Understanding NOAA bridge vertical height

[alanr77]

Just wanted to bounce an understanding off others to see if I am correct.

If the local NOAA chart shows a bridge with 35' vertical clearance and 8' of water, does this mean that;

At high tide, there is 35' vertical clearance. We have a 6' tide here in Tybee Island, Ga so at low tide the bridge would have a 41' clearance.

At high tide, there should be 14' of water under the bridge and at low tide 8'.

Is this information true?

The reason I'm asking is that now I have a boat with a 29' height. The C-27 I am looking at has a 39' height. Thats cutting it really close but I don't want to give up the wonderful slip I have in the marina.

[puddinlegs]

Chart depths are mean lower low water (MLLW), which is a bit lower than mean low water (MLW).

Bridge heights, etc... from Bowditch:

335. Heights
The shoreline shown on charts is generally mean high
water. A light’s height is usually reckoned from mean sea
level. The heights of overhanging obstructions (bridges,
power cables, etc.) are usually reckoned from mean high
water. A high water reference gives the mariner the minimum
clearance expected. Since heights are usually reckoned from high water
and depths from some form of low water, the reference levels
are seldom the same. Except where the range of tide is
very large, this is of little practical significance.

http://www.irbs.com/bowditch/pdf/chapt03.pdf

[Sabreman]

alanr77 - You and puddin are correct. Keep in mind if the mast has an antenna on it, then it's probably going to hit each and every time that you go under the bridge. If you miss the tide even by a little, you will have a serious problem.

IMO, it's too close to live with on a daily basis. Life is stressful enough. Masts should only come down during storms, not motoring to your slip.

[RichH]

If you think that is true .. DONT GO THROUGH FLORIDA!!!!!

The Florida DOT (who controls the bridge depth/clearance 'benchmarks') to avoid FURTHER litigation has in many places, by court order, has raised the clearance benchmarks so indicating that many of the ICW (actual) 65' clearance bridges are now posted at ~60-62 ft clearance at MHW !!!!!!! .... and both the USCG & Army Core of Engineers apparently dont give a damn.

[SVAuspicious]

Originally Posted by RichH

If you think that is true .. DONT GO THROUGH FLORIDA!!!!!

The Florida DOT (who controls the bridge depth/clearance 'benchmarks') to avoid FURTHER litigation has in many places, by court order, has raised the clearance benchmarks so indicating that many of the ICW (actual) 65' clearance bridges are now posted at ~60-62 ft clearance at MHW !!!!!!! .... and both the USCG & Army Core of Engineers apparently dont give a damn.

It's stupid, but what they have done is mark the boards measured from the clearance lights that hang down 3 - 5 feet below the lowest hard point. Apparently some litigants are too stupid to steer away from the light.

[RichH]

I thought that since the 'contract' between the states and the USACOE stipulated 65ft. MHW clearance by specification ... and that those bridges apparently are not to those contract specs., as clearly evidenced by the actual published FLDOT 'clearance boards'.... that those bridges should ALL BE RIPPED OUT and TOTALLY reconstructed to the contract 65ft. MHW clearance .... the state of FL paying ALL the reconstruction costs as its clearly their (judicial) fault by their own FLDOT posted evidence.

.... would be a stunning opportunity for a very messy and costly to defend federal class action suit.

[lshick]

We came through the FL ICW last month and saw no evidence of the reported adjusting of the clearance boards. 65' meant 65'.

[KeelHaulin]

Usually the clearance is reported at MHHW (Mean Higher High Water); not MLLW. So it's a minimum clearance; not a maximum.

You also need to consider the height of the deck; and the height of any antenna or masthead instrument. If 39' is the "I" of the mast; you also need to add the distance from the waterline to the base of the mast on the deck; which might add another 6 feet. Give 3' at the top for your antenna and you are up to a 48' minimum bridge clearance...

OK; looked up the I of a C-27; it's 34 or 36' depending on if you have a "tall rig". I would add 10' to that for a minimum bridge clearance height; which would then be 44 or 46'.

Remember; extreme low tides usually only occur once every 24 hours; and they are not always at an extreme low.

[puddinlegs]

Originally Posted by KeelHaulin

Usually the clearance is reported at MHHW (Mean Higher High Water); not MLLW. So it's a minimum clearance; not a maximum.

You also need to consider the height of the deck; and the height of any antenna or masthead instrument. If 39' is the "I" of the mast; you also need to add the distance from the waterline to the base of the mast on the deck; which might add another 6 feet. Give 3' at the top for your antenna and you are up to a 48' minimum bridge clearance...

OK; looked up the I of a C-27; it's 34 or 36' depending on if you have a "tall rig". I would add 10' to that for a minimum bridge clearance height; which would then be 44 or 46'.

Remember; extreme low tides usually only occur once every 24 hours; and they are not always at an extreme low.

Exactly. I figured that Bowditch made it perfectly clear that bridges where measured MHHW.