Hull Water Loads

Is there any rule of thumb for the water loads on the hull of a sailing yacht? The ce regulations say that an ocean going yacht has to cope with 4m waves and 8bft. What do 4m waves do to a hull?

Some portlights have to be tested to 5psi but that seams a bit on the low end of the scale for a hull.

Basically I want to work out the required strength of the hull structure. I'm an Aeronautical Engineer and have access to all the toys (CAD + FEM). But what loads do you apply?

Thanks,
Karsten

 

Traditional methods of design relate the hull structure to the displacement and service speed, not the conditions. The 'scantlings coeffecient' used by Gerr in "Elements of Boat Strength" is based on the volume of the hull, more or less.

 

Loads

Karsten,

interesting to see that CE regulations are recognised in OZ ;-P.

There are different approaches to the structural layout of boats. First of all there is the classification / CE norm approach. Basically, this means that you employ a cookbook approach. Hence, you open the classification society's rule book, dimension your structures accordingly and that's it.

This will be getting difficult in places where there are no guidelines given (for example the ABS Rules for Offshore Sailing Yachts state that the hull / keel joint must be designed in a way that it won't fall apart).

If you simply want to design a boat four your own convenience I don't see any reason for using the CE norm. In some places it seems too far away from the actual pysics.

On the other hand there is the approach to dimension the scantlings according to the actual loads. Having FE tools at hand seems to be nice for the more involved cases. Problem is: What are the loads. Many folks, especially those involved in racing boats,choose this direction. However, it may take quite a few boats until you will be able to determine the actual loads on the structure correctly. This experience may be reflected (to a certain degree only!!) in the classification society rules.

Cheers

Zed

 

First, you decide how loads will scale. They scale differently for racing craft, pleasure craft, and cargo craft.

Second, you design scantlings for a base boat and build it and test it.

Third, you scale the scantlings for your base boat to your desired boat based on how the loads scale.

Lots of possible answers. Lots of bad answers.

 

Hmm? Trial and error doesn’t sound good. I only wanted to build one boat. The cookbook approach sounds a bit better. What cookbook would you suggest for a 42ft sailing yacht?
Basically the water should act like a damper. If you hit it a low speeds you get a low pressure and if you hit it at high speed the pressure is greater. The pressure load should also depend on the size of the surface that hits the water and the curvature of that surface. Has nobody worked out a scientific approach to this problem?
What about tests (dropping something on a water surface) or putting pressure sensors on a boat hull? Has nobody ever done that?

Cheers,
Karsten

 

Karsten,

The CE regulations, as well as the classification society rules (ABS, LLoyds, Det Norske Veritas, Bureau Veritas, etc.) always relate the loads on a hull to vertical feet (or meters) of water. The formulas used in these rules are designed so that if you assume 4 Meters of water head, for example, then the resulting thickness, moment of inertia, and section modulus for the hull skin and internal structure will be enough to handle all the normal and adverse loads that the sea can throw at it.

The reason for using this approach is because the dynamic loads on a hull are so complicated to model that you really cannot do it reliably in simple calculations. The idea is to use simple formulas for complex problems. The resulting "heads of water" (as in pressure heads used in fluid dynamics) are based on empirical studies of hull structures that have both survived and failed. That is, they are based on vast amounts of experience and study, so that there is a great deal of confidence that the heads--and their corresponding formulas--will work. You cannot take the heads in isolation and expect to use them reliably in equations for structure from other sources. That is, ABS heads will work in ABS equations. Lloyds head will work in Lloyds equations.

The whole concept of heads is to make the engineering easier to do.

 

Karsten,

I can only agree with what has been posted by Eric. The point really is that you cannot simply say that if the boat will survive this and that kind of dynamic pressure (say, by lifting it up with a subsequent bail-out) it will be able to survive anything else. How do you know that there will be absolutely no condition that is worse, ever?

I really do prefer first principle approaches. But dynamic loads on ships in a seaway are simply a pain in the back. Has anybody mentioned yet that the waves are completely different, depending on location? If you are after info on dynamic behaviour I can only recommend this book: Seakeeping: Ship Behaviour in Rough Weather by Lloyd. I costs 50 Pounds. Money well spent.

I spent quite some time assessing yacht behaviour in waves and can only say that if you are searching for something you can spend the rest of your life on, you have come to the right place. (Lloyd did just that!)

Cheers

Zed

 

O.K. you convinced me. So I have to get the Lloyds "Rules and Regulations for the Classification of Special Service Craft". Looks like my Christmas present is sorted.

Karsten

 

Karsten,

There is also a Lloyds Rules for Yachts and Small Craft, which might be more appropriate for your sailing yacht.

Eric

 

Loyd's rules - small yachts - purchase

Eric:

Where can one purchase "Lloyd's Rules for Yachts and Small Craft"?
Thank you.

Robert

 

Lloyds probably has a website, so try googling "Lloyds Register". The last US address I have is:

17 Battery Place
New York, NY, 10004-12195
Tel: 212-425-8050

This was good in 1994.

Home office in London is:

71 Fenchurch St.
London, EC3M 4BS
England
Tel: 01-709-9166

Eric

 

I have the distinct feeling that Lloyds, alomg with most of the other European bodies, has "retired" their rule in favour (if that's the right word) of the EU RCD/ISO stuff.
I looked at their site (www.lr.org) and saw nothing about yachts or small craft any more. Of course, it doesn't make the old rule useless, it just means you can't buy it anymore.

Black Market, anyone?

Steve "hoping it's still out there somewhere..."

 

Regulations

Germanischer Lloyd are still doing the small craft and yacht stuff. Check http://www.gl-group.com

Cheers

Zed

 

Check these two site for draft copies of ISO standards

http://www.nmma.org/certification/technical/technicalupdates/index.asp?bhcp=1

http://sailing.org/page.asp?PageID=...t1H3WyGOCasHNF0ikk1LDBoBSTftGmYfx/frmR5Riuq04

Lloyds have the SSC rules now, which is quite heavy handed for some boats. I have not used it for yachts, but for workboats it's about 10>20% heavier than the old small craft rules. They are however bring out some new rules I'm told, but when that will be I don't know.

 

The German Lloyd rules are what I was after and are free to download. You only have to remember that the Germans use a "," for numbers where the rest of the world uses a ".".

Karsten