G forces on a hull bottom

Is there a chart which gives the approximate impact in " G's " to a deep V bottom when coming down from 3' above 2' waves at a speed of 35 to 50 mph? I am interested in the actual pounds per square foot loading, my boats botton will have to sustain. Or a site that has the subject resolved to a degree? I realize it is only dropping 5'. It is really the vectoral impact of that and the foward speed I am concerned about.

 

Use 1/2*rho*V^2 in consistant units and apply in all three directions. V to include boat speed, vertical speed, and wave celerity and maximum orbital velocity (Use spectral analysis for a 6 sigma wave). Conserative, but works.

 

You could actually get an approximate theoretical figure for this yourself. You'll also want to keep in mind that an operator will only be able to tolerate a certain number of Gs. Anyway, you know how much your boat weighs, you know that the maximum force of impact is mass mutiplied by G (it's actually less, and it changes w/r/t/ time, but play it safe). You also know that the deadrise angle is, and what the surface area of the hull bottom is. Forces always act perpendicular to a surface. That takes care of the vertical aspect of the forces. Then, let's say you're traveling at 50mph, and you encounter a wave. What happens depends on how the wave affects the boat. You can picture what is happening by figuring out how much water is being forced out of the way by your boat what direction and how fast. It's the fwd component of this that is decelerating your boat, and supplying the horizontal component of the g forces.

 

A number or sources (books, technical papers) describe the amount of bottom pressure on the bottom of high speed boats. One of the oldest can be found in "Fiberglass Boat Design and Construction" by Robert Scott, available from the Society of Naval Architects and Marine Engineers. This is not necessarily the most accurate method, nor necessarily the most accepted, but it will put you in the ball park. It is based on the Heller and Jasper paper "On the Structural Design of Planing Craft" of 1960 from the Royal Institution of Naval Architects in the UK.

There has been much speculation and calculation regarding bottom pressures, and how to design the hull structure to them. It is not an easy task. It turns out that a wave impact on a hull is actually very concentrated to a very small area, and this area experiences extremely high loads for a very short period of time--on the order of milliseconds. By the time the structure starts to respond to the load by bending, the load dissipates very rapidly. Structural calculations based on this method of loading assumptions was developed by Allen and Jones in their AIAA/SNAME paper "A Simplified Method for Determining Structural Design-Limit Pressures on High Performance Marine Vehicles", 1978.

Other discussions about bottom loading have been published by Professional Boatbuilder magazine, particularly those articles by Joseph Koelbel, who gives some guidance on how to design boat bottom structures.

Incidentally, the method of bottom pressure calculation and structural design that ABS uses in their High Speed Craft rule is based on Allen and Jones.

Eric

 

Thank you Eric. You are agreeing with Ducane in his book of Design of High Speed Small Boats. My boat is using the data of the vertical 5/16" plywood "I" beams and very slightly convexed surfaces. AA FIR marine ply on 12" X 12" squares of frames and stringers with glued 1.5" X 1.5" triangular gusset blocks should give a monocoupe rigidty for a 2,200# wet, cleaver bowed, 28 degree deep V, 20' X 4' 6" at LWL. Seat of the pants tells me it should be very tough to stress crack at Ducan's 1/3 of the way back from the bow. Still a quick check will allow me to flip the keys to anyone with a piece of mind. Thank you for the Allen and Jones millisecond stress test. I took a stress test in my new boat, unexpectedly. I want to be able to flip the keys to anyone without giving them warnings. Thanks again.

 

I was able to find Allen and Jones in a web site. Sailing yacht @ 20 knots into 15 meter waves results in 35 to 37 psi hull loading. I feel confident that the cleaver bow and 28 degree constant V with no strakes will shear, rather than slam and spray. Thanks very much, Richard Petersen. I have a verification procedure since I am 67. A 35 year old Jersey Skiff or Hydro driver checks it out. Love those empiricals.

 

I have some experience......

It was in 1993 on the way from Falmouth to La Coruna that we fell of a freaky wave, in the Biscaya. Windconditions about seastate 7, sailing at about 5 knots.

Gasoiltank split wide open, watertank ripped open, gasoil all over the place, no further damage in spite of the heavy fall - I guess to be something like 4 to 5 meters. The boat was a 40 footer heavy displacement sloop.

It's not the hull to get worried about - it's the rest attached to it.

Cheerio,

 

Thanks for the wake up and do it right story. I have only had one trip with - G forces, for several hours. That made me a better captain. NO more - G's, PERIOD. My insurance company said run it up on land, I am covered.