The Wind Direction Saga
When I first saw Jengu it was November it was a foul day at Hamble Point and the wind was strong enough to be moving the boat around even though it was on dry land. I didn’t pay much attention to the fact that the instruments said it was blowing 100knots. It seemed plausible.
Fast forward to a slightly calmer day in January and its still saying that its blowing a gale and worse than that the wind is circling….
At that point I didn’t realise how long it would take to work out what the actual issue was.
As discussed in an earlier post Jengu has a Nexus system. The anemometer connects to the NX2 server with a very long cable from the top of the mast. The NX2 server then converts the data to NMEA 0183 for consumption by the other systems on the boat.
We did a visual inspection of the anemometer from the ground over the winter and it looked OK. You can’t go up the mast to check things when the boats in a cradle on dry land.
We checked the GPS and flux gate compass data. If the compass heading was rotating or the boat speed was off then that might explain the odd readings. They all checked out fine, displaying on the B&G Vulcan chart-plotter the expected values. That was a good sign for the rest of the electronics in that the GPS and Flux-gate were connected to the Nexus NX2 server; data was reaching the B&G Vulcan on the NMEA 2000 bus via the NMEA 0183 output port on the NX2 server and an Actisense NMEA 0183-2000 gateway.
Following a brief chat with the support team at Garmin we tried to check the calibration of the server and the anemometer.
This allows the correction of consistent over reading and the adjustment for an offset anemometer. For the Nexus based system and its Garmin branded decedents you can adjust the readings using the Nexus Race software. Sadly for us this didn’t resolve the issue. Unfortunately the NX2 server wouldn’t talk to the laptop…
At this point we were working against the clock to get Jengu ready for the JOG Nab Tower race at the beginning of April. We gave the support guys at Garmin another call. They agreed to check the NX2 Server box itself for a fault…
.. which confirmed the fault with the serial interface for the laptop and that the wind data interface seemed to be OK.
The boat went to the Nab Tower without any wind data… but there was no wind for most of the race so that didn’t matter – we are all dinghy sailors at heart and are used to more basic instrumentation.
With the NX2 Server back in the boat the next thing to check was the cabling. We did that as far as we could without actually pulling the cable out of the mast. We remade a couple of connections in the boat where the cables met at the base of the mast but still not joy.
Somewhat frustrated at this point we decided to bite the bullet and replace the whole wind data/anemometer assembly. Installing a new unit at the top of the mast and connecting it up to NMEA 2000 bus we fitted over the winter with new cabling. We ordered a new B&G Triton anemometer to match the other B&G equipment we had installed.
The engineers set out to replace the anemometer and complete the installation one morning in May; everyone works on their boats April and May on the South Coast so getting the parts and engineers onsite took a couple of weeks, especially as you need two engineers; one to winch the other up the mast.
The engineers first step was to remove the old, presumably compromised cable from the mast.
A bit of history : Jengu got a new mast in 2015 whilst with the previous owner. It would appear that the cables (VHF, Nav light, Anemometer) in the mast are taped together. It probably made sense when the mast went in, its clearly easier to pull three cables taped together up a 60 foot mast, but it does make life difficult if you want to replace one and leave the rest undisturbed.
…there was no way they were going to just get the Nexus cable out of the mast. People started muttering about taking the mast out.
So we went back to Barry and Alex from Garmin Support, they have been immensely helpful all the way through the previous episode with the NX2 server. Could we just replace the unit at the top of the mast and plumb it into the NMEA 2000 network somehow ? The answer was yes and they gave us a list of parts to order including a replacement mast head unit, Nexus/Garmin converter cable and and box of tricks to process the data.
It was a bit of a gamble we didn’t know whether the cable in the mast was any good but given the VHF and Nav lights worked fine it seemed sensible to assume it was OK.
So with a list of parts ordered from Hudson Marine we waited for the day when Andy from MEI at Port Solent (plus colleague to winch him up the mast) could go back up the mast to install the new bits.
To cut a long story short it all worked. Andy replaced the anemometer and discovered that at the point where the wand was clamped to the top of the mast the Nexus data cable had been pinched. The wand had basically severed the outer casing of the cable causing it to short out.
It was a bit of a surprise that the Nexus server was still interpreting the data coming down the mast as valid….only the human eye could tell it was nonsense. So now we have working wind data after a couple of months sailing without it and can proceed to getting the NMEA Data Collection software working on Jengu.
So what did we learn…
We now know where all the cables go in the boat and roughly what they do.
We’ve also traced and removed all the redundant cables that have been left in the boat over the years. I now have enough spare Nexus cable to rewire another boat… if nothing else its saved us some weight.
Go to the top of the mast first. OK so we couldn’t have done that in January but we could have done it in April. It would have told us whether the anemometer was broken and which might have saved a lot of other work.
Label your cabling. We spent a lot of time trying to figure out which cable was important.
If you are going to install a field fit Nexus cable connector you are going to need your glasses. Its got the smallest Allen key I’ve ever seen.
So now all we have to do is persuade the data from the transducers to display on the instruments. But that’s for another post.
Bill/May 2016
Originally published June 9 2016