# Support girth in catamarans??

Discussion in 'Class Societies' started by koanda, Feb 13, 2014.

1. Joined: Nov 2007
Posts: 35
Likes: 0, Points: 6, Legacy Rep: 10
Location: sain

### koandaJunior Member

Hi!! i am trying to calculate the structure of a catamaran with lloyd's rules.

The definition of support girth comes as follows:
2.1.15 Support girth. The support girth, Gs, is the girth
distance, in metres, measured around the circumference of
the shell plate between the tangential points or chines, as
appropriate, of the hull for a mono-hull craft. For multi-hull
craft it is to be taken between the inner and outer bilge
tangential points or chines of the individual hulls. See 2.1.4
and Fig. 2.2.2.

But in a catamaran calse like the one on the picture? what would be that dimension?

1.2? 2.4?

Thank you!!

2. Joined: Oct 2008
Posts: 6,690
Likes: 680, Points: 113, Legacy Rep: 2488
Location: Japan

The 1.20m dim you show

3. Joined: Nov 2007
Posts: 35
Likes: 0, Points: 6, Legacy Rep: 10
Location: sain

### koandaJunior Member

Using only one hull Gs i am having a 'crazy bottom pressure on fw area.

159 Kn/m2! for a 7,2 Lwl, 4 tones cat at 26 knots!

using 2,4 i am having 79.75 kn/m2 which i consider much more logical.

yo mean that is 1.2, but i have to use one, or the sum of the two hulls?
It modifies my values a lot, what do think about that?

4. Joined: Oct 2008
Posts: 6,690
Likes: 680, Points: 113, Legacy Rep: 2488
Location: Japan

That is consistent.

However you're missing the point. It is not about the pressures, but the scantlings. Since your panel aspect ratio, ie frame spacing and stringer spacing shall affect the final scantlings, based upon that pressure!

So work out what thicknesses you need and section modulii

Good luck.

5. Joined: Nov 2007
Posts: 35
Likes: 0, Points: 6, Legacy Rep: 10
Location: sain

### koandaJunior Member

i am having 68 kn/m2 for ISO rules on the same boat...

That is exactly my problem, with that pressure my structure is very strong, and then, very heavy. i am working with 6mm bottom plate, and with that pressure working out the module i am having 50x50x5 with 200mm between reinforcements. Looks to strong for that kind of boat in my experience.

6. Joined: Oct 2008
Posts: 6,690
Likes: 680, Points: 113, Legacy Rep: 2488
Location: Japan

Hmmm..don't confuse ISO with Class rules. Totally different applications. Chalk and cheese.

Agreed, that does sound heavy.

Check the other data inputs into the SSC rules.

Looks like you have a deadrise greater than 30 degrees too, so that pressure should be adjusted accordingly.

7. Joined: Jan 2005
Posts: 2,161
Likes: 259, Points: 83, Legacy Rep: 1110
Location: Philippines

### rxcompositeSenior Member

When using the software, the inputs ask for the breadth or the sum of two hulls for catamaran but when inputting the girth distance or the distance between chines, the rule ask for individual hull.

The accuracy of this distance (Gs) is critical to the bottom pressure derived. I played around with the dimensional input in the software and calcs shows that a longer distance due to a high deadrise or bilged craft reduces bottom pressure.

Panel bottom pressure is also dependent on its distance from the baseline (or keel line). The deeper the panel is, the higher the pressure, thus it is highly dependent on the draught of the vessel. The location of the panel will also affect the pressure. Check where the panel you are analysing is located midship, aft, or forward end.

Lastly, check your desired operational speed as the vertical acceleration will affect the bottom pressure especially when it is in the slamming zone.

8. Joined: Apr 2014
Posts: 1
Likes: 0, Points: 0, Legacy Rep: 10
Location: Perth

### FlatronNew Member

The support girth is the girth distance, in meters, measured around the circumference of the bottom, between the tangential points or chines at the LCG.

For multi-hull craft it is taken for only one hull. This is because the vessel may encounter asemetrical loading where only one hull is supporting the vessel.

1.2m is correct.

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.