[QUOTE=Seancro;132069]That makes sense, that the draft at mid ships would decrease, but to confuse things of course, my Ship Stability for Masters and Mates book defines mean draft as : Draft measured from waterline to keel at a position immediately under the LCF.
I guess it would depend on what Transport Canada’s definition for Mean Draft is…[/QUOTE]
I have that same book here, I think they are calling it “true mean draft” to differ it from “mean draft” .
La Dage says “mean draft” is the draft half way between the fwd and aft drafts which would be (fwd draft + aft draft) / 2
[QUOTE=c.captain;132067]since mean draft is measured at midships but the point of trimming is aft of that point then increasing draft aft will cause the midships draft measurement to decrease.[/QUOTE]
Ok, a big ass is LCF aft of midships, a skinny ass ship would have LCF fwd of midships. It’s all about the water plane area.
If I am thinking straight the LCF should move as displacement changes.
[QUOTE=Kennebec Captain;132083]Ok, a big ass is LCF aft of midships, a skinny ass ship would have LCF fwd of midships. It’s all about the water plane area.
If I am thinking straight the LCF should move as displacement changes.[/QUOTE]
My reading on this is that the LCF is the centroid of the waterplane so since most ships have greater breadth and thus waterplane aft then that point is abaft midships. Yes, LCF moves as displacement changes but it would take a very oddly shaped hull to where the LCF would ever more forward any great degree.
[QUOTE=Kennebec Captain;132076]I have that same book here, I think they are calling it “true mean draft” to differ it from “mean draft” .
La Dage says “mean draft” is the draft half way between the fwd and aft drafts which would be (fwd draft + aft draft) / 2[/QUOTE]
“true mean draft” is the draft at the forward and aft perpendiculars. The draft you read at the draft marks is apparent draft… At equal trim apparent draft = true draft. As the vessel trims they start to differ.
[QUOTE=c.captain;132084]My reading on this is that the LCF is the centroid of the waterplane so since most ships have greater breadth and thus waterplane aft then that point is abaft midships. Yes, LCF moves as displacement changes but it would take a very oddly shaped hull to where the LCF would ever more forward any great degree.[/QUOTE]
Yes, I was thinking about a modern tug which I don’t know how how they are shaped. Or that POS Sunmar Sea.
I’ve got the book here. Chapter 32 is “True mean draft” It says:
"In previous chapters it has been shown that a ship trims about the centre of flotation.It will now be shown that. for this reason. a ship’s true mean draft is measured at the centre of flotation and may not be equal to the average of the drafts forward and aft.
[QUOTE=Kennebec Captain;132093]I’ve got the book here. Chapter 32 is “True mean draft” It says:[/QUOTE]
Then I would be under the impression they are talking about the MMM… Mean mean of means draft… This accounts for the biggest portion of the vessel in the middle, finer at the ends. It is a calculation, as draft cannot be read midships. It would help if they used the proper terms.
Which in my earlier post… The proper term that I referenced should have been true draft, as that is the draft at the perpendiculars corrected from the location of the draft marks that are placed forward /aft of the perpendiculars due to the shape of the hull.
[QUOTE=Ea$y Money;132112]Then I would be under the impression they are talking about the MMM… Mean mean of means draft… This accounts for the biggest portion of the vessel in the middle, finer at the ends. It is a calculation, as draft cannot be read midships. It would help if they used the proper terms.
Which in my earlier post… The proper term that I referenced should have been true draft, as that is the draft at the perpendiculars corrected from the location of the draft marks that are placed forward /aft of the perpendiculars due to the shape of the hull.[/QUOTE]
I don’t think the Barrass Derrett book is a very good one, La Dage is much better. Draft at LCF should be called Draft (LCF), the book has no definitions.
However, according to the book " a ship’s true mean draft is measured at the centre of flotation" - The mean draft ((fwd draft + aft draft) /2) is not going to be measured at the LCF regardless if the hull draft marks are corrected or not. In fact the book uses the term “mean of drafts forward and aft”
This is the whole point of the question. If you shift weights the when the LCF is different location then amidships the draft admidships will change but the draft at LCF will not.
[QUOTE=Seancro;132064]ok, theres another (simpler) question that I’ve lost confidence in over time. If anyone is up for some stability trivia…
When a vessel is trimmed by stern and has its center of flotation 3m abaft amidships, fuel oil is transferred from forward to aft. The mean draft will:
A. not change
B. increase
C. decrease
D. increase or decrease
My first thought was A. Not change. The trim will change, but the mean draft will not because no weight has been added, just shifted.
But then I read a chapter in a book about True Mean Draft and Arithmetic Mean Draft, saying that if the LCF is not amidships and a weight is shifted fore to aft, the AMD will decrease.
I hope it is a simple question and A is the answer, but I’d like to know if I’m missing something…?[/QUOTE]
ccaptain’s ans is correct:
The question is about mean draft which LaDage says is: “That draft midway between the draft forward and draft aft.”
The Barrass book gives definitions in question / ans portion it says:
“true mean draft (or draft at the centre of floatation)”
and
“arithmetic mean draft (or draft admidships)”
Q: The motor tug “Sept Iles” is floating in normal operating condition 50% consumables SW. If its FP tank is pumped full with SW, calculate the new metacentric height (GM) corrected for free surfaces (FS) and the vessel’s new longitudinal center of gravity (LCG).
Yes!!! That is what I ended up with. I finally realized what the free surface inertia was and how to use it, after not finding that term anywhere. I am a little puzzled (with my simple stability knowledge) why there is any free surface effect for the FP tank since it is filled 100%… ? But they give that in the booklet so I tried it out.
[QUOTE=Seancro;132190]Yes!!! That is what I ended up with. I finally realized what the free surface inertia was and how to use it, after not finding that term anywhere. I am a little puzzled (with my simple stability knowledge) why there is any free surface effect for the FP tank since it is filled 100%… ? But they give that in the booklet so I tried it out.
Thanks for confirming my math…[/QUOTE]
Free surface inertia is not found in Barras or LaDage ( I should say I don’t see it), I don’t recall using it. Glad you figured it out. There must be study material with examples you can use?
[QUOTE=Kennebec Captain;132214]Free surface inertia is not found in Barras or LaDage ( I should say I don’t see it), I don’t recall using it. Glad you figured it out. There must be study material with examples you can use?[/QUOTE]
Free Surface Effect virtual loss of GM = i/V x d1/d2 x 1/n² or lb³/12V x d1/d2 x 1/n² … where:
i = the second moment or inertia (m4) of free surface about the center line,
V = the ship’s volume of displacement
d1 = the density of the liquid in the tank
d2 = the density of the water in which the ship is floating
n = the number of longitudinal compartments into which the tank is divided
Hey, buddy, how are you? I am writing SS3 in two weeks and I was hoping I could borrow your specimen exam. Would u mind to send it to my e-mail? It is annapsi2003@gmail.com
it would be a huge help, this is my second attempt. Thanks !
