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Old 17-11-2013, 12:53   #1
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Water Height and Bridge Clearence

Calling all sailing experts I'm looking for an answer to the following question.

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 (in this scenario difference between MHW and MLLW is about 4.7 feet). How low must the water be relative to MLLW if she wants to clear this bridge.
Choices are :
a. - 0.3'
b. 1.9'
c. - 1.7'
d. - 0.7'

I am totally clueless on this one and any help will be sincerely appreciated.
Many Thanks
Randy
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Old 17-11-2013, 13:12   #2
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Re: water height and bridge clearence

38+4.7 = 42.7 is bridge clearance at MLLW.

Mast height 43 - 42.7 = 0.3 feet. Water needs to be at -0.3 feet.

Almost all quality sailing / charting books will discuss this.

The math above doesn't account for wakes, tidal runoff from rivers, differences between real and predicted, etc. All the usual disclaimers.
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Old 17-11-2013, 13:21   #3
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Re: water height and bridge clearence

If you are taking the mast height out of some written specification, do be aware that that things like masthead tri-colour lamps, VHF antennas and so on may well not be included. Further, there can be significant variation in production runs at the spar maker. It is always safer to do a real measurement when contemplating a "near-miss" bridge clearance! The penalty for error is large...

Cheers,

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Old 17-11-2013, 13:22   #4
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Re: water height and bridge clearence

> If one has a sailboat with a 43 foot mast

What's the distance waterline to foot of mast? Is it positive or negative (Deck or keel mounted)?
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Old 17-11-2013, 13:27   #5
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Re: water height and bridge clearence

Based on the original question I am concern if the height of the mast is an accurate dimension above the water line. However with the number given of 48' and a 5.0' difference between clearance but only a 4.7' difference in tidal height. Therefore answer a. - 0.3' would give you 0.0' clearance, so either c. -0.7' or d. -1.7' would give you clearance. Option b. 1.7' I would not consider.
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Old 17-11-2013, 13:28   #6
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Re: water height and bridge clearence

Good point, Stu (nice name!).

Yes, the answer assumed that the mast height of 43 feet was from the waterline, absent any other input, hence, the earlier disclaimers, true for Jim's reply, too.
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Old 17-11-2013, 13:37   #7
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Be sure to add VHF antenna , masthead light, etc to the stated air draft. Or things can get expensive. My swag is +4 feet for antenna and error. That takes me to 55'
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Old 17-11-2013, 15:33   #8
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OK, stupid question, how to measure actual height? By measuring the angle to the top of the mast from a known distance? Or is there a better way?
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Old 17-11-2013, 15:43   #9
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Re: water height and bridge clearence

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OK, stupid question, how to measure actual height? By measuring the angle to the top of the mast from a known distance? Or is there a better way?
You can use a halyard or can go aloft and drop a tape measure.

I determined my by going aloft and measuring to the deck, then took a measurement from water line to the base of the mast.

Also allow for objects above the mast.
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Old 17-11-2013, 15:48   #10
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Re: water height and bridge clearence

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OK, stupid question, how to measure actual height? By measuring the angle to the top of the mast from a known distance? Or is there a better way?


have you ever heard about AIR DRAFT, if not...


Air draft is the distance from the surface of the water to the highest point on a vessel
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Old 17-11-2013, 16:00   #11
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Re: water height and bridge clearence

haul a line up the main halyard, throw ind in the water, lower line, measure from top end to wet spot
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Old 20-11-2013, 16:31   #12
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Re: water height and bridge clearence

I've been wondering about this for a while - if the chart specifies a bridge height eg 12.3M, what point of the tide is that taken from? Then, in looking at tide variations - eg low tide 0.54m, high tide 1.68M - what are the tide measurements taken from -ie what is 0.0M? the lowest tide ever? How does one calculate the relationship between the 2 sets of figures?
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Old 20-11-2013, 16:42   #13
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Re: water height and bridge clearence

Stupid question? Doesn't the bridge piling have markings for the clearance? Almost all of ours do.
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Old 20-11-2013, 17:02   #14
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Re: water height and bridge clearence

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Stupid question? Doesn't the bridge piling have markings for the clearance? Almost all of ours do.
no, they dont
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Old 20-11-2013, 17:12   #15
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Re: water height and bridge clearence

> what point of the tide is that taken from

Chart Datums should be stipulated somewhere on the relevant chart.

Not sure what the standard is in the US, but on Australia charts, heights are generally from Mean High Water Springs while depths are generally from Lowest Astronomical Tide.

For definitions of the above, see:

Tidal Datums - NOAA Tides & Currents
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