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Old 04-19-2017, 08:03 AM   #1
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Measuring Air Draft

Is there a trick to measuring the air draft of a boat?

The usual approach seems to be to read the manufacturer's documentation if available, or the PO's ad, estimate how much that might have changed with the addition or removal of other "stuff," then pass under successively low bridges until you get a sense of when you can make it, and when you can't.

The problem with actually measuring is complicated by the fact that there's no straight line from the top of the anchor light to the waterline. Or from the deck.

I was thinking I could put a long pole horizontally across the deck, then measure from that to the anchor light. Then I'd need to take two measurements from the pole to the water line, and average them, or else get both the pole and the boat perfectly level.

I've never seen anyone do this, or for that matter, any other method of taking an actual measurement.

I wonder if there's a better way. Like last month when I showed up at a newly-purchased boat to help a buddy take it home. I didn't have a 14' pole handy, and we had lots of bridges to pass under along the way.
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Old 04-19-2017, 08:33 AM   #2
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If your boat is the one in your avatar measure the tip of your antennae to the top of the fly bridge. Write down that number. Get your fishing pole out of its locker and lay it across the top of the fly bridge. Let out enough line to submerge a lead sinker about 2" below the surface while the boat sits for a minute or so. Lock down the reel and measure the amount of line and add this to the number you previously wrote down. That is your current air draft.
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Old 04-19-2017, 08:34 AM   #3
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"I was thinking I could put a long pole horizontally across the deck, then measure from that to the anchor light. Then I'd need to take two measurements from the pole to the water line, and average them, or else get both the pole and the boat perfectly level."


That is more or less what we did but had to break it up in 3 segments and utilized a level to ensure the 'pole' was even across the boat.
FWIW - I would not trust the manufacturers spec with the variables in play , remember that the number will change with added or subtracted weight as well as in salt of fresh water.
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Old 04-19-2017, 09:04 AM   #4
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Pythagoras solved this issue many years ago and provided it to Euclid.

Measure with tape or string from your top most point straight out to the water preferably to abreast the beam, without touching anything else on the boat. Don't try measuring straight down but out and away from the boat. Then measure from the spot you are touching the water to the boats hull. Add 1/2 of the hulls width to the second measurement. You have established a right angle in which you now know the hypotenuse and one side. Do the math to solve for air draft.
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Old 04-19-2017, 09:13 AM   #5
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We measured similar to the way you are thinking also. Here's the sketch I made up originally. I have since added a few dimensions but you get the idea.
Attached Files
File Type: pdf Albin draft.pdf (15.4 KB, 120 views)
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Old 04-19-2017, 09:36 AM   #6
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Hi,


I is small and affordable laser level that can measure the height of the boat. In addition you will need a tape measure. Laser shoot a stable and read the dimension of the red line. If you have another guy on the boat even easier.


example



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Old 04-19-2017, 09:41 AM   #7
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Ulysses is right, use Math. If you can't remember how, ask any reasonably intelligent 9th grader.
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Old 04-19-2017, 10:03 AM   #8
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Our airdraft is 23'. I've measured it at least 5 times just to make sure. I've used tapes measures, sticks, measured up/down, you name it and it hasn't changed yet but it's still unsettling, when we go under a bridge that says it's 24' and you look up. There's always that major pucker factor as you're second guessing yourself. Human nature I guess?
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Old 04-19-2017, 10:05 AM   #9
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Please note....


Width of the hull at the waterline is most often much less than the actual beam of the boat.
Many of us have had boats where the 'tallest' fixed point is not on the centerline.
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Old 04-19-2017, 10:12 AM   #10
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Met a fellow who had a story from when he worked at a marina. Somebody came storming into his office demanding he pay for the sailboat mast which had just been destroyed, and that the marina should dig a large pit on the bottom under the bridge so that when boats go under there at high tide, their masts won't get destroyed as well
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Old 04-19-2017, 10:13 AM   #11
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All above advise is good. Make sure you measure when fuel, water and waste tanks are near empty.
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Old 04-19-2017, 10:18 AM   #12
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"Met a fellow who had a story from when he worked at a marina. Somebody came storming into his office demanding he pay for the sailboat mast which had just been destroyed, and that the marina should dig a large pit on the bottom under the bridge so that when boats go under there at high tide, their masts won't get destroyed as well "


How would that work? Wouldn't you need different depth pits for different boats?
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Old 04-19-2017, 10:48 AM   #13
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Quote:
Originally Posted by smitty477 View Post
Please note....


Width of the hull at the waterline is most often much less than the actual beam of the boat.
Many of us have had boats where the 'tallest' fixed point is not on the centerline.

OK this may be true, and if you are unable to figure out the distance of the point of the hull to the center line of the boat then we can try some other math. In this case it might be a 10th grader instead of keeping it simple and on a 9th grade level.

The tangent angle (top of vessel)X distance to Center line plus Tangent angle (to water line) X distance to Center line.

That can be done while you are standing on the dock with your phone and a couple of cheap phone apps.
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Old 04-19-2017, 11:04 AM   #14
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"distance to Center line." - How would you propose getting that distance?
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Old 04-19-2017, 11:07 AM   #15
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this bridge RCCL instructions below target. Exhaust pipes inside the tanks full and full speed dive vessel 1-1.5 meters deeper.

I must say the captain is a pretty cool guy!

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Old 04-19-2017, 11:07 AM   #16
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"distance to Center line." - How would you propose getting that distance?
Tape measure from where you are standing to the center line of your boat. It would be easiest for you to do this with someone else assisting- you hold one end and allow an assistant to walk holding the tape measure to the center line of the boat. Let the assist read the measurement. It can also be done solo if you attach the 0" end on a marked spot that you will be standing on and then walk the tape to the center line and note the distance from said point to the center line.
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Old 04-19-2017, 11:15 AM   #17
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"Tape measure from where you are standing to the center line of your boat."


Ahh - so it cannot be done while standing on the dock and you would need the boat level.
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Old 04-19-2017, 11:23 AM   #18
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"Tape measure from where you are standing to the center line of your boat."


Ahh - so it cannot be done while standing on the dock and you would need the boat level.
No, I believe you are missing something. Can you not stand on your dock and get to your boat with a tape measure if someone is assisting you ? I have yet to find a tape measure with operational instructions, but I am sure that you can find someone that has some experience that can help you figure it out.
Secondly the boat being level will help get an accurate air draft but the math will work whether it is level or not. If the boat is not level the tangent angle will still be to the highest point at that time no matter how the boat might presently be inclined.
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Old 04-19-2017, 11:36 AM   #19
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Here's how we've measured. Find a clearance that is marked well and accurately or that you can mark. This could be a dock or boathouse slip. In our case it's a covered boat slip. The entrance to the slip is a solid and straight beam and we've actually marked along the side the measured distances so at any time we can easily tell the clearance. It varies between 39 and 41'. Then we simply pull the boat under that entrance way and measure the distance from various points of the boat to the top. We might come up with something like tallest antenna 17', radar with other antennas down 21'. We'd then subtract from the current overall clearance, lets say 40' on this day. That would tell us that our air draft with the antenna up is 23' and with it down is 19'. If you found a well marked bridge you could even do it there.

As noted below we've made sure to measure at lightest load and we've also measured at other loads. It's interesting to see the difference as your water draft also changes by the same amount.

It was also interesting to compare to manufacturer's numbers. Of the manufacturer's boats we checked here's what we found. We measured two boats each from two builders and found them to be conservative. Even at the lightest load we could create we had more air draft than they indicated. Their numbers were clearly from a load that was impossible for us to be that light. Must have been all tanks completely empty and no supplies or extras on the boat. Similarly we had two divers measure water draft and even with all tanks full and everything possible plus 12 people on the boat we had slightly less water draft than they indicated. Note that one of these builders did supply a range based on load but even that range was conservative by a couple of inches.

However, we checked one boat from a third builder. Their numbers were from the most optimistic view. Their quoted air draft was from a fully loaded boat in every way and several inches less than the draft we found in our worst case. Their water draft was less than we found in any load situation. They quoted 5'. Out numbers ranged from 5'1" to 5'3".

I must say that on the day we did this, we got quite a few onlookers and those who were very convinced we were crazy, although one of them then asked if we'd check his boat.

Now, once you have a firm grasp of your air draft, you still have one other problem. What does the number you get from the bridge records or tender indicate? Is the bridge accurately marked? A couple of times we've had discussions with bridge tenders that have been very interesting, but pretty much all over the place. We've found well marked bridges where the clearances were clearly based off the lowest area of the bridge. If you stayed centered under the bridge you had a few more inches. Nice, safe. The published numbers were also worse case on those bridges. Conservative. We liked. However, we found one bridge that had no markings and the quoted clearance was based on some really low water situation and the very center of an arched opening. The safe clearance was over two feet less than the number shown on charts and records. Clearly a dangerous situation as someone had found and posted on Active Captain when they lost a light going under.

One other thing we keep in mind. Whatever air draft you come up with does not reflect waves or wakes caused by other boats.
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Old 04-19-2017, 11:45 AM   #20
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"No, I believe you are missing something. Can you not stand on your dock and get to your boat with a tape measure if someone is assisting you ? I have yet to find a tape measure with operational instructions, but I am sure that you can find someone that has some experience that can help you figure it out.
Secondly the boat being level will help get an accurate air draft but the math will work whether it is level or not. If the boat is not level the tangent angle will still be to the highest point at that time no matter how the boat might presently be inclined."


I did not have a dock nor did I have one when we were approaching the bridges on the canals. We were able to make sure that the distances were within about 1/4" by measuring using the simple methods above. It has been pointed out that you cannot just use the waterline distance as the hull is never plumb to the hulls full width - n or is the width even throughout the length of the boat.


"I have yet to find a tape measure with operational instructions, but I am sure that you can find someone that has some experience that can help you figure it out."


It is always a pleasure to exchange posts with folks that are both knowledgeable and sincere in their efforts to help.
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