Steel plate thickness

I have been building composite and cold molded boat for years. I have become interested in metal boats and have read a couple of books on steel and aluminum boat building.

What I am curious about and have not stumbled across is where is the line in steel plate thickness where the sea can stretch the skin around the frames and stringers giving the hull the starved cow look?

Ron

 

Ron,

It all depends on the thickness of the plate and the dimensions between frames. A 1/4" thick plate will not buckle, as we say, with closely spaced frames (longitudinal frames on two opposite sides, transverse web frames on the other two). Increase the distances between frames far enough, and that same plate will eventually buckle. The mechanics of buckled plates is complex, and the amount of curvature of the plate between frames, yield strength of the steel (there are many different grades) and the magnitude of the loads are other factors that affect plate buckling. The loads are the most difficult to define, so it is nearly impossible to say what is the cutoff point of any particular design. Experience with past designs is probably the best gauge.

Navy ships have a tendency to the "hungry horse" look more than other ships, because the Navy has general requirements for fast speed, which means the minimum amount of structure for the most amount of power installed (power to weight ratio). Commercial ships, on the other hand, try to operate at the most economic speeds, and they want their ships to last as long as possible, so by comparison, their plates are thicker, the frame spacing closer, and the loads encountered less severe than on combatant vessels.

I dare say that if you did a search through the technical papers of SNAME or the RINA in the UK, you would find quite a bit of information on just this topic, although I am not immediately familiar with specific findings.

Joseph Koelbel has written a number of articles and papers on ship structures and deformations, and if I were to look, I would start with his writings.

Eric

 

Eric, Thanks for the reply I will look up the sources that you posted.

I have many years experience designing and building boat structures in composites and wood. I can testify to the fact that the numbers without experience in a particular material can get you in trouble.

Learning the ins and outs of metal boat structures should be fun.

Again, thanks for the sources.

Ron

 

It all boils down to the expertize of the designer and the ability of the builder.

I have built hulls with 3mm (frameless) mild steel plate without any distortion. Same with a 38ft (frameless) raduis chine 4mm hull.
Up to 65ft were built with frames with only 4mm hull plating, extemely fair without the dreaded "hungry horse look" disease.

 

Steel plate thickness/ Frameless

Wynand,

In your post in response to my post about plate thickness you refer to frameless construction. Could you elaborate on that.

Your post is exciting I would like to know more about the boats you were refering to.

Thanks,

Ron

 

Ron,

I built three frameless designs.

The smallest was the Tom Thumb 24 (Grahame Shannon design) of which I built seven. It is a multichined design and totally frameless. The entire hull & deck are built of 3mm plate. Hull is built over a "former frame" and then lifted off.
The photo's attached shows the hull's inside without deck and in various stages of completion. Note the fair plating - no fairing used - just paint.
The boat shown was my own TT24 with stretched cabin top and own designed fin keel.

The second design was the Van De Stadt 34 multichine - build a couple of them. The complete hull & deck take about three weeks to complete with welding and shotblasting. Real time saving method with developed plates. The hull shown in photo's has my own creation pilothouse. The interior shows a clean frameless hull. Hull = 4mm and deck = 3mm plate

The third design was the Dix 38 (I commisioned this design 1990) raduis chined and totally frameless. This hull is also built over a former and is very easy to build. The photo's shows the hull being lifted from the building frame former and one where she sits outside - no frames in hull - just the sheer stuts holding the hull in shape. Comes out when deck goes on.
Hull = 4mm and deck FRP!.
BTW, the two other boats are also Dix designs - 57 & 65 ft - both hulls = 4mm and decks = 3mm plate

All these boats were built during 1989 to 1992.

 

Steel plate thickness/ Frameless

Wynand,

Thank you for the information and the photos. Very impressive.

Ron

 

Yes and made worse by the military requirement to fully weld with athe frames and the longitudinals to the plate so even new vessels can have that dimpled look when you sight down the side .

 

another example of frameless assembly (not necessarily frameless structure though) is the origami method. You might want to check out on the Brent Swain designs (http://groups.yahoo.com/group/origamiboats/) and also my own project (http://www.justmueller.com/boats/)

In "origami" the shell is build by simply folding the entire half hull from the sheet, and then adding the structure. Brent designs true frameless, whereas my boat will have both frames and floating stringers.

the assembly is quite scary to watch, there are moments when you think this will never be a boat but actually it all turns out very precise and for ease of building it's hard to beat. Also, concerning the hungry horse, all cutting and welding on the hull is reduced to a minimum with this approach.

My boat has 3mm for the hull, easy to control in origami, and 2 mm for the deck... that will be difficult to weld of course.

 

yago,

I do follow the Origami group. Your web site is great.

Ron

 

The hungry horse look comes mainly from welding distortion, not loads,

However some of it comes from hitting piers and pilings - some people who have steel boats have realized they aren't very delicate, after all, and can stand aggressive handling.