Discussion in 'Multihulls' started by AndrewK, Feb 24, 2009.

1. Guest625101138Previous Member

Gareth
It is great to see someone who can actually do calculations. I have not checked them but will make some comment regarding the relative complexity of the two beam solution.

For starters your beams are cantilevers. Hence their end fixing is a challenge. Think about how you get the bending moment resolved in the beam at the bow where the deck width is nominally zero. So some real complexity here. It will add weight.

The two beams now introduce torsion down the hull. So the torsion has not gone away but it has been shifted into the hull. Same problem resolving it here. Generally this is easier than in a bridge beam but it has to be resolved. If not properly catered for it will cause high stresses around any hatches and ports in the hull. So more complexity here. THis is not a problem with sealed deck beach cats but is a problem in hulls having large hatchways and open spaces internally.

You can see how difficult it for some people to understand the problem. Ad Hoc does not even see there is torsion to resolve. He claims he designs catamarans. Maybe he had a hand in Stafano's boat that shows the classic failing of poorly resolved torsional loads through the bridge structure.

A single enclosed beam located around midships does a nice job. If you can control torsion to limit deflection the other stresses are not much of a problem. In fact it is not all that critical to limit deflection as long as any deflection is not putting stresses into other components. The other large force in a sailing cat is the mast compression so a single beam located near midships caters for this as well.

It is always a good test to build a model of a proposed structure and twist it to see where it breaks. If you have access to good FEA then you can build a computer model.

My paid job is identifying engineering risk and I have learnt through experience that some structural designers do not manage torsional stresses very well. I also know of designs that were adequate in the first pass and then someone decided a hole was needed to pass services through without rechecking what would happen. The hole becomes a high stress point.

If I have time tomorrow I will check your numbers. On the face of it, it roughly agrees with what I have seen in the past but the two beams have not been at the very ends of the hull.

Rick W

2. Joined: Oct 2002
Posts: 216
Likes: 5, Points: 18, Legacy Rep: 53
Location: Cotswolds Waterpark, UK

grobwww.windknife.com

Sigurd,

No torsion in this example, I assumed the hulls were infinitly stiff and the two beams were free to rotate (greased!).

3. Joined: Jun 2004
Posts: 827
Likes: 8, Points: 18, Legacy Rep: 65
Location: norway

sigurdPompuous Pangolin

Rick
grob
Rick
Seems there is some disparity here.
grobs first example has sectional area 0.14 m2.
The second has 0.28.

Extrapolating grobs results, it seems obvious to me that if the section areas of the two beams combined is the same as the single beam, the single one will allow the hulls to flex more. unless we allowed the hulls to twist.
Doesn't that mean your first quote is wrong?
Or is there also a different definition of sectional area of which I am not aware?

4. Joined: Oct 2008
Posts: 7,517
Likes: 1,407, Points: 113, Legacy Rep: 2488
Location: Japan

Grob

I don't know about you, but a deflection of 103mm, even if a rough calc on a catamaran of just 12m is horrendous! To think this is acceptable emphasis my point.

That is the max deflection i use on a cat of 80~90m, not 12m. Cat's I've design of this size 10~20m, i use a max deflection of 10mm, under a large sea quartering load and not a simplistic static gravity load case.

And yes you are correct, everything in Glass and Aluminium design is deflection driven, never stress.

5. Joined: Oct 2008
Posts: 7,517
Likes: 1,407, Points: 113, Legacy Rep: 2488
Location: Japan

"...In fact it is not all that critical to limit deflection.."

To anyone else reading this statement who is designing in glass/aluminium take note. This is not how to design structure. Any quality text book on the subject will tell you that, you don't need me to comment further...

6. Joined: Jun 2004
Posts: 827
Likes: 8, Points: 18, Legacy Rep: 65
Location: norway

sigurdPompuous Pangolin

But? My Tornado 20ft, each bow deflects about 5cm. Never have a problem with it. Is this not normal?

7. Joined: Oct 2008
Posts: 7,517
Likes: 1,407, Points: 113, Legacy Rep: 2488
Location: Japan

Sigurd

You're now slowly getting to the crux of this point and that which i have already mentioned. Every vessel is unique and requires a different solution. Part of that solution is a compromise. But the main thing to factor in is, what is acceptable for the design in its intended service conditions as well as "managing" the deflections from critical to minor.

For example, the way i design my sterns with a waterjet. My colleague and I wrote a paper on a new philioshy, which was done in conjunction and validate with DNV. We "allow" the loads to pass through, which makes the transom flex. It is essentially a diaphragm. Despite the very high loads passing through to duct. The stresses are low because the transom has no stiffness. This is an acceptable deflection.

Only your design and your assumptions and in-service conditions will dictate to you whether your deflections are acceptable.

I use 10mm on cats of this size, because they have been very heavy duty work boats going at some 25knots in Hs 2m seas in the north sea, year round. Clearly a deflection of 103mm is unacceptable in this operation.

If someone is concerned about windows popping out, 103mm over 12m is significant.

Also, to "take out" torsion is very simple. It is a simple couple, therefore it is resolved into 2 equal and opposite forces. You just need sufficient shear area in the attached structure, and then check for local bending and deflections.

8. Joined: Oct 2008
Posts: 7,517
Likes: 1,407, Points: 113, Legacy Rep: 2488
Location: Japan

next time you're on an airplane, look out the window...what is happening to the ends of the wing??..they are displacing vertically up and down. This is another example of an acceptable deflection that has been 'designed'. However, having this deflection over the length of the fuselage is not acceptable, hence it is made stiffer....

see my point?

Only you can decide if the deflection at "that point" of the design is worth a redesign or not.

9. Guest625101138Previous Member

I see you now accept that the load case I provided produces a moment that has to be resolved. Took some convincing but you finally caught on. By the way you cannot take it out it has to be resolved. You cannot simply decree it as not being there as you originally stated - zero moment; what nonsense.

A 100mm deflection over 12m can be quite acceptable providing it is not inducing high stresses at any point - exactly the same way aircraft wings are allowed to flex. If the flexing does not alter hydrodynamic performance it is acceptable. In any event it was a nice round number to do a simple calculation on to compare two structural solutions.

I don't see you offering any simple calculations. I guess you use your design staff to do these. You can only be bothered with the "philosophy" of the design not actual numbers. Seems like you read a few books but did not really understand what was written. You offer silly comments without any useful content. My concern is that people reading your nonsense might take it seriously.

Rick W

10. Guest625101138Previous Member

Is without your weight on it as well? If so set up the load case as I suggested in the posted image and sit in the middle of the tramp if you are game. See how far it deflects then.

With a relatively small cat like the Tornado the moment is limited by the buoyancy in the ends of the hull so it is not quite as bad as the load case I describe. Also when you are running or beating in a quartering see one of the hulls will be more heavily loaded than the other so the moment will depend on where you place your body weight. Sitting aft on the windward hull without a wire will probably be the worst case. The rigging can resolve the moment as well. You could do the test with mast up and also down to compare.

The main point here is that the deflection is unlikely to cause any real problems. You do not have a bridge structure that will stress around openings like big cats have.

On my latest pedal boat I use a single 25mm by 1.2mm wall aluminium tube for the supporting beam. The outriggers are 2m long and I aim to have them deflect torsionally so they pitch over waves to reduce the amount of rolling action. It is somewhat like car suspension. Going windward in 0.2m chop they work like crazy but the boat rides flat. Keeping the boat from rolling reduces the drag on the outriggers so improves efficiency. At the very end of this video, when the boat is approaching the shore, you can sea the way the outriggers pitch:
If they did not flex torsionally the lifting force would last the entire time it takes the outrigger to cross a wave crest. With the flexing the time the peak force applies is much shorter. The alternative to achieve the same result is much longer outriggers with smaller cross section. Anyhow I hope you can see flexing does not matter if it is not causing high stresses anywhere.

Rick W

11. Joined: Apr 2005
Posts: 4,127
Likes: 149, Points: 63, Legacy Rep: 2043
Location: Ontario

marshmatSenior Member

If you design to allow flexing, it's fine. Which was the case thousands of years ago with the Polynesian multis, and copied by Wharram with more modern materials. A flexible cat can work. So can a rigid cat. Where you get problems is when you try to build a rigid cat and it ends up being flexible.

12. Joined: Oct 2008
Posts: 7,517
Likes: 1,407, Points: 113, Legacy Rep: 2488
Location: Japan

Not forgetting, flexing = fatigue

13. Guest625101138Previous Member

The more flexible it gets with age, the more of it is broken.

Rick W

14. Guest625101138Previous Member

WRONG AGAIN. Stress cycling beyond fatigue limit causes fatigue. A spring will happy flex for ever providing it operates in its design range.

Rick W

15. Joined: Oct 2008
Posts: 7,517
Likes: 1,407, Points: 113, Legacy Rep: 2488
Location: Japan