Southampton Boat Show - VIP ticket - Sunseeker/Princess - radar

[marklongstaff]

Having spent time at the PSP SBS it surprised me seeing the big sunseeker/princesses used to run the VIP trips with their radars on almost constantly despite the excellent vis throughout the week, my main point is that the "VIP" guests were around 2 metres from the antennae at chest/head height - on fly bridge.

It was my belief that running radar in the above described situation should be avoided or radar positioned more suitably.

Any comments? Anyone know recommended clearances?

 

[acbruce]

I have always wondered about this. I note the position of radar scanners on most 40-60 flybridge yachts put the helmsman on the flybridge directly in the line of the microwave emissions. Probably not a problem but...

Mine on the Trawler 42 is on the mast above the level of the head of the helmsman, even when standing.

 

[pau1gray]

From my time in the Navy - I've learned to keep away from it. Whether that is correct or incorrect I'd still rather err on the side of caution.
We recently fitted a 4KW radar on the boat at work and after an elf & safety chat - as a company we have decided that when the radar is on then no one goes on the roof as the manufacturers manual was a bit sketchy on this. I understand the theory, but don't want to be responsible for cooking my mates gonads! /forums/images/graemlins/laugh.gif

 

[RogerRat]

The new Squadron 55 has lost its Rad Arch and is directly in line with head height now too.

Not the best improvement.

 

[mjf]

Only likely damage to be sustained is the scanner hitting you if you get too close.

 

[fireball]

Can you justify that - what we're talking about is 2/4kw microwave technology .... ok it is burst cooking as you only get a quick blast each time the scanner rotates, but it is still a blast of microwave - I certainly wouldn't be standing that close to it!!

Yes, we're all getting blasted by microwaves all the time - but we're not so close to the source so the power is far less ...

 

[Major Catastrophe]

I am more worried that the cup of tea I am drinking is getting hotter and I may burn my mouth.

 

[MapisM]

Why? By the time your tea gets hotter, your mouth is bound to have reached the same temperature.

 

[Guest]

Post deleted by danfoley

 

[ari]

Presumably no one objects to holding a transmitting mobile phone against the side of their head though? /forums/images/graemlins/wink.gif

 

[Wiggo]

OK, here goes.

It's a 2kW radar scanner, but if you look at the power consumption from the Raymarine website, it uses 28W when transmitting. How does that work? Well, first you need to understand what a Watt is: it's 1 Joule of energy per second. So if I have a 1kW source, but it only transmits for 1/1000th of a second, it's only used 1 Joule of energy.

Radar sends out very, very short pulses of high power, but the actually energy transmitted is small - in this case 28 Joules per second.

My microwave at home sends out 850 Joules per second, as it's an 850W microwave oven and it runs continuously.

It's the total amount of energy that does the damage, not the power. The radar set cooks you even less, for two other reasons. First, it rotates. A 6* beamwidth means that you are only ever in the beam for 6/360 or 1/60th of the time, so as long as you don't run round and round the scanner to stay in the beam you will only be exposed to at most 1/60th of 28 Joules per second, let's say 1/2 Joule per second. That's 1/2 a Watt.

The second gotcha is that the energy falls off with the square of the distance. The beans in my microwave are 10cm from the antenna and at that range 850W is enough to cook them in 3 minutes. At 20cm, they'd take 12 minutes to cook. At 3.2m (maybe the distance from scanner to helmsman's head), they'd take around 51 hours to cook.

As my radar scanner only puts out 1/1700 of the energy of my microwave oven, and I'm cooking so much further away, it would take approximately 87,000 hours or 10 years to cook a portion of baked beans with a 2KW radar, assuming I left it on the helm console.


The other argument, of course, is that ColRegs says that any available aids to watchkeeping must be used. So a commercial vessel with radar must have it running by law, irrespective of the visibility.

 

[jfm]

There was a really good thread that some radar experts contributed to, but it was in 1999 (showing my age) so I doubt the forum search engine will find it :-/

One contributer talked about repairing a radar on a bench with it switched on, and he got a big sunburn type mark across his stomach. But that was a very old radar

It's importatant to get the maths right becuase the nominal radar power (4kw or whatever) is a very misleading number. Two things:

1. First, there is all the geometry of 4kw and a 2degree beam angle and the inverse square law for dissipation, and the angle subtended by your gonads or your head, etc, and that gives you a percentage (less than 10% unless you are VERY close) of the 4kw that you are actually subjected to

2. Second, you have to adjust the 4kw number. For starters, a Raymarine 4kw open scanner draws 46watts in light winds (more in heavy wind, but the extra is the rotation motor so not relevant here). See page 79 of this (it's a 1Mb+ download, beware). That 46W includes the rotation motor, the heater, and the microwave output. So the microwave output is nothing like 4kw. The reason for this discrepancy is that the microwave sends out only pulses. The pulses are lighter at low range but with the Raymarine 4kw on max range the pulses are 1 microsecond each at 740Hz. So the 4kw is only turned on for 0.00074 seconds, per second. The 4kw is therefore an average of only 2.96watts, before reducing it further for the geometry in para 1 above

All that said, I've never found definitve data on what is a safe/unsafe level, and that was one reason I chose my current boat which sticks the radar right up on a mast, way above people's heads (I do lots of night passages on the flybr, so this was importnat to me).

EDIT - ah, I see Wiggo has written exactly the same Anyone for a Lakesailor? /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

 

[Wiggo]

Sorry, jfm, but after your merciless bashing of TonyJones for poor understanding of geometry you should go and re-read the Physics 101 book on frequency, wavelength etc. 740 Hertz is one pulse every 1/740th second, or every 0.00135 seconds, not every 0.00074. Go to the back of the class and collect an IPC Dunce's Cap on the way.

 

[jfm]

Eh, were you being serious? I didn't see any smileys. I dont have book 101 anymore, thanks. If you have a copy I suggest you read it, and if you don't, buy one and read it. What I wrote on the 740Hz thing was 100% correct. You are computing (with your 0.00135 seconds) the interval between pulses, which isn't the answer. I was computing (with my .00074) the aggregate amount of time per second that the transmitter is turned on, which is the correct approach. So you take the dunce's cap, and keep it for 2 days becuase you also omitted to allow for the bit of the 28watts consumed by the heater and rotation motor and you got your beam angle geometry somewhat mixed up but I haven't time to explain that.

I'll throw in a few smileys to lighten this up, /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif, but you might wanna think twice before saying I dont know what a Hertz is. That's like saying I dont know what a millimetre is, ffs

 

[Major Catastrophe]

So, in a word, it's no good for warming pies then.

 

[ari]

And how does it compare witha mobile phone which, whilst much less powerful is right by the side of your head millimeters from your brain?

 

[Wiggo]

Mea culpa!

I hadn't bothered to read into the spec to that extent, but you're quite right about 1 microsec pulse lengths (that's straight from the product page BTW, you didn't need the manual).

Anyway, that effectively gives you the duty cycle, so you can multiply the rated output by that to get the actual transmitted energy per second.

I'll stand by the beam angle bit to a first order of approximation, but that's probably splitting hairs, although I will definitely concede that I did not factor in motor power or heater power.

My beans will take more like 15 years than 10 to cook, so I'll go and wear the cap while they're cooking.

 

[mjf]

So- like i said, the scanner will cause more harm if it actually hits you.


I have previous here - a while ago I switched on my radar and then went to the F/B and instead of heading to the helm i moved aft of the hatch to look at the stern for some reason and the scanner nearly knocked me back down the hatch as I walked into it!

 

[jfm]

Actually you can be let off the cap cos it is in use by you know who /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

I cant be bothered, and suspect you can't either, with massive analysis on beam angle and it would be trainspottingly dull, but fwiw in case you're curious my thinking was:

1. you need to factor in the beam shape. If it is spherical then inverse square law applies. If it is a flat disc (as indeed it is, approximately) then it's an inverse law to the power 1 not 2. An "inverse linear law" perhaps you'd call it. (I admit I used the label inverse square law in my post but I was being generic on the disipation point and didn't use a power of 2 in my maths)

2. The 6 degree beam width that you used is a red herring. The beam width is inversely proportional to the power per degree of beam width. So if the beam width were 12 not 6 you'd be in it for twice as long but at half the power, so it makes no difference. What you should include is the angle subtended by your body in relation to the radar. And even then, if you are Mr fat and subtend twice as much angle as Mr thin, you are spreading the cooking effect over twice as much body flesh, so actually you need to rate your exposure to the beam as "watts per inch of body width" or some such kackhanded measure. But however you do it, it is wrong to include the 6 degree beam angle anywhere in the maths.

 

[Wiggo]

I'm still claiming the 'first order of approximation' defence /forums/images/graemlins/wink.gif

If the beam width was 180*, you'd only be in it half the time, 90* = 1/4 etc. Of course, a second order approximation would take in the beam shape, as there will probably be a Gaussian distribution across the beam.

The third order would actually be more significant as the vertical beam width is huge in comparison to the horizontal width and most of the radiated energy will miss you, hitting passing seagulls, your G&T and the back of the helm seat.

So, let's agree that it's a little academic as your beans would lose heat to the atmosphere at a far greater rate than they would absorb it from your radar. In fact, refining the approximation above would put the 'time to cook' up to 20+ years or so, hence the beans would have decomposed or been nicked by the seagulls long before they were warm enough to eat.

As this is microwave radiation, and hence not ionizing, any health risk is due to localised heating effects, so I think I will continue to be unconcerned about side effects from my little radar set.