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Old 06-08-2011, 12:07   #1
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About Mast Height and Bridge Clearance

My boat specification states it is 45' 4" above the water line to the top of the mast. I do have a short VHF antenna atop the mast as well.

I'd like to go under the Ft. Walton Beach Bridge which is stated to be 48' at high water.

So far I've only been under 65' bridges. So I'd like to ask if others have experience with this limited clearance. Is this a common occurrence? Should I be concerned? Should I go through at low tide and not worry?

thanks
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Old 06-08-2011, 12:19   #2
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Re: About Mast Height and Bridge Clearance

This is actually the Brooks Fixed Bridge at 50' on the NOAA charts.

It's on the ICW. I thought it was not part of the ICW so I think I'm good to go.

Any input is still appreciated.
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Old 06-08-2011, 12:54   #3
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Re: About Mast Height and Bridge Clearance

You should measure your mast if you don't have some first-hand knowledge that it is the specified height above the water. The VHF whip only needs about half it's height to just bang along under the bridge.
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Old 06-08-2011, 15:49   #4
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Re: About Mast Height and Bridge Clearance

Measure.
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Old 06-08-2011, 16:41   #5
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Re: About Mast Height and Bridge Clearance

I've been working on a way to measure it today.

I'm anchored off Spanish Ante in St. Andrews Bay watching the weekend crowd having fun and drinking some cold beverages.

Seems prudent to measure before I proceed.
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Old 06-08-2011, 17:00   #6
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Re: About Mast Height and Bridge Clearance

Don't know about that particular bridge, but many have tide boards that tell you the actual clearance based on the water level. I have found these tide boards tend to be accurate. I second the suggestion to measure. I have a 50-foot tape measure onboard that has proven to be very useful for measuring for sails too. Many bridges have a bit more clearance close to the center (watch for any lights hanging down), as the clearance figure is supposed to be for the lowest steel or concrete point.
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Old 06-08-2011, 17:13   #7
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Re: About Mast Height and Bridge Clearance

Thanks, I was wondering if the tide boards are considered to be accurate. I have seen them on bridges that were well above my needs.

This will be my first time with less clearance.
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Old 06-08-2011, 18:55   #8
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Re: About Mast Height and Bridge Clearance

As above measure

I've found the tidal clearance boards on the GIWW to be real accurate
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Old 06-08-2011, 19:01   #9
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Re: About Mast Height and Bridge Clearance

A 100 ft tape is good to have around. Get one and haul the free end up the halyard. Stand back and estimate the additional amount. Somehow my mast made it under all the waterway specifed bridges that were supposed to be 1 foot more than my mast (but I had an anttena and tri color light also) but it is scary looking up when the current grabs you!
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Old 06-08-2011, 19:02   #10
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Quote:
Originally Posted by four winds
I've been working on a way to measure it today.
On measuring the mast: Run a long line up the main halyard. Mark the line with a piece of tape where it ends at the deck. Bring the line down and measure it. You might have to measure it in ten foot increments or whatever on your deck since you're at anchor and can't stretch it out on the dock. Just mark the sections with tape as you go. Then measure from your deck to the waterline. Add that to your line measurements. Add in a foot or three, whatever you think your antennae clearances may be. Add a foot or two for safety/miscalculation. You could go up the mast to get the precise antennae clearances and to figure out exactly how many inches there are from the halyard and your line that is tied to it to the very top of the mast, or you could give it your best guess. Looking up your specific antennae may give you the height of it as well.

And then repeat it all again. And have someone check your math. Good luck. We leave our river with just inches of clearance at high tide. I understand your worries.
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Old 06-08-2011, 19:23   #11
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Re: About Mast Height and Bridge Clearance

New VHF Antenn...$100
100 ft tape measure...$20
Fudge Factor...Free
Knowing for sure...Priceless
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Old 06-08-2011, 19:28   #12
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Re: About Mast Height and Bridge Clearance

Run your line or tape measure up the mast and measure to the water midship, this will allow a little extra, about 6". then add what you need to clear the top...
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Old 06-08-2011, 19:31   #13
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Re: About Mast Height and Bridge Clearance

I measured my rig carefully before I took it down the ICW. It paid off when I was able to squeeze under the Indiantown bridge in the Okeechobee Waterway with the VHF antenna going ding, ding, ding, across the steel on the bottom of the bridge.

One way to measure higher than where your haliard goes into the mast is to use two pieces of some tape to hold a three or four foot dowel vertical when the haliard is at the top and tensioned. It's like raising a burgee on a haliard. Then you can tape the long tape measure to the dowel to raise it higher than your masthead.
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Old 06-08-2011, 22:51   #14
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Re: About Mast Height and Bridge Clearance

Thanks for the feedback, everyone. I'm a little late in figuring this out, but I have a plan that combines several ideas.

My spare halyard is currently in a continuous loop through a standing block on deck next to the mast. I'm going to strap or tape a wood handle from my deck brush to it and hoist it up past the masthead. Then row out in the dinghy and use my binocs to estimate its relationship to the vhf antenna. Then mark the halyard with tape at the block. And bring it down while marking every ten feet with tape until it is all measured.

For the rest of the distance to the waterline I'll use the handle or my boat hook lying level on the deck next to the block to overhang the gunwales. Then I'll measure down to the water and add it all up.

Then I'll scratch my head, drink a beer, and do it all again.

Frankly, my heart skipped a beat when I went under the overhead power lines going out to St. George Island just before the 65 foot bridge. I knew they weren't underground like the chart says. And I knew a Coast Guard boat took them out a few weeks ago and killed the power to the whole island for a day or two.

Probably had more than fifteen feet clearance but on approach it sure looked liked it was going to touch.

Thanks again.
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Old 22-05-2017, 08:03   #15
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Re: About Mast Height and Bridge Clearance

What is the mast height of a Bristol 32 please?
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