Measuring needed overhead clearance

Aka 'air draft'.

Just what simple methods are folks using to measure this?

What a timely question, my father just came up with a really accurate method. First you take you bimini top down but leave your antenna up, pass under a bridge and note where your antenna snapped off. Note the clearance height of the bridge and tidal height and the rest is simple math. This method can get expensive.

I used a level and a long 2x4 across the railings on my flybridge with one end of the 2x4 extending past the outside of the boat.
Then I measured to the waterline.
From there I measured up to the top of the mast, bimini top, etc.
Also measured to my "half mast" position.
Made a sketch of the results.

I measured mine in steps. Waterline to rail, rail to upper deck, upper deck to top of mast. The hard part was making a perfectly horizontal shift from the rail to the center of the upper deck.


Ken

Did mine in the building during my refit, with a scissor lift and a 50' tape measure.

I would find a bridge with a pedestrian walkway. Take a tennis ball and attach a string to it. Put a knot in the string every 5'. Have someone lower the ball from the brige where you will go under, till it touches the water. Determine the distance from the water to the top of the railing. Then you slowly drive under the bridge while your helper determines distances from the top of the railing to points on your boat. Obviously the tennis ball won't hurt anything it touches. Subtract railing to boats measurements from railing to water measurement for accurate boat air drafts.

Ted

Great question because it is not as simple as one might think until you actually try to do it. I did this recently starting with a level section above the saloon by laying a fairly sturdy fishing rod across and hanging off enough to be able to lower the fishing line with a moderate weight down until it touched the water.
Measured the length of that line, then took a couple more measurements to get to the top of the radar mast and added together.
I believe this was pretty accurate because it came within an inch of what i thought it was from a discussion with the manufacturer.

Try the horizontal board projecting out to the side of your boat. Drop a plumb bob from the highest immovable point on your boat to the top of the horizontal board for the up measure. Drop the plumb bob from the extended end of the board to the water. That is the down. Up plus down.
Use a spirit level on the board. Clamp the board to available items on the upper deck.
Be careful climbing to the highest point. Remember how you got up there to mount your burgee.

If your boat has a mount for a pennant on the pulpit, you could determine air draft by inserting a long piece of PVC pipe, stretching a line to your highest point. Use a mason's line level to ensure the line is level. Then measure from the pennant mount to the water and measure from the level line to the pennant base. That should derive a very accurate measure of air draft. You could also keep that pipe aboard and use it as a tell-tale when going under bridges, very slowly of course, and perhaps with a few extra inches for a fudge factor.

I tied up next to a tall piling. Took a 12’ straight edge and taped a Sharpie to the end of the straight edge. With a level I used the straight edge to put a mark on the piling. Then just measure from the mark on the piling to the waterline.

Pythagorean's theorem, two people and a long tape measure will get you really close. Use a fake elevation (boathook) at centerline to transfer the triangle to the dock if you have too.

The smart phone app Dioptra, a tape measure and a little bit of geometry will get you pretty close too. I use it often for field estimating elevation.

For a lot of boat measurments though, I cheat. I have scanned scale drawings of my boat into jpeg and pdf. I have found Adobe Acrobat Pro measuring tools pretty close for sketching and estimating.

Regardless of how you measure.... do you really know how high the bridge is anyway? So, you don't need to be real exact..... just add a few feet.

Gdavid said:

What a timely question, my father just came up with a really accurate method. First you take you bimini top down but leave your antenna up, pass under a bridge and note where your antenna snapped off. Note the clearance height of the bridge and tidal height and the rest is simple math. This method can get expensive.

Click to expand...

I tried this method a coupe of weeks ago, but unfortunately the antenna missed the bridge by about three inches per my calibrated eyeball. I will attempt it again in the Fall when Lake Michigan is expected to be even higher.

I have done it by dropping a steel tape to water level, then taping it off at a side rail. Then raise it up to the approximate height you want to measure and use a level or a tablet/phone with a bubble level app and sight from tape, along the top of the level and to the top of the mast, bimini, radar, etc.


Should get you within an inch or so.


David

Or if you happen to know someone with a surveying instrument, you can shoot points at the waterline and anywhere else, and it will tell you the elevation difference within about 1/10th of an inch.

So when you know the height to within 1/10th inch, just add a foot or two and you should be OK!!

twistedtree said:

Or if you happen to know someone with a surveying instrument, you can shoot points at the waterline and anywhere else, and it will tell you the elevation difference within about 1/10th of an inch.

Click to expand...

This would be a good marina-wide event. Have folks sign up for it and get it done as a group effort.

Chrisjs said:

So when you know the height to within 1/10th inch, just add a foot or two and you should be OK!!

Click to expand...

Exactly

Tape a yardstick (antique analog device) to the antenna.
Take a picture of boat - waterline to top of antenna.
Measure length of yard stick in picture.
Measure height of boat in picture.
Divide height of boat in picture by length of yard stick in picture.
Multiply by 3.
This is the height of your boat feet.
Confirm with bridge...

Our preference is finding something where the height is known or can be easily measured such as a bridge or boathouse. Measure the clearance. Subtract from height of bridge or boathouse or other item and you have it. Note though that's only accurate at the existing load.

Seevee said:

Regardless of how you measure.... do you really know how high the bridge is anyway? So, you don't need to be real exact..... just add a few feet.

Click to expand...

Most bridge heights are posted with the tide board alongside ,you can believe it. Heights are minimum, usually at each end of span . Many bridges have additional height in middle of span...

Greetings,
"Heights are minimum..." I've heard the term "low iron" as the lowest point of a bridge from where the heights are measured. As Mr. SoF notes, many spans have extra clearance in the middle which would NOT be "low iron".

High school algebra:


If you are six feet tall and your shadow is 4 feet long.


Your boat mast casts a shadow 20 feet long. How tall is your mast?

AlaskaProf said:

High school algebra:

If you are six feet tall and your shadow is 4 feet long.

Your boat mast casts a shadow 20 feet long. How tall is your mast?

Click to expand...

Well, I was a mechanical engineering student for the first couple years undergrad and I can do that math, but as my radar arch is fast approaching the underside of a bridge, I'm not sure I want to rely on that calculation.

Get a known distance from the highest point and use a phone app that gives the angle in degrees as you sight down the edge of your phone. A little simple trig and voila !!

If you are 50 feet from your mast and you look up at 20 degrees to see it, it is 18 feet above your eye level. Add the height of your eyes above the water, and there you are.

Is a tape measure really that hard or inaccurate for most powerboats?


I have never found it to be...do it a couple times and you will see that the bends and cot completely vertical use (hopefully not severe) is good enough (round up).... I am guessing that most are not going to bet damaging the boat if your measurement and bridge height is that close.


My experiences with bridges "posted" and "charted" heights are all over the place. Some marked high/some low steel, some posted high, some low, some with clear markings and high/low steel mentioned, some nothing at all, some tenders help some say it's all on you.


I wish the USCG had more influence but it seems like local regs/policies override.

I purchased one of the cheapie laser levels, and used that to determine my air draft to the closest inch with a tape measure extended to the water line. I also documented the level of fuel and water, and that the measurement was taken in fresh water, it all can change the level. I would not rely on air draft calculated by geometry.

We have transited hundreds of bridges in our cruising experience, and after a while you can pretty accurately judge a bridge height as you approach it. I know that a 24 ft. span will allow me to pass, anything less, the AIS whip antenna will be twanging on the steel overhead. If I get to 23'-7", I will be breaking stuff.

I am amazed that some owners will contact another owner of a sister ship to determine air draft. My reaction to that inquiry is "measure yours". It's not that complicated, and unless you never pass under a bridge, it's information that you'll use time and again.

I’m confident my measurements are good but like psneld says the heights listed of bridges are all over the map. I hope the NY and Canadian canal systems are better than the ICW.

I am sure you have all considered this but does the amount of fuel and water you have aboard count!

depends on the boat but full fuel and water on my 40 was only a couple inches...a good wake could be worse...

Maerin said:

I am amazed that some owners will contact another owner of a sister ship to determine air draft. My reaction to that inquiry is "measure yours". It's not that complicated, and unless you never pass under a bridge, it's information that you'll use time and again.

Click to expand...

Wifey B: Amen. Doesn't help you to know someone else's draft.