Load Paths in a Catamaran

In the past few years I have come across foam composite bridgedeck catamarans in the 11 - 13m range being built with vastly different laminate schedules and orientations.
I can understand that performance boats will choose lighter laminates than those wishing a more robust and serviceable boats. But I would have expected the orientation of the fibers to be the same.
This has got me wondering about load paths in the inner and outer laminates for the various parts of the structure, do we really know them?
Does any one know of any published data?

Thanks
Andrew

 

I cannot give a direct answer to your question. My thoughts on the worst load cases are:

1. The mast compression. In the extreme it is limited by the buckling of the mast or failure of the standing rigging. If the rigging is tensioned correctly then the load is set by the maximum healing moment generated when flying a hull.

2. The bridge torsion. This load is more difficult to manage. In this case consider the entire displacement shared evenly between the bow of one hull and the stern of the other hull. This case could occur in a quartering sea either upwind or downwind.

3. The above two case could occur in combination or very close to it.

4. There are dynamic loads that could add to the static conditions. The worst I can think of is the solid grounding of one hull at full speed. The loading on the grounded bow could have longitudinal and vertical components.

There are a couple of threads on mast compression loads.

There is a thread on bridge torsion as well.

If you are able to do finite analysis I am sure there would be some interested in the result. I am betting what you see in older designs is based on what didn't break rather than detailed analysis.

Rick W

 

Thanks for your thoughts Rick.
In order to use modeling you need to know the magnitude and direction of loads to start with. There was an article in the Multihull World magazine a number of years back saying that an Australian PHD student was in the process of fitting strain gauges to a catamaran so that all the loads could be determined. I wonder if the work was completed and published?

1. Hull laminates; what I see in use is biaxial 0/90, DB -45/+45, triaxial -45/+45/90 and in all cases the internal support (bulkheads, stringers, shelves, furniture) is the same. So where are the load paths? and which fiber orientation is best? or isn't there are primary load path in which case a light weight quad fabric would be optimal?

2. Bridge deck; biaxial or DB, which is correct? or does it not matter as it is only a walking platform? and the mast and rear connecting beams are taking care of all the loads.

Comparing later designs with one older successful design a Crowther 40 Super Shockwave I see that we have beefed up all of the laminates. Super Shockwave laminates are 180g plain weave kevlar both sides plus 400g biaxal glass on most outer surfaces over 20mm foam core. The amount of UD glass in the mast and rear connecting beam/ bulk head flanges is about 1/2 of that used in more recent lesser beam designs. I am only familiar with one 12 year old S Shockwave and this shows no structural damage other than foam bruising on the internal floors that only had the 180g kevlar laminate. So are we over building things now and therefore wasting money and adding unnecessary weight? Should we be using light weight laminates say 400g if glass and 200g if carbon or kevlar plus extra 400g glass only on surfaces that we walk on plus a strip on the outside hulls where they contact the dock? I think this is what Rob Denney has been saying in a number of threads.

Cheers
Andrew

 

Andrew, have you seen John Shuttleworth's articles on design and building?

http://www.john-shuttleworth.com/QandA.html

 

Andrew
With the basic boat design I have done I find a single layer of 200gsm carbon will take the structural stresses but a layer like this is easy to damage. There are real issues with outer layers tearing from the core as well.

Carbon is not very wear resistant so this needs to be catered for.

Hence there are localised loads and wear resistance that need to be considered when using carbon. A single layer of carbon on 20mm foam will make a good load bearing panel but it will be easily damaged. A tiny scratch in the wrong place could jeopardise the whole structure.

I am sure you could make lighter structures but they could end up being so fragile to unaccounted local loads that they are useless as boats.

Actually I saw some report about a CF racing hull just disintegrated when it hit the wash of another boat.

There is one guy here who suggests taking to a hull with a 14lb sledge hammer to test durability. If it shows no sign of damage after a few hefty blows then you can be confident about its seaworthiness. That is of course if it floats in the first place.

There are some good standards for durability testing of sandwich panels. One mimmics airline trolley casters as these provide concentrated loads. Maybe boating needs a similar test for grounding on rocks or sitting beside a jetty.

So bottom line is that there is more to it than just the main structural stresses. Any boat has to account for a myriad of local load conditions to be serviceable.

Rick W

 

In any catamaran there are 4 main load cases to consider

1) Shear load, that is as if one hull is moving relatively up or down to the other, in-plane.
2) Transverse bending moment, like holding both hulls and trying to pull them together whilst rotating via the vessel CL
3) Pitch Connecting moment, one hull rotating up and the other rotating down. Thus creating the classic "S" shape in the raft structure at the extreme ends. Pivot being midships
4) Torsional moment, same as 3, but pivot being vessel CL.

Each load case imparts loads in different ways and also depends upon the hull geometry.

Such as 3 and 4. If the L/B ratio is close to 1 it produces vastly different results than if the L/B is say 4 or 5.

So each requires investigation and hence ascertaining the direction of the main strain....and then designing the structure accordingly.

 

Rick
I am thinking of serviceable boats, and at the moment I am building one or hope I am. I guess I am mostly thinking about how I could be doing things differently to save materials and weight but still be serviceable. And knowing the load paths is one part of it.
At the start of my project my material supplier recommended 800gsm glass as a minimum laminate over foam to be serviceable.
For my 12m cat I was going to use 750g triax for all external surfaces. Since discovering infusion and seeing how much the laminate is consolidated by the vacuum I beefed up all of the decks (walking & working surfaces to 1,000g. Also added extra 450g DB down from WL+500mm on the outer hulls to help with the docking bumps.

In the next day or two I plan to make the bridgedeck panel, was going to use 600g biax + 450g DB either side of 25mm foam, but is this over the top?
Should I go back to the 750g tri as originally planned (layed transversely as the bottom stiffeners will provide fore and aft support)?

Ray,
I had a look at John Shuttleworth's site, still would like to see more detailed explanations. John seems to have been fortunate to have been involved with a number of high budget projects over the years. Where he has been able to conduct not only laminate testing but tank testing as well. Maybe he will write a book when he retires.

Ad Hock,
Thanks for the explanations. These load paths and conditions would apply to all catamarans, the geometry should only change the magnitude?
My interest is in the modern cruiser, L/B approx 1.7.
Being new to this I would like to see detailed information to know what is happening to both laminate faces in each panel of the boat in different conditions.

Cheers
Andrew

 

AndrewK

Yes, the geometry and displacement change the magnitude.
As for wanting to know what is happening, i think you will be joining a very long que! Unless you have the resources to strain gauge many different configurations and do endless coupon tests, not worth it.

However, there is a good paper on this is in Int. Jour. of Small Craft Technology, Vol.150, part B2, 2008, by R.Loscombe, where he has done exactly that tested many different types.

 

Ad Hoc
How good is the Loscombe article? as it is going to cost me around $80.

 

Wow...$80...if you send me your email address, i'll scan it in and send it to you.

 

Andrew,
In my last 12m cat I was forced by survey requirements to use 1200gsm skin as a miniumum requirement to prevent damage due to impact.
The designer of 'non survey' similar boats only uses 600gsm, but his boats are 'twisty'.
I am happy I used such a laminate for the hull as it proved time and again that damage due to impact was reduced.
There are areas like rooftop where we used 600 gsm DB and it also worked out OK, but that had double compound curvature, so it was stiff by the shape only.

The bridgedeck was 16mm Duflex with 600gsm and it was so flexible that things would rattle on the table as you walked past. If a wave hit you underneath most things would fly off the table.

Unfortunately the core of the hull was 9mm Durakore, so even though I had plenty od glass, my boat was still not rigid enough: all stays would sag under sail and in a seaway the mast had a tendency to whip aroud, not a nice feeling.
Tightening the rig hard, like in monos is a recepy for disaster as the loads then become extreme.

I think you are on the right track, just beef up the underwater sections as well to protect you from the unavoidable debris in the oceans, better to ride over a container than get it stuck in one of your hulls!

I have asked Richard Wood to design the hull and scantling of a 16.5m cat and he has specified 1600gsm quad over 20mm foam. Some designer would say that's far too much, but Richard has the real life experience they lack and in my experience, that is what I would specify on this size boat.

Yes, we want to make cats as light as possible, but not reducing the glass.

 

Stefano,
Did you have any stiffeners under the bridgedeck?
You say 600g DB was enough for the rooftop, what was the core?

 

We had three 100mm half pipes glassed in underneath in which we run haliards, fuel pipes etc, they were not designed to be stiffeners, but did help a little.
In the saloon, the settees were all glassed in, so stiffened the bridgedeck, but after a 1600M trip to Broome, they came apart in places and had to be repaired, hence, there was far too much movement between the hulls.

The roof on the other hand, was very large, about 8x6m, it covered the cockpit and was part of the targa bar. There was a substantial beam over the entrance bulkhead. Some pics here: http://www.boatdesign.net/gallery/showgallery.php/cat/500/ppuser/21040 .
The core was 20mm of 60kg Divinycell; often had 3 or 4 of us working on the main etc and it coped very well.

Hope that helps.

 

Stefano
What members resolved the torsional loads for differenetial pitching between the two hulls?

Rick W

 

Mmm, hard to say, not much in my view.

1. The cross beam was built in composite
2. A "D" section went across after the nets, it was about 800x800 (I might be wrong here). Had hatches and it was used to store anchor rode, spare anchor, fenders etc.
3. Mast bulkhead; I think that was two sheets of 16mm Duflex bonded together and a 20mm ring of UNI glass going right around it.
4. Entrance bhd 16mm Duflex with large door and windows. The corners of the windows started to split after a while.
5. Rear bhd at end of cockpit, same as mast bhd.

Clearly that was not enough as I could 'feel' the boat twisting in a quartering sea.
In defence of the design, I must say that at part from the slits at the corner of the saloon rear windows, and the saloon seats slight damage, after 6 years and 4x1600NM trips along the West Coast the hull was still not showing any signs of fatigue.