radar dome re-location

[markxengineerin]

The Furuno dome is currently on a pole, as shown by red arrow.
It was constantly casting a shadow on the solar panels, and with the new larger panels I'm installing, it plain does not fit.

I'm thinking about A or B for a new location, but not sure about a few things.
Does anyone use A, or is B the only option?
Could B interfere with the genoa in any condition? I roll it up to tack 99% of the time, but haven't tried to heave-to yet. I'm guessing I'd do that with the staysail or no headsails, not the genoa anyway (?)

Does having the dome 30-40' above the waterline actually limit the close range performance? How badly? I've seen many anecdotal statements, but all seemed second hand info rather than a useful concrete statement about what works or doesn't. My main use of radar has been for alarming about non AIS fishing boats approaching <1 mile away, which proved to be very useful in coastal sailing when I needed to make a meal down below, etc. I would usually set the alarm to beep if anything is picked up between 0.5 and 0.25 miles out. I don't remember the exact distance, but at some point really close to the boat, with the pole mount, the sea clutter would trigger too many false alarms. I wonder if the significantly higher mount would help or hurt this situation.

The lack of rearward visibility is something I'll have to live with for A or B, but it's worth it for the extra solar that comes in trade. The other idea I had was a telescoping or folding pole that only goes up when needed (keeping stern location), but I've never seen it done.

[skipmac]

Moved my dome from pole to A. Also have a cutter rig. No problems with snags or chaffing. I think B would put more weight than you want that high on the mast.

[markxengineerin]

Good to know, couple more questions. How far above waterline? Did you ever find trouble with seeing near targets, say 25-50 feet ahead? Useful in fog or finding un-lit channel markers at night.

At the same time I'm switching to synthetic rigging, so that will save some weight aloft, but still much prefer A or even one interval lower (right above whisker pole track) if it's not going to cause other problems.

[Mike Burch]

Somewhere in your radar's manual there should be a note of the vertical beamwidth of the antenna. A bit of quick trigonometry will soon tell you if the close-in performance will be adversely affected.

Not that it's at all important on a yacht, but the radar's long-distance performance will be marginally improved the higher you put the antenna. However, the distance to the horizon varies only as the square root of the height, so position A will get you only about 40% further than at present.

I would think that moving the antenna up the mast would help a bit with the close-in sea-clutter problem, but I wouldn't expect a miracle cure. And I may be talking complete nonsense anyway!

[markxengineerin]

The radar is a Furuno 1724C. The vertical bandwidth from the manual is 25 degrees. If it's parallel with the horizon, then half of that (12.5 degrees) seems to define the cone from radar dome to water. I calculate 180 feet minimum useable range with a 40 foot mount, and 67 feet minimum useable range with 15 foot mount. If this is the right way to do the math, that's a pretty significant difference! I wish I could recall what my "sea clutter throw-away circle" usually was, but I think it must have been about 50 feet.

[tkeithlu]

Quote: " Did you ever find trouble with seeing near targets, say 25-50 feet ahead?"

Markxengineerin has given you the math, clearing showing that you will only see very tall objects at 50 feet. But then, an object that close has to have gotten there, and the taller mounting will pick them up long before they get close to you. Generally speaking the advantages of longer range suggest that taller mountings are better. Avoiding objects 50 feet from you is best done with the Mark I eyeball.

[skipmac]

Originally Posted by markxengineerin

Good to know, couple more questions. How far above waterline? Did you ever find trouble with seeing near targets, say 25-50 feet ahead? Useful in fog or finding un-lit channel markers at night.

At the same time I'm switching to synthetic rigging, so that will save some weight aloft, but still much prefer A or even one interval lower (right above whisker pole track) if it's not going to cause other problems.

Mine is actually a little lower than A, just below the spreaders.

Very useful in fog, finding unlit markers, etc but does not work that close at least with small, low targets. As mentioned, depends on the height of the target but I figure roughly 100-200' practical minimum range.

[markxengineerin]

I have chosen to go with "just below the spreaders". Thanks for all the input, very helpful.

[markxengineerin]

Next question- where to put the radar reflector?