Help Collision Avoidance

[Sailfree]

I try to learn from magazines etc and have done the Radar course but cannot understand why there can be a different result between sea stablised (using boat log input) and ground stablised (using GPS). I am using a Raymarine combined chartplotter and radar overlay.
If a collision occurs it is due to both boats occupying the same piece of water/ground at the same time, therefore their vectors over the ground using GPS should show this.
Contrasting this with heading and speed of the boat and the bearing and distance of other boat. (I recognise that this is equivalent to the Mk1 eyeball - lining up other boat say to a stanchion).
I note that the MAIB report regarding the Moody 47 collision refers to using sea stabilised (ship speed/heading input) and also recent articles in YW & PBO but dispite reading them a couple of times I am still struggling to understand the significance of the need for using only sea stabilise.
Can someone out there please help and explain this simply through text or refer me to a good book/article that does. I have referred to the furnhurst book on RADAR and it seemed more an introductory course .
I will check the inputs on my Raymarine and change them between the need for pilotage and the need for collision avoidance, as it is written that I should but I would also like to understand why.

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[Vara]

I picked up that comment in this months PBO,and have just spent an hour with paper drawing what if vectors and have come to no conclusion except that if the vessels are using different inputs and reacting to each other there may be a conflict.
I look forward to the definitive answer.

<hr width=100% size=1>If it can't be fixed with a lump hammer dont fit it!

 

[cjc]

When In True motion and Sea Stabilised a target (yacht, boat etc) which is stopped will appear stationary.

But in Ground Stabilised mode a stopped target will display movement as given by tide or currents, and fixed objects will appear stationary.

Thus sea stabilised is best for collision avoidance, on the ships I work on we have two radars set sea stabilised for this and two radars set with ground stabilisation as that is better for navigation.

Sea stabilisation is basically showing the water-track and ground stabilisation is showing the ground track.

I dont want to go on too much, and I hope that this answers your question.

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[[2068]]

A lightbulb has just illuminated in my head. Why did the book not explain it that way ?

Many Thanks,
dv.

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[Vara]

Aaaaah,now I understand!
Thanks

<hr width=100% size=1>If it can't be fixed with a lump hammer dont fit it!