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#1 2012-11-26 19:04:29

whatthe
Member
Registered: 2012-05-27
Posts: 6

Vessel Displacement

Your vessel at departure from Brisbane had a draft of 1.6m and had 5,340litres of fuel, 5000 litres of fresh water and 8 passengers embarked.

Your vessel has a crew of 3 persons and uses 120 litres of fuel per hour at a speed of 16 knots and you routinely use fresh water at a rate of 1 litre per person per hour. Your destination is Sydney which is a distance of 188 nautical miles.

a) How long will you be at sea considering a constant speed
b) What will be your draft on entry to Sydney Harbour
c) What draft will the vessel have on its return to Brisbane provided the same conditions remain

Given:
Vessel Tonnes per Centimetre (TPC) in fresh water is 1.05
Specific density of sea water is 1.025
Specific density of fuel 0.78
Assume Brisbane is salt water and Sydney harbour is fresh water

Last edited by whatthe (2012-11-26 20:50:59)

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#2 2012-11-26 20:37:07

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,124

Re: Vessel Displacement

hi whatthe

Let's try to get some formulas to get all this information under control.

(a)

As the distance is in nautical miles and the speed in knots, the journey time should be straight forward. 

T will be in hours.

(b & c)

These will be in Kilograms.

dictionary says draft is the distance from the water line to the bottom of the hull.

So as the weight goes down, the boat will float more so the draft will go down.

That's for fresh water.

Now boats float higher in salt water so I'm guessing that for a salt water harbour you do this calculation and then divide by the sd.

I've never done a draft question before so I hope this is all correct.

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#3 2012-11-26 21:14:08

whatthe
Member
Registered: 2012-05-27
Posts: 6

Re: Vessel Displacement

Thanks for your help! I was on a similar path with the first part which I've been stewing over for days.

Great work on the first Draft question!

Last edited by whatthe (2012-11-26 21:17:44)

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#4 2012-11-26 21:30:38

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,124

Re: Vessel Displacement

Stay on-line.

i think i need to make a correction.

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#5 2012-11-26 21:34:39

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,124

Re: Vessel Displacement

I think I should assume the 1.6 draft is the unloaded draft. so

This is now the amount the boat will sink in the water, so reverse the signs

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#6 2012-11-26 23:05:39

whatthe
Member
Registered: 2012-05-27
Posts: 6

Re: Vessel Displacement

Bob,

I think it should be:

New Draft = 160 - (W / 1.05)cm

You were correct above with subtracting the change in draft, due to the removal of weights onboard, the ship will rise causing the draft to go down.

Rise or fall (cm) = weight removed or added (tonnes) / Tonnes per Centimeter (TPC).

What do you think?

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#7 2012-11-27 00:26:21

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,124

Re: Vessel Displacement

Vessel Tonnes per Centimetre (TPC) in fresh water is 1.05

Given:

So yes, you're right.  It should be divided.  But also you need to convert the Kg to Tonnes.

I change the sign because I reasoned that 1.6 was the unloaded draft.  Then load up with fuel and water and the draft should go up.

Then it gradually reduces as these get used.

So

Hope that works out ok.

As you can tell, I'm making it up as I go along.  smile

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#8 2012-11-27 16:30:10

whatthe
Member
Registered: 2012-05-27
Posts: 6

Re: Vessel Displacement

Thanks again for the help. I was on the right track from the beginning, but had doubts.

Cheers,

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