How to Measure Air Draft Without Climbing Mast

[Dooglas]

I have never done the ICW. The bridges on the Ballard Ship Canal in Seattle have fiberglass wands that are mounted at a right angle to the highest clear point on each bridge. This allows you to approach the bridge and motor under the wand to be sure the top of your mast/antenna clears. Do any bridges on the ICW have something similar?

[kevinnem]

Originally Posted by 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.

random -- but I am a land surveyor, yes, with the total station ( or theodolite it is often called, but technically they are not the same thing) it would take less them 1 min to do.

I wonder if you could measure he length of a known shadow, and then use the mast shadow, and similier triangles? It is a classic why to find the height of a building.

[Jim Cate]

I wonder if you could measure he length of a known shadow, and then use the mast shadow, and similier triangles? It is a classic why to find the height of a building.

Kinda hard to measure the length of a shadow on water!

Jim

[Discoverer564]

I have been lurking and learning for some time now. Thank you all for the great posts, info and instruction. This is my first "2 cents" I thought might be useful.
This is simply the formula and I realize getting the data may be a bit tricky.
Shadows can give the height of an object by taking a yardstick or other object of a known height and measure the length of its shadow. then measure the shadow of the object in question and figure the ratio.
Ex. a 3ft stick has a shadow of 5ft and the mast has a shadow of 50ft.
3 divided by 5 is .6 so 50ft multiplied by .6 would mean your mast is 30 ft.
Shadows can change rapidly so take the mast measurement first and then the yardsticks just after.

Bet some here could tell the time by a shadow also.

Good luck to you

[Dooglas]

The catch with many of the simple, and clever, ways proposed here to estimate the height of an object that you can't easily measure directly is that this particular application requires fairly high accuracy. A 10% error on a 70' mast is 7' - obviously unacceptable. Even a 5% error would be too large. Seems to me that takes us to either a direct measurement or an optical instrument capable of a high degree of accuracy.

[newhaul]

Measure how high of the water to the deck on the coach roof then use your I measurement (use the specs for the sails) its actually quite easy. After you determine how tall the mast is add 2 ft for antennas and Insturments and call it good .

You definitely want to be over on your measure than under.
For your oceanis 41-1 the I measure is
Just shy of 48 ft. So I would say measure the deck height from the waterline then add 50 ft. That will get you there with inches to spare.

[Paul L]

Originally Posted by Dooglas

The catch with many of the simple, and clever, ways proposed here to estimate the height of an object that you can't easily measure directly is that this particular application requires fairly high accuracy. A 10% error on a 70' mast is 7' - obviously unacceptable. Even a 5% error would be too large. Seems to me that takes us to either a direct measurement or an optical instrument capable of a high degree of accuracy.

Depending on your planned ICW route you need to be accurate to a few inches if you are near 64 or 65 feet. Just go up the mast and measure it accurately for the stores load you have onboard.

[CarlF]

If you expect your mast is over 61ft and you're doing the ICW I think it's worth a trip up the mast. You need accuracy to about 4" and "being wrong" can bring the mast down around you.

While up there, it's also a good time to take off the windex, the wind vane and the tricolor light. There's no reason to risk this expensive stuff on the ICW where you won't be sailing much or traveling at night without a motor.

You can leave the VFH stainless whip as it bends nicely making a fun "twang, twang, twang" as it snaps under each bridge girder. This will really get the attention of any guests aboard With my 63' mast this happens with any bridge under 66'

I keep a record of how much the antenna bends (just tipped, 12" contact, etc) along with a picture of the bridge height board for next time.

To get an accurate height, I placed a tight piece of cord across the width of the boat at the mast tied to each lifeline. I then measured from the mast top in the bosun's chair down to where this cord touched the mast. Finally, I measured on each side from the lifeline to the water surface and averaged those two readings (to account for any list). Adding this to the tape from the mast head is a pretty accurate air draft.

[hellosailor]

1. Careen boat, get out tape measure and walk it up the mast.
2. Send up a drone, with a simply plumb line or other cord attached. Mark cord when the drone is level with the top, land drone, measure cord at your leisure.

[wsmurdoch]

As others have suggested, and what I have actually done. (Pig Stick)

From an older thread.

http://www.cruisersforum.com/forums/...ml#post2346411

Here is what we did. I tied the main halyard head-to-tail to make a loop. I duct taped the end of a [8] foot tomato stake to the halyard so that when raised to the top of the mast the tomato stake projected above the mast top. I fastened a steel tape measure to the bottom of the tomato stake. While I raised the tomato stake with the attached tape measure to the top of the mast, my wife stood on the dock away from the boat with a pair of binoculars and told me when the top of the tomato stake was even with the top of the VHF antenna. The rest was easy. I laid my boat hook level across the cabin top and out over the water beside the boat and read the steel tape at the top of the horizontal boat hook. I measured from the top of the boat hook to the water surface. With a little addition we had our air draft. It got us worry free under the railroad bridge on the Okeechobee Waterway.

Bill

[wefleenor]

Trigonometry is your best and cheapest solution. Just keep it simple if you are not mathematically inclined. Use a 45 degree angle, which you can pick up cheaply at your local Target or Walmart, or just fold a square piece of paper from corner to corner. Then walk away from your boat until the angle points from the surface you are on and points to the top of your target. The height of your target is then equal to the distance to your target horizontally, plus any height of your surface above the water level.

Bill
Double-Wide

[markpierce]

I took a photo of the boat. Measured the air draft comparing a known distance on a boat's feature and made a simple math calculation. Added 5% to reduce any anxiety. In my case, needed to consider the VHF antennae and the lightning rod.

[dieseldan]

For those whose mast is too tall for bridge clearance there are a couple ways to squeeze under a low bridge by "heeling." Check out these amazing videos and the discussion.

https://www.youtube.com/watch?v=WGAzt-8minI