Steel hull, magnetism and fun with my logging sensor

[Ulrich G. Kliegis]

Hi all,
I have a Navico (the old original) Corus nav electronics net on my 33
ft sailing vessel. The boat has a steel hull.

Already in the past season, the log sometimes was a bit slow to start.
I thought it was fouled, but then found out it wasn't. This year, it
refused to work so far, i.e., it shows 0 knots no matter how fast we
go.

So, I pulled the sensor out (Navico makes that possible even in the
water by a well working auto-closing valve and a temporary replacement
for the sensor housing).

That allowed me to test the sensor (the rotating wheel variety) in the
dry while being afloat. Surprisingly, the wheel rotated freely, and
when I pushed it or blew at it to make it rotate, the log display
showed reasonable values.

Fine, I thought, maybe just something that got stuck and fell off now.
So, I re-installed the sensor and was curious for the next trip. You
guess it: 0 knots.

The hull around the sensor is level and clean, there is nothing that
could disturb the sensor.

Now, my idea is that the steel around the sensor could be the culprit.
It may have become permanently heavily magnetised (maybe by a
replacement for a hole quite close to the sensor that we welded in
place (after removing the sensor, of course). This increased
continuous magnetic field might 'blind' the sensor coil (flush-insert
while re-reading this: Or is it a hall sensor? That would make even
more sense!) in the log. Or even simpler, the little magnet in the
shovle wheel might have become demagnetized for any reason.

Has anyboy of you experienced such a demagnetized sensor wheel, or a
magnetic saturation by the steel hull? It puzzles me that the thing
works if I pull it into the boat and doesn't if I put it back.
The cables are ok, I've triple-checked them.

Any other suggestions?

Thanks in advance for any helpful hint!

Cheers,
U.

 

[Larry]

Has anyboy of you experienced such a demagnetized sensor wheel, or a
magnetic saturation by the steel hull? It puzzles me that the thing
works if I pull it into the boat and doesn't if I put it back.
The cables are ok, I've triple-checked them.

If the wheel were demagnetized, it wouldn't work pulled out of the
seachest, either. I doubt very much the hull could be magnetized enough
to disturb the magnetic pulses from the wheel. The pickup coil points
straight down at the wheel.

Got a snorkel and mask? I want to make sure that wheel is protruding
into the water flow, unblocked and doesn't have anything near it that
might cause cavitation. Is this new welding that went on towards the bow
from the wheel's location? Does the welding protrude even a bit that
might cause a little eddy current behind it...where the wheel is? There
should be no protrusions or intake/outlets forward of the log seachest
that will disturb the flow over the wheel. Some idiot installed a
seawater intake about 8" forward of the speed log at a local yard. The
captain knows me and my electronic skills so asked me to take a look.
That hole where the air conditioner seawater intake sucks water in at the
dock made a little eddy current while the sailboat was underway and the
wheel STOPPED spinning! He had to bore a new hole in the hull and move
the speed log wheel to the new hole with nothing forward of the speed log
wheel to get his speed back....and it worked better than before because
it was further away from the keel making its own underwater wake near the
wheel!

Doesn't take much of an eddy current to stop the wheel at 3 knots. I
think this is what your problem is if the wheel works in your hand, but
not in the seachest...I doubt the steel hull makes any difference at all.

Larry

 

[Ulrich G. Kliegis]

Got a snorkel and mask? I want to make sure that wheel is protruding
into the water flow, unblocked and doesn't have anything near it that
might cause cavitation.

Larry,
thanks for your comments. All of your suspected reasons do definitely
not apply here, though.
a) my son has dived under the boat and found no fouling, no weed, no
mussels etc. Just antifouling paint, nothing else.
b) The welding was on the stern (back) side of the wheel. And the
welding was sanded down absolutely flat. You would not think that
there has been welded anything when looking at the hull from outside.
And nope, no in- or outlets closer than 4 feet, no protruding
structures at all, and absolutely nothing that has not been there when
the wheel was spinning before. The hull has been sandblasted last
fall, new zinc-epoxy and some professionally done primer. Absloutely
smooth. And of course, the wheel and whole sensor were taken out
before the sandblasting.

not in the seachest...I doubt the steel hull makes any difference at all.

I'll take a little hiker's compass (the hand bearing compass is too
bulky for that test) down to the boat and will try to look for any
directed magnetic fields in the environment of the log wheel.

Cheers,
U.

 

[Richard Lane]

Larry,
thanks for your comments. All of your suspected reasons do definitely
not apply here, though.
a) my son has dived under the boat and found no fouling, no weed, no
mussels etc. Just antifouling paint, nothing else.
b) The welding was on the stern (back) side of the wheel. And the
welding was sanded down absolutely flat. You would not think that
there has been welded anything when looking at the hull from outside.
And nope, no in- or outlets closer than 4 feet, no protruding
structures at all, and absolutely nothing that has not been there when
the wheel was spinning before. The hull has been sandblasted last
fall, new zinc-epoxy and some professionally done primer. Absloutely
smooth. And of course, the wheel and whole sensor were taken out
before the sandblasting.



I'll take a little hiker's compass (the hand bearing compass is too
bulky for that test) down to the boat and will try to look for any
directed magnetic fields in the environment of the log wheel.

Cheers,
U.

Dive on the paddle wheel with a hose to spin it and have someone observe
the display.
Dick

 

[Ulrich G. Kliegis]

Dick,

good idea. The water temperature is still comfortable, 15° centigrade,
that's 59° Fahrenheit.

Thanks,
U.

 

[Larry]

We are at about the same temperature now here in Downeast
Maine as far as offshore surface waters.

"SEA WATER TEMPERATURES ARE NEAR 80."

Water's cooling off in Charleston, too!

Larry

 

[Ulrich G. Kliegis]

Nearby welding suggests warped or expanded plates next to the speedo
sensor. Could it be simply binding, requiring a little relieving of
the area nearby?

Excellent idea, but not applicable here. The welding was just the
closing of a 3/2" hole where the previous mechanical VDO log propeller
had had its home. A pretty well fitting piece of steel was welded in
place, the gap was closed by a welding, done by a professional welder.
Any protruding material was grinded back, the surface is absolutely
smooth.
I just mentioned the welding since I am not sure about the source of
the material, and if it has imported some premagnetisation.

Today, I tested the sensor, though, pulled out of its socket (with a
dummy plug closing it) in the presence of a pretty strong magnet. No
influence. So, that can be ruled out.

Thanks for your thoughts, though.

Cheers,
U.

 

[Larry]

Nearby welding suggests warped or expanded plates next to the speedo
sensor. Could it be simply binding, requiring a little relieving of
the area nearby?

Terry K

Sure. Spin it with the hose as someone suggested.

Larry

 

[Ulrich G. Kliegis]

Is there any metal to metal contact between the transducer and the hull
when it is in place? Some chance that there is a ground fault or
something that is eliminated when it is pulled out?

Absolutely not. The transducer sits totally enclosed in a plastic
housing, that sits in a plastic tube, which again sits in a second
plastic tube that is fixed to the hull. No metal to be touched. No
electrical path. Good idea, but not applicable here.

- U.

 

[Larry]

A 5 knot hose? In and out of the mount?

His suggestion was to point the hose at the paddle wheel UNDER the boat, in
the water, to spin it and have someone watch the display.



Larry

 

[Ulrich G. Kliegis]

My suggestion addresses slight binding in the mechanism somewhere. If
a hose spins it up to 5 knots out of the mount, and only say 1 knot
while in the mount in water, would that suggest the mount is binding
somehow?

Until we know the answer, we don't know the problem.

Answer found, problem solved. I disassembled the probe again last
week, pressed out the little axis, removed the wheel, looked at the
parts with a magnifying glass - and found some residues of some
material that does not belong there (detritus of AF-paint, rest of
algae, whatever?) sitting on the axis. The wheel has a bronze bearing,
the axis is stainless steel. After polishing the axis and cleaning the
bearing with a woolen fathom that I pulled through it several times, I
put the whole kit together again. Now, when blowing some air on it, it
does not only rotate, it literally purrs (pun intended? U guess...).
Anyway, today was the reality test, a short trip out of the firth -
and the speed looked very true all the time.

Problem solved, no ion storms, no mysterious powers, just simple dirt
in the mechanics. But very well hidden.
Terry, your remark triggered me to look at it one more time again -
thanks, it helped.

Any thank you to all other contributors as well.
Now, three more weeks until the boat is craned out

Cheers,
U.

 

[Larry]

Anyway, today was the reality test, a short trip out of the firth -
and the speed looked very true all the time.

Obviously, more sea trials next weekend will be needed to make sure you
have it corrected.....

We'll all fly to Germany on Thursday, have a few local beers at the best
beer hall near the marina, load out the boat for you on Friday so the
"extensive sea trials" can begin early on Saturday morning.

By Sunday or maybe Monday, we all should have a pretty fair idea of the
condition of this log system and can have a little "Post Sea Trial
Evaluation Conference" back at the beer hall, once we've cleaned and gotten
your boat back in pristine condition at the dock.

Our "extensive sea trials" over, proper reports written and properly
translated into High German for filing, we'll head on off to the airport
for Lufthansa to take the evaluation teams back to their respective
countries and homes....after paying customs, of course, for all that German
booty we bought between the boat and airport...(c;

Larry

 

[Ulrich G. Kliegis]

rec.boats.electronics:

....

Obviously, more sea trials next weekend will be needed to make sure you
have it corrected.....

Sounds all very logg-ical...

But you'll have to correct your image of beer halls in Germany (hey,
another reality test!). They are rather found in the south, Bavaria
etc., but due to the lack of coasts there, there are no marinas either
(some people call their daughters Marina, but I consider that as
overcompensation of frustrated charterers). So, the proper place to
enjoy some beer here in the north will rather be in a 'Kneipe'
(pronounce K-nígh-pay), more like a pub. There is one some miles up
the coast here that serves over 100 different beer brands, logg-ically
called beer-academy. A proper place for studies.

Cheers,
U.

 

[Larry]

rec.boats.electronics:

...

Sounds all very logg-ical...

But you'll have to correct your image of beer halls in Germany (hey,
another reality test!). They are rather found in the south, Bavaria
etc., but due to the lack of coasts there, there are no marinas either
(some people call their daughters Marina, but I consider that as
overcompensation of frustrated charterers). So, the proper place to
enjoy some beer here in the north will rather be in a 'Kneipe'
(pronounce K-nígh-pay), more like a pub. There is one some miles up
the coast here that serves over 100 different beer brands, logg-ically
called beer-academy. A proper place for studies.

Cheers,
U.

Thank you for setting me straight. I suppose we could have the various
evaluation meetings around a nice fireplace after the event...

We'd better hurry....It's getting colder!

Larry