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BONUS QUESTION: Can you estimate this vessel's GM? Its beam is 25.8 meters.

The first half about 4 minutes of the video shows how a Log Ship is loaded. The last 4 minutes shows how the vessel is forced to roll and has a view across the vessel's bridge so you can time the rolling period.

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  1. Friends of STABILITY AND TRIM FOR THE SHIP'S OFFICERJanuary 27, 2018 at 5:15 PM

    I hope everyone enjoyed the video.

    As you can see once you get a big vessel rolling in still water with the lines slacked a little it will roll a long time. It is best to observe about 6 complete rolls from port to starboard back to port. By standing on one bridge wing and watching the opposite bridge wing you can really detect the motion of the vessel even though it is rolling less than 2 degrees. (I remember when we did this the average rolling period was about 24 seconds.)

    Keep in mind if a vessel is initially stable it will return to its original position after it has been disturbed which is proof alone that the GM is positive.

    So if the Average Rolling Period is 24 seconds and knowing the beam = 25.8 meters, we can simply calculate an approximate GM including the actual Free Surface Effects the vessel has by doing the following:

    GM = (0.8 x Beam in meters / Average Rolling Period in Seconds)^2
    GM = (0.8 x 25.8 meters / 24 Seconds)^2 = 0.86 x 0.86 = 0.74 meters

    To find the KG of the vessel all you need to do is read the drafts and find KM
    KG = KM - GM. If KM = 11.0 meters KG will = 10.26 meters.

    Here is a useful trick:
    Knowing KG you can find the distance to the top of the deck load. Now assume 100 MT and use that distance to determine the loss of GM per 100 MT loaded on top.

    GG' = loss of GM/100 MT loaded = W x D / Displacement
    Keep in mind you will be assuming or pretending the Weight = 100 MT and knowing the Depth from the keel to the top of the load is 20 meters from the Capacity Plan, you can calculate the distance, D, = Depth to the top of the load = 20 meters - (KM - GM) = 9.74 M the added weight will shift G upwards.

    Here is a practical example:

    Your have 1,000 MT of draft left to load and you do a rolling test and find your GM = 1 meter. The Master will sail with a minimum GM of 0.70 M or 70 cm. How much more cargo can you load on top? Well if you do the rolling test and calculations and find that you will be loosing 3 cm/100 MT loaded on top that means if you load all 1,000 MT you will loose 30 cm or 0.3 meters of GM, so you can load all 1,000 MT and have the 70 cm GM!

    Now that you know you can load the 1,000 MT vertically you need to use the trimming table to figure where to put the cargo to get the sailing trim required.

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