How to Measure Air Draft Without Climbing Mast

[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.

[Chotu]

Measure the distance up the forestay using a halyard.

Measure the distance from your forestay on deck to the base of the mast on deck.

Use A(squared) plus B(squared) = C(squared) to find mast height from deck.

Drop tape measure over the side of the boat from height of where your mast is on deck.

Add (height of mast) to (height from water to mast base on deck).

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

[derfy]

Haul up the end of a tape measure. Spin halyard is best. Rig a down haul line rather than relying on the tape. Measure at the toe rail at the middle shroud.

Add

Toe tail to water line

Antenna height, or highest fixture .. windex, etc

[Steven UK]

Sextant...? Or use the angle function on your iPhone (or other smart device) and sight down one edge, measure the distance to the boat and the angle and do some trig... I used this method when deciding not to fell a tree close to my house recently.

[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.

[Chotu]

Of course mine is how I did it on a boat with in mast furling and no main halyard to speak of.

Of course using the main halyard is way easier. Ha ha ha

[Snore]

I use three methods on deliveries. The first two are not 100% accurate, so if it close I don’t go ICW.


Method 1, take sail P dimension + 6” SWAG to top of mast + SWAG of antenna or windex height + measure from tack of sail to waterline.

Method 2, hoist tape up mast + SWAG to top of mast + SWAG of antenna or windex height + measure from tack of sail to waterline.

Method 3, what the broker or owner tells me. Even with this I will cross check using option 1.

In reality, the only way to accurately measure is sending either a tape measure or a line that you measure up the mast + measuring masthead instruments by hand+ measure from the base of the mast to the water.

If this is your boat, you owe it to her and yourself to have a 100% accurate measurement.

[Howler]

Thanks for the replies.

Forgot to mention that the boat has in mast-furling so the main halyard is inaccessible and doesn’t go to the top of the mast. And the spinnaker halyard is short of the top of the mast too.

The sighting plus trigonometry methods are interesting when there’s tolerance for a few percent of error.

Conclusion: Going get the bosun’s chair out and climb the mast before hitting the ICW! Yardstick to measure the antenna above the mast, and a weighted tape to measure down to the boom. Air draft = height of stuff above the mast + height from masthead to boom + height from boom to waterline. Will do it before filling water tanks and loading gear removed for winter. That way it should be a worst-case scenario.

Thanks again.

[tanglewood]

Do you know a surveyor? Or maybe a builder with a “total station”. That can measure the height to within about 1/10th of an inch.

[Howler]

Total station idea is good! Will look for a local builder. Thx.

[SV Bacchus]

Originally Posted by Howler

Total station idea is good! Will look for a local builder. Thx.

Yes, that would be a great idea but.. Bosun's Chair with a tape and a yardstick would be very accurate and probably much cheaper.

[Gruz]

Get

Inclinometer

Blume Leiss

Sextant

or school protractor




then use formula


tangent (sextant or protractor) angle x distance = heights


Calculate the height by multiplying the horizontal distance by Tangent of the angle.

so let say your distance from the boat is 12 m and angle to the top of the mast is 46 deg

tangent 46 deg = 1.03553 x 12 = 12.4 your mast height from water line. (air draft) 12.4 m

if your eyes are not on the sea level add your heights + slip heights


so 12.4 + let say 3 m wil give you your height (air draft) from sea level to the top of the mast

15.4 meters or 50.52 feet

[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.

[KJThomas]

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.

Use that sextant, that has been kicking around, gathering dust, in the locker.

[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?