# effect of double bottom on floodable length calculation?

Discussion in 'Boat Design' started by sirhamid, Dec 3, 2013.

1. Joined: Jul 2009
Posts: 40
Likes: 1, Points: 0, Legacy Rep: 38
Location: middle east-persian gulf

### sirhamidJunior Member

hi all
what is the effect of double bottom on calculation of flood-able length of a vessel?:idea:

2. Joined: Sep 2011
Posts: 6,238
Likes: 301, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

Speaking from memory for a rapid response, I seem to remember that if the height of the double bottom is less than 1.5 m (or a ratio of maximum width), has no effect on stability after damage. It all depends on the height of damage that has to be considered in accordance with the regulations.
Where the double bottom has a height greater than said minimum height, can be considered as the compartment that suffers damage while those above it do not reach their failure through the bottom. Maybe they can damage through the side.

3. Joined: Jul 2009
Posts: 40
Likes: 1, Points: 0, Legacy Rep: 38
Location: middle east-persian gulf

### sirhamidJunior Member

guess the accident on side at water line.

4. Joined: Sep 2011
Posts: 6,238
Likes: 301, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

In that case, the double bottom is not flooded and therefore the volume of the flooded compartment is smaller, which is very good.
Beware of the side damage, or at the bottom, does not affect the two compartments. The regulations call for both dameges to be studied, bottom and side.

5. Joined: Oct 2008
Posts: 6,643
Likes: 650, Points: 113, Legacy Rep: 2488
Location: Japan

The reason for using a double bottom is to improve the damage stability of a vessel. So, whatever statutory requirements you must comply with in those set of rules shall be a set of criteria. Part of the criteria, shall be the extent of the damage, vertically, athwartships and longitudinally. If you place the double bottom outside of this "zone" of damage, the double bottom greatly improves you final stability post damage.

6. Joined: Aug 2004
Posts: 2,803
Likes: 365, Points: 83, Legacy Rep: 2040
Location: Port Orchard, Washington, USA

### jehardimanSenior Member

Be very careful here, because an unflooded double bottom will usually not improve damage stability! This is because the effect of an unflooded double bottom will raise the KG faster than the KB with the same loss of waterplane inertia resulting in a lower damaged GM (assuming standard arrangements where the double bottom volume is small compared to the compartment).

Under the new SOLAS probablistic damage rules, damage is assumed to extend from the keel to 12.5m above the waterline and to 1/2 the beam, rendering double bottoms moot for collision. Double bottoms are to improve hull girder strength and limit grounding/bilging damage, not flood control for side collision.

http://www.imo.org/blast/blastDataHelper.asp?data_id=24855&filename=281(85).pdf

Last edited: Dec 3, 2013
7. Joined: Oct 2009
Posts: 4,738
Likes: 332, Points: 83, Legacy Rep: 1485
Location: Midcoast Maine

### DCockeySenior Member

My impression is a primary political/popular (in contrast to technical) reason for mandating double bottoms on tankers is the idea that a double bottom may prevent oil spills in the even of a grounding or allision with a rock.

8. Joined: Oct 2008
Posts: 6,643
Likes: 650, Points: 113, Legacy Rep: 2488
Location: Japan

You must be referring to significantly larger vessels that I design. I would have to dig out old notes from when I was a student (and had hair) to investigate that further, but I defer to your judgement on that, as its been over 25 years since I did big ship stuff.

On all the HSC vessels I design a DB assists in the raking damage, which is where the hull is damaged for 55% of its length from the FP and/or 35% of its length anywhere. Without a DB almost all vessels fail the damage stability requirements. Monohulls struggle and result in much larger than necessary, or very poor motions. Since to counter such damage, the beam needs to be wider and an increase in freeboard to prevent the deck line/opening of progressive flooding from being immersed. It does generally raise the VCG from a non-DB version simply to pass the onerous damage stability that being the raking damage rules, i.e using DBs outside of the zone of damage.

Last edited: Dec 4, 2013
9. Joined: Sep 2011
Posts: 6,238
Likes: 301, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

Remember that is not the immersion of the deck but the margin line.
I think if the double bottom has a vertical dimension less than the height of the damage, it will have no influence on the stability after damage.

10. Joined: Aug 2004
Posts: 2,803
Likes: 365, Points: 83, Legacy Rep: 2040
Location: Port Orchard, Washington, USA

### jehardimanSenior Member

Actually, they changed the "margin line" to the "flooding deck", i.e. where the ship downfloods, read the IMO resolution I linked. IMO really needs to get a full copy of SOLAS on the net, but I think there is money to be made selling copies.

11. Joined: Sep 2011
Posts: 6,238
Likes: 301, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

Sorry, I'm not up to date. Is gone the margin line?
There may be missing in the probabilistic method. In the traditional method, I think not ????

12. Joined: Jul 2009
Posts: 40
Likes: 1, Points: 0, Legacy Rep: 38
Location: middle east-persian gulf

### sirhamidJunior Member

Dears

13. Joined: Sep 2011
Posts: 6,238
Likes: 301, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

The method of floodable length:
- Not considered asymmetric flooding.
- Only consider the effect of transverse bulkheads that extend to the bulkhead deck.
-Not considered neither longitudinal nor transverse stability.
Does this answer your question?. If not, I recommend you read the SOLAS as far as damage stability concerns ..

14. Joined: Jul 2009
Posts: 40
Likes: 1, Points: 0, Legacy Rep: 38
Location: middle east-persian gulf

### sirhamidJunior Member

dear TANSL
if the bulkhead or any suggestion about place not effect on F.L but why we can consider the permeability factor for place and changed the flood-able length?

15. Joined: Sep 2011
Posts: 6,238
Likes: 301, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

You can outline the floodable length curves for various permeabilities, 85 and 96 for example. Subsequently, for each real compartment, once assigned its permeability, you can deduct from these curves what is its maximum allowable length.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.