[aa3jy]

about Annapolis.. From this months BoatUS 'Seaworthy'..

Fluxgate Flummox


Seaworthy would never recommend using an autopilot when passing under a bridge due to the potential for boat, ship, and barge traffic, and to the likelihood of changes in wind and current near such a large obstruction. But if that won't convince you, maybe this will. Operating under autopilot while passing under the Route 50 bridge on the Severn River near Annapolis last spring, a Sabreline 36 suddenly turned hard to starboard and drove right into the bridge abutment. The boat was traveling south at about 7 knots with 10 knots of northerly breeze, and she was about 70 feet from the bridge abutment when she turned. The incident happened so quickly that her captain had no time to react. Similar cases have been reported elsewhere when captains relied on autopilots to pass under bridges. The metal in the bridge structure can easily disturb the magnetic fields that the fluxgate compass uses to determine the boat's heading and keep it on course. When navigating under bridges or in the vicinity of natural magnetic anomalies, usually noted on charts, do not rely on the autopilot. Hand steer until you are well beyond any potential disturbances.

 

[MarkofSeaLife]

I don't always have my auto pilot on under bridges but I usually do.

And I have never had a problem, certainly not by magnetic anomalies (must be a Florida bridge near the Bermuda Triangle).

Sometimes there can be a bit of current swirling around bridge pylons.

But i have learned that other people on the internet are always right. So I think you should NEVER use the auto pilot for anything anywhere anytime, but a wind vane is fine for bridges. NEVER go under a bridge without your life vest (inflated) and your life line lashing you to the wheel so when you turn right you have to lie down.
But most importantly NEVER GO UNDER A BRIDGE IF A BLACK CAT SWIMS ACCROSS YOUR PATH!

If it does throw a fender over your port shoulder.

Mark

 

[gamayun]

Same thing has happened to others going under the the new span of the Bay Bridge in San Francisco. Maybe it has something to do with a different type of steal?

I don't trust my old AP right now so any time I'm near enough to something that I'd like to not hit, I take control.

 

[travlin-easy]

Probably a good idea not to carry lots of change in your pockets, wear that rodeo belt buckle, or use a handheld VHF near the auto-pilot either. Personally, I've never had a bridge or even a metal ship effect any of my compasses.

All the best,

Gary

 

[SHNOOL]

... any time I'm near enough to something that I'd like to not hit, I take control.

^This is probably your best policy honestly. But the topic makes perfect sense actually... as is the comment about using a portable VHF, or anything magnetic near the fluxgate compass. This was a very real concern of mine when locating my speakers. I looked for magnetically shielded speakers for just this reason. Imagine ruining your autopilot just because you located speakers to close. Ugh.

 

[MarkofSeaLife]

The other thing about bridges is I am allways looking up anyway. You've gotta admit you do too. Because we all know bridges might say they are X feet high but when you go under them they look like they are 2 feet lower than the top of your mast.

So if you are not watching where you are going the AP is much better

Mark

 

[SVAuspicious]

The other thing about bridges is I am allways looking up anyway. You've gotta admit you do too. Because we all know bridges might say they are X feet high but when you go under them they look like they are 2 feet lower than the top of your mast.

Agreed. The only bridge I don't look up at is the Verrazano Narrows Bridge. At 228' I'm pretty sure I won't hit it. The Chesapeake Bay Bridge at 186' is close enough that I keep an eye on it.

 

[gamayun]

^This is probably your best policy honestly. But the topic makes perfect sense actually... as is the comment about using a portable VHF, or anything magnetic near the fluxgate compass. This was a very real concern of mine when locating my speakers. I looked for magnetically shielded speakers for just this reason. Imagine ruining your autopilot just because you located speakers to close. Ugh.

This reminds me about the time my son put his laptop next to the bulkhead containing the fluxgate compass and the AP would not hold a course for longer than about 5 seconds. After the 3rd time of 90 degree turns while I was trying to set the sails, I went below and figured it out. I'm now more careful about people putting their bags near the port settee. This whole issue might just be a new versus old equipment problem. A new controller and compass are definitely on my list of things to upgrade next on my 27-y-o boat.

 

[sailak]

This reminds me about the time my son put his laptop next to the bulkhead containing the fluxgate compass and the AP would not hold a course for longer than about 5 seconds.

On my boat it's a single cup hot water maker than runs off the inverter. Turn it on while AP steering and it's an immediate turn of about 65 degrees --- which is correct when it's switched off.

 

[MedSailor]

My first boat had a main ships compass that read North as the starboard side of the boat in all conditions. Once I removed the VHF speaker from right next to the compass, navigation was improved.

 

[SimonV]

The other thing about bridges is I am allways looking up anyway. You've gotta admit you do too. Because we all know bridges might say they are X feet high but when you go under them they look like they are 2 feet lower than the top of your mast.

So if you are not watching where you are going the AP is much better

Mark

I try not to look up, it's going to be too late anyway. I have faith in the charts when it comes to bridges and power lines, for power lines the heat of summer starts to concern me. The admiral heads below for bridges and power lines.

 

[ccriders]

I've had a few occasions when the AP took off on its own course. In these instances there were low flying training aircraft doing approaches, so I just assumed it was their radar. Maybe I need to see if I'm doing something that disrupts the AP.
But I would never sail under a bridge on AP, it's just too scary all by itself. The only bridge I have to contend with is 48 feet verticle and my air draft is 45 feet. I just cannot not look up and always convince myself maybe the bridge has subsided and the tide is higher than the gauge says it is and oh my god I'm going to hit and then, whew, got lucky one more time!
Isn't sailing fun?
John

 

[CLucas]

Probably a good idea not to carry lots of change in your pockets, wear that rodeo belt buckle, or use a handheld VHF near the auto-pilot either. Personally, I've never had a bridge or even a metal ship effect any of my compasses.

All the best,

Gary

Foil hat okay?

 

[ccriders]

Hmm,
Maybe my artificial hip?