Discussion in 'Multihulls' started by superyachtmobil, Jun 12, 2013.

1. Joined: May 2013
Posts: 10
Likes: 0, Points: 0, Legacy Rep: 10
Location: australia

### superyachtmobilJunior Member

Does anyone know if there a method of working out the forces and loading exerted between the two hulls of a catamaran.

2. Joined: Aug 2002
Posts: 16,464
Likes: 1,489, Points: 123, Legacy Rep: 2031
Location: Milwaukee, WI

### gonzoSenior Member

Yes, but it is rather complicated. The calculations are way beyond the scope of a short answer. What is your level of engineering and/or math?

3. Joined: Jan 2005
Posts: 2,715
Likes: 563, Points: 113, Legacy Rep: 1110
Location: Philippines

### rxcompositeSenior Member

Cats have two sides per hull that is defined. Outer and inner with the inner divided into two parts. Above waterline and the haunch. Plus the bottom. Which one are you interested in?

4. Joined: Feb 2005
Posts: 3,900
Likes: 199, Points: 63, Legacy Rep: 971
Location: Coastal Georgia

### SamSamSenior Member

One way is to calculate them, another way is to build a model and figure out solutions for the problems that develop when you stress it.

5. Joined: Aug 2002
Posts: 16,464
Likes: 1,489, Points: 123, Legacy Rep: 2031
Location: Milwaukee, WI

### gonzoSenior Member

It seems to me that he is asking about the forces on the joining structure between the hulls.

6. Joined: May 2013
Posts: 10
Likes: 0, Points: 0, Legacy Rep: 10
Location: australia

### superyachtmobilJunior Member

Sam Sam - Thank for the idea about the model - It is half built and that where I am now asking myself about the forces that will twist, compress and tension the central structure joining the 2 Hulls.

Gonzo is correct that its the forces on the joining structure that I am interested, compression,canter lever and tension, Is there a simple diagram out there of the forces exerted on a line diagram, for say pitching , slamming or stresses induce when one hull rises out of the water.

It would seem to me that the new Americas cup cats rely on a slightly flexible method of joining the hulls so that the stresses are absorbed prior to any structural failure.

7. Joined: Aug 2002
Posts: 16,464
Likes: 1,489, Points: 123, Legacy Rep: 2031
Location: Milwaukee, WI

### gonzoSenior Member

No, it is a very complex diagram, since there are forces in every possible direction. There is also tension and compression at different times too. Unless you are designing something for a high performance application, use similar successful boats as a guideline. If your design is way off, question yourself why you calculated such different scantlings.

8. Joined: Jan 2005
Posts: 2,715
Likes: 563, Points: 113, Legacy Rep: 1110
Location: Philippines

### rxcompositeSenior Member

9. Joined: May 2013
Posts: 10
Likes: 0, Points: 0, Legacy Rep: 10
Location: australia

### superyachtmobilJunior Member

Many thanks everyone for the input, I will digest the threads my design is somewhat in a grey zone as we have foils, hence we have a displacement leverage to work out, then factor in the foils.

Once again thanks for the direction on this

10. Joined: Apr 2010
Posts: 620
Likes: 97, Points: 28, Legacy Rep: 319
Location: EU

### Manfred.pechSenior Member

11. Joined: May 2013
Posts: 10
Likes: 0, Points: 0, Legacy Rep: 10
Location: australia

### superyachtmobilJunior Member

Thanks Manfred - Thats perfect exactly what I need to get a real feel for the forces - The John Shuttleworth page looks good,

12. Joined: Feb 2002
Posts: 2,319
Likes: 303, Points: 83, Legacy Rep: 1673
Location: Port Gamble, Washington, USA

### tspeerSenior Member

There are a great many loading cases that have to be considered. The obvious ones are with both hulls in the water and equally loaded, and with all the loading on one hull and the other one clear of the water. But there are others as well, such as the weight taken on the bow of one hull and the stern of the other (wracking from diagonal wave), and with one or both bows immersed and the sterns in the air (near pitchpole). Basically imagine all the possible ways the craft can be supported and take the worst cases.

You also have to consider dynamic effects, such as the sudden deceleration from driving into a wave and the attendant hydrostatic pressure on the bows. And waves can have the effect of pinching the hulls together or prying them apart.

The loads on the rigging also have a substantial effect on the loading of the hulls and beams, resulting in wracking of the hulls relative to each other.

The Shuttleworth illustration would be for one load case out of many.

13. Joined: Oct 2008
Posts: 7,560
Likes: 1,436, Points: 113, Legacy Rep: 2488
Location: Japan

Indeed there are, which is why such matters should not treated lightly by those unfamiliar with such calculations and loading scenarios, such as the OP.

Exactly....you (the designer) must establish and evaluate all of them, not just one.

14. Joined: Jul 2003
Posts: 3,047
Likes: 327, Points: 93, Legacy Rep: 1306
Location: Thailand

### AlikSenior Member

For a power cat, we usually check 3 cases:
- Transverse shear
- Transverse bending
- Twisting around transverse axis

#### Attached Files:

• ###### cases.jpg
File size:
55.9 KB
Views:
673

15. Joined: May 2013
Posts: 10
Likes: 0, Points: 0, Legacy Rep: 10
Location: australia

### superyachtmobilJunior Member

Thanks, Thats a good graphic for a starting point, Once I have completed the scale model I will shear and bend the model, am planning to build flex into the design to absorb some of the kinetic forces

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.