# Difference between GM Solid & GM Fluid

#1

Dear Colleagues:

Could you please assist me with the difference between the GM Solid and GM Fluid? I just received an email from my HO requesting keep the differences between them near to “0”…Is my first time that I have listened someting like that.

#2

Take a look at this: Stability Help

#3

GM (solid) = (KM – KG)
GM (fluid )= GM (solid) – FSC
FSC = Free Surface Correction
With a tank with dimensions L and B FSC = (i di/ W)= (LB^3 x di)/ (12 x W )

With an empty tank the FSC = 0 which means:

GM (solid )= GM (fluid)

I hope that HO do not want you to sail with an empty fuel tank…

#4

you mean you have never encountered the requirement to maintain all bunker tanks either full or empty except for no more than two which may be left slack? that is a standard as old as ships carrying oil for fuel!

#5

I wonder what the interest of the office boys is in this matter. Normally speaking I would say to them please go…well, OK. Unless they do the stability calculations ashore and they know things.

The only reason I can think of is that the initial GM is almost near the legal limit of 0.15 m and that with a load in the tank the FSC will it bring down below this value. What is your GM?

#6

I’ve never calculated stability using the terms GM Solid and GM Fluid. We calculate GM then correct for free surface to get GoM, corrected GM.

Given that the OP is unfamiliar with the terms as well I’d guess that it’s a burst of static from the office. The fix is to turn the squelch up a bit.

#7

Yes that is probably the cure. It is a somewhat strange request from the office and I am puzzled for the reason why they should ask this. Maybe somebody fresh from the academy showing off…

#8

The thing is that aboard ship a single requirements can’t be added without compromise elsewhere. In any case the procedures should be changed rather then a single message.

Things are done in accordance with the GM, stress requirements in the stability book and operational requirements such as draft and/or air draft restrictions, trim and list, fuel burn requirements.

Some are non-negotiable, for example min GM and max stress but there are also often sharp limits to how much room there is to play around with tanks levels.

An example is the conflicting requirements to keep ballast tanks either empty or pressed up along with the requirement to do ballast exchange.