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Collision avoidance – ARPA , CPA or …what?

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


2 comments to Collision avoidance – ARPA , CPA or …what?

  • wackyracer

    Came across this here:


    Use of VHF as Collision Avoidance Aid

    7. There have been a significant number of collisions where subsequent investigation has found that at some stage before impact, one or both parties were using VHF radio in an attempt to avoid collision. The use of VHF radio in these circumstances is not always helpful and may even prove to be dangerous.

    8. At night, in restricted visibility or when there are more than two vessels in the vicinity, the need for positive identification is essential but this can rarely be guaranteed. Uncertainties can arise over the identification of vessels and the interpretation of messages received. Even where positive identification has been achieved there is still the possibility of a misunderstanding due to language difficulties however fluent the parties concerned might be in the language being used. An imprecise or ambiguously expressed message could have serious consequences.

    9. Valuable time can be wasted whilst mariners on vessels approaching each other try to
    make contact on VHF radio instead of complying with the Collision Regulations. There is the further danger that even if contact and identification is achieved and no difficulties over the language of communication or message content arise, a course of action might still be chosen that does not comply with the Collision Regulations. This may lead to the collision it was intended to prevent.

    10. In 1995, the judge in a collision case said “It is very probable that the use of VHF radio for conversation between these ships was a contributory cause of this collision, if only because it distracted the officers on watch from paying careful attention to their radar. I must repeat, in the hope that it will achieve some publicity, what I have said on previous occasions that any attempt to use VHF to agree the manner of passing is fraught with the danger of misunderstanding. Marine Superintendents would be well advised to prohibit such use of VHF radio and to instruct their officers to comply with the Collision Regulations.”

    11. In a case published in 2002 one of two vessels, approaching each other in fog, used the VHF radio to call for a red to red (port to port) passing. The call was acknowledged by the other vessel but unfortunately, due to the command of English on the calling vessel, what the caller intended was a green to green (starboard to starboard) passing. The actions were not effectively monitored by either of the vessels and collision followed.

    12. Again in a case published in 2006 one of two vessels, approaching one another to involve a close quarter’s situation, agreed to a starboard to starboard passing arrangement with a person on board another, unidentified ship, but not the approaching vessel. Furthermore, the passing agreement required one of the vessels to make an alteration of course, contrary to the requirements of the applicable Rule in the COLREGS. Had the vessel agreed to a passing arrangement requiring her to manoeuvre in compliance with the COLREGS, the ships would have passed clear, despite the misidentification of ships on the VHF radio. Unfortunately by the time both vessels realised that the ships had turned towards each other the distance between them had further reduced to the extent that the last minute avoiding action taken by both ships was unable to prevent a collision.

    13. Although the practice of using VHF radio as a collision avoidance aid may be resorted to on occasion, for example in pilotage waters, the risks described in this note should be clearly understood and the Collision Regulations complied with.

  • brian

    Actual the idea of safePass zones came from a University in Holland I believe.

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