How to Measure Air Draft Without Climbing Mast

[Cadence]

Originally Posted by Howler

There are numerous threads on the importance of checking local charts etc. for actual bridge heights on the ICW.

But is there a reliable way to measure a boat's true air draft without dropping or climbing the mast?

I'd be reluctant to rely solely on the specification published by the manufacturer, which probably doesn't take into account variability caused by masthead gear like a VHF antenna or wind instruments.

You could probably ball park it by hauling a tape measure up on your halyard.

[scottydaum]

I work in the Concert Industry and were are putting lighting, and sound rigs into the air everyday.
Most of our Technicians these days use Laser Measuring tools to get their heights.
They are available at most Big Box Building Supply stores, and are not terribly expensive. Many are bright enough to use in daylight.
Pay attention to the "accuracy range" stated on the device. In general the cheepo ones may have a +/- range while the better ones are very precise.
So you shoot the Laser to a point on the masthead from the cabin top to get the number, then do as others have mentioned to get the distance from cabin top to deck height, then from toenail to waterline. Adding the Antenna height.
The advantage of these tools is if the boat is pretty steady and the water is flat someone can actually stand on the bow and measure up to measure a scary bridge. Then you still have to do the math to get total height.
On the AICW there are a couple of bridges that the real clearance is a little suspect but those are mentioned in most Cruising Guides, and in different Forum discussions. It's only 2 or 3 as I recall. Most have accurate water level "Gauges" that state the clearance, though some may be poorly maintained, and a little hard to read. Checking the Local Notice to Mariners also will keep you up to date. You can subscribe for free online to weekly updates about all Aids To Navigation ATONS.
If you purchase the Tool, and get good at the process, I am guessing you will have many friends, and will recoup the investment in free beer for measuring other peoples boats. ENOY!

[ebs001]

61feet 11" above DWL

[Uricanejack]

I just read the specs for the boat. Mast hight.

[Paul L]

If you are close, then you need to measure accurately, say to +/-1 or 2 inches. Go up the mast and measure from the top of the hard stuff to the base of the mast. Also measure from the mast top to the top of hard stuff in case you decide to remove the hard stuff to get a little more clearance latter. Then stick a pole out the beam from the mast base with a level on it. Measure from the pole to the water. Combined that will give you the airdraft for the current load. Less fuel, water, crew and the airdraft goes up.

[bobnlesley]

Originally Posted by Paul L

If you are close, then you need to measure accurately, say to +/-1 or 2 inches...will give you the airdraft for the current load. Less fuel, water, crew and the airdraft goes up.

A valid point, if you're talking of a small boat and bring an extra big/heavy friend aboard to help with the measuring, might that in itself reduce mast height by an inch? Similarly, water salinity can make a difference, we were definitely an inch or two deeper (so must've been lower by an equal amount) in much of the ICW compared to out in the ocean.

[Dooglas]

Originally Posted by Chotu

Look up a standard VHF antenna length. Add this as the instruments are always lower than the antenna.

Okay, I'll bite. How tall is a "standard" VHF antenna? West Marine lists VHF antennas from 1 to 10 feet. (having said that, the one on my last sailboat was a 3 foot SS whip)

[smith8273]

The method that I read for measuring trees and such, is get a stick of a known length, stand it upright and measure the shadow that it casts. Measure the shadow of the big stick, and do the math.

[billknny]

Originally Posted by Jammer

You can use a yardstick (or other suitable stick) that is couterweighted and rigged so you can hoist it with the top of the stick going well above the hoist point. I've thought of doing this with a camera to confirm the height under a bridge while approaching it, but it would also work for measuring the clearance required by a boat.


You would stand ashore well away from the boat and use binoculars or whatever to determine how high the stick is above the topmost instrument.

What Jammer is refering to is called a "Pig Stick" and is traditionally used to haul the owner's burgee to the top of the mast. It would be the easiest and most accurate way of measuring the total height of the mast without going up.

Google "pig stick" and see lots of ways to rig it.

[WanderingStar39]

I think I would use the laser. I 'measured" mine by carefully coming up to a bridge gauged at 52' A cyclist on the bridge thought I'd make it. As we got close, the crew sought shelter, and I looked at the scratches on the bottom of the bridge. I made it, close. The crew concluded that our fixed height is 51.5'
This all occurred a couple of years after having the sticks pulled for paint. The yard measured the main at 54', it's keel stepped.

[Jenneau Owner]

Why not just pull up a long piece of chord on you Spin halyard and the pull it back down and measure it. Add height of mast step above water line.

You should be able to acurratley guesstimate the small dist from mast head to Spin fitting, or use topping lift?

Depending on your size of boat and if there are only a few inches to spare be very careful taking into account tide heights and even barometric preasure ! If it’s that tight should you be doing it!🤪

[boatman61]

Originally Posted by zengirl

Does all this mean that in the specs I have for my boat the mast is only measured from top to deck and not waterline?

I would say its from the waterline.. 49ft on a 32ft boat would be exceptionally tall.. especially for a freestanding mast.

https://sailboatdata.com/sailboat/freedom-32

[waterman46]

Originally Posted by derfy

If you are off by 1 degree, how much error in mast height is it?

I reckon about 2 ft if you are shooting around 45 deg at a 60 ft mast.

How accurately do you know the height of your sighting device above the water or deck when you are 60 ft away? Another significant error source.

On the ICW you should know the mast height within .5 ft if you are over 60 ft. The nominal 65 ft bridge clearance varies a lot with changing water levels. A rough approximation is appropriate only if you have lots of margin for error. You don't have much margin there.

If using a sextant, you need to accurately know your distance from mast, and height of eye. Assume you will measure angle formed by toerail to top of antenna. With sextant exactly 100 ft from mast and sextant at height of toerail, if toerail to top of antenna is actually 60 ft, then a 1 degree error in sextant will cause a 1.4 ft error in your result. I would not use this method as it depends on placing sextant at very accurate distance from mast.

[Kenomac]

Just run a tape measure up the mast using a halyard.

[thinwater]

a. I'd feel a little funny not having a spare halyard that went to virtually the top. Different strokes.


b. Although measuring to the nearest inch is a worthy goal, the more important factors are probably the state of the tide, wind effects on the tide (can be major), surge due to weather, the arch of the bridge, any obstructions hanging under it, and inaccurate bridge markings. Some bridges have tide gauges, some do not. You really need to give yourself considerable leeway unless you are sure of these factors.