Air Draft vs. Bridge Height vs. Heel - SailNet Community
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post #1 of 13 Old 02-07-2012 Thread Starter
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Air Draft vs. Bridge Height vs. Heel

Come laugh at the newbie!!

There is a drawbridge between my new mooring and the big blue sea with a closed height of 19'; my little 18' boat has an airdraft of 23' 2", I'm calling it 25' with the 'vane and a safety margin.

My trig was all rusty but I found an online calculator that showed if I induce a heel of 40 degrees I should be able to slip under the bridge without having to wait for it to be raised.

I don't need my math checked (but you're welcome if you want) I just want to know if this is a done thing. With a displacement of only 1500# it seems like it wouldn't be too tough to get it to heel over. I'd practice in open water to see how handling is affected and I'd need crew to do the heeling while I drive. If I heel over to starboard that would keep the engine in the water.

I don't see much risk unless it was really crowded; since the bridge only opens on the half hour it seems like I could save some time.

Do people do this?

Is there some cool name for it? Sailing has the best terminology of any sport.

The bridge is being replaced soon with one that will have a height of 21' that makes it even more tempting.

Zen Again

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post #2 of 13 Old 02-07-2012
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40 degrees is a long way over.. even on a small boat maintaining that angle may be difficult and if something goes wrong while you're under the bridge...

There is a video out there of a rather large boat using ballast bags hung from the rig, so it has been done.
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post #3 of 13 Old 02-07-2012
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40 degrees is way, WAAAAY over! What most people consider an extreme angle of heel when they are sailing is probably no more than half that. I think I would just wait for the bridge to open.
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post #4 of 13 Old 02-07-2012
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Will you still have proper steerage and propulsion at 40 degrees of heel?

The mentioned video is this one: Boat Balls - YouTube

How will you attach the water bags to your masthead? If the sheaves are fixed, will the halyard jump out of the track and jam? How long will it take to rig, and will the time (and risk) savings be worth it?

Have you had the boat over that far before? I know our Contessa likes 15-25 degrees of heel, but our narrow hull likes it over that far before it locks in. I wouldn't feel comfortable at 40 degrees... and inanimate objects in the cabin and the animate objects in the cockpit wouldn't like 40 degrees either.
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post #5 of 13 Old 02-07-2012
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Every half hour is very fortunate if you ask me. How much time are you going to save?
Not worth the risk in my opinion.

Does the mast step have a tabernacle? See where I am going with this? If you’re in such a rush to get out, it might ne easier to drop the stick on an 18’ boat and re-step it on the other side of the bridge.

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post #6 of 13 Old 02-07-2012 Thread Starter
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I guess 40 deg is pretty far.

I'd feel pretty stoopid if I got stuck under there!

A more moderate heel of 20 deg only shaves 1.5' off the air draft.

So, it's good to know what it would take but not worth the risk for such a small gain.

Thanks for talking me down off the roof.

Zen Again

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post #7 of 13 Old 02-07-2012
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Quote:
Originally Posted by CarbonSink62 View Post
the bridge only opens on the half hour it seems like I could save some time.
Lucky you. My bridge is plenty high but I can only buck the tide twice a day.

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post #8 of 13 Old 02-07-2012
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Forty degrees is awfully close to halfway broached. If you're going to do that on a regular basis, put the video on utube, sell advertising space and tickets for the live performance, because folks don't get to see that very often.

You probably CAN do it, but wouldn't it be more fun to dynamite the bridge?
(Now kids, don't do that at home, dynamite is strictly for professional use under adult supervision!)
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post #9 of 13 Old 02-07-2012
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Your trigonemetry is correct.

The heel angle needed for the 19 ft bridge clearance is given by...

a = invcos{19/25} = 40.53 deg

...and for the 21 ft bridge clearance...

b = invcos{21/25} = 32.86 deg.

Those are first estimates, as the mast will not pivot perfectly around a single point as the boat heels.

Those are quite aggressive heel angles.

It's a risky business though. If you get it wrong, you will smash the masthead and fittings on the bridge. You may even bring the mast down.
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post #10 of 13 Old 02-07-2012
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Heeling to that degree is not a viable option on an ongoing basis.
You mention the big blue sea, might you have a tide, that could range up to 8'+ in your neck of woods. Bridge clearance is published
based on mean high water...so if assumptions above are close you
may have about half time that you clear...??
If close remove vane, but I would bribe a buddy with a dinghy/powerboat and check out first with a few 10' lenghts of pvc.
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