Design area factor

[Kastally80]

Hello guys,

I am working on a 12 m planing boat.
I am using ABS, Bureau Veritas and Iso Rules for the scantling of the hull, which are very similar.
In the design pressure formula we need to calculate the Design Area Factor (K2 for Bureau Veritas, Kar for Iso and Fd for ABS) which depend on the panel's dimensions.
According to each rule, this factor should not be less than 0.25 for Kar, 0.5 for K2 and 0.19 for ABS, but in my case, I mostly get lower values than these.
What factors should I take into account then? these minimum ones? or should I better increase them?


Thanks a lot for your help.

[Eric Sponberg]

You should use the minimum numbers as stated in the rules. If you use a lower number for Fd, your design pressure will be lower, and that might take you outside the practical bounds of the science that this design procedure and philosophy are modelled on.

By the way, my copy of the ABS Rules for High Speed Craft says that the minimum value for Fd is 0.4, not 0.19. I have copies of the latest updates, and I don't see any other minimum value allowance for Fd other than 0.4. Which set of rules of the ABS rules are you referring to?

Regards,

Eric

[Kastally80]

Hi Eric

Thank you very much for your answer. I actually mistyped the minimum value for Fd according to ABS Rules: it should be 0.4 indeed.

I am aware that these are minimal figures but I was just wondering whether there were cases where one should use higher factors...

Alik

[Eric Sponberg]

As you are aware, you determine Fd by the ratio of Ad/Ar where Ad is the area of panel under investigation, and Ar is the reference area as determined by the boat's displacement and draft. The smaller this ratio is (so the smaller the panel area is) the higher Fd is, and so the higher the pressure is. Also, the larger the panel is, the lower is Fd and the lower is the pressure. This sounds almost counterintuitive, but it all has to do with how fast the panel reacts to peak impact pressures. Small panels react quickly and so see high pressures; large panels react more slowly and so see smaller pressures.

This science is based on a short paper by R.G. Allen and R.R. Jones, structural engineers at the David Taylor Model Basin, and published by the American Institute of Aeronautics and Astronautics, 1978. The paper is Publication 78-754 and is titled: "A Simplified Method for Determining Structural Design-Limit Pressures on High Performance Marine Vehicles". It was presented at the AIAA/SNAME Advanced Marine Vehicles Conference in San Diego, CA, April 17-19, 1978. It is commonly known as "The Allen and Jones Paper."

ABS and all the classification societies bought into this method of pressure calculation pretty quickly (within about 10-12 years). Some say it has its flaws and does not predict bottom loads accurately. For that discussion, search the articles by Joseph Koelbel on bottom design for high speed craft which have been published by SNAME and Professional Boatbuilder magazine. However, it is the current science.

I hope that helps.

Eric

[Kastally80]

thank you very much for your help Ecric