Horizon Distance

[Gus Orviston]

I thought I would start (not be the expert) a discussion on computing the distance to the horizon. Mentioned in an earlier post was the functionality of VHF being line of sight, and a how far that actually was.

As with nearly everything ... it depends, are you looking from your boat to the horizon, or are you looking from your boat over the horizon at land, or some other navaid, bouy, light. etc...

I have a couple of attachements that are large here, but I want to be sure they are readable. The first is a definition of the distances, Horizon or LOS, Geographic, and computing the range. Second is bowditch ,it will quickly give you reference from your sight height to the horizon (not a navaid, just 0 elevation on the horizon as shown in the first drawing).





Next is to determine the geographic range of a light. Which is somewhat nonlinear and requires a bit more thinking.


Combine both Geo range and horizon to get the computed range to a light. There are many more applications...here is the URL I borrowed the pics from. Suffice it to say there is no one answer from one moving object to another of varying heights.

gus

http://www.unc.edu/depts/nrotc/class...VisNavAids.ppt

[ 07-10-2003, 11:55 AM: Message edited by: Gus Orviston ]

[Pitch Pocket]

The visibility of a light is not the same as distance VHF will operate. VHF will bounce and increase it's distance from certain atmospheric conditions (cloud cover). A light's maximum range can only be diminished due to those same atmospheric conditions.

Someone at sea often will need to know whether or not he can see a particular light from his position offshore.

When approaching shore (making landfall, arrrg), you should be able to calculate what time you expect to see the light on your track. If you don't see it when you expect to, it could indicate a serious error in positioning.

For example, the Bowditch chart shows that a light 100' tall has a geographic range of 11.7 nautical miles. If you were the observer in a boat on the fly bridge at say 20' you could see 5.2 miles. Adding those two distances you should be able to see the light on your track 16.9 nautical miles away. If you are on your course and estimated your position 15 nm's off and don't see the light, then you're in trouble......or at least off course.

[Miss B Haven]

Steve- Unless your in the CR soup, then you just wish you could see ANYTHING! Gimmee a duck or a seagull on the water please, anything for a litle depth perception! :shocked:

For daylight, same thing can be said for land forms and bouys. Use the binoc's to help magnify and scan the horizon. They can't see over it but will really help id that first siting which you can use for range info if you know the heights (from the charts).

[Pitch Pocket]

Been there. Time to pray to your black magic plastic box when it's soupy like that.

If you don't already have a plotted course when the fog sets in, it's too late. You are forced to guessavigate using soundings and compass.

That's why god invented chart plotters!

[Miss B Haven]

Steve- LOL "That's why god invented chart plotters!"

and I thought it was Al Gore???? :grin:

[Gus Orviston]

guessavigate no bueno para mi

A couple of times out in the fog we listened for soundings....thought we heard japanese , I asked my brother..." you didn't put a bigger fuel tank in did ya?"

...Yep,,,got my courses in my handheld...in the process right now of extracting them and putting them in my CHART PLOTTER right now :grin:

adios muchtacos :grin:

[Pilar]

Great stuff!

I give VHF a range of about 15 miles. After that it gets pretty scratchy.

[Miss B Haven]

Great post, not bad for a guy who has a boat called FunGus anyway!

Speaking of Bowditch... one of my confessions. Have had the book for about 5 years and haven't read it yet. :blush: :depressed: It's got to get pulled out of the boat this winter and go right next to the throne! :grin:
The nightstand won't work, tried that the first time.