mast-mounted optics and bridge clearance

BobM · Mar 1, 2011

I'd get something like this ($100 laser rangefinder), then hop in the dinghy to check the height that day. You could also potentially do it from a distance using the pythagorean theorem and a level. Perhaps the best thing would be to build yourself a jig on something like your bow rail because you need absolute level and you need to know the distance to the water accurately. Even then you will introduce a lot of error. A dinghy ride is the better option.

http://www.amazon.com/CST-Berger-TL...ref=sr_1_1?ie=UTF8&s=hi&qid=1298986759&sr=1-1

BobM · Mar 1, 2011

There are also quite a number of cheap wireless indoor/outdoor surveillance cameras out there. Some can be directed remotely to pan and tilt. You could measure the distance from fully hauled height from the fully hauled height of the camera on a halyard pretty accurately. You could then have an idea of whether or not you could make it from the mast head without all the fuss and bother of a permanent installation. I was shopping for something like this for a cat shelter to use to monitor their charges just yesterday and I am sure you could do something for under $100. I should do it just to be able to inspect my mast head from the deck.

BobM · Mar 1, 2011

I don't know if you found this, but it has a wealth of good info.

http://jfaughn.com/crew_boat_projects/projects/tenntom/tenntom.htm

All U Get · Mar 1, 2011

BobM said:
I don't know if you found this, but it has a wealth of good info.

http://jfaughn.com/crew_boat_projects/projects/tenntom/tenntom.htm

Hi Bob,
Using your link I believe there were two bridges at 52. Using a 53' mast height and 12 degrees heal, I get 51.8'.
http://www.1728.com/trig.htm
With 53.5' that Snotter said he had, I get 52.3 at 12 degrees. Too bad there isn't a bumper bar like in the parking garages. I suppose you could check with a helium baloon from your dingy to be sure. The gas is fun at parties, too.
All U Get

Stu Jackson · Mar 1, 2011

It's HEEL, unless

his mast hits the bridge, then it's HEAL.

JVB · Mar 1, 2011

Here's another variation. Make a 90º triangle out of pieces cut from a wooden broom stick. Attach 2 or 3 sail slide slugs to the vertical side of the triangle. Attach the laser pointer to the horizontal side of the triangle but offset away from it about a foot. Hoist the triangle with the main halyard so that the laser light is shining forward horizontally over the top of the mast. It's a simple do-it-yourself project. And nothing is left out in the weather between uses.

Or bend a piece of metal conduit into a "?" shape. Put the pointer on top and sail slugs on the vertical section. Raise with main halyard.

JVB · Mar 1, 2011

Use a sextant to get the angle between water surface and span you need to pass under. Use a laser range finder to get the distance to the bridge. The sine of the angle multiplied by the distance will be exactly the span height if the measurements are taken at water level. A small error is introduced if you are higher than water level when you take the measurements.

Business opportunity: Combine a cheap calculator, an angle from horizontal or vertical measuring device, and a laser range finder to automate the procedure outlined above.

All U Get · Mar 1, 2011

Stu Jackson said:
his mast hits the bridge, then it's HEAL.

You're right Stu, I looked it up on line and got both spellings. Note to self (Never trust the internet). I agree, the anchor light is at risk. There might be some local knowledge about the water flow from the locks that might make it work. I go under a fixed bridge in NH and have to wait for dead low tide. Once my VHF antenna touched a power line sagging under the bridge. Judging by the scrapes on the beams, some didn't make it.

All U Get

All U Get · Mar 1, 2011

Found these posts.

http://www.youtube.com/watch?v=OEx504WaE0I

http://www.youtube.com/watch?v=eJAnqIwQVfA&feature=related

All U Get

txtowman · Mar 1, 2011

The laser pointer will be difficult or impossible to see on a sunny day, I use a laser distance measuring device a lot it's hard to see.

Has anyone thought of a bladder with air to lift one side along with weighting the other?

55 gallon barrels of air will provide #500 of buoyancy. And #400 pounds of ballast if full of water.

How much is required to achieve enough heel?

JVB · Mar 2, 2011

Mount a machine gun on top of the mast. Fire a burst. If it hits the bridge then keep firing until you have sufficient opening to sail under. NOT

I've done some Internet searching and found that a green laser pointer with about 50 mw output, an On/Off switch, and a pulsed beam (about 30 pulses/second) to prevent overheating when left On for several minutes is what you should look for. That tends to take you to models intended for gunsights that are powerful enough to see in daylight.

http://www.amazon.com/Green-Laser-Sight-average-filter/dp/B0017000CA

Ron20324 · Mar 3, 2011

JVB said:
Business opportunity: Combine a cheap calculator, an angle from horizontal or vertical measuring device, and a laser range finder to automate the procedure outlined above.

My Caddy has a gadget that beeps if you back up towards someone/something. Perhaps get one of those but mount it up high and pointing forward...???