# GZ for Tug Boat

Discussion in 'Stability' started by Merida, Sep 24, 2014.

1. Joined: Jun 2014
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### MeridaJunior Member

Hello Everyone!
I am from Myanmar. I am now doing a project on tug.
These are the principal particulars of my design tug.
LOA = 30.7 m
LBP = 27.5 m
B = 9.8 m
D = 4.75 m
design draft = 3.35m
When I draw the GZ curve for the tug(as in the picture), it has no range of stability ( i mean, even when the angle of heel is 90 deg, the value of righting arm is still positive). Is that possible? Does it mean that even when the tug heels to 90 deg, it won't capsize?( i think it's nonsense). Could someone please explain me this?

#### Attached Files:

• ###### GZ curve for FULL LOAD.png
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2. Joined: Oct 2013
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### NavalSArtichokeSenior Member

This is incorrect. It means, if your calculations are correct, the range of stability > 90 degrees.

What is the range of stability of a self-righting lifeboat, for instance?

Regardless of what your GZ curve shows, most tugboats have vents of some sort to allow the engine room to be ventilated and which cannot be closed while the engine(s) is running. The location of these vents is a point of downflooding, and stability criteria are evaluated only up to the point of downflooding and not beyond this angle of heel.

Having a GZ > 0 at an angle of heel = 90 deg. is not impossible, if you think about it.

If GZ = KN - KG sin (theta), where theta = angle of heel, then when theta = 90 deg., GZ = KN - KG, which for positive GZ, suggests that G is located below B when the vessel is upright.

Incidentally, a submarine must maintain G below B in order to remain stable underwater, since its KMT = KB when submerged.

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### MeridaJunior Member

As u said, first of all, I need to decide the location of the vents(which is down-flooding point) and only after that, I should calculated the GZ curve. Is that so?
And one more thing. I am now using Maxsurf Stability Software to generate the floodable length curve but I am not sure which data is to be entered in the floodable length criteria(as in the picture). Could you please give me a piece of advice?

Thank you.....

#### Attached Files:

• ###### q1.png
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### NavalSArtichokeSenior Member

The values of the calculated GZs are not affected by the downflooding points, only the evaluation of the area under the GZ curve from zero degrees of heel to the location of the Max. Righting Arm or the angle of downflooding, depending on which stability criteria you want to apply to this vessel.

In the US where I work, most tugs are not required to meet any stability criteria unless they work offshore or have a load line. This is why I am confused about your performing a floodable length calculation on a tug. Normally, such calculations are intended for passenger vessels, which must meet certain subdivision and damage stability criteria. In any event, floodable length calculations have been superseded for passenger vessels and certain cargo vessels in the IMO regulations by the various SOLAS damage stability criteria (including the probabalistic damage stability regulations).

Floodable length calculations assume the vessel does not heel due to flooding, but merely trims either forward or aft depending on where the center of damage is located. To account for the effects of heeling to one side, you must perform some sort of damage stability study.

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### MeridaJunior Member

Oh..I see....Thanks

I am just preparing my project for graduation. I have included the tank capacity, Hydrostatic curve, Cross curve, Weight and CG Estimation, and Intact Stability Calculation and Checking in my project book. Is there anything you wanna complement?
Now, I am trying to include the floodable length curve and Damage stability check. What do u think? Is it necessary or not?
Thank you

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### NavalSArtichokeSenior Member

I don't know anything about what your project requires you to complete.

I'm just saying, given how tugs are regulated in the US, floodable length/damage stability studies are not normally performed in the design stage.

You have to read the statement of requirements for your project and decide what your instructor wants to see.

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### TANSLSenior Member

I do not think it mandatory to study the stability after damage, in the case of tugs. What is necessary is to study two specific cases of tugs:
1. Bollard pull
2. Tateral traction
NOTE: I do not see normal that the AVS is greater than 90 °. What forms has the boat?

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### MeridaJunior Member

Thanks for ur information senior
I can include anything I want in my project as long as the theory is correct...
there is no restrictions....

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### MeridaJunior Member

Hello TANSL,
I am not clear what u mean...do u mean Lateral traction?? What is the worst loading condition for a tug?? And if i find the angle of equilibrium in that condition, what value do i need to compare with that?? I mean , is there any restriction on that angle in that case?
The tug is displacement hull. It has Cb of .541.

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### TANSLSenior Member

Sorry but I only have information in Spanish. The formulas and figures, maybe help you to find the necessary information.
Simply calculate the heeling of the boat when pulling from a fixed point and when the boat drags sideways.
The condition will be not submerge the margin line.
A priori it is not know the worst loading condition. We must study them all to see which of them an angle of heel greater occurs
Forget some windows, in attached file, which are from the software that performs these calculations.
Sorry, also, I can´t express myself correctly in English.
I hope, despite all odds, I helped.

Rectification: the condition to be met is that the heel does not exceed the angle of flooding, without ever being more than 10°

#### Attached Files:

• ###### Tiro y Arrastre.pdf
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Last edited: Sep 29, 2014
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### MeridaJunior Member

Thanks for ur precious help Senior!

I will try to study that pdf.

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### Morteza NavalNew Member

Hello to all,
I want to compute the GZ values for both crest and trough situations in longitudinal waves.
Does any one know how to compute that by autohydro software?
Actually I worked with Maxsurf before, but I haven't any experience about Autohydo....