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17-11-2013, 12:53
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#1
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Registered User
Join Date: Nov 2013
Posts: 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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17-11-2013, 13:12
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#2
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Registered User
Join Date: Jan 2011
Location: Cowichan Bay, BC (Maple Bay Marina)
Posts: 8,486
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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.
__________________
Stu Jackson
Catalina 34 #224 (1986) C34IA Secretary
Cowichan Bay, BC, (Maple Bay Marina) SR/FK, M25, Rocna 10 (22#) (NZ model)
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17-11-2013, 13:21
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#3
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Moderator
Join Date: May 2008
Location: cruising SW Pacific
Boat: Jon Sayer 1-off 46 ft fract rig sloop strip plank in W Red Cedar
Posts: 17,406
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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,
Jim
__________________
Jim and Ann s/v Insatiable II, back in Port Cygnet after adventures in the big smoke.
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17-11-2013, 13:22
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#4
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Senior Cruiser

Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43 and OPB
Posts: 10,945
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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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17-11-2013, 13:27
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#5
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Marine Service Provider
Join Date: Feb 2011
Location: St Petersburg, FL
Boat: 18' Banno USA Old World
Posts: 78
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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.
__________________
Absolute,
"Suckin Sludge & Havin a Gas"
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17-11-2013, 13:28
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#6
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Registered User
Join Date: Jan 2011
Location: Cowichan Bay, BC (Maple Bay Marina)
Posts: 8,486
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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.
__________________
Stu Jackson
Catalina 34 #224 (1986) C34IA Secretary
Cowichan Bay, BC, (Maple Bay Marina) SR/FK, M25, Rocna 10 (22#) (NZ model)
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17-11-2013, 13:37
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#7
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Marine Service Provider
Join Date: Jan 2011
Location: St. Petersburg, FL
Boat: Tartan 33 and OPB
Posts: 2,999
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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'
__________________
"Whenever...it requires a strong moral principle to prevent me from deliberately stepping into the street, and methodically knocking people's hats off- then, I account it high time to get to sea..." Ismael -a link to my delivery website is in my profile—
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17-11-2013, 15:33
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#8
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Moderator

Join Date: Oct 2013
Location: Jacksonville/ out cruising
Boat: Island Packet 38
Posts: 31,464
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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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17-11-2013, 15:43
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#9
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Marine Service Provider
Join Date: Feb 2011
Location: St Petersburg, FL
Boat: 18' Banno USA Old World
Posts: 78
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Re: water height and bridge clearence
Quote:
Originally Posted by a64pilot
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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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.
__________________
Absolute,
"Suckin Sludge & Havin a Gas"
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17-11-2013, 15:48
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#10
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cruiser
Join Date: Nov 2013
Posts: 31
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Re: water height and bridge clearence
Quote:
Originally Posted by a64pilot
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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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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17-11-2013, 16:00
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#11
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Eternal Member

Join Date: Jul 2011
Location: Australia
Boat: Lagoon 400
Posts: 3,650
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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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20-11-2013, 16:31
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#12
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Registered User
Join Date: Aug 2010
Location: sydney, australia
Boat: 38 roberts ketch
Posts: 1,308
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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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20-11-2013, 16:42
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#13
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Registered User
Join Date: May 2011
Location: Portland, OR, USA
Boat: Hallberg Rassy 35'
Posts: 1,200
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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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20-11-2013, 17:02
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#14
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Registered User
Join Date: Aug 2010
Location: sydney, australia
Boat: 38 roberts ketch
Posts: 1,308
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Re: water height and bridge clearence
Quote:
Originally Posted by jkindredpdx
Stupid question? Doesn't the bridge piling have markings for the clearance? Almost all of ours do.
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no, they dont
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20-11-2013, 17:12
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#15
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Senior Cruiser

Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43 and OPB
Posts: 10,945
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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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