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Old 06-10-2021, 11:48 AM   #1
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How to measure air draft

Thought it would be easier to find a procedure on this. Perhaps my Google Fu is weak, but a fair amount of search time produced no consensus. Here's what I have in mind.

My marina has floating dock pilings that are taller than my boat. I have a laser level that I can place even with the top of the anchor light atop my radar mast. The laser level will "paint" a red line on the piling even with the high point on my anchor light. A tape measure and either a tall ladder or an assistant will let me measure the height above the water surface. Sound reasonably accurate?

I get that loading impacts air draft, but I need to establish a technique so that I can begin assessing the impact of loading. Thanks.
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Old 06-10-2021, 11:58 AM   #2
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Personally, I'd just measure from the waterline to the deck in line with the tallest point of the boat (or each point you want to measure if you want to check antennas up vs down, etc.). Then measure from that part of the deck up, etc. And add a little if necessary to account for the worst case of the boat being lightly loaded and higher in the water.
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Old 06-10-2021, 12:03 PM   #3
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Personally, I'd just measure from the waterline to the deck in line with the tallest point of the boat (or each point you want to measure if you want to check antennas up vs down, etc.). Then measure from that part of the deck up, etc. And add a little if necessary to account for the worst case of the boat being lightly loaded and higher in the water.
This is bit how I measured mine, adding sections. No need to be 1/16" precise.

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Old 06-10-2021, 12:42 PM   #4
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That makes sense. I readily get caught up in the 1/16" thing.
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Old 06-10-2021, 01:07 PM   #5
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I took a 12’ straightedge and a Sharpie taped to it. Held the straightedge level and put a mark on the piling. Measured down from the mark to water. I didn’t have a laser level with me but same concept. Worked perfectly.
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Old 06-10-2021, 01:27 PM   #6
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Originally Posted by HeadedToTexas View Post
Thought it would be easier to find a procedure on this. Perhaps my Google Fu is weak, but a fair amount of search time produced no consensus. Here's what I have in mind.

My marina has floating dock pilings that are taller than my boat. I have a laser level that I can place even with the top of the anchor light atop my radar mast. The laser level will "paint" a red line on the piling even with the high point on my anchor light. A tape measure and either a tall ladder or an assistant will let me measure the height above the water surface. Sound reasonably accurate?

I get that loading impacts air draft, but I need to establish a technique so that I can begin assessing the impact of loading. Thanks.

I think that's an excellent way to do it. I've always done as others and measured a section at a time from the waterline up, wherever there is a vertical span that can be accurately measured. The challenge is where there are cambered deck and other boat-like shapes. But your method bypasses all that.
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Old 06-10-2021, 02:22 PM   #7
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Originally Posted by HeadedToTexas View Post
Thought it would be easier to find a procedure on this. Perhaps my Google Fu is weak, but a fair amount of search time produced no consensus. Here's what I have in mind.

My marina has floating dock pilings that are taller than my boat. I have a laser level that I can place even with the top of the anchor light atop my radar mast. The laser level will "paint" a red line on the piling even with the high point on my anchor light. A tape measure and either a tall ladder or an assistant will let me measure the height above the water surface. Sound reasonably accurate?

I get that loading impacts air draft, but I need to establish a technique so that I can begin assessing the impact of loading. Thanks.
Basically the main way to do it. Use a laser light or board or anything that gives you the height on a known vertical object, then measure to the water.

Even better if you luck into it and you can pull into a slip with covering or pull under something that is horizontal is to measure your clearance, then measure it's distance to the water.

We have a slip in a covered building so have the luxury of measuring clearance at the entrance, then subtracting.

Within normal realm of loading you're talking 6" or so difference. Extreme cases you might get up to a foot total variation, but that's rare.

Normally I wouldn't be concerned about even a foot as I refuse to pull under a bridge with less than a foot clearance and force them to open. One good wave and you wipe out. However, for the Chicago bridge which is fixed, I had to know exact and be prepared to squeeze. Luckily when we hit there, the water was lower than normal.
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Old 06-10-2021, 02:34 PM   #8
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Normally I wouldn't be concerned about even a foot as I refuse to pull under a bridge with less than a foot clearance and force them to open. One good wave and you wipe out. However, for the Chicago bridge which is fixed, I had to know exact and be prepared to squeeze. Luckily when we hit there, the water was lower than normal.
I agree, 1 foot is about my minimum comfortable clearance considering that boats move. So if I base my measurements on an unloaded boat (meaning we'll almost always be sitting lower than that) and have a foot of buffer, even if I'm an inch or 2 off, I'll still have plenty of clearance in the real world.
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Old 06-10-2021, 02:52 PM   #9
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I've been thinking about this a little bit lately.
Your tall pilings make it a lot easier for sure....kinda how I measured the height of my motorhome
The issue I see in measuring segments and adding them up is that while with some boats this might be relatively easy and close, some boats add in a lot more of those little 1/16 inch errors. 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. Could very quickly and easily get several inches off. I had a hard enough time getting a repeatable number using my laser from the top of the motorhome's AC unit shooting over to the shingles on my garage just a few feet away, then down to the gutter's edge, then down to the driveway. My laser level is garbage, so there's that, but still....

just more food for thought, if you have a way to measure the angle, you can stand back and sightline it for a double check.....at a distance of 20ft away, 1inch vertical is about 0.238°, or about 4-1/4 inches per degree if I'm thinking clearly this afternoon....
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Old 06-10-2021, 03:21 PM   #10
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Haven't given this much thought; always accepted the number I was given by the previous owner, BUT:


The method I (and probably many of you) was taught in the 6th grade is to compare the length of shadows. i.e. if a piling 6' above the water casts a 6 foot shadow, and my mast casts a 20 foot shadow...


Obviously, you want to wait til the shadows fall on the dock...or move the boat accordingly.


You in the PNW may need to wait a while for a shadow.
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Old 06-10-2021, 04:43 PM   #11
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Tie up under a bridge on a rising tide. When you hear a "crunch" read the number off of the bridge marker.

If you had a sextant you could triangulate to the top of the mast and dust off your high school trigonometry. SOHCAHTOA!
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Old 06-10-2021, 05:03 PM   #12
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Water line to deck around 4 ft
Deck to ceiling around 7.5ft
Same on next level
Mast about a ft taller than me so another 7.5ft
Round it up to 30ft.
Close enough.

Next week's lesson "How to count fingers"
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Old 06-10-2021, 05:22 PM   #13
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Quote:
Originally Posted by HeadedToTexas View Post
Thought it would be easier to find a procedure on this. Perhaps my Google Fu is weak, but a fair amount of search time produced no consensus. Here's what I have in mind.

My marina has floating dock pilings that are taller than my boat. I have a laser level that I can place even with the top of the anchor light atop my radar mast. The laser level will "paint" a red line on the piling even with the high point on my anchor light. A tape measure and either a tall ladder or an assistant will let me measure the height above the water surface. Sound reasonably accurate?

I get that loading impacts air draft, but I need to establish a technique so that I can begin assessing the impact of loading. Thanks.
How can you get more accurate that that? I'd say you slam dunked it.
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Old 06-10-2021, 05:25 PM   #14
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I get that loading impacts air draft, but I need to establish a technique so that I can begin assessing the impact of loading. Thanks.
So if it's to determine immersion rate due to loading why not simply measure waterline to top of antifoul?
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Old 06-10-2021, 05:52 PM   #15
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Originally Posted by Simi 60 View Post
Water line to deck around 4 ft
Deck to ceiling around 7.5ft
Same on next level
Mast about a ft taller than me so another 7.5ft
Round it up to 30ft.
Close enough.

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On some of the rivers and lock systems here you really want it to be more precise. Particularly if you want to do some of the lock systems like the western part of the Erie Canal. But if it works for you where you are then more power to you.
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Old 06-10-2021, 07:15 PM   #16
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On some of the rivers and lock systems here you really want it to be more precise. Particularly if you want to do some of the lock systems like the western part of the Erie Canal. But if it works for you where you are then more power to you.
It was said tongue in cheek hence the wink but really, if you need more acurasy the same applies with a tape measure

Water to deck
Deck to ceiling
Take out light fitting for ceiling to deck (or roof)
Add on deck thickness
Plus height of mast
But really, why so accurate?
Passing boat bounces you around several inches.
Walking side to side and for and aft affects some vessels.
Exact measurements are constantly changing

And, ss the op seems to be after immersion rates, measuring waterline empty vs loaded is surely easier.
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Old 06-10-2021, 08:52 PM   #17
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No problem. Some places air draft doesn’t really matter. Where I live it doesn’t but on some of the rivers it does. I don’t know about down under.
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Old 06-11-2021, 10:35 AM   #18
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HeadedToTexas, let us know the results of your calculation. As a H38 owner I am curious. (It would be helpful to provide a calculation to the top of the pilothouse or the fairing around the flybridge or the top of the flybridge rails, since that should be more-or-less standard among H38s or Mariner 37s.)
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Old 06-11-2021, 02:45 PM   #19
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HeadedToTexas, let us know the results of your calculation. As a H38 owner I am curious. (It would be helpful to provide a calculation to the top of the pilothouse or the fairing around the flybridge or the top of the flybridge rails, since that should be more-or-less standard among H38s or Mariner 37s.)
Don't assume anything is standard. Different day, different place. Measure your own boat.
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Old 06-12-2021, 06:27 PM   #20
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The method I (and probably many of you) was taught in the 6th grade is to compare the length of shadows. i.e. if a piling 6' above the water casts a 6 foot shadow, and my mast casts a 20 foot shadow...
I like that one. Might have been taught it in 6th grade, but had since forgotten it if so.

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How can you get more accurate that that? I'd say you slam dunked it.
Thank you!

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Originally Posted by Simi 60 View Post
So if it's to determine immersion rate due to loading why not simply measure waterline to top of antifoul?
While I might just be obsessive enough to take a series of waterline measurements to establish the effect of loading on air draft, that was not the purpose of this exercise.

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On some of the rivers and lock systems here you really want it to be more precise. Particularly if you want to do some of the lock systems like the western part of the Erie Canal. But if it works for you where you are then more power to you.
Traveling the western section of the Erie Canal and some nearby low bridged tributaries is exactly why I'm asking!
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