Radar Location

[Guest]

I have just purchased a JRC 1500 Radar and I now have to deside where to locate the scanner/redome. I know that the higher the scanner the better it is. My sailing boat is 30 ft medium to heavy displacement with a 36 ft mast and the radar weight is 4.5 kilos. I dont like the idea of drilling holes in the mast to fit a radar support. However, I can attach a long pole, say 2 metres/6 feet, on to the transom steel frame which is 2.5 metres high. Therefore, the scanner/radome will be at a hight of approximatelly 4.5~5 metres / 15 feet. Locating the radar on the stern of the boat instead on the mast, would that create any problems?. Thank you in advance.

 

[longjohnsilver]

Transom mount should be just as good, on my motorboat it's not as high and still happy with performance. It won't "see" as far, but so what, how often do you use 20 mile + scale anyway, most of the time it's 4 miles or less.

Also not having it on the mast means when heeled over you get better performance cos it's not pointed towards the sea so much.

 

[Guest]

Radar mounted on the mast gives a greater range (I believe the formulae for distance in miles to the horizon is 1.44 X square of height in feet) but unless you intend sailing in big waves, which would reduce the height when in troughs, the pole idea is OK. The mast would cause a blind spot ahead but as the boat is constantly yawing it will have little effect. Ensure the radome is well above head height or your chances of being a father will be reduced!!

Jaxon

 

[david_bagshaw]

As a motor boater with a mast mounted radar, and having a roof mounted one on the previous boat, go for the mast, better radar vision, less sea cluttter MUCH less likely to get damaged docking , feeding a rope round etc,

David
www.yachtman.co.uk

 

[brianhumber]

Fitted a JRC lasr year. After thought mounted on the mast low down BELOW the cross trees. Does not get in the way of the geny, no worries over weight high up on the mast. Works great, still get a 16m range and on shortest range picks up pot flags in a flat sea- great for St Vaast if going down the coast at night with no wind.

 

[Guest]

transom pole mount does not cause any problems. the 1500 comes with 15 meters of cable is this long enough for your mast mount to display?
Apart from the drilling the cable almost always has to be fed from the bottom of the mast due to the display plug,and there is the waterproof deck gland as well.
You can get special conduit to mount on the mast for cable runs.
You also might need a guard to save the forsail from fouling or wearing.
In your owners manual you will find the ranges expected versus hight.
Given that most of your time you will proberbly use the 3-6 mile range a transom pole mount will do.
There are lots of mast mounted units around but I have always found the time I wanted mine was was a when I can't see a thing due to rain fog etc and most of the time when in that situation I have been rolling badly, with little way on, I know a mast mount picture is not to good when heavy rolling as the set is constantly moving targets around. which is why on my second installation I opted for a pole.
like usual the proof of the pudding -------- ask other owners at your moorings first before you start.

one last thought, use the set all the time and play with the settings till it and the interpretation of the screen is second nature. learn how far off are two objects before they become two instead of of one, practice pratice practice, on judgeing the speed and heading of other vessels. all the above best are practiced initially moored fore and aft
regards Adobe

 

[johndf]

I likewise mounted mine on the mast, just above the crosstrees in my case, but not more than 5 metres above the waterline. It works perfectly well there, so I'd have thought a transom mounting would be fine.

 

[Guest]

Thanks for the replies. However, what about performance?, would a radar located at the transom have a blind spot due to mast and sails being in front of it and in the way?

 

[Twister_Ken]

Eerr? On most boats, when the mast heels the transom does too.

 

[DaveA]

my radar is fitted on the transom on a 6 ft pole ,it was fitted by the previous owner- I find it works no problems at all ,the mast has no effect. I do however ensure no one stands close in its beam when its on- (health precaution)

 

[PeterGibbs]

Why do you not want to drill holes in the mast? This is, after all, the best position for the radome. Contrary to rumour the dome does not foul foresails.

Siting it about 17-20 feet above deck will give a suitable horizon. Lower, and it will just reflect waves (pretty but not what you paid good money for!) Higher, and there is no gain to offset the greater difficulty of getting to it once a season and servicing it (assuming the mast stays up)

The useful horizon of your radar is 16 miles. The normal range position will be about 6 miles for navigation purposes. At 16 miles the range is stretched.

Siting the radome either on the mast or transom will incur no shadow penalties.

PS if you site on the mast and take the mast down in winter, DO cover the radome fully so that no rain can get in whilst the dome lid is on its side: if you don't do this water will accumulate in the dome and it's good bye to an expensive piece of gear.

If you use a pole mount on a smaller vessel be advised it will be vulnerable to all kinds of damage, and will be difficult to fix firmly.

Siting the screen is the bigger problem, as you no doubt know. You will only use your radar perhaps 2-4 times in a season, unless you intend to cover many night miles. Rather than clog up the instrument panel / navigation table, do consider having the screen mountable under the hood (you have one?) with flexible leads stored behind the instrument panel. Install it when the need arises(I do this) This way you can actually see the screen AND work the important range and VBL controls and see the images whilst at the tiller / on deck. This you cannot do if the screen is mounted sideways below: believe me.

PWG

 

[jfkal]

Need some advise on dimensions of the tube for a stern mount option and some good tricks on how to secure that tube there.

 

[HaraldS]

The height you suggest for your stern mounted variant would be plenty high. Staying 2.5 to 3 meters above deck is certainly safe from a radiation perspective. If the hights you mention are above deck, then an extra pole extending up from your transom frame by half a meter to a meter would be fine.

Don't assume the extra height will buy you a lot. Height differnces of the targets you are worried about are more than the height difference you can influence. Big ships raise 20 and more meters and if only a few meters of their construction are above horizon, your radar will pick them up, most likely before you see anything on a clear day. Very flat land is hard to pick out anyway, and rain clouds you can always get.

So I wouldn't give the height a too big weighting. There are also disadvantages in going higher, in that the acceleration forces increase when the ship is rolling. For a solid mounted radar that doesn't matter too much, but if you think about a gimbled attachment, then it does. You will need damping, but the higher you go, the harder it will be to find a good compromise.

A gimbled bracket or a manually adjustable bracket are not a bad idea. With 30 degrees vertical beam angle, the JRC will cover more heel than most, but still once you heel around 15 degrees you will start getting blind abeam. Being able to offest the unit is a nice feature.

Main mast, versus post at the stern:

On the main mast you will have a better performance forward looking, but a list of other drawbacks: Quite bad looking back with the mast so close to the scanner. Higher acceleration forces. More likely contact problems on connectors, especially on deck stepped masts, and more so if you take the mast down in the winter. The scanner can easyly get damaged in that proccess too. I would also think the blocking efect of wet sails would be more.

On the stern post you will have worse performance in the direction of the mast, but since the scanner antenna is wider than the mast (about 2 versus half a foot in your case), most of the power will still go by the mast. Some people say that because of the reduced performance through the mast, the stern post should never be in the centerline and if possible offset to starboard, so that you have the best performance in the sector where you have to give way.

The direct reflection from the mast doesn't matter, since the receiving circuit is still off to surpress the immediate vicinity. Indirect bouncing however does and you can get ghost echoes. They show up in the direction of the mast for very strong targets that are relatively close to the left or right of the mast. With some practice you will be able to tell those from the real ones.

If you have a spinnaker pole on the mast, try to hide it behind the mast for the radar antenna. If you have two of them, like I have, then that's a bit more difficult to do. But I found my 3ft antenna goes by this whole block quite ok.

 

[HaraldS]

My tube is 100mm diameter, goes through the deck and has another fixing point about a meter below deck. It stands free above deck and is strong enough to support a little 100kg crane as well as a 21kg radar antenna about 2.6m above deck. It is of some aluminum alloy and comes from Italian maker Nemo. I can send you a photo if you want to see what this looks like.

 

[jfkal]

Thanks Harald,

a photo of the deck joint as well as the one below would be great. Is the tube fixed (laminated?) to the deck or is there a flexible gasket allowing the whole setup to move and flex?

Please send pictures to joerg.kalisch@dexteritas.com

Thanks again for your help.

 

[harvey]

Either will do as well since at the mounting heights you're talking about you will get the range thats available in the set anyway. The really important thing is to make sure that you put the display where you can see it and play with it whatever the weather and that means demountable under your spray hood. There's no point having it mounted down below in foggy conditions when you will want to be up on deck. You will also want to have the radar on in good vis for practice so that you can get accustomed to interpreting the picture.
Pip Pip

 

[ParaHandy]

I also purchased and fitted a JRC1500. Spent weeks trying to decide where to put it. Height (within reason) is not so important. In theory, with a radome 6mtrs above surface you can see an object 9m away which is at least 6mtrs high. The higher you are, the poorer will be your close range discrimination.

The advice from adobe may well determine your course of action. If you purchased less than 15mtrs cable, put it on the transom.

I decided to put it on the mast (at about 16' from deck). I doubted I could reinforce the deck adequately to support a 6' pole (eg somebody falling against it) and the installation was not so much easier. It's tidier up the mast and radiation hazard much reduced. The additional holes drilled thro' the mast should make no difference to it's strength (use industrial rivet gun). I decided to cut the cable having read in MBM of one owners experience of doing so (JRC1000) as it would make feeding the cable v. much easier. Otherwise you will be forced to drill 25mm hole to feed the connector - not just thro' the deck but at every other bulkhead. I used a simple connector box to join the 12 core cable (2xCoax + 8 singles) and it works fine.

Major difficulties are: feeding the cable through the mast (but worth the effort - drop a plumb line and fish the weight out) and the user manual is only adequate.

 

[brians]

Do you have the 1000 or 1500? Would you recommend JRC - they seem, very simple to use compared to other makes.

 

[brians]

Mast mounted would have a blind spot behind - just as bad in Southampton Water !

 

[ZEBEDEE]

I do not think that you will have any problems with your scanner mounted aft. The formula for Radar range in miles is 1.23 x Sq Rt of antenna height height of target therefore if your scanner is 10 ft high and target is 15ft high range = 1.23 x sq rt 25 which = 1.23 x 5 = 6.15 Miles. your mast will also be less of a problem from this position, horizontal beamwidth of most small boat Radars is about 4 degrees so will easily see around your mast.
Also with scanner lower the stability of the boat will not be affected as much as if you had it up the mast, where you would get a blind arc

<P ID="edit"><FONT SIZE=-1>Edited by ZEBEDEE on Mon Feb 4 15:49:51 2002 (server time).</FONT></P>