# Speed Through The Water



## Fred Field (Mar 24, 2013)

The recently started thread "Negative Slip" jogged my memory about a subject that was never satisfactorily explained to me, being an engineer that is.

When I retired, 2005, ocean going vessels over 15, 000 tons were required to have an 'ARPA'. This was supposed to be manufactured in accordance with a defined 'performance standard' the exact name of which escapes me. Part of this standard required that ARPA have an input from a 'speed through the water' recording device. 
Can anyone tell me why? 
I know several owners were caught by this 'requirement', particularly in the US, and after GPS devices largely took over the SOLAS requirement for a 'Speed and Distance Indicator'.


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## trotterdotpom (Apr 29, 2005)

The ARPA displays the course and speed of ship targets relative to your own ship. In order to compute that accurately it needs course and speed data from your own ship. That's what I recall from yonks ago. I suppose a little picture of the target and the name of its insurance number are displayed nowadays.

John T


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## Fred Field (Mar 24, 2013)

trotterdotpom said:


> The ARPA displays the course and speed of ship targets relative to your own ship. In order to compute that accurately it needs course and speed data from your own ship. That's what I recall from yonks ago. I suppose a little picture of the target and the name of its insurance number are displayed nowadays.
> 
> John T


I appreciate that. 
However I would have assumed that the speed of your vessel, 'over the ground' and similarly the speed 'over the ground' of your 'targets' would have been more 'use' to the computing mechanism. I do realize that the 'closing speed' of the targets is all that the ARPA can actually compute.

I actually wondered if it had anything to do with a time when 'speed over the ground' could only be computed from 'fixed' marks (ie on land) and the only reference for your vessel was a 'speed through the water' measuring device once out of sight of land? That boils down to Regulations not having caught up with technology, as in when DF's ceased to be required under the communications chapter of SOLAS but where still required under the navigational aids chapter even though the vast majority of transmitters had been shut down.


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## trotterdotpom (Apr 29, 2005)

As far as I recall (not always that reliable these days) the speed input was from the Log and assume that gave speed through the water. I don't remember the radar calculating speed from land targets, but maybe it did. Probably they do now.

With a bit of luck, someone with up to date knowledge will answer soon.

John T


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## Cisco (Jan 29, 2007)

No... the technology ignored the reality... a big cross tide and the arpa could be saying be saying that tother ship was showing red when in fact.... if one used the Mk 1 eyeball and looked out the window... it was actually showing green.....


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## Cisco (Jan 29, 2007)

PS... thats why you should use both *course* and speed through the water....


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## Michal-S (Nov 30, 2010)

Water track is important to determine aspect, i.e. the relative angle at which our ship is seen from the other. It is crucial to tell the right of way. For example, two ships proceeding in opposite directions and fighting a current from side will see each other as opposite going when ground track is applied whereas they will find each other as crossing with water track. Prudent navigator will find a difference irrelevant and will react properly in either case but there are more complicated situations around port approaches and at anchorages with vessels moving, drifting, anchoring, just getting underway etc.
*Besides, if ships are to collide they will bump each other at water speed not speed over the ground.


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## Ken Wood (Sep 6, 2006)

Michel has it spot on. Actual course and speed over the ground is irrelevant. ARPA will calculate the course and speed of a target RELATIVE to your own vessel which will give the correct aspect of the target from you. As Michel says, if you are going to bump into another ship, it matters not a jot what your ground speed is, only speed through the water.


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