Judging object height clearance

[jssailem]

So right Jack. It was the key that empowered HP calculators of the time. Not with it a few bugs, but it enabled me to put my slide rule in a drawer where it has gathered dust.

Makes me wonder if my HP is in a box somewhere. I'll have to check when I get home.

 

[Gunni]

I have done this exercise with field mapping techniques using a theodolite on a leveled tripod on land and yes the trigonometry is straightforward once you know the distance of one side of the triangle and the angle to the target. The problem in your exercise is that you moving in 3 dimensions and you have little room for error in a circumstance where the distances are small, and the angle is jiggling. You method would be useful for a large scale approximation where you knew that you were 30 miles off an unknown island and the angle to the top of a misty mountain was X.XX. From that you could determine that you had made St. Vincent, and passed St. Lucia.

This concern is best resolved by following the protocol to have updated charts, NTMs, understanding the Rules of Navigation, keeping a watch monitoring radio coms, and having the good sense to integrate them all. In other words, good seamanship. Do that and you won't be hitting anything above or below the water.

 

[Johann]

Maybe this:

https://www.amazon.com/Bosch-GLM-10...coding=UTF8&psc=1&refRID=EH5ZWSCA3YMZ1X6AV794

Send the dinghy ahead to survey...

 

[Brian D]

Every boat should have a slide ruler. LOL

Good point all around. The key is good seamanship. But like I said, it is just another mean to help find the end.

5° was an arbitray number to help keep the final result down. But at 1000' a few degrees may change the results but probably not by much.

 

[centerline]

the the first problem lies in the GUESSwork of determining the exact boat length away from the overhead structure.
if, standing at the helm, and you had a mark on the mast as the correct height so that your line of sight passing thru the mark was at the predetermined angle, and assuming the boat was at the predetermined distance, you COULD compute the height of an object crossing your line of site beyond the mark on the mast.

but on the water its very hard to determine distance, especially when there is no reference point other than the bridge itself.... if you where near one of the abutments, you would have only a slightly a better reference point to do your guessing from.... and then the second problem is if the water is choppy, its harder to judge, and IF there is any wind or current pushing you towards the bridge, 2 boat lengths or even 5 boat lengths is MUCH to close if you are unsure if you have the proper clearance....

 

[Johann]

5° was an arbitray number to help keep the final result down. But at 1000' a few degrees may change the results but probably not by much.

I doubt you could get as accurate as 1 deg, but 1 deg error at 1000' is about 17'.

 

[Brian D]

Yeah, if your eye is 10' above the waterline, then you would have to add 10' to the results. One would have to compensate.

Distance x Tangent(elevation angle in degrees) = height (and error if looking for that number)

Again, it was all theory, conjecture, speculation, what if, and what not. Just bored like I said. Unfortunately I do not have a bridge to try it on. Closest is Mission Bay and I know for fact I cannot pass under it. Then there is the Coronado Bridge that stands way about the waterline.

Just playing with numbers.

 

[DayDreamer41]

Well I wouldn't say that you can't pass under it, it just takes some good ole ingenuity,,,,,,

 

[Bill Roosa]

You guys sure like to do things the hard way. I have found that if you run parallel to the bridge in close proximity to it you can judge the distance pretty accurately. Also if you try to go under at a VERY low angle to the roadway you don't stand much chance of de-masting yourself. Also GO SLOWLY.

 

[Rich Stidger]

I have thought about hoisting a wireless video camera to the top of my mast. The camera would be mounted on something like a pig stick and would be gimbaled to remain horizontal. Then you would have your eye level with the very top of your antennas. I am planning a trip south through the ICW with all of the 65 foot bridges. Mast mast is 62.5 feet plus antennas and masthead light of another 2 feet. To say I am terrified is an understatement.

 

[LloydB]

Rule of thumb.

 

[Bill Roosa]

how about mounting a gimbaled laser pointer on the mast head so it projects a line level with the mast head......
OR
one of those laser levels that rotates
or
microwave flux capacitor tuned to the bridge I-beam frequencies mounted in such a way that the reflection of the microwaves creates a positive feed back loop in the flux capacitor causing an avalanche breakdown that lights a light
OR
read the chart and abide by it.... Na that's way to non-nerdy
or......
use a sextant turned 90 degrees and measure the included angle from a known distance.......Oh wait you have to read the chart to know what the actual clearance is....Yea that is not nerdy enough cause it is just reading text on a chart.....totally unacceptable

 

[Bill Roosa]

hey Don
You can practice on the door frames in your house. get a stick that is 7' 2" and do you trig. Since door opening are 7' the thick (mast) will not go through so you can test the math. Probably going to need another stick to keep the "mast" at the proper distance from the "helm"

 

[rgranger]

how about mounting a gimbaled laser pointer on the mast head so it projects a line level with the mast head......
OR
one of those laser levels that rotates
or
microwave flux capacitor tuned to the bridge I-beam frequencies mounted in such a way that the reflection of the microwaves creates a positive feed back loop in the flux capacitor causing an avalanche breakdown that lights a light
OR
read the chart and abide by it.... Na that's way to non-nerdy
or......
use a sextant turned 90 degrees and measure the included angle from a known distance.......Oh wait you have to read the chart to know what the actual clearance is....Yea that is not nerdy enough cause it is just reading text on a chart.....totally unacceptable

 

[Rich Stidger]

Bill Roosa- Your usual helpful ideas fail me today. The problem with the laser pointer is that I could not be sure it would be visible in the daytime- how would I know if I clear or if I just missed the dot? Of course I can read a chart, but with a total mast height of about 64.5 feet I think I am justifiably concerned about a possible clearance of just 6 inches. And if for some reason the tide or waves or the height boards are not exactly right, my day and my passage could turn very bad. Of course I can read a chart, but it is the unknown variables that have me concerned. Unfortunately any angle measurements require you to know the distance off accurately. So I thought my suggestion of a level camera might be a possible solution.

 

[rgranger]

..... So I thought my suggestion of a level camera might be a possible solution.

To be fair... I think that is a good and inexpensive way to judge bridge clearance. A wireless camera with blue tooth to your phone would be all you need. Maybe a simple blue tooth web cam would work. Once you get it all figured out. Market it the Bridge Spy or something cleaver. If you used a wide angle cam. it might also be a good way to see whats on the other side of the genoa

I always get spooked when I pass under a bridge, even when the chart says I have 5' or more... I still think I can see my mast about to hit right before I get to the edge. It is a clench moment for me every time even bridges I've passed under before.

 

[shemandr]

And of course there are the bridges which are arched. So maybe you can pass in the middle easily, but the container ship there is a disincentive. How far to the side can you go? A real time, device which can tell you the actual clearance from a a 1/4 mile, would be worth something. Especially if you are going under a lot of bridges, like on the ICW, and don't want sphincter issues.

 

[rgranger]

@DayDreamer41

That is a great video. I've watched it many times. Gets you thinking at least. I suspect for a lot of us, just having a crew member hang off the shrouds might be enough to pass under a close call bridge where you don't want to lose your windex or such.

 

[DayDreamer41]

@rgranger, talk about doing the math to establish required clearance, there was some serious gusinta's to figure out how much weight to hang at what angle to achieve a heal angle to reduce clearance required. I just can't imagine attempting that for the first time.

 

[Allan12210]

Every boat should have a slide ruler. LOL

Good point all around. The key is good seamanship. But like I said, it is just another mean to help find the end.

5° was an arbitray number to help keep the final result down. But at 1000' a few degrees may change the results but probably not by much.

Two things:
1) I keep my circular slide rule in the nav table at all times. You never know when there will be an EMP these days.


2) Use a sextant or other angle measuring device set to a predetermined angle (say 20 deg) from horizontal while moving at a constant speed. When the bottom of the bridge is exactly in line with that sight, start your stopwatch. Reset the angle to double the first angle (now 40 degrees). As soon as the boat has moved enough to double the angle stop the clock. Using the formula Distance = Speed x Time you will know how far you traveled. That is the exact same distance from you to the bottom of the bridge (Isosceles triangle with 2 equal sides). That distance is also the Hypotenuse of the Right triangle formed with the vertical distance from the bridge to the water and the base back to you. Solve for the height with Sin (your larger angle) = Vertical height/distance traveled. Add the distance of your eye above the waterline.