Water Height and Bridge Clearence

[natew]

Easy way for a 45' mast to clear a 28' bridge, we do it all the time.

So you mean all boats can't pull this trick off?

In all seriousness definitely don't forget to include any masthead attachments you may have. Also take a careful look at the bridge and figure out wether it is curved at all or not as I think most will end up having some kind of curve and most likely the height measurement is from the highest point.

Also I would be concered if the bridge doesn't include any markings wether or not the info you have is accurate. Where did you get the info? Have you also double checked that with any boaters in the immediate vicinity?

[Hudson Force]

I guess the question has been answered above, but here's an added bit of advice.
Do you expect to return from this trip under the bridge? I have logged my passage under a couple of high risk bridges with notes and photographs of the fender boards during my passage. For example, I know that I can pass under a particular bridge southbound if I can see half of the bolt head on the second fender board from the top on the port side and I will need both of the top fender boards viewed above the water line on the starboard side if I plan to clear the bridge when northbound. This, of course, only works until the fenders boards are repalced! Don't miss the chance to make clear notes of your observations when you do manage to clear the bridge!

[JayCall]

Charlie, don't know how it is done Down Under but here in the States, it is measured from Mean High Water. On the tide charts, the +/- numbers I believe are deviations from MHW or MLW as appropriate. This a -1.0 low tide would be (in OZ) 1 meter lower than the MLW mark.

[sandy stone]

Not exactly an answer to your question, but - if you're getting the bridge clearance from a nautical chart, don't assume that the figure is correct.

[Stugots]

Originally Posted by RandyBr

If one has a sailboat with a 43 foot mast , she wants to sail it under a bridge span in Santa Monica bay that has a 38 foot clearance at MHW

I'm racking my brain... what bridge in Santa Monica Bay matches this description?

[rgscpat]

People should know how to use the basic trick of measuring shadows and using proportions... assuming the boat's shadow can be cast ashore to a point whose height above the water is known.

[Gerrycooper56]

One bridge in particular on the St.Johns river shows the bridge clearance at 65', taken from the official channel say bridge center. The span over this part of the bridge is quite wide to allow passing traffic, however on either side of the center are smaller spans which can fit traffic in either direction. Because the spans don't need to be so thick as they are shorter, the bridge clearance actually increases.(because the road height has to be the same!)

[GordMay]

Greetings and welcome aboard the CF, Randy.

[Capt.Don]

Originally Posted by Stugots

I'm racking my brain... what bridge in Santa Monica Bay matches this description?

Me too! The only thing I can think of are Heim/Ford bridges in LA Harbor/Cerritos Channel.

[jkindredpdx]

I'm thinking one could stand on the dock with a sextant (or protractor) and use basic trig to calc the true height of the mast + antennae etc. from the waterline... couldn't one do the same at a safe distance from the bridge?

[charliehows]

heres what i found;
A typical days tide dat here (australia) looks like this;
Fri 22 Nov L05:20 H11:46
L18:28
Sat 23 Nov H00:32 .....etc...

It is measured from a level called Chart Datum, which is approximately the level of the lowest astronomical tide (LAT). This is the lowest level that the tide falls to under the influence of the pull of the moon, sun and planets, however, high pressure and strong winds can occasionally make it fall below this. the lowest tides usually occur just after the full moon around the time of the spring and autumn equinoxes.

The use of the lowest water level as the basis for charts (maps of the seabed and surrounding coasts) means that the water is almost always deeper than shown on the chart and sailors simply have to add the rise of the tide to the depths marked on the chart. This is easier and safer than sometimes having to add and sometimes having to subtract figures.

As an example, many modern ports are dredged to maintain a depth of 10 metres below chart datum, so when the tide is 6.3m the actual depth of water is 16.3m. At low water on a spring tide the depth will only be around 10.1m, but will rarely fall below this.

what i haven't found yet is how the height of bridges is measured - i always assumed it was above the highest tide but havent been able to find an explanation yet.

[a64pilot]

Yes, but you have to know the exact distance to the bridge, all you need is a plumb bob, a cheap protractor and a piece of wood and an assistant, to read the degrees on the protractor.
Used this as a kid to determine how high the model rockets I built went.

[charliehows]

according to NSW maritime;

"Bridge heights on maps are measured at the Mean High Water mark, so you should allow for higher than average tides at certain times of the year. Also consider your vessel may require more room when unloaded."

#%@!^##!!! how do you know what MHW is at any particular place?

[Don1500]

Originally Posted by Gerrycooper56

One bridge in particular on the St.Johns river shows the bridge clearance at 65', taken from the official channel say bridge center. The span over this part of the bridge is quite wide to allow passing traffic, however on either side of the center are smaller spans which can fit traffic in either direction. Because the spans don't need to be so thick as they are shorter, the bridge clearance actually increases.(because the road height has to be the same!)

But the water could be shallower.

[Hudson Force]

Originally Posted by Gerrycooper56

One bridge in particular on the St.Johns river shows the bridge clearance at 65', taken from the official channel say bridge center. The span over this part of the bridge is quite wide to allow passing traffic, however on either side of the center are smaller spans which can fit traffic in either direction. Because the spans don't need to be so thick as they are shorter, the bridge clearance actually increases.(because the road height has to be the same!)

'good point, this is not unusual. The well known Julia Tuttle Bridge in Miami on the ICW is similar,- 56' MHW at center and 57'MHW at the side intervals and for the same reason.