Measuring vertical clearance

[Jammer]

I have ongoing problems determining vertical clearance of obstacles around me.

• I had a near miss with an uncharted overhead obstruction at the lower St. Anthony Falls lock. A crane gantry had been added to the lock when some of the hydroelectric equipment on the nearby dam was upgraded, and the charts and ENCs were never updated.

• The high bridge in St. Paul has a power line next to it that is about 1/3 the height of the bridge (see photo; the powerline is most visible above the bridge footing to the right in the photo). How high is it today (powerlines sag when hot)? Published clearance is over 60 feet but it is rare for traffic under the bridge to be that high. When was it last surveyed? Who knows..

• The operators of the Duluth Aerial Lift Bridge do not open the bridge all the way except for commercial traffic. Typically they open it until their gauge says 75 feet. My mast is 62'. There are no markers on the structure. If they say it's 75 feet but it's not and I hit it I am sure that there will be plenty of blame to go around and make sure everyone gets a full serving.

• Some of the Minneapolis-area bridges are charted to relatively lower (50'-ish) clearances because they are charted based on the width of the barge channel. Nearly all bridges are arched at least somewhat and these particular bridges have a steep arch. There's more clearance in the center, but how much more?

For these and other vexing questions I am tempted to purchase a Nikon Forestry Pro II Laser Rangefinder that I came across. It's a laser rangefinder with an accelerometer built in that measures tilt. Point it at a powerline or bridge and push the button, and it does the measurement and trigonometry for you and gives you the height to the nearest tenth of a foot. All you have to do is add height of eye.


$350 street, not cheap, but much cheaper than a new rig.


I have also purchased a backup camera with the idea of installing it at the top of the mast but have not done so -- obviously a high-quality installation is not straightforward.

[Jim Cate]

That sounds like a neat gadget, Jammer. But I wonder: will it actually get a response from something as small as a high voltage wire at a useful distance? Pretty small cross section to lock on to...

Jim

[Kettlewell]

In my experience along the East Coast the official NOAA charts are generall pretty accurate as to heights, with a few exceptions that have been off by a foot or two. If you are that close to the published height you should proceed very slowly. Waterways tend to have established height minimums for the entire waterway, which makes sense if you thing about it--if a boat can clear one 55-foot bridge you don't want them to get trapped just beyond. It's the side channels that can be less defined as to vertical clearance.

[Jammer]

Originally Posted by Jim Cate

That sounds like a neat gadget, Jammer. But I wonder: will it actually get a response from something as small as a high voltage wire at a useful distance? Pretty small cross section to lock on to...

Well, it says so on the outside of the box:

The Forestry Pro II’s Target Priority system is capable of reliably picking up and ranging extremely small or thin targets such as distant power lines

The power utility industry has been using laser rangefinders for years to measure sag and vegetation clearance.

[Jammer]

Originally Posted by Kettlewell

If you are that close to the published height you should proceed very slowly.

At what speed is it safe to hit a bridge with the mast? Asking for a friend.

[thinwater]

Sounds smart.


Remember that the allowable clearance on power lines includes a buffer related to voltage. Might be 500 kV.

[rslifkin]

Originally Posted by Jammer

At what speed is it safe to hit a bridge with the mast? Asking for a friend.

If you've got a camera on top, slow enough that you can go for a fistful of reverse when the camera shows a possible hit coming.

I'd expect a very gentle bump won't hurt anything, but I wouldn't want to hit a bridge at even 2 - 3 kts.

[thinwater]

Originally Posted by Jammer

At what speed is it safe to hit a bridge with the mast? Asking for a friend.

I'm guessing that depends on what it hits and how much damage is acceptable. The upper furler swivel and instruments are rather fragile. It's not like touching something with a bumper. It will rain plastic bits with any contact.

[Jammer]

Originally Posted by thinwater

Sounds smart.


Remember that the allowable clearance on power lines includes a buffer related to voltage. Might be 500 kV.

The ones I'm concerned about right now are all 115 kv or less. OSHA says stay 15 feet away.