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Old 20-10-2020, 07:49   #1
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hitting bridges.

An idea , what do you think? A laser lite on top of your mast . If it shines on the bridge, stop. Knots
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Old 20-10-2020, 13:51   #2
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Re: hitting bridges.

How about just checking your charts for the air draft and doing the arithmetic?

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Old 20-10-2020, 13:54   #3
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Re: hitting bridges.

Air draft is not an exact science. Charts work fine when there’s plenty of room for error, sometimes you don’t have that luxury.
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Old 20-10-2020, 14:14   #4
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Re: hitting bridges.

Might work in a pond, but even the slightest pitch caused by small waves would give a very erratic reading.

Why do I think we're being trolled?
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Old 20-10-2020, 14:21   #5
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Re: hitting bridges.

sounds like a good idea to me. As someone who’s “next boat” list contains two with masts right around 64’10”, i’ve yet to figure out what it will take for me to be comfortable with those 65” bridges.
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Old 20-10-2020, 14:21   #6
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Re: hitting bridges.

If it were a laser rangefinder it might be useful. Would also detect the natural pitching oscillations, and as a bonus show range to collision.
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Old 20-10-2020, 16:11   #7
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Re: hitting bridges.

Go very slow. If the bridge height markers show anything under 67 feet, we slow to under 2 knots.
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Old 26-10-2020, 20:08   #8
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Re: hitting bridges.

I've thought about this a lot because I'm on the Mississippi. Bridges are generally between 60-63' above normal pool. Pool levels vary seasonally and with rainfall upstream. There are gauging stations. The gauging stations do not use standard datums -- each one has to be interpreted individually. In conditions of high flow the river level is not flat between gauging stations so you have to interpolate, but doing so is not an exact science. Some bridges have bridge boards, and sometimes they're legible.


Sometimes powerlines appear to be below the bridges.


There exist uncharted overhead obstructions.


Ultimately, doing the math doesn't work. Right now we're 41' above the water, so it doesn't matter much, but the sort of boat we would like to have in a few years will be high enough that we have to get this right.


With today's technology I believe the best we can do is a camera at the top of the mast and a laser that projects an "X" that is aimed straight forward. Unlike a laser "dot" if you can see part of the upper half of the X you can confirm that the laser is working and that the clearance is sufficient. I haven't experimented and don't know what the practical range is like but even 20' would be better than nothing since in most cases the current isn't that bad and you can hold the boat against it and creep along.


On the other hand a camera along may be enough using the horizon as a reference (not always visible inland) or by whether you can see the underside of the bridge (though some aren't level and could fool you). With careful adjustment the centerline of the image could be set to be level with the horizon and then that could be used. Someday I'll experiment.
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Old 26-10-2020, 20:29   #9
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Re: hitting bridges.

Quote:
Originally Posted by Jammer View Post
With today's technology I believe the best we can do is a camera at the top of the mast and a laser that projects an "X" that is aimed straight forward. Unlike a laser "dot" if you can see part of the upper half of the X you can confirm that the laser is working and that the clearance is sufficient. I haven't experimented and don't know what the practical range is like but even 20' would be better than nothing since in most cases the current isn't that bad and you can hold the boat against it and creep along.


On the other hand a camera along may be enough using the horizon as a reference (not always visible inland) or by whether you can see the underside of the bridge (though some aren't level and could fool you). With careful adjustment the centerline of the image could be set to be level with the horizon and then that could be used. Someday I'll experiment.

Define "straight forward" on a boat that is being affected by water movement / wind / waves / etc and weight distribution (tankage, crew positions etc).



How often and for how long would your projector be horizontal? How would you know when it was?
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Old 26-10-2020, 20:57   #10
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Re: hitting bridges.

Quote:
Originally Posted by Jammer View Post
With today's technology I believe the best we can do is a camera at the top of the mast and a laser that projects an "X" that is aimed straight forward.
A camera setup does seem handy on a number of levels. Air draft, having a better vantage point for spotting obstacles in the water, checking blind spots (maybe?). Would something like a FLIR M332 work?
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Old 26-10-2020, 23:05   #11
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Re: hitting bridges.

Quote:
Originally Posted by StuM View Post
Define "straight forward" on a boat that is being affected by water movement / wind / waves / etc and weight distribution (tankage, crew positions etc).

How often and for how long would your projector be horizontal? How would you know when it was?
It seems reasonable that the oscillations would be somewhat regular, if the boat were otherwise in trim.

There are laser distance measuring devices on the market that are capable of 100+ feet. If one of these was mounted horizontal at the top of the mast, it would register range to the bridge as the bow of the boat pitched up, and should register infinity as the bow of the boat pitched down.

As for interpreting the output, if the device registers infinity for more than 50% of the cycle, or a finite number for less than 50% of the cycle, one could presume the mast will clear. And a computer (or someone quick at math) could estimate by how much.
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Old 27-10-2020, 14:22   #12
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Re: hitting bridges.

Quote:
Originally Posted by StuM View Post
Define "straight forward" on a boat that is being affected by water movement / wind / waves / etc and weight distribution (tankage, crew positions etc). ... How often and for how long would your projector be horizontal? How would you know when it was?

The perfect is the enemy of the good


If conditions are calm and you can make a close approach under control you can get under the bridge safely with very little clearance.


If conditions are poor you may have to wait for better conditions
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Old 27-10-2020, 15:30   #13
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Re: hitting bridges.

Agree with Jammer. If the boat is pitching and rolling, and so forth, maybe that is not he time to cut it close. But, I also think that you could use gimbals, or even a level, such that when the level indicated correct trim, you could check the laser. Somehow, I don't think this is a big technical problem. The problem may be low demand.
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Old 27-10-2020, 15:33   #14
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Re: hitting bridges.

Quote:
Originally Posted by Knots View Post
An idea , what do you think? A laser lite on top of your mast . If it shines on the bridge, stop. Knots
So you are in the fender boards under the bridge and .............. oh no too short

What are you going to do now?
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Old 27-10-2020, 15:57   #15
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Re: hitting bridges.

Not a bad idea if it's bright enough. Many bridges, once you get close, the current may drag you through... dont ask how I know.... OK, I'll tell you anyway.
Headed to FL from the Chesapeake, I sailed outside and got pasted south of Hatteras spending the worst night I ever had offshore.
SO next day, tail between our legs, we went in to the waterway for a bit/ Our mast was 65 ft. The bridges are 65 ft more or less. Our first bridge was the one at Crab Cut. I was approaching slowly.... suddenly realized that in the narrower cut near the bridge the current was running like 5 knots! I went into reverse and all that did was turn the boat sideways due to current. So under the bridge we went, sideways, VHF antenna rattling on the girders... Probably safer abeam anyway.... if the mast hit the boat would heel...
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