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Old 03-01-2016, 12:30   #1
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Calculating ACTUAL bridge clearance

Hello,

I'm trying to understand how I can calculate actual bridge clearance for a specific bridge, in this case the Gil Hodges Bridge in Jamaica Bay, Lower New York Harbor. I feel like it shouldn't be this complicated and maybe I'm missing something. The charts list the bridge clearance as 55ft above MHW, but all I know from tidal predictions is the height of the water level above the baseline datum (MLLW).

The piece I'm missing is how to calculate how high MHW is above the baseline datum. In my Chapman Piloting book it says to add half the value of the mean tide range to the mean tide level. This makes perfect sense, but I cannot find these mean tide ranges or mean tide levels anywhere. The one shown in the book is called Table 2: Tidal Differences and Other Constants, but it does seem to be readily available except in sample form.

On the other hand, datums are published online by the NOAA here, including the MHW for some locations nearby, but something seems wrong. The Brooklyn Bridge and the Battery are only a tenth of a mile apart but they are showing MHWs of 1.79 feet above the station datum for the bridge and 8.02 feet above datum for the battery. A couple miles south in Great Kills Harbor it says the MHW is 25.64 feet. These drastic differences don't seem possible.

Can anyone point me to a more reliable way to calculate Mean High Water for a specific location? Or is there a simpler way of calculating actual bridge clearance from the predicted height of tide?

Thanks!

Jack
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Old 03-01-2016, 15:35   #2
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Re: Calculating ACTUAL bridge clearance

Your link is broken. It should be
Station Selection - NOAA Tides & Currents

Those figures are correct. Ignore the absolute height above datum, you just need to look at the difference between MHW and MLLW.

You are confusing chart datum MLLW with station datum for the various stations, which is frequently very different (The pages are labelled "Elevations on Station Datum"

From https://tidesandcurrents.noaa.gov/datum_options.html

"A fixed base elevation at a tide station to which all water level measurements are referred. The datum is unique to each station and is established at a lower elevation than the water is ever expected to reach. It is referenced to the primary bench mark at the station and is held constant regardless of changes to the water level gauge or tide staff. The datum of tabulation is most often at the zero of the first tide staff installed."

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Old 03-01-2016, 16:24   #3
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Re: Calculating ACTUAL bridge clearance

Thanks for the help.

Let's see if my thinking about this is correct. I found a complete version of the tide ranges and was able to look up the location of the bridge. The mean tide level is 2.7', the mean range is 5.0', and the spring range is 6.0'.

So if I add half the mean range of 2.5' to the mean tide level of 2.7', I come up with a MHW of 5.2'. The listed clearance of the bridge is 55' above MHW. Today's higher high tide was predicted at 2:13am at 4.4' (above the baseline datum, which I presume is equal to MLLW). If I also had a mast height of exactly 55' and I tried to pass under the bridge at this time, I would in theory have an extra .8' of clearance, because the high tide was not as high as MHW. Do I have this right?

The reason I ask... I'm looking for a new boat and I have a place I'd like to keep it, but some of my favorite boat options may be cutting it too close on this bridge. I'm not sure how comfortable I would be with a 54' mast on a 55' bridge height at high tide. It's a drawbridge but the hours are not good, so I guess I could either be looking for a slightly smaller boat or a slightly different marina than I had in mind.
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Old 03-01-2016, 16:29   #4
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Re: Calculating ACTUAL bridge clearance

I'm curious as to why you're so concerned about calculating this? Especially when your boat has an air draft of 38' or so. Or are you trying to figure it out exactly, for another boat?
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Old 03-01-2016, 16:59   #5
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Re: Calculating ACTUAL bridge clearance

Your calculations are correct. Your use of the words "in theory" is also unfortunately correct.

In addition to a different boat or a different marina, You could also be looking at timing it so that you pass under the bridge at low tide. There are many places when people base their arrival/departure on tides - frequently because they need high tides to get in or out, it's just that you have the opposite problem.
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Old 04-01-2016, 09:06   #6
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Re: Calculating ACTUAL bridge clearance

Two more things for you to consider

Firstly builders data doesn't usually include the hardware on top of he mast - VHF antenna, wind vane head or lights.

Secondly the tidal height varies by about 1cm/mb difference from 1015 - eg if the pressure is 1000mb the tide will be around 15cms (6") higher. 1030 6" lower. These are UK figures and I don't know what the US equivalent is.
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Old 04-01-2016, 09:10   #7
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Re: Calculating ACTUAL bridge clearance

Quote:
Originally Posted by OrangeCrush View Post
... The reason I ask... I'm looking for a new boat and I have a place I'd like to keep it, but some of my favorite boat options may be cutting it too close on this bridge. I'm not sure how comfortable I would be with a 54' mast on a 55' bridge height at high tide...
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Originally Posted by UNCIVILIZED View Post
I'm curious as to why you're so concerned about calculating this? Especially when your boat has an air draft of 38' or so. Or are you trying to figure it out exactly, for another boat?
Indeed, he's looking at a new boat.
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Old 04-01-2016, 09:15   #8
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Re: Calculating ACTUAL bridge clearance

Calculate overhead clearances from HAT (Highest Astronomical Tide) as this is the theoretical highest possible tide therefore the lowest possible clearance. This should be given on the chart by the bridge/power line/overhead obstruction.

Don't use MHW or MLW for anything as this is not the highest or lowest tide but the average HW or LW. LAT for depths and HAT for clearances always.

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Old 04-01-2016, 09:40   #9
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Re: Calculating ACTUAL bridge clearance

Assume as on the ICW the Bridge has a water level (Air Draft) sign on the pilings. I say that because I have 64' mast and ICW bridges are SUPPOSED to have 65' clearance. Well they don't!!!!!!!!!
I just knocked off my Tri color passing under a bridge which had signage that clearly said I was good to go by an easy 6". I got under ALL of the bridge without even scrapping my VHF which I have done and can hear. Just as I got to the far side I heard a clunk and my Tri was left hanging by its wiring and a rusty 6' bolt with a speck of concrete landed on my deck. Go figure. By the way I had JUST passed under that same bridge going to opposite way 1 hour before.!!!!!!!!!!!!!! Russ
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Old 04-01-2016, 09:46   #10
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Re: Calculating ACTUAL bridge clearance

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... I just knocked off my Tri color passing under a bridge which had signage that clearly said I was good to go by an easy 6".
Are you certain that you passed under the apex?
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Old 04-01-2016, 09:50   #11
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Re: Calculating ACTUAL bridge clearance

Why all the calculatin'?



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Old 04-01-2016, 09:51   #12
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Re: Calculating ACTUAL bridge clearance

Keiron,

Thanks but I believe you are confusing the European system with the US way of doing things, which as usual is perhaps a bit less logical.

HAT would make a lot of sense as it is a more conservative reference point to use, but in the US they generally use MHW on for charted bridge clearance figures. This means that even though the clearance is listed at 55ft for the bridge I'm talking about, on a spring tide there will actually be less than 55ft of clearance. Depths are calculated from MLLW, which is the average of the lowest of the two low tides in a day, but again it is not the lowest tide possible. This means that you could run aground with a 5 foot draft where the chart says you have 5 feet if it was around the time of the full moon.

I am not trying to defend this system but I did read someone make the argument that weather alone could make the tide higher or lower than it should be, so captains should be aware of the need to adjust their calculations anyway, and that using the MHW and MLLW is more accurate even if it's less conservative. Then there is the question of rivers, which are even more variable depending on seasonal rainfall.

Let's hope those tide boards that show clearance at the base of the bridge are accurate at least...

Jack
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Old 04-01-2016, 10:13   #13
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Re: Calculating ACTUAL bridge clearance

So can I ask everyone's opinion on the following question?

Would you consider buying your "dream boat" if it had a 55' mast and you needed to pass under this 55' bridge every time you went out sailing? Assume you have confirmed the height of the mast down to the inch and you have confirmed the accuracy of the tide boards at the base of the bridge. 95% of the time the bridge clearance will be fine. If it is one of those infrequent moments with less than the posted clearance you can time it so that you pass at a lower tide or give the drawbridge operator 8 hours of notice so they can lift the bridge if it's a weekend.

I'm a bit tortured by this because I'm realizing some of the boats on my short list will not fit under the bridge at all times.

Corbin 39 (56 feet of air draft, probably gotta rule this one out)
Tayana 37 (55 feet of air draft, hate to say it but prob a no-go as well)
Valiant Esprit (54 feet of air draft)
Alajuela 38 (54 feet of air draft)
Baba 35 (53 feet of air draft)

These are among my favorite options and I hate ruling any of them out, but I'm also not sure it's worth the stress of having to think about clearing the bridge every time I go out for a day sail. Obviously it will not be much of a factor on longer cruises, but I will no doubt be doing both...

Any opinions?
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Old 04-01-2016, 10:19   #14
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Re: Calculating ACTUAL bridge clearance

Neat....but shall be interesting to see how I can get this work on my cat.


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Why all the calculatin'?



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Old 04-01-2016, 12:20   #15
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Re: Calculating ACTUAL bridge clearance

Quote:
Originally Posted by OrangeCrush View Post
So can I ask everyone's opinion on the following question?

Would you consider buying your "dream boat" if it had a 55' mast and you needed to pass under this 55' bridge every time you went out sailing? Assume you have confirmed the height of the mast down to the inch and you have confirmed the accuracy of the tide boards at the base of the bridge. 95% of the time the bridge clearance will be fine. If it is one of those infrequent moments with less than the posted clearance you can time it so that you pass at a lower tide or give the drawbridge operator 8 hours of notice so they can lift the bridge if it's a weekend.

I'm a bit tortured by this because I'm realizing some of the boats on my short list will not fit under the bridge at all times.

Corbin 39 (56 feet of air draft, probably gotta rule this one out)
Tayana 37 (55 feet of air draft, hate to say it but prob a no-go as well)
Valiant Esprit (54 feet of air draft)
Alajuela 38 (54 feet of air draft)
Baba 35 (53 feet of air draft)

These are among my favorite options and I hate ruling any of them out, but I'm also not sure it's worth the stress of having to think about clearing the bridge every time I go out for a day sail. Obviously it will not be much of a factor on longer cruises, but I will no doubt be doing both...

Any opinions?
You would be well advised not to cut it so close, especially given all the variables. My air draft is just over 65' (including instruments, lights, antennas) and although I routinely pass (very slowly and carefully) through ICW bridges with reported 65' clearance, I recently lost all the masthead accoutrements when a powerboat came by with a 2ft wake. Expensive!
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