Aluminium Structures Q

Hi, my Structure leturer has given the class the following question:

Prepare a paper examining the design pressures used for structural design of a high speed monohull vessels operating in the planning mode.

Part of the Q includes:
outline the theoretical pressures based on first principles eg: displacement, speed, vertical acceleration.

Any idea on what he's on about, nobody in the class seems to know!

 

ask TOM SPEERS in here somewhere

 

Sounds like he wants exactly what he's saying. You are to calculate the static and dynamic pressures on the hull bottom from a Newtonian kinetics standpoint. Since this is rarely done in practice, it's an exercise to get you to see the connection between the bare physical laws, the situation at hand, and the empirical and theoretical methods you usually use in boat design.
My experience is that when a prof is unclear, it's always ALWAYS best to ask them to clarify what they want.

 

you know Marsh. I understand all you say, but I was not blessed with a math brain so it severely limited my design capabilty. And over the years made me feel quite inadaquate All my doctor mates, science friends, some in Russia with degres in Molecular biology , say to me, oh we would rather do what you do, like hell they would, swallowing paint fumes, welding fumes, sawdust has not been easy on the bod

 

marshmat,
thanks for the reply however i can't seem to find this info anywhere ( i just may be looking in the wring place), any ideas on where to fin d the appropriate equations.
Also, i can't ask the prof as he only available on friday's and the assignment is due on friday!

 

Rather irresponsible of him.... unfortunately this is all too common.
In engineering, "from first principles" usually means to start with the basic equations of classical physics. Force=d/dt(momentum)=mass*d/dt(speed)=mass*acceleration, pressure=force/area, etc that you'd find in your first-year physics text. (Or Wikipedia if you must.)
Now don't take my word as gospel as I may be reading your question wrong (we are from opposite sides of the world after all). But from what I see, you're supposed to start with the speed and displacement of the boat, from which you can figure out the momentum of the boat and the change in momentum of the water it must force out of its way, from which you can determine the force (and thus pressure) that water applies on the hull as it is redirected. Then I'd look at the vertical accelerations, how much force they involve (assuming the whole weight of the boat is accelerated) and how much of the hull area that force would act on.
Again, I'm not sure if this is what your prof wants, just how I would interpret the mathematical part of the question. My own physics work lately has involved a bit too much quantum-wave mechanics and so I'm a bit rusty on the classical (read: practical) stuff.

 

M_artis: There has been a lot of research on bottom pressures and there are any number of ways to calculate them, starting back in the 1960s by Englishmen S.R. Heller Jr. and N.H. Jasper in their classic paper "On the Structural Design of Planing Craft", published in the Quarterly Transactions of the Royal Institute of Naval Architects, July, 1960. This paper was used directly for the Small Craft Engineering class at the University of Michigan, Dept. of Naval Architecture and Marine Engineering, published as class notes, "Small Craft Engineering--Structures", Publication #121, October, 1971. This publication may still be available from UofM, but I am not sure.

The authors of those notes were Peter A. Silvia, Robert J. Scott, and Constantine Michalopolous. Robert Scott went on to publish essentially the same material in his classic book, "Fiberglass Boat Design and Construction" which is still available through the Society of Naval Architects and Marine Engineers (SNAME). www.sname.org. This book covers the Heller and Jasper method of calculating bottom pressures of planing power boats.

In 1978, another very significant paper was published by the American Institute for Aeronautics and Astronautics (AIAA) and SNAME by R.G. Allen and R.R. Jones called "A Simplified Method for Determining Structural Design Limit Pressures on High Performance Vehicles.", paper #78-754. for the Advanced Marine Vehicles conference in San Diego, CA, 17-19 April, 1978. This classic paper and method of calculating bottom pressures has been used by the major classification societies such as ABS and Det Norske Veritas. The ABS High Speed Craft Rule is based entirely on this science. You can download a complete version of the High Speed Craft Rule from ABS for free at www.eagle.org. The science is pretty direct and based on good first principles engineering, therefore a pretty worthwhile method, particularly if the classification societies use it.

Finally, another notable contributor to the problem of bottom pressures has been Joseph Koelbel who wrote a two-part article for Professional Boatbuilder magazine that appeared as "Structural Design for High Speed Craft" in the October/November 2000 and the December/January 2001 issues. He discusses the pros and cons of the different methods of bottom pressure calculation and structural design. These were summarized further in Koelbel's presentation at IBEX, 2001, "Design for High Speed in Rough Water." You can probably get a copy of the lecture notes from Mr. Koelbel by writing him in Springfield, VA, at joekoelbel@aol.com. His business phone is (703) 321-1712.

I hope that helps. Good luck.

Eric