Measuring Air Draft

[CPseudonym]

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.

 

[koliver]

Years ago I saw one of these, cheap at $30, so I bought it: https://www.amazon.com/Davis-Mark-3-Sextant-Marine/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.

 

[Bacchus]

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.

 

[jleonard]

This thread cracks me up.

I had no Earthly idea figuring out the air draft of a boat could be so complicated. ?

 

[ulysses]

Years ago I saw one of these, cheap at $30, so I bought it: https://www.amazon.com/Davis-Mark-3-Sextant-Marine/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.

 

[Bigsfish]

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.

 

[rwidman]

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.

 

[ulysses]

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.

 

[rwidman]

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?

 

[dhays]

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.

 

[dhays]

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.

I like this idea. I never would have thought of it yet it is simple, and if done carefully, would be very accurate.

 

[Bacchus]

I like this idea. I never would have thought of it yet it is simple, and if done carefully, would be very accurate.

You bet...and out there in the PNW I'll bet you could find a logger to assist! [emoji1]

 

[O C Diver]

Measure from the building's ceiling down. When my boat was in the building for the refit, I made a number of measurements from the ceiling down. The ceiling is 25' off the floor. Then measure from the floor to the water line.

The other way is to get a long 2x4 board. Rest it on the cabin roof to over the water, blocked level with a carpenter's level. Then measure from the bottom of the board to the water. That will give you a know height of the cabin roof, to measure up from.

Ted

 

[Bacchus]

Measure from the building's ceiling down. When my boat was in the building for the refit,

Ted

Ted
Not many have the opportunity that you had... although if one gets the air draft wrong they may need a refit [emoji1]

 

[Capt.Bill11]

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?

Tape measure.

It's worked for me for years.

 

[High Wire]

Its not rocket science

I'm more worried about the markings of bridges being accurate. Plus the latest quote from a bridgetender "I am FORBIDDEN to divulge weither or not there is additional clearance at the center".
No problem, open the bridge!

 

[BandB]

I'm more worried about the markings of bridges being accurate. Plus the latest quote from a bridgetender "I am FORBIDDEN to divulge weither or not there is additional clearance at the center".
No problem, open the bridge!

Actually I think it's good that he's forbidden from saying that. I'm only going to go under a bridge in lieu of opening if there's clearance through most of the bridge. I'm not going to take the additional center height and bank on staying in that narrow area. I'm not taking chances with bridges. Too many things can go wrong.

 

[Bigsfish]

Most bridges in my area (SE Florida) have a sign stating that the center is 5 feet higher than the number showing on the marker. This is due to the curvature of the span.

 

[CaptTom]

Most bridges in my area (SE Florida) have a sign stating that the center is 5 feet higher than the number showing on the marker. This is due to the curvature of the span.

Same along most of the FL Gulf ICW. Sometimes it's a 4- or even 3-foot difference. We found the markings to be fairly consistent, although admittedly we had no way to confirm their accuracy. Which is what started this thread.

The bridges in that stretch also have signs warning of severe fines for requesting an opening you didn't need. I think that, combined with the reluctance to incur liability by having bridge tenders make judgement calls, suggests to me that there is probably a strong motive to keep the clearance gauges pretty accurate.

 

[Bigsfish]

Capt Tom

Also if the marking are off and the skipper of the vessel contacts the bridge the skipper can't sue. Case law has stated that those marking are just a rough estimate and the skipper has to use due diligence.