# 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: Doug Royer
Date: 2005 Jan 19, 13:11 -0800

```Bill,will do.
I'll collect my workpapers tonight and post the proceedure tomorrow.
I used the exam question only as an example of consumption.It may have been
poorly chosen as those exam questions are only built to allow the examiner
to make sure the cadet/student knows the theoretical material and in
actuality may not have a great bearing on actual shipboard proceedures.

Doug

Since I see only the results of your computations, not the input and
calculations, I may well be talking out of school here.

1.  I do not know if fuel consumption is in direct proportion to rpm.
Potential variables, load or no load. Perhaps you or the list can help me
here.  Common sense says my car will consume more fuel maintaining a
velocity of 50 mph up a grade than it will maintaining 50 mph on a level
road (ignoring wind velocity and direction).

2.  Even if fuel consumption vs. rpm was a straight-line graph, Chapman
indicates screw turns vs. speed is *not* directly proportional.  It is quite
possible that when the speed was reduced to 14 kt, if fell on a different
part of the speed-to-screw-turns curve.

If memory and Hobie racing (also displacement hulls) experience serves me,
when speed increased by 2X, hull/water drag is increases by 4X (proportional
speed increase squared). Light wind to weather, skipper and crew as far
forward and to leeward as possible to keep the transom(s) from dragging.
Heavy air, skipper and crew as far aft and windward as possible to keep the
bows from submarining.  Triple the wind velocity equals approx., 9X force on
the sail.  Triple the speed means approx. 9X increase in hull drag.  The
boat simply tries to trip over itself, stuffing a lee bow and sending a crew
on the wire for a spin around the forestay. Painful.

As fuel consumption is part of the passgage/navigation equation for power
craft, I would love to hear more about the methods you used to come up with

Bill

> As for Bill's question about turns and fuel consumption the easiest way
for
> me to show how much fuel these larger,laden vessels consume would be to
take
> a question out of the C.G. exam for 3rd mate,unlimited as an example.
>
> Q: The speed necessary for reaching a point at the designated time is 18.0
> kt.The pitch of the prop is 23.9 ft.How many rpm will the shaft have to
turn
> with a - 2 % slip to reach the point in the given time?
> My calculated rpm = 77 rpm
>
> If the fuel consumption at the above shaft rpm is 215 barrels/day what
will
> be the fuel consumption if the vessel's speed is reduced to 14.0 kt.?
> My calculated consumption = 102.3 BBLS/day
> The correct answer = 101 BBLS/day
>
> Close enough to pick the correct answer out of the supplied answers but my
> calculated rpm is apperantly high as shown by the 102.3 barrels instead of
> 101 barrels.So,in real life, you can see that a fair amount of money would
> be lost if calculated incorrectly.

```
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