# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Barrels was DR thread from Nov-Dec '04
From: Bill B
Date: 2005 Feb 1, 20:13 -0500

Doug

Not clear what the old and new displacements are relative to a a couple of
counts.

How many pounds in a ton for this formula (I am assuming with displacement
we are not talking gross or net tonnage as a factor of square feet of cargo
capacity)?

a.  Is there a baseline displacement based on the displacement of an empty
vessel plus fuel etc?  which is already factored in before we add or
subtract cargo, or

b.  Is the new displacement the total weight of the ship, fuel, cargo etc
plus/minus the added or subtracted cargo?

Tangentially, in practice does a captain run close to hull speed (assuming a
non-perishable cargo and sea conditions permitting) to save time, or back
off to decrease fuel cost?

Put another way, is the earning capacity of the vessel per day so large that
fuel cost be damned, only time is money; or is the margin slim enough that
he has to calculate the cost/benefit ratio of speed (time is money) vs. fuel
(oil is money)?

Bill

> Regarding:
>
> w = pld displacement
> W = new displacement
>
> Can you further expound on the meaning of the above?
>
>
>
>
> w = old displacement
>
> Say the vessel,laden,displaces 20,000 tons. The transit is from port A to
> port B. While in port B the vessel unloads cargo. Say the off loaded cargo
> was 5,000 tons. The vessel now displaces roughly 5,000 tons less water.
>
> W = new displacement
>
> 20,000 tons old displacement minus the offloaded cargo's displacement.
> 20,000 tons - 5,000 tons = 15,000 tons = new displacement.
>
> If the sea conditions are such when transiting from port B to another port
> as the conditions were  entering port B and turns are made for the same
> speed the displacement procedure is accurate for calculating fuel
> consumption.

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