How to measure air draft

[sledge]

I think the OP has a good idea. Getting this within a few inches is also very useful in the south florida ICW; it's an endless and agonizing series of bridges of various heights

FYI: we hung a weighted line off the end of a long boat pole laying flat on the upper house deck until the weight touched water, measured that line length, then physically measured upwards from deck to top of mast.

 

[HeadedToTexas]

FYI: we hung a weighted line off the end of a long boat pole laying flat on the upper house deck until the weight touched water, measured that line length, then physically measured upwards from deck to top of mast.

That's a good approach too. Parallax worries me about the camera and app methods as does sighting the elevations and accurately measuring the angle on the trigonometry methods. I trust the science (being a scientist), but recognize the limitations of measurement. A simple tape measure on a string is well within the limitations of measurement.

 

[Seevee]

Another issue is knowing the height of a bridge.... accurately. What the height board says and what it really is could be several inches off.... or perhaps more.



The straight edge with a level and a string is hard to beat. Easy and pretty accurate.

 

[catalinajack]

Plumb bob.

 

[HeadedToTexas]

Okay folks, this took longer than I expected. The laser level point I shot from the top of the bridge windshield on our Mariner 37 Seville was 12’ 3” above the water with both fuel tanks at ½ and the fresh water tank at ¼. The top of the bimini rises another 33” bringing the air draft total up to exactly 15’. The radar mast rises 9’ 11” from the bridge deck and the laser level shows bridge deck at 99” above the water with the tanks as listed above. That adds up to 218”, or 18’ 2”. Your mileage may vary.

 

[Landfall]

More importantly, when did air draft become a thing. Clearance, bridge clearance or height seemed to work for generations.

 

[Art]

More importantly, when did air draft become a thing. Clearance, bridge clearance or height seemed to work for generations.

It's pretty simple... Over time, things [descriptive words] change: Descriptions such as male and/or female worked for millennium!

 

[slowgoesit]

Seems easy enough to measure up to a point on the gunnel using a plump bob..... but getting from there over and up might go across all sorts of curving and sloping surfaces.

skyhawk, careful now, bob may take offense to your characterization . . .

On your measurement, I like the trig solution, but as stated, you need an accurate method of determining angle . . . . and sextants appear to be few and far between nowadays

 

[Benthic2]

You can measure angles with an app on your phone. Judicious use of duct tape and a cheap laser pointer and you've got a modern day, high tech sextant. I would practice on a few things you could verify, like the height of your home's chimney, or a second floor window before actually risking a sunbrella & aluminum vs steel encounter.

 

[Medic]

Whoops. Not my fault, but was my responsibility

The Portage Canal Lift Bridge between Hancock, Michigan, and Houghton, Michigan is lifted by request. It is a cut on Lake Superior that makes going around a large peninsula unnecessary.

I had gone under the Lift Bridge when raised a number of times in my Nordhavn 55. On one occasion when there was a lot of traffic and an emergency vehicle expected "sometime in the next 20-30 minutes or so", the Liftmaster said he had lifted the bridge for me in the past and assured me that it was simply not necessary. I would clear easily enough.

I'm sure you are familiar with the grating that makes up the roadway often on the opening sections of the lift bridges. I was the only boat needing passage and the water was absolutely calm. Well, I moved in very slowly. One of my antennas was soon clanging away as it was hitting the roadway. I slowly backed out. No damage to the antenna or anything else but I was going no farther.

He lifted the bridge a few minutes later after checking with the police and being told the ambulance would still be another 20-30 minutes at least. After passing through I thanked him just like I would normally do. He didn't answer back.

I should not have let him talk me into the passage in the first place. That was ultimately my decision.

I called Nordhavn and got my approximate air draft number when light, and when with full everything. My measured guesstimate and the numbers provided were off by about 4 inches so I went with the highest.

Lessons learned: know you air draft, and don't let a bridge Liftmaster make a decision as to whether or not you can safely pass when closed.

 

[Portage_Bay]

More importantly, when did air draft become a thing. Clearance, bridge clearance or height seemed to work for generations.

Interesting question. I've been using that term for at least 20 yrs. I learned it from my boss at the time who before being desk bound was a ship's master. It was a new term to me at that time. Your question got me to pull my copy of Bowditch 1966 and Dutton's 1969 off the shelf. I didn't find air draft in either of those antique tomes. I don't know when it came into common use.

 

[markpierce]

Sinking bridges have advantages. Seen one in Greece in operation, but here we only have lifting and obsolete swivel bridges.

 

[markpierce]

Liveaboard limitations can favor recreational boaters versus those wanting low-cost housing. In many places, one can't find a berth.

Wondering ... One percentage of marina liveaboards dump their waste there.

 

[markpierce]

Sorry. My last two posts belong to another thread.