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Old 04-19-2017, 10:48 AM   #21
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Wouldn't it be easier to get a ball of twine...climb to the highest position on your boat and reel out the twine to water level. You could then measure everything above the highest point you climbed to with a measuring tape....combine and then bammo...you have your air draft...I'd add a little bit to that number just to be safe...

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Old 04-19-2017, 10:50 AM   #22
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"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?"

absolutely +1 - we have seen this as well in the NE.
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Old 04-19-2017, 11:05 AM   #23
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Go to a marina or dockside restaurant that has a balcony over the water. Bring a carpenters level and a tape measure. Get on the balcony and set level on something about the height of your boat. Level it and sight your boat mast down the length of the level. Measure from level down to the water surface. Get a helper on the boat to put a piece of blue tape on mast at sight level. Measure on boat from blue tape to top of whatever mast.

Something like that, depending on boat and marina, etc.

You can't rely on bridge markings. Can't say "well that one was marked 22', and we had 2' clearance, so we are 20'." Some bridges marked super conservative, some have arcs on lower structure, tides, etc.
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Old 04-19-2017, 12:17 PM   #24
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Good stuff.
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Originally Posted by jleonard View Post
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.
The boat that brought up this question was, coincidentally, an Albin 40.

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Originally Posted by ulysses View Post
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.
I'm thinking something like this may be the easiest way. I'd sort of forgotten about math

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Go to a marina or dockside restaurant that has a balcony over the water...
That's another easy answer, but not always practical when time is an issue.

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You can't rely on bridge markings. Can't say "well that one was marked 22', and we had 2' clearance, so we are 20'." Some bridges marked super conservative, some have arcs on lower structure, tides, etc.
This is pretty much what we did on the Albin, and it didn't inspire a lot of confidence in me. Granted it wasn't my boat, but I didn't want to spoil a perfectly good boat ride
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Old 04-19-2017, 03:36 PM   #25
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[QUOTE=Ski in NC;545415]Go to a marina or dockside restaurant that has a balcony over the water.
QUOTE]
That would be nice, ,however.....
If that was my plan I would still be looking as I have never found what you describe (in the last 11 years that is).
Just sayin'.

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The boat that brought up this question was, coincidentally, an Albin 40.
If it is the same era as mine (1980s) my measurements will keep him out of trouble. I made very conservative measurements.
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Old 04-19-2017, 04:07 PM   #26
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Some captains cut it real close:

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Old 04-19-2017, 09:24 PM   #27
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I did by myself years ago, like J Leonard showed in post #5, it took just a few minutes. I added 6" to the total and made stick on labels for the upper and lower helms.
It is nice when bridges have tide boards and clearance marked. So far so good.
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Old 04-19-2017, 09:52 PM   #28
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My method in not proper but it works for me. I get my kid to stand on the hardtop and put his eye level with the highest point. I then take a tape and run it up from the side until he says it is level with the highest point. I measure that to a rail, I then measure from the rail to the water. Add those together and then add a couple of inches. Pray your correct when you go under that first close bridge. LOL

We can do the measurement in a few minutes.
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Old 04-19-2017, 09:59 PM   #29
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Our boat was in a covered slip when we bought it. I took a 12' straight edge and stood on the hardtop. With a level on the straight edge and a Sharpie taped to the end of the straight edge I made the straight edge level and at the top of the radar dome. Them I made a mark on the piling supporting the roof over the slip. Next just measure from the waterline up to the Sharpie mark on the piling. Now that I had the air draft to the top of the radome all I had to do was measure the anchor light above the radome. By doing it this way, I could keep the boat from heeling if I stood on one side to measure to the waterline which would skew the measurement.
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Old 04-19-2017, 10:19 PM   #30
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This thread cracks me up.

I had no Earthly idea figuring out the air draft of a boat could be so complicated. 😂
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Old 04-19-2017, 10:28 PM   #31
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This thread cracks me up.

I had no Earthly idea figuring out the air draft of a boat could be so complicated. 😂
🤔 You must be new around here.
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Old 04-19-2017, 10:43 PM   #32
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Years ago I saw one of these, cheap at $30, so I bought it: https://www.amazon.com/Davis-Mark-3-.../dp/B0014476FI

Ulyssis suggested using a grade 9 math student or an app for your phone. Using a sextant is so much more fun.

Find some place where you can see your anchor light, and also get an accurate measurement from your eye level to the water surface. measure the angle with the sextant. Measure the distance to the centerline of your boat. Do the math. Back before the "new math" this would be a problem for grade 10, but your I-phone should be able to help you.
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Old 04-20-2017, 06:01 AM   #33
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Use the old Boy Scout / logger method to est ht of a tree to fell.
Stand away from the object...abeam og the boat. ..
With a stick or pencil in hand and arm extended place tip on highest pt and place a finger at bottom...waterline.
Now rotate extended arm 90* and place finger at stern of boat..note where tip lands...have a helper stand there if necessary
Now measure distance from stern to helper or noted location...
Easy peezey air draft...don't even have to climb aboard or get to highest pt.
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Old 04-20-2017, 06:31 AM   #34
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Quote:
Originally Posted by Capt.Bill11 View Post
This thread cracks me up.

I had no Earthly idea figuring out the air draft of a boat could be so complicated. 😂
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Old 04-20-2017, 06:46 AM   #35
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Years ago I saw one of these, cheap at $30, so I bought it: https://www.amazon.com/Davis-Mark-3-.../dp/B0014476FI

Ulyssis suggested using a grade 9 math student or an app for your phone. Using a sextant is so much more fun.

Find some place where you can see your anchor light, and also get an accurate measurement from your eye level to the water surface. measure the angle with the sextant. Measure the distance to the centerline of your boat. Do the math. Back before the "new math" this would be a problem for grade 10, but your I-phone should be able to help you.

Exactly. Math can be your friend. Tangents by sextant should beat phone apps.
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Old 04-20-2017, 07:28 AM   #36
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Or you could try the old time honored approach. Start with a six pack and extend your arm Holding the can Out at eye level. Is the height more than
The can. If no you can quit as you have the air height. If yes drink The can and place on the deck, dock or grass. Get another beer and repeat. She the first six pack is gone get another six pack and repeat. Continue until you no longer care and just call it :& feet so you will be safe.


Your welcome.
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Old 04-20-2017, 07:43 AM   #37
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Quote:
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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.
So how do you mark the spot on the water and how do you measure from there to the boat?

I think the OP has the most practical plan.
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Old 04-20-2017, 08:06 AM   #38
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So how do you mark the spot on the water and how do you measure from there to the boat?

I think the OP has the most practical plan.
Pike pole and tape measure is one option. Baton in the mud and tape measure is another way. Homemade buoy and tape measure at slack tide is a third way.
Your particular mooring situation may provide other options such as using a piling in the adjoining slip or even another vessel that might be moored alongside. You can be imaginative and use some problem solving skills to address the issue. It is not an insurmountable problem.
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Old 04-20-2017, 09:46 AM   #39
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This thread cracks me up.

I had no Earthly idea figuring out the air draft of a boat could be so complicated. ��
For most of our boats, there's no direct path from the highest point straight down to the water so we cannot use use a tape measure.

What is your suggestion?
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Old 04-20-2017, 10:08 AM   #40
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So how do you mark the spot on the water and how do you measure from there to the boat?

I think the OP has the most practical plan.


In my situation it is easy as my boat is in a slip on a floating dock.
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