target boat speed (speed through water) should be for a given wind speed, and know that if we are sailing at that target boat speed we are close to the desired true wind angle. But, you still need a reliable boat speed source for this method, and the method is far less precise than having the true wind angle actually displayed on your console to steer by. It's easy to believe, except for practiced racing sailors, that access to true wind angle can reduce the time to your upwind destination by 10% or more.

Stay tuned for Part III and I will close this series by discussing one method for bumping up boat speed precision just a bit more, and by extension, true wind angle precision, along with turning your CS4500 into a lean, mean, NMEA 2000 machine. There is also a Airmar H2183 solid state compass evaluation in the works that this series leads into.

I would just add, especially for the benefit of power boat owners who get very little value from true wind information, power boat and sailboat sailors can both benefit (if you have a good heading source) from obtaining an excellent ground wind angle and speed from your chartplotter, and if your chartplotter is capable, current set and drift. The ground wind angle and speed is a measurement of the wind blowing over a point on the earth as if you anchored your boat momentarily. This isn't so obvious when you're moving at 15 knots, but knowing this throughout the day and how it might have recently changed, can effect your estimates on when you might arrive at a destination and what the sea conditions will be when you do.

 

ws_with_fog_touchup (Medium).jpg

Comments

Lucid, Dan!

But is the set and drift arrow in figure three going the wrong way? And can you not take the heading from the PB200?

{Editor: drawing fixed on 11/3/2009}

Posted by: Sandy Daugherty at October 27, 2009 12:23 PM | Reply

Good call, Sandy, and Dan's humble editor missed it too! The set & drift arrow should be pointed the other way. Because COG & SOG are the vector sum of a boat's motion thru the water and the water's motion.

Posted by: Ben at October 27, 2009 12:42 PM | Reply

Does the sonic speedo get screwy data from fouled hulls which which might shed rubbish? That is what give them inaccurate readings if anything? Can the be calibrated?

What about the direction of the flow - must it be normal to the two ceramic sensors? What about the effect of a keel if you place it on the side and slightly aft of the leading edge?

What are the outputs of the Airmar units? Is it universal? Sorry to be so dumb here,

Posted by: SanderO at October 27, 2009 5:11 PM | Reply

Sandy : The arrow is way wrong. I was so proud of this diagram, I will have to fix this! Thursday when I get home for sure. PB200 heading and GPS COG/SOG suffers a bit on the mast, Airmar has a new version on the way with additional axis of correction. The motion corrected wind is still A+++ in my book. I have a water level H2183 instead which I have an evaluation in front of Ben for comment. He is a a tough editor and mentor (no ego snacks), so don't expect it soon.

Ben: Wow, you missed that. I am still a novice, got to keep a sharp eye on me !

SanderO: Within reason I have an answer. The CS4500 has worked well in the first year when the bottom wasn't cleaned during the season (and was on the 2nd year of a 2 yr bottom coat) it wasn't pretty at season end !, the 2nd year when it was cleaned every 8 weeks, and this last season when it was cleaned every 2 weeks and I could almost eat off it. No improvement over the three years, all seemed about equal in performance, although this last year I had the CS4500 under a microscope so to say as I was writing Part III of this series and testing with an ST70 speed pod and under said microscope noticed some jitter that some instruments dampen (like the ST60) and others don't (like the data window in the e-series). It's a small annoyance, but if you race boats you might want to hold out for the next version with the built in damping and more if your instruments don't do it now.

Posted by: Dan Corcoran (b393capt) at October 27, 2009 11:55 PM | Reply

On sailboats, adding the leeway vector completes the picture.

Great article!

Posted by: far_south at October 28, 2009 2:44 AM | Reply

I am currently trying to order one of these cs4500 from Gemeco and have been told they do not have any; they are going to try to build one for us. (I do not understad that half of the firms have nothing in stock and they expect boaters to purchase their equipment...execpt they do not have any ready to purchse)
We already have the PB200 ready to install once boat goes into yard. We are going to add these along with two P79s for depth to a total retro fit of 1996 40' Beneteau that was hit by lightning.
This boat will have the new Simrad NSE-8 (got to test it out at IBEX...Wow) networkd with all IS20 Instruments, AIS 300 (now much lower priced) from Simrad and of course an AP2802 set up for auto pilot with a AP24 second head at the nav station down below. Simrad HD2k Radar will round out the set along with their BSM-1 depth sounder/fish finder thrown in for good measure.

Will also have the black box from Mastry Engine to convert the 50hp Yanmar engine data to yet another LCD screen rather than the original Yanmar control panel which is dead. Data will show up on a Lowrance HDS-7 and also function as a total separate back up GPS unit.

I for one cannot wait to see real true wind on boat instruments; this will be a real plus as the original RayMarine instruments never worked right after one week of use.

Posted by: Jeffrey at October 28, 2009 7:20 AM | Reply

I am currently trying to order one of these cs4500 from Gemeco and have been told they do not have any; they are going to try to build one for us. (I do not understad that half of the firms have nothing in stock and they expect boaters to purchase their equipment...execpt they do not have any ready to purchse)
We already have the PB200 ready to install once boat goes into yard. We are going to add these along with two P79s for depth to a total retro fit of 1996 40' Beneteau that was hit by lightning.
This boat will have the new Simrad NSE-8 (got to test it out at IBEX...Wow) networkd with all IS20 Instruments, AIS 300 (now much lower priced) from Simrad and of course an AP2802 set up for auto pilot with a AP24 second head at the nav station down below. Simrad HD2k Radar will round out the set along with their BSM-1 depth sounder/fish finder thrown in for good measure.

Will also have the black box from Mastry Engine to convert the 50hp Yanmar engine data to yet another LCD screen rather than the original Yanmar control panel which is dead. Data will show up on a Lowrance HDS-7 and also function as a total separate back up GPS unit.

I for one cannot wait to see real true wind on boat instruments; this will be a real plus as the original RayMarine instruments never worked right after one week of use.

Posted by: Jeffrey at October 28, 2009 7:25 AM | Reply

In order to consider changing over my B&G speed paddle and air instruments I need to know if they are compatible with the earlier NEMA standard used by my older B&G computer/autopilot. How can I find this out?

Posted by: Len Bertaux at October 28, 2009 8:31 AM | Reply

Len,
You should be able to use the CS4500 and may be able to display wind information from the Airmar sensor but B&G will not use nmea wind in any calculations so it will be essentially useless for you.

They have a support number where someone could probably give you a better explanation than I.

try them @ 800 628 4487

Posted by: bobc at October 28, 2009 5:37 PM | Reply

Raymarine has started reselling the CS4500 in combination with their DSM product.

Posted by: Dan Corcoran (b393capt) at October 29, 2009 10:10 PM | Reply

I spoke to an Airmar Tech support person a few weeks ago who told me they have a new, Depth, Speed, Temp transducer with N2k output that will be introduced this coming spring. I believe he said it may be called a DST-900. This is exactly what I'm looking for since I am in the process of abandoning all NMEA 0183 devices, (with the exception of a feed to the autopilot).

Posted by: Richard C at November 4, 2009 9:55 AM | Reply

Updated Entry - The true wind diagram has been updated to correctly reflect the direction of set & drift in figures 2 & 3.

If you look back at Part I, there is also some new comments about how true wind is valuable to one sailboat owner while racing, to the extent he dosn't even configure apparent wind to display on his instruments.

Posted by: Dan Corcoran (b393capt) at November 4, 2009 3:34 PM | Reply

Dan,

Most serious racers will prioritize in this order:

TWD True Wind Direction is the most important number they have, period.

Boatspeed, and targets

Target Speed for upwind sailing

Target TWA for downwind sailing

TWS True Wind Speed for sail selection and to establish Targets

Heading as a gut check on TWD

Apparent Wind Angle, occasionally for steering but only upwind and also as an aid in determining wind shear

Apparent Wind Speed, almost never as it is irrelevant to any decision making

Posted by: bobc at November 6, 2009 10:04 AM | Reply

I am curious. Has this series of articles and/or the diagram above helped anyone? Has it encouraged anyone who day sails to see value in using true wind to judge the performance of their sailing and sail trim settings? Has anyone who sails become a convert from believing that GPS SOG is good enough, to now keeping your paddlewheel clean and using real boat speed ?

Posted by: Dan Corcoran (b393capt) at November 29, 2009 2:39 PM | Reply

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