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Old 21-07-2011, 02:53   #1
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GPS Antenna Flush on Reverse Transom ?

Greetings all,

I'm just about to install my Maretron GPS antenna which I bought almost a year ago (!) and can't install it in a "proper" location, due to fears of kicking it etc (narrow/crowded boat) and I don't have a radar arch.

Also, the usual option atop the aft rails for me is a no-go as the idea seems a little flimsy to me... cabintop is also unavailable due to wiring restrictions.. which leaves unfortunately few options for me to pursue.

However, I do have a reverse transom. It is at a 40 degree angle from the horizontal plane so I'm thinking of mounting it directly on the transom, somewhere near the middle, off to the side of the backstay. This way, there's nothing that can foul on it and it will see most of the sky (albeit at an angle).

Actually I have a need to mount another GPS antenna as well (one for the AIS) - and am planning on mounting that right next to it as well.

Any thoughts about this - and why I should/shouldn't do it? Thoughts about following seas/being pooped ripping off the antennas while on passage etc?

Thank you in advance and looking forward to your thoughts!
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Old 21-07-2011, 04:08   #2
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Re: GPS Antenna Flush on Reverse Transom?

Not a good idea in my opinion. GPS position accuracy relies on good satellite geometry which means the satellites being used to calculate the fix are at relatively evenly spaced angles from your antenna. Some deviation from perfectly horizontal for you antenna is ok but 40 is a bit steep.

Eric
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Old 21-07-2011, 06:00   #3
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Re: GPS Antenna Flush on Reverse Transom?

I agree with Eric. It needs to be horizontal and have a clear view of the sky all round...insofar as possible.

Put it on the stern rail. There's no reason it should be "flimsy". I have two of them on my stern rail which have survived multiple hurricanes, snowstorms, and many thousands of sea miles.

You can see them here:

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Old 21-07-2011, 06:06   #4
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Re: GPS Antenna Flush on Reverse Transom?

It could be mounted on an antenna ratchet mount to make horizontal or have a fiberglass or teak platform made to make horizontal.
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Old 21-07-2011, 06:50   #5
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Re: GPS Antenna Flush on Reverse Transom?

Be aware that having a GPS at the stern will maximize the error of every anchor alarm I've seen (except for Vesper's new AIS system) a significant amount. I know it seems counter-intuitive but the math is more complex than you realize.

As a specific example, if you have the GPS 40' back from the bow, anchor in 10' of water, put out 50' of chain, and figure that an 80' anchor alarm will be good to warn of movement in the tight anchorage, you'll find that when you swing with the tide, your anchor alarm will go off. Even though your anchor hasn't dragged an inch, it'll show you 50 + (2 * 40) = 130' back. That will make your 80' alarm go off at 3:00 am. The best approach is to have your GPS as far forward as practical.
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Old 21-07-2011, 06:55   #6
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Re: GPS Antenna Flush on Reverse Transom?

Can it be mounted on the coaming? Mine is mounted on the coaming, see just below the blue line on the right.

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Old 21-07-2011, 07:02   #7
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Re: GPS Antenna Flush on Reverse Transom?

You can find GPS mounts that clamp on to the backstay which should avoid all the fouling issues.

Jeff's points regarding anchor alarms are well taken, and part of why I don't use them any more. I like zoomed in tracks on the chart plotter better.
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Old 21-07-2011, 07:08   #8
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Re: GPS Antenna Flush on Reverse Transom?

Unless he has a cat or tri the antenna will never be horizontal, close maybe on the fore and aft plane, but never side to side.
When I was installing my GPS, watching signal on screen, had antenna laying on cockpit sole and the screen showed me in my slip, not recommending there.
I have wheel steering and mounted mine on the pedestal guard rail, out of the way and no wires run all over the boat. Any where you can mount it horizontally and SECURE will be good.
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Old 21-07-2011, 08:00   #9
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Re: GPS Antenna Flush on Reverse Transom?

Mine is simply mounted up against the overhead. Right at the nav station. Inside the boat. Works fine. No tangles. No windage. No ugly.
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Old 21-07-2011, 08:00   #10
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Re: GPS Antenna Flush on Reverse Transom?

The "rules" for GPS antenna locations were invented when GPS receivers were much less sensitive. And fiberglass is pretty transparent to GPS signals. I'd experiment with mounting it inside the cabin or even under a side deck. I have my AIS antenna as well as the puck GPS I use with the laptop below and both see just as many satelites as the main external one on a rail. What you don't want is big hunks of metal close to the GPS antenna that block it's view of a big part of the sky.

And think again about the rail mount. I agree that it looks flimsy but you don't hear about problems. Mine's back there and it has survived just fine.

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Old 21-07-2011, 08:33   #11
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Re: GPS Antenna Flush on Reverse Transom?

Mine has been on the aft rail since mounted by the previous owner in about 1998 or so. No problems I'm aware of except aesthetics.

It's mounted such that it will move if bumped to save it from damage. Occasionally I correct it's orientation.
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Old 21-07-2011, 09:31   #12
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Re: GPS Antenna Flush on Reverse Transom?

Quote:
Jeff's points regarding anchor alarms are well taken, and part of why I don't use them any more. I like zoomed in tracks on the chart plotter better.
I like track displays too - it is a good way of viewing where you've been and a great idea to keep running. But how do you use one to wake you up in the middle of the night? That's where those single point anchor alarms fail.

Quote:
I'd experiment with mounting it inside the cabin or even under a side deck.
This is really good advice. Modern GPS's do pretty well inside. It's often possible to have an external antenna on one too.
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Old 21-07-2011, 10:34   #13
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Re: GPS Antenna Flush on Reverse Transom?

A co-worker just brought in his Nuvi 1390 GPS to show me this morning. It had no problem at all picking up good signals in my shop which has a gridded drop ceiling, above that, a mass of electrical conduit, air ducts, sprinkler system piping, and a wooden sloped roof several inches thick with layers of roofing material on top. My shop has no exterior facing walls and the exterior facing walls of the building are aluminum panel siding. I don't know what kind of antenna this thing has in it but the display is only about a half inch thick and I was holding it vertically. Amazing sensitivity!

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Old 21-07-2011, 13:50   #14
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Re: GPS Antenna Flush on Reverse Transom?

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Originally Posted by ActiveCaptain View Post
I like track displays too - it is a good way of viewing where you've been and a great idea to keep running. But how do you use one to wake you up in the middle of the night? That's where those single point anchor alarms fail.
It won't wake me up. Agreed that the alarm part of anchor alarms is of value except the false alarm rate is too high, in part due to the issue you identified in an earlier post. The only way I have come up with to use an anchor alarm effectively is to set it when the anchor goes down (when lots of other things are going on) and setting the alarm radius to a bit more than scope.

Instead, I turn track on and zoom in as much as possible. The "smile" shape of the track is confidence inspiring. After all my time hanging on my ground tackle the number of time my middle-aged prostate drives me out of bed over night is plenty to keep an eye on things.

Of course in the US, my iPhone with Charts & Tides and ActiveCaptain (*grin*) lets me check any time I wake up without having to get out of bed.
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Old 21-07-2011, 14:07   #15
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Re: GPS Antenna Flush on Reverse Transom?

I have never set an anchor alarm. When I anchor I always check the land and try to find two prominent landmarks (trees, rocks, houses, dark shapes, anything) in a row, or close to being in a row, like a range. If they change significantly I'm dragging. That's a lot easier than taking bearings. During the night even in the most benign conditions I'll get up a time or two and look around. When it blows my wind generator (we sleep in the aft cabin) tells me to get up to check things out. I don't think I could hear the beep beep of an alarm when the wind starts honking.
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