How to calculate a sail catamaran hull pressure?

Thanks Alik.

Could anyone point me to Alik's article in Professional Boatbuilder please, I cannot seem to find it there.

Thanks

 

Alik's article "Impact on Cats" is in the October/November 2015 issue of Professional Boatbuilder, and starts on page 80.

 

Well, you're in good company since the Allen & Jones paper you cite, only used 2 monohulls for their paper

It depends what one wishes to do with said loads. If one wishes to design a boat for the exact conditions that were used to be tested and no more and a vessel of said parameters, that's a perfectly logical conclusion.

However to extrapolate the findings to vessels of different size and configuration, would be a case of the kettle calling the pot black, with regards to Class rules

All Class rules have their upper and lower limits. LR for example clearly state that below 24m vessels should be on a case by case basis, as they recognise that some design parameters may throw up inconsistencies. However, Class has a huge database of vessel, not 2 or 3, to confirm this and hence take notice when such occurs.

But one must also recognise that Class rules are intended for longevity. Thus Class rules have plenty of their own in-house fudge factors to ensure that if a vessel is designed with their rules, the structure will not fail, assuming the design parameters have not been exceeded. Thus using a sample of just 3 and then finding out in say 6months time you experience a failure, that is not necessarily a fault with the results/findings, it is merely a result of a small sample and limited variables of said sample. Thus to extrapolate from 2 or 3 vessels to all different types and configurations of vessels is a long stretch to make.

I have plenty data of my own that suggests Class rules are often too conservative. But my sample set is significantly less than that of say LR, for that exact design parameter and operational requirements, and thus would be taken into context of others. However on some occasions I have demonstrated plenty of my own evidence and validation which has prompted a change.

Just like any scientific research it is a constant WIP and where necessary changed owing to new data. Just because new data conflicts with existing, does not automatically mean it must become the new default. Class put their name on the boat and thus need to ensure consistency across a wide spectrum that performs day in day out for many years. Only a larger database of in-service conditions and many different type of vessels can provide that assurance.

Whenever I come across such anomalies, i present a case to Class. That's all one can do, unless one wishes to start ones own Class rules for everyone to use and all the guarantees that must come with it

What also needs to be taken into account, for me anyway, i care little about the design pressure (except for localised structure in the absence of any other hard design data to use). I only care about the final scantling, is it sensible for a boat of that size/speed/operational area etc. Whereas designing to a minimum, i.e. to just pass rules, is not the way to design structure either. Many other factors come into play that would suggest Class rules should be the minimum requirement, and not considered as a maximum.

 

2AdHoc

Right now, there is a discussion of DNV-GL LC& HSC Rules. Have You already contributed?
I wrote quite a few comments, mainly to the composite structures where they want to impose min laminate weight (typical DNV approach, but not in current GL rules) that I believe is an overkill for smaller craft. Hope they listen...

 

Unfortunately, these 'databases' are not transparent, there are not too many published data and they are not likely to cover small craft. Besides, most of classification societies use Savitsky-Brown formula for accelerations; if they have their own database why don't they use it? I believe same happens with pressure where results from 1960-70s have been used repeatedly; just look at pressure distribution.

 

It does depend how the min laminate weight is used, since there are also minimum thickness requirements for steel and aluminium too, no different.

The min laminate weight must be coupled with the stress/deflection requirements not just a stand alone requirement, then it yields sensible results. Well, it has for me in the past.

But when designing smaller vessel, any rules, Class or otherwise, will at times yield erroneous results. Keep plugging away at them..it's the only way to affect some change, positively

Because most of it is using design data supplied by many companies for validation. They would not be in a postilion to publish property information on clients' technical design data. But where they find something new that must be shared by all, they do this in the usual route via IACS and also conferences. They attend and review data that is published by many others too...just to ensure nothing has slipped by. As noted it is a constant WIP.

 

This is quite simple - for single skin laminate DNV (and new DNV-GL rules proposal) requires 4200 g/m2 of glass on bottom. This is 10mm of thickness, for CSM. Will it work on 6-8m boat? The answer is NO. Similar, they require min weight for sandwich which gives about 5mm of thickness - another overkill. But what is the core, and if it is coremat why do we still need these 5mm?

 

Well, I would suggest you find better laminaters then.

Our laminaters get 6.7mm on that minimum, not 10mm. And is consistent with an aluminium vessel of that size, i.e roughly half the thickness of the composite = 3.5-4mm, which is what would be used for an Ally boat. I know others get generally 8-9mm on that 4200mm min requirement.

This is the biggest issue when using composites, consistent QA to obtain clear and consistent values.

 

No, You have to find better laminators If You consistently getting 6.7mm with 10 layers of CSM450 hand layup, likely that some layers are stolen. See ISO12215-5 on layer thickness

 

Yes AH, I think you better find good laminators.. The CSM is for bulking and they should getting 1 mm per layer (ISO, LR rule). If they are rolling it too much or laying wet on wet, the laminate will be thinner and there will be less resin by content.

 

Alik, can you give us a snippet of the new DNV rule they are proposing? The 10 mm all CSM might work for a 6-8 m boat but I think this a step in the wrong direction. ISO do use CSM as a base laminate guide but allows the user to use other combination of WR and Uni with their laminate analysis method.

ISO laminate classic analysis method is the same as LR. Although they differ in their approach, the end result of the calc is the same, except that LR woulld not permit all CSM in the boat. It requires 50/50 % minimum combo of CSM/WR.

 

DNV-GL does not force to use CSM, this is just sample I used to illustrate absurd requirements to min laminate/thickness. The problem is: we were designing a lot of small composite craft under GL (commercial and patrol) and the GL2012 Rules are perfect for that. The DNV HSCLC Rules are not friendly to craft below 24m, and now those unfavorable requirements are merged with GL Rules to make the document impractical. Generally, DNV Rules are step backwards for small craft - towards heavily overbuilt and extensively engineered structures.

I also expect engineering time with all these innovations will be 300% from what we have now, due to numerous and mandatory global strength checks for ALL composite boats. Some of those cases can be done only using FEA... At GL2012 Rules, global longitudinal strength for small craft was not checked, it is never a limiting case usually it complies by default. A small boat client is not ready to pay for such extensive calculations... hope the DNV-GL guys understand that and would not destroy their own market.

 

I agree with you Alik. Unless DNV comes up with its own composite analysis method applicable to small crafts, it will lag behind ISO and LR.

 

In what way are they not friendly to vessels below 24m?...Im not familiar with GL rules, thus what's the difference?

 

In brief, for composite crat: exaggerated requirements, more unnecessary engineering required due to more load cases, min laminate criteria, laminate testing. These requirements so not consider limited design/construction budgets for small craft, but do not add any additional safety.