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Old 09-12-2010, 07:59   #1
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Backstay- vs Pole-Mounted Radar

Hi all.
I'm thinking of installing a radar and I'm interested in finding out what are the arguments for and against a backstay vs a transom pole mounting. Also, I'd like to know with a backstay mount, can one still tighten or loosen the backstay without affecting the radar mount?
Any views would be much appreciated.
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Old 09-12-2010, 08:25   #2
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Originally Posted by Srah 1953 View Post
Hi all.
I'm thinking of installing a radar and I'm interested in finding out what are the arguments for and against a backstay vs a transom pole mounting. Also, I'd like to know with a backstay mount, can one still tighten or loosen the backstay without affecting the radar mount?
Any views would be much appreciated.
The radar antenna mount should have a way to level the radome - this is handy if you're on a heeled sailboat and want the radar to illuminate the horizon rather than the sky.

Radar mounts built onto the backstay do a couple of things: place a side-load on the backstay, may make it difficult to inspect the backstay for damage, and position the radome on center-line.

The downside to having the radome on centerline is that an aluminum mast blocks the image directly ahead of the boat; it's odd to see a buoy ahead of you, see it on the radar display, then point the bow directly at the buoy and the spot on the radar display disappears - somewhat disconcerting that the one thing you really want to see (something dead ahead of you) is hidden from the radar.

Backstay mounts may incorporate a typically expensive (but nice) automatic hydraulically-dampened leveling mount that cost more than the radome itself (I don't understand why one would bolt a radomes to a fixed mount on the mast). Struck me as simpler and more bullet-proof to have some sort of person-operated leveling mechanism.

I like to separate equipment such that a failure of one will not impact using the other; if I want to replace the backstay I do not want to undo the radome, and if the rig comes down I would lose the radar (I'm not sure why that matters, but it is an unecessary connection between the two).

I first went with a Garhauer vertical pole set on the transom offset to starboard, and used their manual screw-lozenge leveling mount for the radome; it worked just fine. Later I added a wind generator and solar panel to help the boat's charging circuit. To do this I removed the Garhauer pole and built an aluminum compression/tension 'H' frame to carry them and put the radome on the starboard vertical pole of the 'H'. Garhauer made up a custom version of their leveling mechanism that I installed on the aluminum 'H'. It's all worked well to date.

- rob/beetle
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Old 09-12-2010, 08:55   #3
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Photos? Dimensions?
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Old 09-12-2010, 09:07   #4
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Some argue about the need for the additional weight/complexity/expense of a self-leveling radar mount. Typically, the radar isn't in use much while sailing with a heel, but in any event the angle of view of the beam/receiver is fairly wide so only slightly affected unless heel is large.. The view straight ahead and behind isn't much affected by heel at all.

I'm no expert on this, just regurgitating the arguments of others that make sense to me!
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Old 09-12-2010, 12:24   #5
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Quote:
Originally Posted by SoonerSailor View Post
Some argue about the need for the additional weight/complexity/expense of a self-leveling radar mount. Typically, the radar isn't in use much while sailing with a heel, but in any event the angle of view of the beam/receiver is fairly wide so only slightly affected unless heel is large.. The view straight ahead and behind isn't much affected by heel at all.

I'm no expert on this, just regurgitating the arguments of others that make sense to me!
Our experience is that there is a big difference in the vertical beam width between different model radars. To wit: our elderly Furuno 1720 has a rotor made from a circuit board with antenna elements "plated" on it. Its vertical beam width is large enough that heeling has little effect. On our previous boat we had an Anritsu radar which used a cute little parabolic reflector type antenna, and this had a very sharp vertical cutoff angle. This one was blinded severely when heeled more than about ten degrees.

So, the requirement for a leveling mount varies with the specific radar in question... check it out for yours.

Cheers,

Jim and Ann s/v Insatiable II lying Church Point, NSW Oz southbound
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Old 09-12-2010, 13:08   #6
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Originally Posted by JDGreenlee View Post
Photos? Dimensions?
Here's a picture taken by Latitude 38 (thanks, LaDonna!) of Beetle coming in on approach to the Golden Gate.

The image shows the basics of the 'H' frame: vertical pole to starboard and to port, a single horizontal brace, and a pair of forward set of legs. What you can't really see in the picture is the 'X' brace (a pair of tension wires) that stabilize the 'H'.

I stole the design idea from a picture I saw many years ago of a BOC race boat with a terrifically simple 'H' that looks much like what's on Beetle. What I wanted was the lightest possible structure that would support the gear, and what we came up with is pretty darn light (around 50 pounds total) in Aluminum, I'm sure it could be done even more lightly in carbon - but carbon was completely beyond budget.

I can pull the dimensions tonight from the engineering drawings, if you're interested.

http://sfbaysss.org/TransPac/transpa...beetlegate.jpg

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Old 09-12-2010, 13:19   #7
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Originally Posted by SoonerSailor View Post
Some argue about the need for the additional weight/complexity/expense of a self-leveling radar mount. Typically, the radar isn't in use much while sailing with a heel, but in any event the angle of view of the beam/receiver is fairly wide so only slightly affected unless heel is large.. The view straight ahead and behind isn't much affected by heel at all.
As a shorthander, I operate the radar pretty much continuously (on a 5 minute watch mode) while offshore - so I do use the radar a lot while sailing, rather than just when it's foggy and motoring along in the flat water (which also happens around these parts a fair bit).

The vertical beam angle of a Furuno 24" radome is 20 degrees (according to Furuno's current brochure) which provides 10 degrees above and below horizontal. If the radome heels more than 10 degrees it's no longer painting the horizon; based on my own empirical evidence (e.g., seeing the ship while playing with the radar) the radar is far more likely to pick up the ship on if the radome is kept horizontal, rather than leaving it fixed in the same plane as the boat.

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Old 09-12-2010, 13:21   #8
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Thinking practical issues we've gone with a pole mounted randome on both our last boats.

We always had to drop our backstay when the boats were lifted. A combination of the size of the lifters we typically found around the Med, and our need to be lifted backwards. Meaning anything else added to the backstay gave us something else to worry about at that time.

Our boats typically rolled more than the specified spread of our radar beams, but our fixed randomes have never once given us a issue in picking up objects despite using both when heeled. So personally I really do have to question the values in having the randome adjustable.

It's not just added weight aloft of IMHO questionable value, but KISS really does work on a sail boat.

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Old 10-12-2010, 09:49   #9
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My original question arose because I'm buying a new boat and the only option was a fixed mast mount, so I'm investigating the alternatives. The magazine Yachting Monthly did a test which showed that beyond a heel angle of 10 degrees there was a serious diminution of performance. {of course taking on board the comment made, this could vary by brand). While, judging by comments made, some people have not found this to be real problem, it would seem that, if one can afford it, a self-levelling mount is a useful addition.
To go back to my original question, are there benefits to a backstay mounting?
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Old 10-12-2010, 10:45   #10
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Talked with a couple on a 40 footer who were having a frame mount built for their radar. They'd had a backstay mount antenna but it ended up doing 360s around the backstay when they got into nasty weather off the Oregon coast. Don't remember who the manufacturer of the backstay mount was but these people weren't impressed with it's durability.
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Old 10-12-2010, 13:15   #11
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I have the 24nm Raymarine radar on pole mount at port quarter.
I did originally wonder about performance whilst healed. To be honest - I have seen no loss of performance.
For example - when heeled to stbd - I don't seem to have lost the ability to acquire and track tgts off the port bow, or abeam to port.
I haven't checked the tab data - but I suspect there is a 15deg + vertical beam width.
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Old 10-12-2010, 14:55   #12
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I use my radar on a pole mount aft; while I was in San Francisco in 1994 I had Spencers (?) make a manually controlled leveling mechanism for me. I used a Furuno 1621 LCD radar on that mount until 2 years ago when the radar died and was replaced with a new Furuno Navnet 3D system.

When I first installed the radar I used the leveling mechanism and got a bit of a lesson in the obvious: when heeled over most of the signal on the weather side goes up into the sky (reducing returned signal), and most on the lee side blasts down in the water (reducing signal and increasing noise). The modern radars compensate for the disparate performance so all appears well with adequate results. But I can assure you that the radar performs noticeably better when it is level - something that was easy to test with a manual control. That said, I have mostly put up with merely adequate performance instead of climbing aft to manually adjust the level every tack. Only on some long offshore passages have I messed with keeping it level. If there were a good, inexpensive and reliable self-leveling mount I would buy it, but not for $1000+. BTW one of the problems of a mount that is swinging is that it puts wear and tear on the cable connection between the (fixed) backstay and the (moving) scanner, so factor in somewhat reduced reliability - and try to minimize cable movement. This should be less of an issue, at least for repair, with modern ethernet connections but the older cables were a nightmare to work on.

One of the values of a pole mount is that the radar is available with the mast down. If you don't plan on cruising in canals/rivers with the mast down this isn't an issue. If you do, then you might want to consider it. I use the pole for mounting other antennae as well as a solar panel. The brace for the solar panel also serves as a crane for lifting the outboard. I put the wind generator on a completely separate pole as I don't think the extra vibration would do the radar's mechanism any good - probably being overly cautious.

I hope this helps.

Greg
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Old 10-12-2010, 16:18   #13
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On a boat that is trimmed properly....the backstay will sometimes be too loose to support the radar in any reasonable way. Use a pole.

I've found leveling swivels to be not worth the effort. Reducing heel has many advantages. This is one of them.
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