Arrival Draught

[Capt Bill Davies]

This one will be of interest to all and hopefully encourage input from the Bulk Carrier fraternity or any other of the membership.

Back in the early 70s I recall reading an article in the Nautical Institute periodical of the time with the title ‘ Does it move or doesn’t it’. The article addressed the above phenomenon and gave a series of solutions, which beggared belief and certainly was the cause of much embarrassment.

The above question was asked.

‘How often have loaded in various ports Narvik, Seven Island, Port Etienne etc to a certain draught aiming at arriving at the discharge port Even Keel?’
The following solutions were tendered.

1. Noticing the Burtons and Nories tables moved slightly forward in the chartroom bookcase when underway it was considered that the individual ‘cones’ of ore in the holds were following suit!!!
2. Was the vessel trying to align itself in the earths magnetic field and ‘dip’ in line with the ‘flux dip’ in Northern latitudes.
3. Were the several inches of ‘ballast strippings’ left in the double bottom tanks moving forward when underway causing a change of trim which did not reverse when the vessel stopped.
4. Did the ‘G’ change within the ship?

I will refrain from commenting on each point as I am sure all the readers will form their own opinion.

Any comments, opinions or solutions.


Brgds

Bill

[John Cassels]

Does it move or does'nt it ? . thought you were talking about the C of B .

Is your subject pertaining to change of trim with change of density or is this another question ?.

[Capt Bill Davies]

You are right. Unfortunately, the author of the article never acknowledged this shift in B wrt G in all the subsequent months when other 'fanciful' explanations were given.

[j.sabourn]

Ref. both your posts change of trim and squat etc. I can remember up the Great Lakes in the 70s as mate on a 30000 ton grain carrier, loading at various ports also different types of grain, so actual loading was a headache. As owners wanted every ton of grain out of the Gt. Lakes ports(higher freight) and as the max draft in the welland canal max draft was I beleive 25 ft. 6 inches, this was strictly enforced and heavy fines imposed if even half an inch over. I can remember arguing that temperature changes, hogging and sagging etc. to blame for half an inch, was never fined so must have put up a good argument. As the trim was constantly changing due to fuel burnout etc. was a constant job keeping vessel on an even trim. Was always a relief to get clear of Welland canal and the lakes and top off the cargo in Montreal or Three Rivers. Was a constant headache getting in and out of lakes with a Deadweight cargo. Regards J.Sabourn

[alf corbyn]

as a steward i had nothing to do with trim but i would have thought that if you went full speed ahead the cones would have shifted slightly aft. i only sailed on one ore carrier and the ore acted like a pendulum. alf

[John Cassels]

Oh my goodness Alf ,please don't say things like that , pendulums move !!!!!.

You're giving me bad dreams again !.

[Capt Bill Davies]

as a steward i had nothing to do with trim but i would have thought that if you went full speed ahead the cones would have shifted slightly aft. i only sailed on one ore carrier and the ore acted like a pendulum. alf

Good afternoon Alf,

Let me explain further as I do not think my opening post did so sufficiently.

We are talking about an Iron Ore Carrier loading in many of the load ports associated with this commodity. Invariably the vessel will be loading and sailing in Salt Water. The sailing draught will reflect the arrival draught in the discharge port and usually it is desirable to arrive even keel.

The problem the author of the article I mentioned had, was that he was always arriving ‘ by the head’ at the discharge berth at Ijmuiden, Netherlands and he offered four possible causes. However, none of these causes were the reason.

When the vessel arrives in Ijmuiden she will not be in Salt Water (SW). She will be lying at the discharge berth in water that will have a density closer to Fresh Water (FW), which we will call Brackish (in between FW & SW).

Quite clearly when our vessel is at the discharge berth and lying in this brackish water she is sitting deeper in the water and her underwater volume is increased (enlarged ). What happens is that the Centre of Buoyancy ( B ) shifts position relative the Centre of Gravity ( G ) which in turn creates a ‘trimming lever ‘ ( BG). This trimming lever puts the ship ‘by the head’

With respect to your reference to pendulums. You will be referring to the GM on these vessels which will always be large hence the rather rapid tendency for these vessels to return themselves to the upright when inclined.

Hope the above helps

Brgds

Bill

[John Cassels]

Thanks Bill , you certainly explained it better than I could .

With your permission I would just add for Alf 's ( or any other interested party)that this shift of
the C of B is perhaps more simply explained by the fact that a large bulkcarrier has a more
"fuller " afterbody than a forebody hence the upthrust ( buoyancy) will have a greater effect abaft
the C of B.as she sinks slightly going into brackish water.

Still not as good as your explanation though.

[j.sabourn]

very stiff is I believe the term. J.Sabourn

[j.sabourn]

Alf, know what you are trying to say, re pendulums. Was a number of years on ore carriers. Most ores were very heavy and thus gave you a very low centre of gravity, causing the vessel to be very stiff. You will probably have noticed that when the ore was poured into vessel it was left mostly in a heap and was never flattened, this was to try and raise the centre of gravity. However saying that there were certain concentrates, Nickle I think was considered dangerous due to the water content coming to the surface instead of going the usual way down to the bilges, this caused a free surface effect and unable to get rid of the water, I believe there was a ship lost due to this. These cargoes were usually levelled off in hold,pipes inserted into cargo and daily temperature readings taken by dropping thermoters on line down these pipes. I believe you are referring to the violent pendulum type effect of a very stiff ship. Cheers J.Sabourn