Judging object height clearance

[Bill Roosa]

I'm with Allan. The chart usually has a horizontal clearance number which you can use to determine your distance off or alternately set the sextant so that you know when you reach a certain distance. Then quickly turn the sextant and measure the angle and your golden (if you know trig)
example
Horizontal clearance = 100 ft
vertical clearance (suspect) 50 ft
set the sextant for about 1/4 mile out with Tan(theta)= HC/(5280/4) or invtan(HC/(5280/4)=theata to set the sextant to 4deg 20 minutes. As you approch the bridge monitor the increasing horizontal angle between the bridge piers till it gets to 4 deg 20 min. Quickly turn the sextant to measure the vertical angle accounting for the horizon not being the line you measure to. Lots of practice here as finding that spot in the water directly below the bridge is a skill. The shadow of the bridge may be a helpful reference. Once you have the vertical angle you can use VC=(5280/4)*tan(vertical angle). this would be great sextant practice BTW In this example you would want to see an angle => invtan(50/(5280/4))=2deg 10 minutes 9 sec.
Now if you could show me how tu use my E6B to to trig....... that would be useful in the extreem

 

[Bill Roosa]

Alternately you could use a land mark to determine you distance off from the bridge and set the sextant for your mast height at that distance. If the angle is greater your OK in not you can use my parallel approach method

 

[RWSlater]

I see what I started here with my ICW bridge clearance post.

 

[Gunni]

Rich;
Think you are too anxious about this clearance issue. Your mast and masthead light are well below the 65' limit, your antenna should be a flexible stainless whip. A good quality antenna, securely mounted can handle brushes on bridge structure. And then you can also time your passages during low water. You should also consider picking good weather routing and going off to avoid the whole ditch rodeo...especially the Florida scene. You have the boat for it.

 

[Benny17441]

It has been my experience that the human brain using the depth of field of the eyes and a data base of distances and heights acquired through the years can intuitively calculate and upgrade in real time a decision whether a sail boat can pass under a bridge or not as it approaches. The closer to the bridge the more accurate the solution will be. It can also take into account the conditions of winds and swells or traffic that may affect the clearance. As a final check I would read the markers at the entrance to the bridge and compare to the known mast height+antenna. It would be foolish to attempt a pass with a limited clearance for the conditions, and this is where the antenna provides a safety margin for any unavoidable and unanticipated closing of the gap.

 

[jssailem]

Had no idea my slide rule comment would resonate with so many fellow sailors.

Send the dinghy ahead to survey...

Great idea. Easy if you have a couple of crew aboard, a really long stick, or one of those fancy laser do hickeys. If lucky they'll comeback and provide a favorable report. Or sit on the bridge and have a good laugh as they urge your forward to tempt your fate.

run parallel to the bridge in close proximity to it you can judge the distance pretty accurately

That works great if you have a calm quiet water way. Not sure so easy to do when dealing with a 4 knot current carrying you towards the bridge in a limited water way with traffic. I guess it is all about boat size, seamanship, and a bit of luck.

 

[Allan12210]

Now I'm really in to this. Obviously a slow day. In my earlier post I used the concept of doubling the observed angle to determine the hypotenuse of the right triangle that includes the vertical height of the bridge. The height could then be calculated.

Instead, I thought how about determining the minimum amount of time to double the angle while traveling at a constant speed. If the angle doubles too soon, you won't make it. If it takes longer, there is room.

In my case I need 55 feet of clearance. Determining the hypotenuse to a 10 degree observation angle by:
Sin(10)=55'/Hypotenuse where Hypotenuse then is 316.73'
Backing up to where the observed angle is 5 degrees, how long should it take to travel 316.73' at 1 knot.
(316.73' * 60 min/hr)/6076 ft/hr = 3.13 minutes.
If the angle doubles from 5 to 10 degrees any earlier than 3.13 minutes at 1 knot, the bridge is too low.

 

[Brian D]

Scarecrow: The sum of the square roots of any two sides of an isosceles triangle is equal to the square root of the remaining side. Oh joy! Rapture! I got a brain! How can I ever thank you enough?
Wizard of Oz: You can't.

 

[Captn Scotty]

Anybody ever try radians? angle = 57.3 x Dis.off
Range
Dis.off = Range x angle
57.3

 

[Captn Scotty]

That didn't display the way I typed it, sorry

 

[NotCook]

Don't ask me - I always think I'm going to hit the bridge, even when there is 200' of clearance above the masts!

 

[NotCook]

his concern is best resolved by following the protocol to have updated charts

The problem I've found is not so much with fixed bridges, but with draw bridges. Why is it the operators won't open the bridge more than a few degrees? I find it necessary to turn and line up to pass directly through the open slot (when the bridges is at a bend in the river), to be sure that I am not going to the the only partially raised bridge, and wind to boot.

 

[Gunni]

Why is it the operators won't open the bridge more than a few degrees?

Because they can. It provides entertainment! Reason #37 why I will sail extra hours to avoid a tended bridge.