Mast Height, Bridge Clearance, Tides.

[bastonjock]

i had to laugh at that one DWT

gave me a real chuckle

[nodee]

Actually, whiile I haven't been down the ICW further south than central VA, I was thinking that the tidal change could affect bridge clearance in that it would seem possible that in waiting for a low enough tide to clear the mast on a marginal bridge height, one could run out of depth to traverse under the bridge. Don't know if any of the southern bridge channels are that shallow but it does seem like a possible scenario. Sort of reminds me of the old joke about the long eared mule that wouldn't clear the barn door.

[Tropic Cat]

Originally Posted by btrayfors

Rick,

1. Silting and dredging has nothing to do with bridge height clearance.
2. If the ICW water level drops, that would increase the bridge clearance, not decrease it.

Bill, yer right about water level, I was 1/2 asleep when I wrote that, my apologies as I reversed it. Winter months are fine, summer is not. I have photos taken of the water marks at the base of some of the bridges showing the decreased depth.

Though I think dredging has a lot to with things. Water level, is water level, how much clearance there is under the water level, is a function of dredging. When the Feds created the ICW they dredged it's entire length.

[Hud3]

Originally Posted by DWT

So is the Mean High Water (MHW) and highest astronomical tide (HAT) the same thing?

DWT,

It may be done differently in other parts of the world, and/or different terms may be used for tidal levels used, but here's the U.S. convention, according to NOAA's Office of Coast Survey:

Vertical bridge clearances on charts are referenced to Mean High Water in tidal areas. Therefore if the existing height of tide is below MHW, there will be greater clearance. If the existing height of tide is greater than MHW, there will be less clearance. The USCG provides bridge clearance information to NOAA.



Source: Charted Bridge Clearances

[muskoka]

Originally Posted by DWT

So is the Mean High Water (MHW) and highest astronomical tide (HAT) the same thing?

Admiralty charts use highest astronomical tide (HAT) which is the theoretical highest tide possible under normal weather conditions. Mean High Water Springs (MHWS) is different - it is the average of all highs over a given period of time.

In any event, the chart will describe where the overhead clearances are measured from - I think US charts use MHWS as the reference for overhead clearance whilst Admiralty charts use HAT.

Here's a quick reference: Definitions of tidal levels and other parameters

Cheers

[keger]

Originally Posted by DWT

So is the Mean High Water (MHW) and highest astronomical tide (HAT) the same thing?

They are not the same. MHW is what NOAA uses. HAT is what Admiralty charts use (nice and conservative). Just check your chart datum. It will be stated either way in the fine print somewhere.

[bastonjock]

one of the bridges that i occasionally hop under has a tidal drop of 7.6 meters

Before the great loss of life in 1953 on the East Coast of the Uk,Admiralty charts used mhws as their datum.We get surge tides over here.

Ive often thought that a small lazer mounted on top of the mast would be of great benefit if you are doing a lot of bridge hopping,the margine for error on a 59ft mast going under a 60ft bridge on a tidal river/estuary

[svcattales]

Originally Posted by DWT

Mast Height, Bridge Clearance, Tides.
We are looking at a 45 to 47 foot catamarans for our next boat. Mast height with some of these boats are too high for the ICW’s and I’m wanting to do this route plus where we are at in the Florida Keys we have a 65’ bridge to get under to get to the ocean side. One of the boat we were looking at was 64.5’ high. So my questions are…..
When they say the clearance for the bridge is 65’ is that at low tide?
Could we at 64.5’ cruise the ICW?
Thanks!

DWT, We went thru the same thought process before buying our cat. Our decision to switch from mono to cat occurred in Chesapeake Bay so we paid careful attention to all bridge heights between Norfolk-Fl Keys-Tampa Bay as we headed south then north in our monohull. Although the bridges are listed as 65' on the charts, we found a few that were less than 65 when we looked at height scales at the base of the bridges. I recall one that was less than 63'. Unfortunately, I don't have the log handy from my mono to give you more details, but the experience did point us toward a mast height of about 60'. Even at 60' you can easily add another one to three feet with wind indicators and antennas.

I don't think we passed under these "low bridges" at unusually high tides or that would have stuck in the memory banks. You can probably cruise the ICW with 64.5' but you may have to wait for the tides to pass the problem bridges. That would be a "royal pain" for me because anchorages are few & far between in some areas and playing the tides could make for very short or very long days in the "ditch."

[DWT]

Originally Posted by svcattales

DWT, We went thru the same thought process before buying our cat. Our decision to switch from mono to cat occurred in Chesapeake Bay so we paid careful attention to all bridge heights between Norfolk-Fl Keys-Tampa Bay as we headed south then north in our monohull. Although the bridges are listed as 65' on the charts, we found a few that were less than 65 when we looked at height scales at the base of the bridges. I recall one that was less than 63'. Unfortunately, I don't have the log handy from my mono to give you more details, but the experience did point us toward a mast height of about 60'. Even at 60' you can easily add another one to three feet with wind indicators and antennas.

I don't think we passed under these "low bridges" at unusually high tides or that would have stuck in the memory banks. You can probably cruise the ICW with 64.5' but you may have to wait for the tides to pass the problem bridges. That would be a "royal pain" for me because anchorages are few & far between in some areas and playing the tides could make for very short or very long days in the "ditch."

Greg,

It is those bridges that are 63' that have me worried. And waiting for a low tide might be doable on one or two but would not want to do it all of the time.

Has anyone else come across a bridge not 65'?

[camaraderie]

The Fairfield Bridge at mm114 in NC is listed at 65' but widely reported at 64'. I've been under twice with 62'+antenna and never scraped....though tides are wind driven and it would not be totally unusual to get less clearance if the wind has been blowing in the right directions for a couple of days. This is perhaps where the rumors come from.The good news is that you can take the alternate ICW inside the outerbanks past Manteo and through the Pamlico to the Nuese River to avoid this bridge entirely and actually have a better trip and do some sailing.
I have been through a number of 65 ft bridges showing 63 ft. on their boards with no problem and I believe many of the boards are simply inaccurate. No problems on ANY bridges all the way to the Julia Tuttle and have been through most several times.
I think you can count on 65 feet UNLESS there are Spring Tides or high water due to flooding or winds.

[emcmia]

As I rarely sail north of Miami without going offshore, I can report that the Julian Tuttle (I-195) bridge is only 55 ft. That being said, even with a masthead height of 67' (plus nav lights and antennas), I typically have no problems getting under the those bridges advertising 65' clearance. The trick is to identify the highest point (which is not necessarily in the channel center) and to go through VERY slowly. I typically furl all sails and motor through while steering only with throttles and gears. Lastly - when clearance is really critical, tides, wind and wakes should not be ignored.

[category4jay]

Originally Posted by keger

They are not the same. MHW is what NOAA uses. HAT is what Admiralty charts use (nice and conservative). Just check your chart datum. It will be stated either way in the fine print somewhere.

Can someone tell me why in the world I would ever use a high tide chart based on Mean High Water? Or conversely a depth chart showing depths calculated from Mean Low Water? I would think that charts should be based on the worst case scenario given the situation - both height to the bottom of a bridge and depth to the bottom of the sea/river? I assume that would be Admiralty charts which, I assume, reflect depths and heights using High Actual Tide and Low Actual Tide data. Am I out to lunch on this or what?

[Tingum]

Bridges are also rated at low steel, meaning the lowest part you could manage to hit. The rise of the bridges arch adds some height to what is on the chart and measure board.

[Cheechako]

I went down much of the waterway from Wrightsville Beach, NC to Lauderdale with about that height.. nearly 65 ft. my 3 ft VHF antenna banged on some bridge girders.
Some bridges have enough current under them that once you are committed, you aren't going to change....

[Robin3]

Originally Posted by DWT

Greg,

It is those bridges that are 63' that have me worried. And waiting for a low tide might be doable on one or two but would not want to do it all of the time.

Has anyone else come across a bridge not 65'?

We live by and overlooking Seabreeze bridge in Daytona and often walk over it for exercise. The clearance gauge routinely shows 63 ft even though it is supposed to have 65ft. I questioned it locally and was told there IS 65ft in the centre of the span, plus that when built it was incorrectly measured when the clearance gauge was installed . Fortunately our air draught is only 48ft so we have a little bit of wriggle room for our masthead aerials!Our last boat we sold in the UK had 7ft draught and needed 65ft air draught, not very ditch friendly at all.
Maybe we should drop a measured lead line over the centre when we walk it one day, as a service to fellow mariners.