Radars on foremast on some cargo ships...

[Dеnis]

I wonder, why do some general cargo ships with aft accommodation have 1 of the 2 rotating radar antennas on top of the foremast instead of having both above the bridge?   Ships like Deo Volente, Arctic Rock, Marietje Deborah & etc.  What are the rules for placing those radar antennas?  I would guess, even if placing it on foremast, it still should be above the bridge level...  Though I have seen some big ferries having a rotating radar on the front of accommodation below the bridge. 


Another question regarding placing the magneting compass above the bridge on cargo ships.  Sometimes it can be seen standing on some platform above the bridge, often just on deck above the bridge - why's that?  What if there's an obstruction behind it like a tall funnel or something?

Regards,
Denis

[MO Roy]

Hi Dennis,

In the past it was quite common for general cargo and multi-purpose ships with lots of derricks/masts and high cranes to have a radar antenna in the foremast.
Nowadays it's not so common anymore besides some special ships.

Regarding radars fwd on ferries, they are only used during docking / undocking. Some big container ships had them also, even some had them on the stern as well. But as they cost extra money it's quite seldom to see them now on big and even bigger containerships.

Magnetic compasses in principle have to stand clear of surrounding metal structures, which is quite difficult in practice. There is of course always metal in the vicinity like a funnel, mainmast etc. Thats why they are being compensated with small iron bars and balls. This is done during sea-trials when the ships turns 360 degrees so it can be compensated for every direction.

Cheers,
Roy

[Dеnis]

Hi Roy, thanks for your reply.  Well, how would you explain why they put 1 radar antenna on foremast on this ship - http://www.shipspotting.com/gallery/photo.php?lid=2626512 ?  Not like the possible deck cargo would be obstructing it, was the radar placed on the aft with the other one.
Out of curiosity I compared the profiles of those ships I mentioned & all of them have the front radar above the accommodation level, but below the aft radar level.

Another thing I noticed is that those ships that have other equipment on top deck standing close to the magnetic compass would usually have that compass slightly raised above that stuff.

[MO Roy]

Hi Dennis,

The ship you are referring to is one of those ship types that can have obstructive deck-loads so that there would be a big blind-angle for the radar above the bridge.

I'm not sure about class-rules but I guess in such a case it suffices to have only one radar in the foremast.
Maybe members that are working in classification societies can confirm this.

Once again ferries that have a radar on the bow well below bridge level should still have two radars on top of their bridge.

Cheers,
Roy

[Emiliyan]

Hi Dennis,

I was a few years ago on one container vessel with 3 radars - 2 above bridge and 1 fwd! According documentation fwd radar was with big "blind"spot sector aft to prevent radiation over accommodation and its primary target was to detect small crafts ahead of vessel as she was initiallyy designated to sail around/close to Japan.
Nowdays you may see on some "monster"container ships with radars on fwd mast + 2 or 3 above bridge + aft mast(below) exactly for that reason - similar to ferries, but main idea is to detect small crafts which the main radars cannot due to some cargo restrictions.

On that picture/link which you attacched - I believe the idea is the same - due to possible cargo obstruction is better to have "an eye" fwd ... and most probably has blind sector aft!

If the Class allows - why not .... even in some points is better if no have "side effects" 

Best Regards