IMOS - Max Lift Calculations

🔑  This is a Key Topic, with high value for all users.

Max Lift is the maximum quantity a vessel can lift in the port in the voyage that has the most restrictive deadweight and draft limits. This amount can be calculated based on the density of the cargo, the vessel's deadweight, and the weight of bunkers and everything on the vessel.

1. On an Estimate column or details toolbar, click 

   

    or 

   

    and then click Auto Calculate by Max Lift Qty. The following information is used:

   • From the Vessel form:

     • Capacity fields vary with Type Code, for example:

       • Type Code T (Tanker): Capacity M3 or Capacity Bbl

       • Type Code B (Bulk): Capacity (Bale) or Capacity (Grain)

     • Vessel DWT (taking into account the season and the salinity of the berth)

     • Constants and Safety Margins

   • Bunkers ROB

   • Cargo Name

     • Cargo Class Dry or Unspecified: Stowage Factor (M3/MT or Ft3/MT)

     • Cargo Class Liquid: Density (API/Specific Gravity)

     • Cargo Class Dry: The Capacity Basis default is Grain, but you can change it on the Cargo Name form.

2. Quantities appear in Estimate details:

   • On the Draft/Restrictions tab, in the Cargo and L/D Qty fields.

   • In the Cargoes grid, in the CP Qty field, depending on configuration flags.

3. You can override the calculated lift value. 

4.  To recalculate, clear the field and then press Enter. The new value appears blue.
   

To see the inputs to the calculation, on the Draft/Restrictions tab, hover over the value in the Max Lift field.

Max Lift Calculation

In determining the Max Lift, three possibly restricting quantities are calculated:

• Max Available Deadweight

• Max Deadweight Given Cubic Restrictions

• Max Deadweight Given Draft Restrictions

The Max Lift is taken as the most restrictive of these values.

You can see which restriction is in effect on the Itinerary Details form.

Max Lift = Capacity divided by Stowage Factor

Max Available Deadweight = DWT minus Bunkers ROB on Arrival minus Constants and Margins

If the result from the Max Lift calculation is higher than the Max Available Deadweight, then the Max Lift is the Deadweight itself (the maximum the ship can lift).

Max Available Deadweight

Max Available Deadweight = Vessel Deadweight − Bunkers and Constants

Vessel Deadweight

The deadweight comes from the baseline chosen for calculations:

• Vessel DWT, SW Summer Draft, and TPC/TPI on the Vessel form.

• Information entered on the Vessel DWT/Draft tab.

If an exact baseline does not exist (meaning Deadweight, Draft, or TPC is missing) backups are used depending on the relevant loadline:

• Tropical Fresh Water -> Tropical Salt Water -> Summer Salt Water

• Winter Fresh Water -> Winter Salt Water -> Summer Salt Water
  Note: Winter Fresh Water is not an available line, so a backup is always used.

• Summer Fresh Water -> Summer Salt Water

When Summer Salt Water is used as a backup for Winter/Tropical loadlines, the draft and the deadweight are adjusted as follows:

• Tropical SW Draft = Summer SW Draft + 1/48 * Summer SW Draft
  
  Tropical SW DWT = Summer SW DWT + (Tropical SW Draft − Summer SW Draft) * TPC *100 CM/M
  
  

• Winter SW Draft = Summer SW Draft − 1/48 * Summer SW Draft
  
  Winter SW DWT = Summer SW DWT − (Summer SW Draft − Winter SW Draft) * TPC * 100 CM/M

Bunkers and Constants

Bunkers and Constants = Sea Constants + Fresh Water + Other Constants + Bunker Margins + Arrival ROB Bunkers + Bunkers Received

Sea Constants, Fresh Water, Other Constants, and Bunker Margins come from the Vessel Consumption tab:

• In the In Port Consumption Table (Per Day) grid, the Margin column.

• Under Variables and Safety Margins, Constants Sea, Fresh Water, and Others fields.

ROB Bunkers and Bunkers Received come from the Voyage/Estimate.

Example

Let’s consider the calculation of the Max Available Deadweight for the vessel seen in the screenshots below:

Suppose we are sailing to Bergen in winter, so our loadline is Winter Salt. We arrive at Bergen with 200 MT IFO and 150 MT MGO. We also buy 150 MT IFO and 100 MT MDO at the port. Because we do not have information on the Winter Line, we will need to use the summer line. Therefore, our calculation will look as follows:

Adjusted Vessel Draft = 15 M − 1/48 * 15 M = 14.6875 M

Adjusted Vessel DWT = 80,000 MT − (15M − 14.6875 M) * 70 MT / CM * 100CM / M = 77,812.5 MT

Bunkers and Constants = 20 MT (Sea Constants) + 30 MT (Fresh Water) + 40 MT (Other Constants) + 110 MT (IFO and MDO Bunker Margins) + 350 MT (IFO and MDO Arrival ROB Bunkers) + 250 MT (Bunkers Received) = 800 MT

So, we get:

Max Available Deadweight = 77,812.5 MT − 800 MT = 77,012.5 MT

Other Notes

Unlike the Max Deadweight Given Draft Restrictions calculation, this portion of the Max Lift Calculation does not account for possible differences in salinity between the baseline used and the itinerary.

Max Deadweight Given Cubic Restrictions

For Gas Carriers and Tankers:

Max Cubic Deadweight = 0.99820701 MT/M³ * Specific Gravity * Capacity in M³

This formula is based on the fact that the density of water at 20 degrees Celsius is 0.99820701 MT/M³ and the following two physical relations:

Mass = Volume * Density

Specific Gravity = Density / (Density of Water)

Specific Gravity

The conversion between API and Specific Gravity is given by the following formula:

Specific Gravity = 141.5 / (API + 131.5)

Capacity

Example

Suppose that we are shipping OIL, with an API of 40. This gives us:

Specific Gravity = 141.5 / (40 + 131.5) ≈ 0.825072

If we then ship this cargo on a Tanker with a capacity of 80,000 M³, we calculate the Max Cubic Deadweight as:

Max Cubic Deadweight = 0.99820701 MT/M³ * 0.825072 * 80,000 M³ ≈ 65,887.4

For Other vessels (except LNG Carriers):

Max Cubic Deadweight = (Capacity in MT * 35.3146667 Ft³/M³) / (Stowage Factor in Ft³/MT)

35.3146667 is the number of cubic feet in one cubic meter.

Stowage Factor

Capacity

The capacity used depends on the Capacity Basis set on the cargo. Unspecified defaults to Grain in the calculation.

Example

Suppose we are shipping COAL, with a Stowage Factor of 100 Ft³/MT and an Unspecified Capacity Basis. If we then ship this cargo on a Bulker with a grain capacity of 2,100,000 Ft³, we calculate the Max Cubic Deadweight as:

Capacity in Ft³ = 59465.28 M³ * 35.3146667 Ft³/M³ ≈ 2,100,000 Ft³

Max Cubic Deadweight = 2,100,000Ft3 / (100 Ft3/MT) = 21,000 MT

Other Notes

This calculation is not performed:

• For LNG Vessels.

• If there are different cargo densities specified in the itinerary.

• If Capacity or Specific Gravity/Stowage Factor is missing.

Max Deadweight Given Draft Restrictions

This calculation happens in one of two ways, depending on whether we can determine the deadweight from the Deadweight Table in the DWT/Draft tab of the Vessel form.

Notes

This calculation is only performed when we have a draft specified on an itinerary line.

Calculation Based on Deadweight Table

Notes

The Max Deadweight Given Draft Restrictions are not determined if:

• There are less than two lines in the Deadweight Table.

• Every draft in the Deadweight Table is greater than the itinerary’s ESWD.

Calculation

First, the draft in the itinerary is converted into the Equivalent Salt Water Draft, as follows:

Equivalent Salt Water Draft (ESWD) = Itinerary Draft * (1 − 0.92 (1.025 kg/l − Itinerary Salinity))

In this equation, 1.025 is the density of salt water.

Then, if there is a line in the Deadweight Table with draft within 0.005 M of the ESWD, we take the deadweight from this line.

If not, we order the lines by draft and find the closest line with a draft greater than the ESWD (call it LG), and the closest line with a draft less than the ESWD (call it LL). We then extrapolate the deadweight through a linear equation as follows (recall that the equation of a line is y = mx + b):

m (slope) = (LG.Deadweight − LL.Deadweight) / (LG.Draft − LL.Draft)
b (intercept) = LL.Deadweight − m * LL.Draft
Table Deadweight = m * ESWD + b

An easy way to imagine this is to think of a graph where the y-axis is deadweight and the x-axis is draft. LG and LL give us two points on this graph, and we use them to build a line. This line will then have a point corresponding to the draft we are looking for. The deadweight at this point is the result of our calculation.

Following this calculation, the Bunkers and Constants (which are calculated as described in the Max Available Deadweight section) are subtracted to get:

Max Deadweight Given Draft Restrictions = Table Deadweight − Bunkers and Constants

Special Case: ESWD is Greater than Every Draft in the Table

If the largest draft in the table is greater than the vessel SW Summer Draft we simply use this line’s deadweight as the Max Deadweight Given Draft Restrictions. If the SW Summer Draft is greater or equal, then we use the SW Summer Draft and the Vessel DWT as the point LG, and extrapolate from the resulting line. (If the difference in draft is less than 0.005, we simply use the deadweight from the line in the table.)

Bad Data Handling

If we end up in a situation where LG.Deadweight > LL.Deadweight, we deal with the bad data by assigning Max Deadweight Given Draft Restrictions to be the LG.Deadweight if ESWD ≥ LG.Draft and LL.Deadweight otherwise. We do not do a linear extrapolation based on a negative slope.

Example

Consider the following vessel:

Suppose that we arrive at a freshwater port with a draft of 13 M. Then, we get the ESWD as follows:

ESWD = 13 M * (1 − 0.92 (1.025 kg/l − 1.000 kg/l)) = 12.701 M

Then looking through the table, since we do not have a row with a draft within .005 of 12.701, we take the row with draft 13M as LG, and the row with draft 12M as LL. From these two points we extrapolate the Max Deadweight Given Draft Restrictions as follows:

m = (66,500 MT − 60,200 MT) / (13 M − 12 M) = 6,300 MT/M

b = 60,200MT − 6,300 MT/M * 12 M = −15,400 MT

Table Deadweight = 6,300 MT/M * 12.701M - 15,400MT = 64,616.3MT

Then, supposing we have 200 MT worth of Bunkers and Constants, we get:

Max Deadweight Given Draft Restrictions = 64,616.3 MT - 200 MT = 64,416.3 MT

Calculation Based on TPC

If we fail to get a deadweight from the Deadweight Table calculation (for one of the reasons given under Notes in the relevant section), we calculate the max deadweight based on the vessel’s TPC. The equation for this method depends on whether the salinity on the itinerary line matches the baseline salinity we are using for our Max Lift Calculation (see the Max Available Deadweight -> Deadweight section for information on baselines).

Salinity Matches

Max Deadweight Given Draft Restrictions = Max Available Deadweight − (Adj Vessel Draft − Itinerary Draft) * (TPC * 100 CM/M)

The Max Available Deadweight calculation is discussed in a separate section.

Example

Consider the example we have from the Max Available Deadweight section. Recall that our vessel has a TPC of 70 MT/CM and adjusted draft of 14.6875 M. Moreover, we have:

Max Available Deadweight = 77,012.5 MT

Now suppose that the port has a draft of 14 M. Then we get:

Max Deadweight Given Draft Restrictions = 77,012.5 MT − (14.6875 M − 14.0 M) * (70 MT/CM * 100 CM/M) = 72,200 MT

Salinity Does Not Match

Note that this calculation works poorly if Lightship is not specified on the vessel. There are warnings generated when this calculation is done without a Lightship value.

Salinity Ratio = (Baseline Salinity − Itinerary Salinity) / (Itinerary Salinity)

Max Deadweight Given Draft Restrictions = (((Itin Draft − Adj Vsl Draft) * TPC * 100 CM/M) + (Adj Vsl DWT) − (Salinity Ratio * Lightship)) / (1 + Salinity Ratio) − Bunkers and Constants

Example

Consider the example we have from the Max Available Deadweight section. Instead of going to a port with a Winter Salt loadline, suppose we go to a port with a Winter Fresh loadline. Let’s say this port has a draft of 14M. Our vessel would still have a TPC of 70MT/CM, an adjusted draft of 14.6875M, an adjusted deadweight of 77,812.5MT, and 800MT worth of Bunkers and Constants (See Max Available Deadweight for how these values are obtained). Moreover, let’s say that our vessel has a Lightship value of 11,000MT. Lightship is specified in the Details tab of the vessel form.

Given these details the calculation will yield:

Salinity Ratio = (1.025 kg/l - 1.000 kg/l) / (1.000 kg/l) = 0.025
Max Deadweight Given Draft Restrictions = (((14 M − 14.6875 M) * 70 MT/CM * 100 CM/M) + (77,812.5 MT) − (0.025 * 11,000 MT)) / (1 + 0.025) − 800 MT = 70,151.2 MT