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VHF – Low or High Power on Transmit?

By Russ, on February 27th, 2007

VHF radios that conform to the ETU spec have two power settings 1 or 25 watts for fixed sets and 1 or 5/6 watts for handhelds. If you were calling for help especially in a MAYDAY situation you would naturally choose high power so that the Coastguard and as many vessels as possible would here your call.

ht640 525 awardBut using high power for routine calling severely restricts the number of people who can use the few channels available at any one time – in other words several 1 watt conversations can take place within a given area but only one 25 watt conversation. Misuse of the high power setting is a bit like walking into a bar and shouting at your friends – eventually everyone will have to do the same and that is counter productive.

Marinas can only reply on 1 watt if they comply with their license, race management uses channels M and M2, and inter ship is channels 6,8,72 and 77 – so it is a good idea to use the right channel and use only low power – that means you should only be using high power on channel 16 – Coastguard.

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February 27th, 2007| No comments - be the first!|Tags: .

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Hydroptere – 47.2 knots

By Russ, on February 27th, 2007

The fastest sail boat in the world is the Hydroptere. This is an incredible trimaran and if you watch the video on the site below you will see that the hulls barely touch the water – meaning that the whole vessel is virtually flying.

Last month Hydropteré became one of only a handful of sailboats to Hydroptere break the 50-mph barrier, with a run that topped out at 47.2 knots (54 mph) in 25-knot winds. That’s just a knot and a half shy of the absolute sail-speed record of 48.7 knots – but that’s held by a windsurfer, so it’s arguably safe to say that Hydropteré is – for the moment — the world’s fastest sailboat.

http://www.hydroptere.com/accueil/images/videos/var/lang/FR/rub/19.html

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February 27th, 2007| No comments - be the first!|Tags: .

Collision avoidance – ARPA , CPA or …what?

By Russ, on February 24th, 2007

If you have a Radar it will probably be set to use ARPA to assist you in deciding whether you are on a collision course with another vessel . Many yachts have now added AIS to their nav electronics and can overlay the AIS targets on to the Radar and chart plotter display to add more information. So far the logic is that you calculate the Closest Point of approach (CPA) and Time to Closest Point of approach (TCPA) and make a judgement on which way to turn according to the Collision Regs.

SafePass is an idea from Fred Pot of SeaCAS. In this scheme the system calculates various CPA/TCPA results and plots “safe” and “dangerous” zones on your current heading to make it even easier to decide which new heading may be appropriate for your vessel. This depends on the other vessel not changing course and also on the circumstances of the impending collision with regard to the Collision Regs of course.

By the way one of the simplest, most practical and down to earth descriptions of how to use your radar can be found on Bill Dixon & Pat Watt’s personal web page

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February 24th, 2007| 2 comments - be the next!|Tags: - .

What is the Automatic Identification System (AIS)

By Russ, on February 24th, 2007

Courtesy of the excellent US Coasd Guard Navigation Site…

http://www.navcen.uscg.gov/enav/ais/default.htm

Picture a shipboard radar display, with overlaid electronic chart data, that includes a mark for every significant ship within radio range, each as desired with a velocity vector (indicating speed and heading). Each ship “mark” could reflect the actual size of the ship, with position to GPS or differential GPS accuracy. By “clicking” on a ship mark, you could learn the ship name, course and speed, classification, call sign, registration number, MMSI, and other information. Maneuvering information, closest point of approach (CPA), time to closest point of approach (TCPA) and other navigation information, more accurate and more timely than information available from an automatic radar plotting aid, could also be available. Display information previously available only to modern Vessel Traffic Service operations centers could now be available to every AIS-equipped ship.

With this information, you could call any ship over VHF radiotelephone by name, rather than by “ship off my port bow” or some other imprecise means. Or you could dial it up directly using GMDSS equipment. Or you could send to the ship, or receive from it, short safety-related email messages.

The AIS is a shipboard broadcast system that acts like a transponder, operating in the VHF maritime band, that is capable of handling well over 4,500 reports per minute and updates as often as every two seconds. It uses Self-Organizing Time Division Multiple Access (SOTDMA) technology to meet this high broadcast rate and ensure reliable ship-to-ship operation.

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February 24th, 2007| No comments - be the first!|Tags: - - .

Dual Band AIS – Raymarine AIS250

By Russ, on February 24th, 2007

At last Raymarine have produced an AIS receiver – not only that but it combines three other cost saving attributes :

Ramarine AIS250-275-2061. It has a built in multiplexor with 2 inputs and 2 outputs which can all be set to different speeds. Remember the C and E series Raymarine chart plotters only have one NMEA port and that can only be set to one speed. If you dedicate this port to AIS then you have to set it to 38,400bd and if it is dedicated to your normal NMEA instruments it would be set to 4,800bd. This new unit from Raymarine would avoid that choice having to be made and would make the Brookhouse multiplexor that I installed last year redundant since that was primarily used to enable me to run NMEA0183 at 4,800 baud (normal instruments) and at 38,400 baud (AIS signal). Of course the Brookhouse also gives me USB connection for a PC so that is a bonus that the Raymarine AIS does not offer – yet. Cost saving for a UK install – approximately £200. On the otherhand I have greater resilience by using a seperate multiplexor.

2. It has an antenna splitter. This means that you dont need a completely new antenna, antenna mounting, and cable run just to install an AIS. Your main VHF aerial can now be used for AIS, FM, and VHF – Cost saving for a UK install – I would say about £250. Again, I would counsel against using splitters, and going for the greater resilience of a separate VHF aerial for the AIS

3. It is dual band receiving on 161.975 MHz and 162.025 MHz but only one channel at a time (just like my NASA unit – oh well….)

http://www.raymarine.com/raymarine/ProductDetail.asp?site=1&Section=2&page=1718&product_id=4004

Only gripe – no status lights ! – why dont they learn !

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February 24th, 2007| 2 comments - be the next!|Tags: - - .
 
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