How to figure Mean High Water on chart?

[arsenelupiga]

that is al mumbo jumbo method relying on charts that we know are inherently inaccurate.

what you do is buy yourself binocular with degrees engraved. Go japanese, they are the best.

Stop before the bridge.

Measure on your map exact distance of the boat from the bridge.

Use binoculars and count number of degrees from surface to bottom of the bridge - usable height that you need to go thru.

Use high school math to calculate the available height from distance to the bridge and angle taken with binocular.

For confidence, do dry run or two before.

Works like charm every time without complications.

[SV__Grace]

Originally Posted by boatman61

What I am saying is it matters not what the depth is you have 75ft air draught at MHW of 10.96.
If you need more, go through before or after HW but your air draught will depend on the tidal range not how deep the channel is.. unless the chart states your boats draught will run you aground at LW.
If the difference is 9ft between H and L thats all you have to work with.. 75ft to 84ft. Simple.

First, the 10.96 figure is not the actual depth. As illustrated in the screenshot above it is the number relative to MLLW (also the zero tide level).

When vertical clearance is close, and a 65' vertical boat going under a 75' bridge is pretty close, in a tidal range of 14 feet (-2 to +12 is typical for this area) it seems to me that depth matters.

I'd love a quick, easy way to figure if I have enough vertical clearance in tight situations.

Here's what I have now, with the help of the info in this post-

Step 1: Identify vertical clearance listed on chart, in this example, 75'

Step 2: Look up the MHW differential for that location from NOAA, in this example 10.96'

Step 3: Find the depth on the chart under the bridge (MLLW), in this example 24'

Step 4: Add the MHW figure from NOAA to the MLLW number on the chart to get the highest anticipated water level (depth) for that location. In this example 35'. Now I know that the 75' vertical clearance number is at 35 feet of depth.

Step 5: Check the current tide level for actual anticipated water depth (or when trip planning, the tide level at the anticipated time of passage), let's say mid-tide, +5 feet, 29 feet of depth.

Step 6: Do the math (I hate math ), in this example I now know I have 81' of clearance (75' plus 6' below MHW) and am good to go.

This seems pretty cumbersome, but it works, which is better than I had before.

Got an easier solution that hopefully uses less math?

[JPA Cate]

The arithmetic will get easier with practice. Not being sarcastic, but if you do it routinely, you become more used to it and it comes faster.

In our experience, usually the air draft for electrical transmission lines is taken from the bottom of the sag, sometimes you can sneak by at the higher end of the wire, but watch the depths! High temperatures makes for greater sag.

Ann

[SV__Grace]

Originally Posted by SV__Grace

Got an easier solution that hopefully uses less math?

Answering my own question, without doing math or looking up the MHW figure from NOAA, I suppose I could look at the 75' MHW clearance number on the chart and be (mostly) confident that my 65' vertical boat would be fine unless it's a super, super, super high tide.

Am I on the right track?

[SV__Grace]

Originally Posted by arsenelupiga

that is al mumbo jumbo method relying on charts that we know are inherently inaccurate.

what you do is buy yourself binocular with degrees engraved. Go japanese, they are the best.

Stop before the bridge.

Measure on your map exact distance of the boat from the bridge.

Use binoculars and count number of degrees from surface to bottom of the bridge - usable height that you need to go thru.

Use high school math to calculate the available height from distance to the bridge and angle taken with binocular.

For confidence, do dry run or two before.

Works like charm every time without complications.

AHHHRRRGGG, MATH!!

This does seem like a great strategy and I do have binoculars with range finding feature, but given my aversion to math I haven't figured out how to use it.

I'm willing to learn if the formula is easy enough, do you know what it is?

[boatman61]

Originally Posted by SV__Grace

First, the 10.96 figure is not the actual depth. As illustrated in the screenshot above it is the number relative to MLLW (also the zero tide level).

When vertical clearance is close, and a 65' vertical boat going under a 75' bridge is pretty close, in a tidal range of 14 feet (-2 to +12 is typical for this area) it seems to me that depth matters.

I'd love a quick, easy way to figure if I have enough vertical clearance in tight situations.

Here's what I have now, with the help of the info in this post-

Step 1: Identify vertical clearance listed on chart, in this example, 75'

Step 2: Look up the MHW differential for that location from NOAA, in this example 10.96'

Step 3: Find the depth on the chart under the bridge (MLLW), in this example 24'

Step 4: Add the MHW figure from NOAA to the MLLW number on the chart to get the highest anticipated water level (depth) for that location. In this example 35'. Now I know that the 75' vertical clearance number is at 35 feet of depth.

Step 5: Check the current tide level for actual anticipated water depth (or when trip planning, the tide level at the anticipated time of passage), let's say mid-tide, +5 feet, 29 feet of depth.

Step 6: Do the math (I hate math ), in this example I now know I have 81' of clearance (75' plus 6' below MHW) and am good to go.

This seems pretty cumbersome, but it works, which is better than I had before.

Got an easier solution that hopefully uses less math?

I kinda figured it was not seeing as a depth of 35ft had been mentioned.
I get that it is the range of the two heights the lower being 0.0.
Figured if I therw in using it as water draught it might get clearer..
If you have 75ft at 10.96 MHW then at 9.96 you have 76ft and so on until you get to 0.0 MLW which gives you 85.96ft air draught for half an hour before it starts decreasing again.
No complex math needed.. Tide tables Yes.
Seemed to work for me on the ICW.

[boatman61]

Originally Posted by SV__Grace

Air Gap info is fantastic but not available everywhere, darn.

Most of us don't use paper charts anymore and the electronic charts don't have that info- double darn!

From what I see there are maps and bridge/lock lists for the ICW Atlantic and other places readily available online.. just download it.
Charts will give you the depths.

[requiem]

Originally Posted by SV__Grace

AHHHRRRGGG, MATH!!

This does seem like a great strategy and I do have binoculars with range finding feature, but given my aversion to math I haven't figured out how to use it.

I'm willing to learn if the formula is easy enough, do you know what it is?

A mil subtends a meter at a kilometer.

You are 800 m from the bridge; your binoculars tell you the air gap is 30 milliradians:

0.8 km x 30 mils = 24 m air draft

The other question of course is... how accurate is your distance to the bridge? If you're really 850 m away, you gain another 1.5 m of clearance. Probably safer to measure the angle before the range.

[Lodesman]

Originally Posted by SV__Grace

First, the 10.96 figure is not the actual depth. As illustrated in the screenshot above it is the number relative to MLLW (also the zero tide level).

When vertical clearance is close, and a 65' vertical boat going under a 75' bridge is pretty close, in a tidal range of 14 feet (-2 to +12 is typical for this area) it seems to me that depth matters.

I'd love a quick, easy way to figure if I have enough vertical clearance in tight situations.

Here's what I have now, with the help of the info in this post-

Step 1: Identify vertical clearance listed on chart, in this example, 75'

Step 2: Look up the MHW differential for that location from NOAA, in this example 10.96'

Step 3: Find the depth on the chart under the bridge (MLLW), in this example 24'

Step 4: Add the MHW figure from NOAA to the MLLW number on the chart to get the highest anticipated water level (depth) for that location. In this example 35'. Now I know that the 75' vertical clearance number is at 35 feet of depth.

Step 5: Check the current tide level for actual anticipated water depth (or when trip planning, the tide level at the anticipated time of passage), let's say mid-tide, +5 feet, 29 feet of depth.

Step 6: Do the math (I hate math ), in this example I now know I have 81' of clearance (75' plus 6' below MHW) and am good to go.

This seems pretty cumbersome, but it works, which is better than I had before.

Got an easier solution that hopefully uses less math?

Do you have access to tide data? If you get tides over 10' greater than MHW, then you might not have enough clearance, but otherwise I would think you'd be fairly safe. Take bridge clearance + MHW - actual HOT = vertical clearance.

[Dsanduril]

What the depthsounder says has nothing to do with whether or not you will clear the bridge - unless you are trying to use the depthsounder to estimate the height of the tide using the difference between chart soundings and what you measure. Shoaling (or dredging) will make no difference to available air draft, that is measured from a fixed sea level (MLW) and cares not what the depth of water is at any point (though your keel might).

It is pretty simple as Boatman says, you probably have a little more than 10 feet to play with. On a negative tide you might get a bit extra, on a spring high tide is when you really have to watch out. With a 10-foot mean tide you probably have 14-15 foot spring tides and those will put the available clearance at less than the charted value by several feet.

[SV__Grace]

Originally Posted by requiem

A mil subtends a meter at a kilometer. You are 800 m from the bridge; your binoculars tell you the air gap is 30 milliradians:
0.8 km x 30 mils = 24 m air draft

Thanks for this, but it's a bit over my head and I would need it translated into Imperial units for the U.S. to make sense to my math impaired brain.

Is "milliradians" the vertical number I see inside the binoculars?

Interpreting the formula you used above, is it Distance x the Vertical number I see in the binoculars?

If so, that would be easy, even for me.

Does is matter what unit of distance is used? (feet, miles, nm, etc)

COOL RESOURCE ALERT- the Google machine helped me find this cool online Binocular Range Calculator.

I changed the height to feet, the distance to nm, and the angle to degrees, which is what I assume I'm seeing inside my binoculars.

Then I entered 5 degrees and 1 nm, and presto, it gave me a height of 530.2 feet.

So it seems like the formula of Distance x Angle (number in binoculars) gives the height no matter what unit of measurement is used.

Am I correct about that?

To address the comments of questions of a few others:

- In my case I can accurately measure distance on my B&G Zeus as well as using radar.

-I use the charted depth and tide info, I don't rely on my depth sounder for going under bridges.

- Thanks to this thread I learned a new term "Air Gap" and figuring out MHW was the key to that. I'm grateful for the assistance of this community for helping me solve that problem!

[SV__Grace]

Originally Posted by SV__Grace

COOL RESOURCE ALERT- the Google machine helped me find this cool online Binocular Range Calculator.

Update- I added the above calculator to my iPhone home screen (like a bookmark) and used it to figure the mast height of a nearby sailboat. It's easy and cool to use, but requires internet connection.

Playing with it I figured out by trial and error that the reticle in my West Marine branded binoculars (with no users manual) uses Millirads and not degrees.

According to my radar, that boat is .14 nm away, the reticle height shows 70 (millirads), and entering those numbers and units into the calculator gives me a mast height of 59.55 feet, which makes sense for that boat.

Next time we go ashore I'll test using my own mast height, which I know to be 65 feet.

THIS I can do, enter two numbers and no math needed, that's what calculators are for!

[requiem]

The units must only be consistent, and it comes down to the definition of a radian. Consider a wide slice of apple pie where the length of the outside crust is the same as the other sides of the slice; the angle of the wedge is one radian (~57 degrees).

This property is why radians, and not degrees, are so useful for this; they are tied by definition to both the distance and the height.

Thus, the equation is really "X units (range) * Y radians = Z units (height)"

You can use feet, miles, whatever. But, a radian is pretty large, so milliradians are used instead (1/1000th of a radian). Here's an example how that works out, and notice how the range changes to balance the switch to mils:

"3,000 yards (range) * 4 milliradians = 12 yards (height)"

(Things get really ugly if you use degrees, or mix units like miles and feet. Only do that if you have an app or calculator. Kilometers and meters are ok, since the 1000x factor is built-in.)


The manual for your binoculars will tell you what the marks measure, likely they are spaced every 5 mil.

[Cadence]

Radio the bridge tender. The sky is the limit when I'm open. ;-)

[Donald Bryden]

First when approaching a bridge where there is possible conflict with air height, look for a bridge gauge. Some have them, unfortunately many do not. If no gauge and if you know the time and height of high water, you can use the Rule of 12s to proximate the sine curve for tide height to the closest hour. Other considerations affect tide height as well, and they have been highlighted in other replies.