initial stability of a ship

jehardiman,
I'm afraid I do not understand anything of what you say. I do not know what the initial stability and final stability are. Perhaps what you mean is stability at small angles and large angles of heel. I know what are the criteria of stability and how to make a boat the meet them. I know what the initial GMt is, but do not know what you mean when you speak of initial stability. Because a ship must meet a certain value of the initial GMt but also with other things. I know that transversal stability is only achieved when G and B are in the same vertical. I know that a huge weight on deck does not have to lead to negative GMt. I know that a boat can be heeled, put the deck under the water and not tip over. I know there are self righting boats, boats with positive dynamic stability to a heel of 180 º. I know what are the free surfaces of the tanks and the effect they have on the initial GMt and on GMt for each angle of heel. I know that three identical cubes, homogeneous, with different densities, float exactly the same but with different drafts.
I know many other things about naval architecture, and despite everything I know, I am not able to understand what you say because, I think, or do not explain well or you mess.
Cheers

 

Then you have just proven my comment. A balsa (~0.10) cube will float with a face up because GM is positive, an dense pine/ light oak cube (~0.55-0.6) will float with one corner up because it has negative initial stability (i.e. GM negative) so it will roll to a stable position, a teak/ebony (~0.9-0.95) will float with a face up because GM is once again positive.

In this demo KG and Iwp are unchanged, what changes is the height of KB and the volume. So for a unit cube trying to float face up...

KG 0.5
Iwp 0.083333333

Density KB BM GM Initial Stability
0.1 0.05 0.833333333 0.383333333 Yes
0.2 0.1 0.416666667 0.016666667 Yes
0.3 0.15 0.277777778 -0.072222222 NO
0.4 0.2 0.208333333 -0.091666667 NO
0.5 0.25 0.166666667 -0.083333333 NO
0.6 0.3 0.138888889 -0.061111111 NO
0.7 0.35 0.119047619 -0.030952381 NO
0.8 0.4 0.104166667 0.004166667 Yes
0.9 0.45 0.092592593 0.042592593 Yes

All will eventualy have ultimate stability and float, but won't float face up. What's great about this demo is you can just toss the cubes into water. You have to really try to get the balsa or ebony to float other than face up and you cannot get the oak to float face up. It is an important lesson you need to review.

 

Dear Jehardiman, I do not know if you do not read what I write, or you read it but do not understand. I have discussed about three identical cubes, HOMOGENEOUS.
And now, here, I end this absurd argument. We speak different languages.
Cheers.

 

...Solid cubes are homogeneous...

HOMOGENEOUS:
1: of the same or a similar kind or nature
2: of uniform structure or composition throughout <a culturally homogeneous neighborhood>
3: having the property that if each variable is replaced by a constant times that variable the constant can be factored out : having each term of the same degree if all variables are considered <a homogeneous equation>

 

Stability - submarines and others

Fellow contributors

The Submerged Submarine

Free surface on a submerged submarine's tanks is real, but of negligible practical significance. For the submerged vessel, the main ballast tanks are flooded. External fuel tanks (diesel boats) are water compensated; no free surface (not quite true - the "free surface" is between diesel SG 0.83 and SW 1.025). This leaves only the trim and compensating tanks, fresh water, and internal fuel tanks. These are almost permanently part full, but at varying levels, and small effect on stability.

The submerged stability is an issue for the designers, not the operators. The operators deal with the immediate issue of balancing weight and buoyancy forces, and maintaining the total vessel LCG at submerged LCB - "catching a trim".

Transient conditions (diving/surfacing) could be interesting, but who cares if the fully enclosed boat lolls (or heels on external forces) briefly?

I spent over 10 years professionally supporting submarine refitting but never had to do the stability numbers.

Understanding Ship Stability

We have the separate issue of understanding ship stability. A basic classroom course will take several hours; book study or personal mentoring will be similar. Then comes the understanding and fluency through project exposure; beyond answering a few (relatively simple and idealised) exam questions. We are never going to achieve this necessary level of coverage in a forum discussion.

For those lacking, and seeking the understanding I strongly recommend:

- Go out and find one or more of the standard texts, study and understand the relevant sections.

- If at all possible go out and get some face to face mentoring from a suitably qualified and experienced practitioner.

I come from a position of preparing vessel stability particulars for approval by regulatory authorities, and limited Naval Architecture lecturing. I am aware that there is much knowledge in ship and boat stability that I don't have.

There are significant issues that don't follow across the size range of vessels. However, somewhat incredibly, there are a number of stability criteria (refer IMO and its national implementations) that scale quite effectively over a wide range of sizes - inshore/trailable to ocean going.

Sailing vessel stability is another game again.

Naval Architect
Peter Edmonds Marine Design
Perth, Western Australia