Restricted draft at discharge port

Loading a Grain Ship for a Restricted Draft at the Discharge Port

Over the years the size of ships has grown. During World War II thousands of Merchant Vessels were built for shallow draft ports which limited their size to about 10,000 Displacement. They were general cargo vessels with deck cargo. Stability was a limiting factor, not draft.

Now ships have grown and exceeded the size of the Panama Canal. My personal experience is with the Panamax Size vessels which are approximately 225 meters long, 32.26 meters in beam with a depth of 18 to 21 meters. Some are lightly built with a Light Ship weight under 10,000 tons. Others are built heavier with Light Ship weights over 12,000 tons. These larger ships that I dealt with were dry bulk carriers loading grain from Seattle and Tacoma to China, South Korea, and Japan.

Because the cargo was underdeck, we never really had to worry about Stability other than complying with the Required Grain Stability Regulations because they were “specially suitable” for grain. Draft, Trim and Hull Strength on the other hand was the Chief Mate’s biggest concern after dealing with the required Grain Stability Calculations.

The Approved Loading Computer took care of checking the Hull Strength easily.

The real challenge was determining the sailing drafts required so that the vessel arrived in China with a maximum cargo limited by a draft of 13.0 meters and zero trim.

Ideally, the ship’s trim and drafts are to be controlled by using the grain cargo and not ballast water. Because taking water ballast would result in carrying less cargo. So, if possible two slack holds were used to trim the vessel at the completion of loading to produce the required sailing draft that would result in the ship arriving at the destination at the required draft and trim allowing for the bunkers and fresh water consumed enroute.

Where to start to plan the loading?

Because the limiting draft and trim for the voyage is at the discharge port, we should start there. Keep in mind this method could be for any limiting draft during a voyage such as a Panama Canal.

In this case the arrival draft and harbor water density at the discharge port will define the LCG and the Displacement. Do not confuse the Ship’s Displacement from the Hydrostatic Data and its actual maximum weight of the ship if the harbor water is not the same assumed for the Hydrostatic Data.

Estimating the consumption enroute.

The bunkers consumed will be needed. The Chief Engineer should be consulted and provide the best estimate of consumption of bunkers from Departure to Arrival including maneuverings in Pilotage Waters. Remember to allow for a “sea margin” of bunkers onboard for the voyage. The consumption and production of fresh water should also be considered; however, most ships can make as much as they use. Caution should be made not to make too much fresh water. Another consideration is not to overlook the holding tanks for gray water. This information needs to be applied to the required arrival condition at the discharge port using the LCG and the Weights of the Tanks involved that were just mentioned using the Weighted Average method I mentioned in previous News Letters. This will result in the maximum displacement and required LCG of the ship at the Loading Port.

Now to calculate the Required Sailing Drafts.

Knowing the Required LCG, Displacement and Harbor Water Density at Loading Port we can first calculate the True Drafts. If the ship has been keeping a record of the vessel’s Hull Deflection in the Loaded Condition that too should be consulted.

Entering the Hydrostatic Data, the Drafts will take us to the corresponding Displacement for the Assumed Water Density and the LCB. Do not forget to correct the Draft for the Harbor Water Density. On a Panamax every centimeter of draft represents about 70 metric tons of cargo. We can use this information to calculate the ship’s weight in metric tons and the trim required to calculate the true forward, after and mean drafts. At this point we can also make an allowance for hull deflection. Hull deflection will reduce the cargo we will be able to carry. At this point you can actually calculate the Maximum Loadable Cargo due to a draft restriction during the voyage.

Once we calculate the True Trim we can use the distance from the Forward Pperpendicular, FP, and After Perpendicular, AP, to the ship’s draft marks to calculate the mean Forward and After Apparent Drafts which are the ones that can actually be read. Do not forget the Apparent Trim will not be the same as the True Trim. Apparent and True Drafts and Trims are something you learn with Draft Surveying.

The next task is once you know this, how are you going to top off the vessel?

Topping off the vessel.

In previous Newsletters I described how to make a Trimming Table. This is useful to use to find which cargo holds will be of assistance to adjust the ship’s LCG to the Required LCG. For a Panamax I have found No. 2 and 6 Holds work nicely when topping off the ship. Always load into the slack hatches last! Aboard Handy size 5 cargo Hold ships, I have found No. 2 and 5 work best because No. 4 Hold is too far forward on this size ship.

Now the Grain Elevator will have all the cargo weighed and ready to load for the topping process. They will stop with about 1,500 Metric Tons to go. It is the ship’s turn to do a Draft Check. Note: this is not a Draft Survey. They will only give you about 30 minutes to do this check before trying to charge the ship extra port fees or fines for delaying the loading. All you need to do is to check the drafts and determine the Actual LCG of the vessel. The remaining cargo should not put your vessel overdraft, but you can use TPC to check that quickly. You can find the Actual LCG by calculating the Trim backwards! The difference between the Actual LCG and the Required Sailing LCG is the shift, GG’, you need to make. The tonnage to go, reported by the Grain Terminal, is the weight you have available to load for example between No. 2 and No. 6 Cargo Holds on a Panamax. So you can solve for the tonnage required using simultaneous equations as follows:

Let x = No. 2 Hold Weight to Load

Let y = No. 6 Hold Weight to Load

Shift of G Required = LCG Actual – LCG Required

x + y = Total Weight to Load

(LCGx)x + (LCG y)y = (Shift of G Required) (Final Displacement)

Just in case the Grain Terminal did not give you what you asked for.

I always ask for half the amount calculated for each cargo hold immediately. Then you have time to recheck your calculations. When these parcels of grain run out you can briefly check the drafts to confirm everything is going correctly. Any small changes can be made at this time to control the final trim of the vessel. Then proceed to finish the loading of the cargo without delay.

Would you like to take an online course that covers this topic that illustrates this method? William is considering writing a detailed course addressing this.

Do you have questions, feedback or something to add? Let him know.