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  #1  
Old 12-03-2008
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Real world experiences using radar -- how far can your radar really "see"? MARPA?

I'm doing some online shopping for a radar for our Beneteau 50. It looks like there are 24 nm, 48 nm and 72 nm radars. If I'm only mounting the radar 15 feet off the deck, will I be wasting money/windage/weight if I get a 48 nm range? The 48 nm ones tend to be 24 inch radomes instead of 18 inch.

Note: I know about the line-of-sight calculations and have seen a site or two that helps you calculate it. But I heard that radar bends, so those equations aren't completely accurate.

What's your real-world experience using radar, distance-wise. How far away can you "see" ships? (And did MARPA work for you?)
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Old 12-03-2008
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The 24" radomes will have better resolution than the 18" since the beam will be narrower generally speaking. Just remember that radar is good for distances but not very accurate for bearings. The new HD ones from Garmin are very nice, at least from what I've seen and heard. I don't own one, but installed two of them this past season.

As for mounting it...be aware that the higher you mount the radome, the greater effect boat pitch and rolling will have on it. Mounting higher also increases the effective range, but that works at both the near and far ends... and if it is mounted too high, you'll lose the ability to use it close in to the boat—where it is probably more important.
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  #3  
Old 12-03-2008
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Quote:
Originally Posted by Bene505 View Post
I'm doing some online shopping for a radar for our Beneteau 50. It looks like there are 24 nm, 48 nm and 72 nm radars. If I'm only mounting the radar 15 feet off the deck, will I be wasting money/windage/weight if I get a 48 nm range? The 48 nm ones tend to be 24 inch radomes instead of 18 inch.

Note: I know about the line-of-sight calculations and have seen a site or two that helps you calculate it. But I heard that radar bends, so those equations aren't completely accurate.

What's your real-world experience using radar, distance-wise. How far away can you "see" ships? (And did MARPA work for you?)
I would estimate my radar as being about 25 feet off the water or so. Here is a pic, you can guess for yourself. I have the MARPA function, but have never used it in a real world situation.



One of the buggest plusses of a larger radar is not how far out it sees, but the resolution. The larger domes have a narrower beamwidth that allows better clarity. How this affects a boater is that it might pick up two ships/boats instead of one - especially on the horizon. As the distance goes further out, this beamwidth becomes increasingly more important (think of a triangle with the point being your boat).

Now how does this affect the sailor? Not a lot.

In order to get the maximum range via line of sight, you have to mount your radar higher up the mast. However, the higher you mount it, the more susceptible its losses/disruptions are to heeling. The more it heels/shakes, the less worthwhile any of that clarity is. Does that make sense?

In a magazine, it would seeem that for the modest amount of money more that a 4kw is versus a 2kw, it makes sense to get the 4kw for better clarity and distance. In the real world, in my experience, it makes no difference. The only way it would matter is if the seas are perfectly flat and you are motoring. If that is the case, why are you even using the radar??? The exception to this is to get a radar leveler in which case the clarity and distance of the 4kw would really become apparent. However, now you have really increased your cost, install, weight, and power.

Here is where radar has become very useful for us: At night - especially down the ICW. That is where overlay is especially helpful. You will then be able to see markers that are supposed to be there via the chartplotter/radar, and the ones that are NOT supposed to be there! Radar has avoided several collisions or close calls for me in those instances. It has also helped me to see storms that were coming out at sea and make preparations (unlike what others have said, I have never been able to steer around them).

For those in fog prone areas (which Florida is not), the radar's benefits are obvious.

I think you would be better off spending less money on the 24 and going with it. If you want to spend a bit more, get the leveller. But I think you would be fine without it.

THose are my opinions,

- CD
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  #4  
Old 12-03-2008
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Yes and no...

Personally I don't see much benefit to a 48 mile dome over a 24nm or 36nm, in the case of the 18" Garmin, for on water distance. I prefer an 18" dome size for obvious reasons on a sail boat and currently Garmin packs the most performance into an 18" dome. The GMR 18 is a 4kw 36 mile unit but it's 18" isnead of 24".

As you go bigger in dome the screen resolution does get better because the beam width is narrower. I have used very large, and very expensive, 72 mile radars on mega yachts and large fishing vessels and the screen resolution is great but you still can't see beyond the curvature of the earth. Anyone who claims they can see anywhere near 24nm, 36nm, 48nm or 72nm, on water, is full of bull excrement.

What you can see with a bigger dome is weather but with the quality GPS weather subscriptions these days why bother adding all that weight & expense?

Beyond that the new High Def 18" domes are amazing! Furuno & Garmin have them & I believe Raymarine will. My Garmin was very, very sharp right out of the box and after a little tweaking matched resolutions and target displays of units much larger, and more expensive, that I have used.

I use radar a lot, being from Maine, and probably have well over 2000+ hours, perhaps more, (was a commercial fisherman for about 8 years in my younger days) and literally thousands of miles of time spent in the fog with radar. My suggestion is quite the opposite of dogs. Get the dome on the mast and get it high.

The junk Dog and I banter about with close in targets is just that junk, especially when it comes to the real world!

I have picked up kayaks at close range and many other close in targets with a dome on the mast. In my life there have only been a few instances, perhaps one or two, out of thousands of hours in thick fog, where I had so little vis that a lower dome would have even mattered. In contrast I have had hundreds of times when more range would have been better.


A dome, regardless of it's rated range, 10' off the water will see a 10' tall target at about 7.6 miles.

A dome 20' off the water will see a 10' tall target at roughly 9.2 miles.

Most mast mounts are roughly 25 feet high or more!!

Never under estimate the distance you'll need. We have a ferry up here called the "Cat" that runs at over 50 knots. A 6nm dome visibility is NOT enough in pea soup for a vessel traveling 50+ knots trust me.... If you sail in shipping lanes or high traffic areas you'll want as much distance as you can get.


The higher the better!!!

The first photo bellow shows our old boat with mast mounted radar. The second photo shows my neighbors boat which I could pick up clear as day with that mast mount. If you have not seen a target on your radar before it gets that close you have MUCH bigger issues like going back to the owners manual big!

Even with my spar mount on my Catalina 310 I could make out this neighbor, the C&C just behind my boat, clear as day and he has no reflector...


Our current boat has a pole mount radar and I will eventually move it back to the spar for better range performance. Keep in mind that a pole mount ten feet off the water, in ten foot seas, will lose targets and range performance every time you are not on the crest of a swell of a wave and when you get to the bottom of a 10-12 footer you will have zero target tracking ability until you begin to come back up and reach the crest again where the best performance will be. The has happened to me more than once with low domes.
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Last edited by Maine Sail; 12-03-2008 at 11:42 AM.
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  #5  
Old 12-03-2008
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May not qualify as real world but a good review if you haven't seen it already.
Boat marine Product Review/Test; Garmin GMR 18 marine radar review
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i have a 24nm raymarine and just finished a run from miami to woods hole mass and back - i did not use radar a lot - main use was in fog, at sea especially at night and to use marpa to track big boats and how close we were going to get -
the radar is mounted on a quest pole and gimballed - i would est it is 12-15' above the waterline -
marpa worked well not so much to miss big boats in a narrow area - such as delaware bay or cheaspeake bay or those barges running close to shore at night - gives me peace of mind that i know how close we will be and if i think they will be to close can call them and determine their course and notify them that i will change mine - and i like peace of mind
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in miami for repairs before heading to the bahamas
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Old 12-03-2008
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Quote:
Originally Posted by davidpm View Post
May not qualify as real world but a good review if you haven't seen it already.
Boat marine Product Review/Test; Garmin GMR 18 marine radar review
Good read, but poor quality photos?

Here is a shot of mine, right out of the box, and before I even tweaked any of the settings. HD is sharp!!
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  #8  
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Range is basically a function of height and power. There is no substitute for either. Dog's comments on rolling are certainly valid though I am not as enamoured as he on using radar for close-in work.

You would not be wasting your money on the more powerful set as you'll reap better overall performance within the range constraints of your mounting height.

Now expected range is a bit of a touchy subject and it's not good to raise false expectations. I've used very high powered radars that could reach out over one hundred miles. It should be mentioned that this was with an antenna height well in excess of 100 feet over the waterline of the vessel. And I've picked up returns at that long range but only a certain type of return. This holds true for less powerful radars as well. The returns I got were high coast lines and mountains. But what image of the coastline was I seeing? Certainly not the beach edge. Perhaps the bluffs some distance back? Perhaps the tree lines even further back yet? Another words, all this technology was sufficient to tell me I was still in the Mediterranean Sea and south of Sicily. An idea which i already had some knowledge of. (g) Practically speaking, on a good quality commercial ship-mounted radar, 24 miles is the limit of the practical working scale for collision avoidance and navigation not much further depending on the uniqueness of the landmark being scanned.

On a sailboat you can expect substantially less but should understand that radar propagation varies tremendously with atmospheric conditions from day to day, or even time of day. I've picked up the southwest coast of Alaska at a range of 350 miles, even though my radar's max scale was 48 miles, under super refraction conditions. For six hours the ghost of the refracted signal was clear as a bell allowing me to see I was pointed directly at the Cook Inlet. Neat but, of little navigational use practically speaking.

A more powerful radar will punch through rain and mists more capably. It's easier to dampen the return via rain or sea clutter adjustments than to lack for return from a weaker unit. Range is important because you'll pick up larger targets at a greater distance. Larger targets are generally speaking the ones that are going to kill you, both due to size and speed. Things like ferries and container ships. You'll likely notice that some deep laden tankers do not show at the same range as, say, a container ship but they tend to be slower and you'll have more time to deal with them as they appear. You may only pick up a non-radar reflector equipped sailboat at six miles, if you're lucky, but your relative closing speed allows you ample time to act even then. Your biggest worry offshore, in my opinion, is the high speed ferry or container ship. At twenty five to thirty knots they can be upon you in no time. While you're unlikely to be able to take much evasive action in their regard, you are going to be able to reach out via radio and alert them to your presence in a timely matter. Because you won't show up on their screen nearly as well as they do on yours'.

MARPA, or automatic collision avoidance systems, are neither. The auto acquisition is of questionable utility as it is as likely to acquire and track a wave as it is a ship. And collision is only avoided by proper interpretation of the data received. One of the problems will be induced by the amount of yaw your boat is experiencing as well as the accuracy of the compass signal you're sending to the radar unit. You have to stabilize the radar image via some type of compass input for a MARPA system to work. Or, alternatively, you have to hold your course within a degree or two, and realize that, a change of course will require a complete recalculation of the data, about six minutes minimum. If you're yawing significantly, the radar will be near useless for collision avoidance or much else without some form of image stabilization via course input.

While not answering your specific questions, I hope this has helped. Personally, I think that radar is the greatest navigational and collision avoidance device invented in the last century or so. I'd take a good radar unit over six GPS units, Loran, or even my trusty sextant. The only thing I'd rank higher is the lowly fathometer, in terms of necessity for safe navigation. And a compass, of course.
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Old 12-03-2008
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Have to agree with sailaway.
Rather then power the best antanna is the longest, as the horiztonal beam width reduces

Pole mounting is fine, I have had mast and pole mounted, would go back to mast as wave motion causes more clutter when pole mounted.

MARPA ( mini automatic radar plotting aid) is very questionable without a really good stable heading sensor, preferably a GPS compass ( wihich is not your average GPS, rather a RTK unit) or failing that a really good solid state heading sensor. Otherwise the yaw in your small boat confuses the vector calculation and what you see is the other boats vector line swinging all over the place along with huge changes in velocity. but hes not actually doing that its your boat yawing as sailaway says the recompute time on modern chartplotters tends to make the system then lag behind whats really happenning. However with a good heading sensor its is an amazing system

The current config I have is a 4 foot open array garmin and a Hemisphere GPS compass as a heading sensor and the result is just amazing.
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Old 12-03-2008
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Wow, excellent replies! Time to hit the rep power button.

I have the heading sensor as part of the Raymarine Autopilot. So I guess I'll have to get a Raymarine radar? (Or are the Garmins ones interoperable somehow?) I really want to take advantage of MARPA.

Second question, why don't they make an elevation-adjusting array for use on sailboats. The elevation would cycle up and down as the array spins around and around. That way, heel wouldn't matter.

Last edited by Bene505; 12-03-2008 at 03:46 PM.
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