The Harbor Column:

I am predicting an excellent weekend to be on the water, with only a very slight chance of rain this morning. The skies will clear up with sunshine and the air temperatures will reach the mid-70s. The swell heights are dropping and the swells at Point Conception are anticipated to be less than 8 feet, and not building until Monday.

My advice is get out on the water, but expect foggy conditions in the mornings due to the warming air temperatures along the coast.

I received a few e-mails from last week’s column about the curvature of the Earth and why the curvature would affect boaters, besides just not being able to see something over the horizon. Well, the curvature of the Earth and the rotation of our planet have an effect whether you are a boater, jump on a jet to fly to the East Coast, or watch the sunset in the evenings.

However, specifically in boating, the curvature of the Earth affects the radar and VHF radio signals. Both of these electronic devices operate basically by line-of-sight, except VHF signals will bend slightly over the horizon toward Earth off the atmosphere.

Yes, sometimes VHF radio transmissions can travel hundreds of miles over the horizon by a phenomenon called Tropospheric skip, which I explained in detail in my book on using your marine radio, which I wrote a while ago. Normally, your boat’s radio has a range of about 15 miles, which depends on how high you install your antenna.

The radar is an acronym for “radio detection and ranging.” It works by sending out a signal that bounces back when it hits an object. How long it takes the reflected signal to return back to the radar determines the distance the signal has traveled, thus measuring how far away the object is. Naturally, the radar’s signal cannot bend over the horizon to bounce off an object in a straight line back to the unit.

So, one e-mail asked why do radars have a 36- or 48-nautical mile setting? Ah, but the radar will show how far away that coastal mountain range is, which you can visually see over the horizon. Then you can use the radar’s distance and direction to determine your vessel’s approximate position on your charts. Charts — remember those rolled up paper things all boaters should have onboard?

MIKE WHITEHEAD is the Pilot’s boating columnist. Send marine-related thoughts and story suggestions to [email protected] or go to www.boathousetv.com .