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Old 07-12-2008, 08:05   #1
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bridge clearance

i need to know the bridge clearance for 27' catalina
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Old 07-12-2008, 08:30   #2
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I don't know if some came in tall rigs or if it may have chanced with some model years, but I've seen a few that have an advertised bridge clearance of 38'3"

1980 Catalina yachts Catalina for sale in Chicago, IL: Cruiser (sail) - SailboatTraderOnline.com

1984 Catalina 27 Sail Boat For Sale - www.yachtworld.com
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Old 07-12-2008, 09:44   #3
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Our club's tall rig Catalina 27 is given at about 41 feet in our club's literature. This heighth includes clearance for an antenna.

Even though it's a pain I suggest you measure it yourself. You don't know if you have the tall or standard rig, antenna, lights, or someone could have put a different mast on it, or you could get bad advice from the web. The first time after we had acquired an Ensign that it went under a bridge, the mast came down. Turns out the mast clearance obtained from the web wasn't right.

John
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Old 07-12-2008, 10:15   #4
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One quick and dirty way to find your boat's air draft (bridge clearance is the distance between the water surface and the lowest point on the bridge) is to go through a drawbridge with a scale on it. Not all draws have this, but some do. With a scale and a little luck, the bridge operator should be able to come up with a good working number for your air draft. Keep in mind, though, that operator won't be on your boat if he gave the wrong value for the air draft.

If you're going to do the measurement yourself, the toughest part is getting the distance from the top of the cabinhouse to the water - heeling and general imprecision can cost you a foot or more. Remember, too, that hauling a tape to the top of the mast will still miss the antenna, Windex, masthead light, AWI sensor, etc. Add at least 18" to keep all of that gear on when squeaking under a bridge.

Finally, if you're sailing in a tidal area, remember that clearances are given relative to Mean High Water (loosely, the average high water mark). Of course a low tide will probably add to the clearance, but an unusually high tide takes it away. Plan accordingly.
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Old 07-12-2008, 10:20   #5
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Quote:
Originally Posted by j8502 View Post
i need to know the bridge clearance for 27' catalina
If I wanted to be really sure of the clearance (and I would, before going under any bridge where the clearance is even close), I would measure it rather than rely on information from an internet forum. It is easy enough to do by hoisting a long tape measure (50' will do in your case) up the mast. Don't forget to add allowance for the coach roof height above waterline, vhf antenna, plus another 2-3 feet for safety.
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Old 07-12-2008, 11:01   #6
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If you know anyone with a theodolite they can very quickly give you an accurate measure of the height.
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Old 07-12-2008, 12:22   #7
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Or... use a sextant. Row off from the boat in a dinghy and find your distance off in feet (see below). Measure the angle from the waterline to the masthead (just bring the masthead image down to the waterline image). From there, do the math (angle-side-angle in a right triangle):

Mast height = tan(sextant angle) * distance off

For example, if you row off about 50' and get an angle of 40 deg from the water to masthead, the mast is about 42' high. tan(40) = .839 and .839 * 50 = 41.95 (call it 42').

It'll help to be as close to the water as possible, to avoid problems with height of eye correction, when getting this angle (besides, being lower in the dink means you won't fall out of the dink, sextant in hand ). As to distance off, get a fix on the boat, make it a waypoint, and start rowing. The distance from the dink to the waypoint needs to be turned into feet. Divide the distance (in nm) by 6076 (well, 6076.115, if you want to be anal about it) and there's the distance off in feet. Many handheld GPS' will average a fix over time. As long as you're not drifting around, let the fix settle out a bit beforesetting the waypoint or measuring distance off. Also, up to a point, further distance off (say, a couple hundred feet) will reduce the impact of imprecisions in the fix. The number is still somewhat approximate (GPS errors, sextant errors) but it will be useful for deciding whether or not to chance going under that 40' bridge when the numbers suggest a 45' air draft...
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Old 07-12-2008, 13:44   #8
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There you go. Not many of us know owners of theodolites but we all know sextant owners just looking for an excuse to use them, right?
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Old 07-12-2008, 15:21   #9
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You can do this with known lengths, shadows, and algebra pretty easily.

The ratio of the height of your mast to its shadow is equal to the ratio of the length of a yardstick to the length of its shadow (measured at the same time).

So... Wait till your masthead's shadow is in a convenient place. Measure from the base of the mast to directly over the tip of the shadow. This line needs to be level. You might even want to use a level to make sure you're even with the ground. Add the radius of the mast to this measurement.

Once you've got that number (let's call it L), get a yardstick (Y), hold it straight up and down (use a level), mark the tip of its shadow, and measure from the base of the yardstick to the tip of the shadow (S).

Don't dilly dally between the two measurements. The sun moves quickly!

Now that you have three known lengths and a known ratio, it's just a matter of plugging it in to H / L = Y / S (or H = (Y * L) \ S).

So let's say L is 60 feet, S is 40 inches (and Y is 36 inches obviously). Don't pull a NASA and forget to convert feet to inches!

H = 36 * (60 * 12) / 40
H = 36 * 720 / 40
H = 25920 / 40
H = 648 (inches)
H = 54'

Y and S can be any known lengths.

Most accurate time to do this is when you have a nice long shadow.

Here's a pic to illustrate:


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Old 07-12-2008, 15:29   #10
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Math is fun but it would be a lot easier to tie the end of tape measure to a spare halyard, haul it up the mast, toss the tape measure into the drink and take a reading where it stops being dry.
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Old 07-12-2008, 21:59   #11
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Well, yeah, but... that's assuming you can get that cabinhouse top (or mast butt) to waterline measurement accurately. Measuring the stick itself is as easy as hauling a piece of string to the top of the mast and measuring the length of string used. The other is a touch trickier.
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Old 07-12-2008, 22:27   #12
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i need to know the bridge clearance for 27' catalina
That's OK! Some people don't know how tall their house is either.

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