Radar how high

[jwieneck]

I am pulling my mast this coming week to install radar on my 1998 376. I am wondering if there is any rule of thumb about how hight to place the dome . I have a two spreader mast. At first I was going to put the dome in just above the first spreaders but that puts the dome below my steaming deck light combo. The steaming and deck light combo is about half way between the first spreader and the second spreader. If I put it just above the steaming deck light combo it seems a little high. Just wondering what is normal ?

 

[Bill Roosa]

The things to consider are:
high enough to not send the radar beam into the crew in the cockpit/on deck
high enough to give a good "radar horizon"
low enough to not have mast movement interfere with the radar image.
up in the spreaders is high enough for crew safety
top of the mast is too high as it tends to move around so much that the radar screen is difficult to read. if not mounted on a gimble to keep the antenna horizontal it sends the beam into the sky on the windward side and into the water on the leeward side. as the boat angle of heel changes the screen display changes radically!!!
I believe WM has an adviser article on this subject BTW.

 

[Don S/V ILLusion]

Not sure there is one "right" answer but you didn't mention what size radar you have. It makes a difference in placement for two reasons: weight aloft and range capability.

The weight consideration is self-evident but regarding the range, it makes no sense to place a 16 nm range radar higher than necessary as you will never reliably exceed the range (power) limit of the radar regardless of how high it is.

 

[Rick]

On the assumption that you have a 2K radar and since one any bigger probably spins, we have that solved. Also the max range of a 2k radar is generally about 16 miles and that is for tanker sized targets. The curvature of the earth is around 8 inches per mile so to be able to see out to the max range of your radar, you need to put it up at least 10 feet on the mast or so. The gotchas are as mentioned above. What you want to see typically is close to you, bouys, jetties, other boats so to put your radar up so high that it cant paint a target in the 1/8th to 1/4 mile range does not make sense IMHO. So up around your steaming light isn't a bad place to start. About 8 feet up from your water line would be ideal but a bit difficult to mount unless you do a pole mount and your jib would probably beat the heck out of itself every time you tack. Also you need to take into account the angle of the mast past vertical due to the design of the boat and rake of the mast. Adjust the level accordingly.

I haven't found myself healed over under sail that many times when the visibility is so low that I need the radar, so a gimbal mount is just a boat bucks option, and Bill I don't mean to belittle your advice at all. My radar world is typically under motor in a sort of 'Hooper and Brody going to find the feeding grounds of Jaws" way.

You can find specs about it from the manufacturer.

Good luck

 

[jwieneck]

The radar is the 418 HD. Just above the first spreader is about 18 to 20 feet. The steaming and deck light is closer to 30 feet. I originally was just above the first spreader but I could shadow the deck light some. I am installing spreader lights also so that is not a big deal. I would think this gives me a good look up close and pretty far. In my experience 10 to 12 miles is plenty of range for the radar.

 

[Mulf]

Two thirds way between lower and upper spreader

That is how my dealer installed the radar dome on Dreamboat. That is, 2/3 up from the lower spreader towards the upper spreader. Your 376 is proportionately similar boat to my 410.

 

[Don12364]

Time to Collision??

At 20 feet scanner height your horizon is almost 5 nautical miles distant.
At a closing velocity of 30 knots; i.e. 6kts of your boatspeed plus 25kts of the other vessel the time to collision will be about 10 minutes. Then add time to the horizon distance for the other vessel too.
If, in fog, one doesn't look at one's radar screen for more than ten minute intervals you are asking for trouble.
You should know that ships do not slow down in fog. In the busy shipping lanes of the English Channel a ship ran down and sank a yacht in fog so thick that those on the bridge could not even see their own bow. They didn't stop because they saw it on their radar after they had passed bye. It sank a few minutes later. They were travelling at 27 knots. Their normal cruising speed in clear weather was 26 knots but having left port an hour late they were trying to make up for lost time.

Another one did the same in clear weather at night just off the Isle of Wight and drowned all the crew. It also saw the yacht on screen in the track it had just covered and just before the yacht sank. It didn't stop either.

There are better ways of navigating than the use of radar but the coastline should be visible on screen whilst you are still ten miles off.

I have my small 12" scanner only 10' high and find this is more than adequate.

 

[Rick Webb]

Somthing not Being Mentioned

The height of the object reflecting your radar beam. If your radar antennae is 20' high and the object your scanning is 20' above the horizon you have doubled the distance you radar can see. There comes a point where the object is visible to the radar but out of it's effective range. Look around at the other boats in also the cable is only so long and will need to reach from the helm to the dome.

 

[jwieneck]

That is how my dealer installed the radar dome on Dreamboat. That is, 2/3 up from the lower spreader towards the upper spreader. Your 376 is proportionately similar boat to my 410.

It seems with radar positioning there are as many options and perpectives as there are boats. Dreamboat I have decide you are about right. I am going to place the scanner just about the steaming/deck light combo. This is just about half way between the first spreader and the second spreader. Should be high enough to see all and out of the reach of the jib for the most part.

I have used radar as crew on many boat out in the Atlantic and understand the limitations. With the radar, AIS, and always have a watch person on watch I am about as prepared as I can be. Thanks to all for your feedback. I will let you know how the installation goes and how the radar preforms after our trip to the Dry Tortugas and Key West. Cheers