Remounting my radome.

[Jolly Roger]

Every start of the hurricane season in Florida, I remove my Raymarine radome from high up my schooner’s mainmast, because I’ve seen them damaged in a hurricane. Bringing it down and re-hoisting it is an absolute PITB! not to mention dangerous. So this time I’m thinking of buying a pole, and mounting it on the stern.
My question is: would a ten-foot pole, mounted on my transom, six feet above the waterline, reduce the 45-foot range significantly, compared to 35 feet up the mainmast? There is no easy way to test it.
Anyone done this?

[Cheechako]

There's a formula. Not sure what it is. But I don't really understand what you are asking. "a ten-foot pole, mounted on my transom, six feet above the waterline, reduce the 45-foot range significantly, compared to 35 feet up the mainmast? "
Are you just asking how far you can see with the dome 10+6 ft off the water? What does that have to do with "45 foot range"?

Rough range estimate:
16 feet up ~= 5.5 miles
35 feet up ~= 9 miles
45 feet up ~= 11.5 miles
(Based on RF and earth's curvature only)

I guess:
-what are the odds of being hit by a hurricane directly with a lot of force?
-might the radome survive anyway in most Cat 1 hurricanes?
-Will the boat tip over if hit with cat 2-5, thus rendering larger issues than a radome?

[Jolly Roger]

Sorry for the misunderstanding. My radar has a 45-mile range. It is up the mast, about 40’ feet from the waterline. What will it be reduced to, if mounted 16’ feet from the waterline, on an aft pole?

Re hurricanes. We’ve had some very near misses recently in Cape Canaveral, like Dorian last year, which sheered the radome clean off a nearby boat, with 100mph winds. It also blew three boats over on the hard, so windage is important down here, even on dry land.

Laura is coming up early next week as a possible cat 1, and originally routed to pass directly over Orlando, but now they say it will shift more westerly. My boat is stripped bare already, better safe than sorry.


I mention other factors against mast mounting, like windage, weight, removal and replacement twice a year, etc.


I just wanted to know what effect it would have on the range.

[Cheechako]

I got those range estimates on line (see above). They seem low to me though. Maybe someone else will chime in. Don't user manuals have a chart?

[Discovery 15797]

Your radar has a max range setting of 45 nm, but that is not necessarily how far the radar "sees."

The distance the radar can "see" is a function of the antenna height above the water and the height of the target being painted.

Here is a good calculator to determine radar target visibility.

But, as Cheechako stated...removing the radome seems a bit overkill.

[Jolly Roger]

Not having a doctorate in mathematics, I found that calculator a bit daunting, but I did ask for it I suppose.
It would seem: with a target height of 15 feet, (something between a sailboat and a commercial fisherman), and the radar 40 feet up the mast, the range is 12.5nm. But with the mounting at 16 feet it is still 9.6nm.
I find that a bit hard to believe. Maybe I didn’t do it right.
If I’m honest, the main reason for wanting to shift it is removing and replacing it twice a year. I’m getting to the age when I will need a rigger to do it, which will be about the same cost as installing it on a pole, once.

[Bycrick]

A rough estimate of "distance to the horizon" can be made by taking the square-root of the height in feet. That number will be an approximation of the horizon distance in miles. So an antenna at 10’ above the water will see the horizon at a little more than 3 miles. An antenna at 40’ will have a horizon at a little more than 6 miles. To double the horizon distance, you have to have about 4 times the height.

The radar horizon computed mathematically is about 10% greater than the "optical" horizon.

The maximum distance that radar can detect a target is based on the sum of the radar's horizon and the target's horizon. An antenna 40’ above the water searching for a ship 100’ tall is unlikely to detect it until it is closer than about 16 miles, ie 6 miles for the radar horizon and 10 miles for the ship horizon.

The "range" specification on the radar is simply how far away it might detect a big enough target that was completely unobstructed. Just because a radar has a range setting of 48 miles says absolutely nothing about what you might be able to actually detect at 48 mile even if the target were actually visible.

So about the only things you’ll ever see on a sailboat radar on long ranges are a high, relatively straight-up-and-down islands or heavy rain.

[smac999]

If a hurricane managed to damage a radar attached to a mast. I would think you’d already have damage on the entire boat.... and the radar would be a minor concern at that point.

Seems silly to remove

[chris mac]

My understanding, and please correct if I'm wrong, is you will have the same range regardless of height for detecting storms, but slightly reduced for detecting other vessels(or landmass)

[Jolly Roger]

Thanks Bycrick, all things considered, that answers my conundrum. I will re-mount the radome, (when I put it back after November), on a ten-foot-high pole on the transom.
The Garhauer styles seem to be the best value, and I might be able to attach my outboard crane to the same pole.
Thanks everyone

[CassidyNZ]

Originally Posted by Bycrick

So about the only things you’ll ever see on a sailboat radar on long ranges are a high, relatively straight-up-and-down islands or heavy rain.

Or the superstructure of a container ship (could be 35m above sea level) which by the calculator in Discovery’s post #5 could be seen by my radar (4m above sea level) at a distance of about 20nm

That said, a post-Panamax container ship would cover that space in about 45 minutes

I would prefer to depend on AIS to see him. I very rarely even turn my radar on. If I ever sold the radar, I would advertise it as “near new” even though it’s at least 20 years old