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Bridge Clearance?

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JJM
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USA
170 Posts

Initially Posted - 08/13/2011 : 09:42:08

I would like to know if my standard rig '04 WK could get beneath a 34 1/2 foot bridge? I know that current specs say that for a WK that from W/L to masthead is 33.3 feet. But does that include the masthead light? I also have a windex and antennae up there.

Has anybody actually cleared a bridge of similar height? I don't want to try it if it is ill-advised.

Appreciate any feedback!

John
"Enchanted Becuma"
C250 '04 WK #746

Edited by - JJM on 08/13/2011 11:11:03

John Russell
Master Marine Consultant

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USA
3444 Posts

Response Posted - 08/13/2011 : 11:47:04

I wouldn't try it. How tall is the antenna? Mine's to long to expect it to bend and whip back o the other side. I know the windex won't bend. Remember the measurements on the charts are based on low chart datum. The actual depth of the water will vary from that based on a lot of variables. Lake Erie is currently 36"+ over chart datum which makes bridge clearances 36" lower.

Maybe somebody with local knowledge might have the answer about how high the bridge actually is at low tide, for example.

If you really need to get to the other side, check out some of the threads about lowering the mast. There's somebody here that has to do it while underway every time he goes sailing.

Edited by - on

TakeFive
Master Marine Consultant

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2272 Posts

Response Posted - 08/13/2011 : 12:55:58

I believe that NOAA's charts in tidal waters show bridge clearances at MHW or MHHW. It's depths that are shown at MLW or MLLW. The chart notes should say that explicitly, so you must check those to be sure. Depending on where his bridge is, that could buy him some add'l clearance if he goes at low tide.

Edited by - TakeFive on 08/13/2011 14:12:05

Stinkpotter
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Djibouti
9081 Posts

Response Posted - 08/13/2011 : 12:56:47

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by John Russell
 ...Remember the measurements on the charts are based on low chart datum.<hr height="1" noshade id="quote"></blockquote id="quote">On tidal waters, [url="http://www.nauticalcharts.noaa.gov/mcd/learnnc_bridge.html"]NOAA bridge clearances[/url] are from mean high water, not low. (Depths are from mean low.) But remember that spring tides (new or full moon) will swing higher (MHHW) and lower, and weather can cause abnormal levels as well. So if you're in a tidal area, you might be OK at low tide, but it doesn't sound good at high. In other words, you might be able to go out but not go home. If you're close enough that the difference between MHHW and MHW is significant, then you're too close--variances from MHHW will getcha--not just the antenna, but maybe the mast!

Also, in tidal areas, it's common for a bridge to have a "scale" on one side that shows the clearance from the current water level (not including boat wakes and such).

Edited by - Stinkpotter on 08/13/2011 14:24:59

John Russell
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USA
3444 Posts

Response Posted - 08/13/2011 : 14:48:19

My memory ain't what it used to be. I sit corrected. I still wouldn't do it. Too close.

Edited by - on

delliottg
Former Mainsheet C250 Tech Editor

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USA
4479 Posts

Response Posted - 08/13/2011 : 14:53:41

As Dave & Rick said, overhead clearances are measured from the high water mark, and depths from low. Since we're currently at a full moon and making a guess at Port Hueneme for tides, it looks like you've got about 6' swings to work with right now. Assuming it's somewhere close to my guess, there are some small negative tides where you should be able to get underneath your bridge at low tide. Can you check out the bridge in question before you go to look for the scale Dave mentioned? It'd give you a better idea of the actual clearance and give you more confidence to make the run through the bridge.

You can estimate your mast height with a 50' fiberglass measuring tape attached to a halyard and run to the top (don't forget a messenger line to bring it back down with) and measure to the base of the mast. Remember that the tape won't go all the way to the top of the mast, just close, so round all your measurements up. It's better to estimate too long than too short. Add the height of your anchor light from it's specs online, and add the height of your antenna to that (probably 3' if it's a whip antenna, and remember the antenna is probably mounted down the mast a little bit), and then add the height from the waterline to the tabernacle to get your overall clearance height. Remember that this is all estimates to get you a ballpark figure for your clearance. Obviously some of these measurements overlap, which simply adds to your overall measurement, making a conservative estimate that's deliberately too long.

Next time your mast is down you can get precise measurements for all of these.

Edited by - on

Stinkpotter
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Djibouti
9081 Posts

Response Posted - 08/13/2011 : 14:58:34

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by John Russell
 My memory ain't what it used to be...<hr height="1" noshade id="quote"></blockquote id="quote">I'm guessing they never taught you that stuff in the USAF--unless it was for flying under bridges...

(...like I do in MS Flight Simulator.)

Edited by - on

TakeFive
Master Marine Consultant

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2272 Posts

Response Posted - 08/13/2011 : 15:16:54

The easy way to remember is that NOAA gives the most pessimistic/conservative estimate for both depth and bridge clearance.

Edited by - on

JJM
Navigator

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USA
170 Posts

Response Posted - 08/13/2011 : 15:32:47

Thank you all for your replies.

Dave, come to think about it - there is a scale on the side of the bridge that should provide me with the clearance at any given time/tide. Now all I have to do is get a good number for the height of the mast with the masthead light, and antennae factored in...Then decide if I can get beneath it safely. Also - I am going to search for the person who has to lower his/her mast everytime out.

Edited by - on

Davy J
Master Marine Consultant

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USA
1511 Posts

Response Posted - 08/13/2011 : 17:10:25

<blockquote id="quote">quote:<hr height="1" noshade id="quote"> I am going to search for the person who has to lower his/her mast every time out.<hr height="1" noshade id="quote"></blockquote id="quote">
That would be me.

My bridge clearance is 11.9'. However, I am set up for this.

Edited by - on

frog0911
Master Marine Consultant

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USA
1349 Posts

Response Posted - 08/13/2011 : 18:26:23

I use 38 feet for my number. I have basically the same clearance problem so the bridge depth counter is my gage. I get the 38 feet from "I" = 29.0 plus 3 feet antenna plus almost 6 feet of freeboard.

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Stinkpotter
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Djibouti
9081 Posts

Response Posted - 08/13/2011 : 20:14:32

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by frog0911
 I use 38 feet for my number.... I get the 38 feet from "I" = 29.0 plus 3 feet antenna plus almost 6 feet of freeboard.<hr height="1" noshade id="quote"></blockquote id="quote">Probably conservative, since I don't think the freeboard plus cabin trunk of a C-250 is 6', (the cabin sole is below the waterline)... But who wants to count inches when moving a mast propelled by over two tons under a fixed bridge? Not me! I've estimated my "air draft" on Sarge at 9.5' to my anchor light just over my radar dome... When I go through the Mystic railroad bridge, which based on tides is from 7-10', I always, always ask for an opening.

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delliottg
Former Mainsheet C250 Tech Editor

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USA
4479 Posts

Response Posted - 08/13/2011 : 23:49:59

On our tall rig, the measurement from waterline to top of antenna is 38.83', so I call it 40' which only gives me a foot & change of conservative height. Anything below about 45' of clearance, I'd be asking for an opening, especially if it were in a tidal area. Fortunately there is only one bridge we ever have to worry about, the railroad bascule bridge right next to our marina. When it's down, it only has 7'-10" of clearance. Pretty much no one is getting through. It's rarely down for more than 15 minutes or so at a time, so it's no big deal.

The measurement from waterline to top of the cabin at the tabernacle is 3.33', so a six foot estimate is very conservative for our freeboard.

Edited by - delliottg on 08/13/2011 23:55:11

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