Expert advice on where to mount a raydome
In the May 2006 issue of Yachting Monthly, a reader’s question about the position of a raydome is answered in brief by one of YM’s panel of experts. Here is the full version of that question and answer.
I’m looking for some advice regarding where to fit a raydome on my yacht (an X-382). I’m planning to fit a Raymarine C series with an 18″
raydome and would like to mount it on the mast of my X-382. Due to the configuration of the rig – triple spreader with a baby stay and high pole uphaul – the lowest point on the front of the mast where it could be mounted without interfering with the rigging is at the level of the second spreaders, about 12 metres up. I’m not keen to put it that high because of the obvious problems associated with putting that much weight at that height and the amount the scanner will move around in even the slightest swell. I’m also not keen on putting a pole on the pushpit.
I have seen some people who mount their radar scanners on the side of their mast. Doing that would allow me to mount the scanner much lower, at around the level of the first spreaders. I would obviously expect some loss of performance on the side that the scanner was looking through the mast. I would be prepared to accept some loss but I don’t want to mount the raydome and then discover that I can’t see a thing. Any advice? Richard Clark, via email
Jason Sidaway, Raymarine’s Senior Product Support Engineer Radome Scanner Installation replies: Mounting of the radome scanner on the mast is very popular and very much typical for yacht installations for 2 main reasons: 1) safety and 2) range, but there are important considerations to take in account when mounting the radome scanner on the mast.
In terms of safety, this offers an ideal location as the transmitter is well above head height avoiding mechanical danger (not so much for a radome scanner as it is enclosed) and electromagnetic contact removing the potential for compromising personal health on-board when the radar system is operated for long periods and minimising exposure to the crew.
With respect to range of the radar system, since radar basically operates at line-of-sight, the suggestion of higher the mounting position, the longer the range performance, but this is not always the case, remembering that a yacht will be moving and the boat will be ‘heeling’ . Therefore the radar antenna will be as well. Increased height will offer improved long-range performance but at the same time, increases the minimum short-range detection circle as well as exaggerate any rolling or pitching motion of the vessel.
Therefore, a compromise as such is necessary in attempt to mount the radar scanner as high as possible but take into consideration, the movement of the vessel… A simple equation can be used to calculate the distance to the radar horizon which is approximately 1.2 times the square root of the height of the scanner. For example, a scanner mounted 20′ above the water-line will provide the radar of 5.4 miles range before the radar beam is ‘blocked’ by the earth’s curvature.
Immediately, you may say this is contrary to the marketed maximum range scale of the radar but there are two factors to take into consideration with regards to radar range: 1) height of the antenna and 2) height of the target, so the same equation can used to determine the targets radar horizon, by using the sum of the distance to the targets radar horizon to the maximum radar horizon to derive the maximum detection range for that target.
So same example, a scanner mounted 20′ above the water-line will provide the radar of 5.4 miles range, radiating a target that is 20′ feet tall, can detect that target at a maximum range of 10.8 miles, also remembering that atmospheric and weather conditions can either increase/decrease the range.
The specified maximum range of a radar antenna is determined solely by the ability for the radar system to transmit a pulse to a maximum range and detect a returning echo, the specified maximum range can be compromised and affected by antenna height, target height, weather, horizon etc etc
As already mentioned the main problem for sailboat installations for radar is ‘heeling’, all Raymarine radar products have a vertical beamwidth of 25 degrees, meaning that 12.5 degrees of the beam is directed downward toward the water and 12.5 degrees directed upward with the centre aimed at the horizon. If the boat is heeling in excess of 12.5 degrees port or starboard, the lower beam will be directed above the horizon and essentially rendering the radar blind in that direction. Additionally, all of the radar energy will be directed into the water, resulting in increased sea clutter… from this aspect, you will heed the benefit a ‘gimbal’ style mast mounting, which will abolish this problem, maintaining the radar antenna parallel to the waterline.
Another factor to take into consideration is ‘blind spots’ which can be evolved as a result of detection of masts and booms, if the antenna is mast-mounted, the radar will block the sight of the radar in the direction of the mast, on a typical mast mounted installation, the radar can be subjected to a blind spot of 1 to 15 degrees wide (this is obviously unavoidable) depending on the mast thickness. However, because this is directly astern, it is relatively of lower concern to the user, as generally the higher risk is forward, port and starboard of the vessel. This is potentially where a backstay installation is preferred with ‘some’ distance between the antenna and the mast, there will be a small blind spot in the direction of the mast.
Additionally, this is also a reason why ‘some’ scanner installations are located aside of the mast on the spreaders as a combination of the movement of the vessel and the intelligent software used within the Raymarine radar products, it is very unlikely to detect a blind spot visually on the radar display.
Therefore, personal view is that a radar antenna mounted on the backstay using a gimballed mounting is the best choice for a sailboat, although you will sacrifice range performance by mounting the antenna lower and at the same time increasing short range detection, and you will minimise the probability of blind spots BUT remembering the safety aspect with an antenna mounted overhead.
Typically, a radome scanner when mast mounted will be installed on the forward facing of the mast and in some cases, is protected using a radar guard to prevent the sails and rigging from ‘hitting’ or ‘snagging’ on the radome scanner.
With the cable run, a high number of sailboats are removed from the water and the mast is ‘stepped’.It is recommended that the cable is passed through a waterproof deck gland and a suitable junction is applied to the cable using a suitable waterproof junction box, located inside the boat.
CONCLUSION: the preferred mounting would be on the cross-trees as initially suggested by the customer, offsetting the antenna on the cross-trees should not present a hindrance to the performance of the radar system; in terms of mounting the scanner to the mast, Raymarine do supply a standard mast mounting bracket which would not facilitate the required mounting, suggestion would be to contact Scanstrut (www.scanstrut.com) for recommended mounting solutions but the probability is that a purpose-fabricated bracket will be necessary.