hdpe hull strip planking /molding

[gabdab]

Hello,
is it possible to adopt a strip planking method of construction for hdpe hulls ?
I guess that is an extension of what repairing such a hull (hdpe) would require (patching the hull?).
I know the proper method is rotomolding , but wonder if making a hull by aligning sheets (welding) is possible.

[PAR]

The short answer is yes, unfortunately once you get a handle on this material's physical properties, you'll see why it's not an easy, nor desirable option. Do you're self a favor and look up the physical properties of some of the common materials used in hull construction, then see where HDPE fits in.

[gabdab]

I will surely do check it later, no time now.
By the way I am looking into a hard chined hull , since I am not sure on the process to bend hdpe sheets (if possible).

[TeddyDiver]

Just a quess, but you happen to have a large stock of HDPE ?

[gabdab]

nope
Is that hard to find or expensive ?

[Cheesy]

The coeficient of thermal expansion is an important one to look at...
On the other hand its commonly used on high end kite boards, printed and then bonded to the glass/wood core

[marshmat]

Yes, welding HDPE is possible.

To save you some trouble, here's a few of the numbers PAR suggested should be compared.

Elastic modulus
Aluminum 70 GPa
Mahogany ~11 GPa
E-glass/epoxy ~20 GPa
HDPE 0.8 GPa
(rounded off somewhat, of course... don't use these values for design.)

Or in simpler terms... HDPE is over 20 times less stiff than typical E-glass / epoxy laminates, for the same thickness. It is extremely difficult to make a panel of HDPE stiff enough for boat hull purposes without also making it so thick as to be heavy and hard to work.

HDPE is a decent material for ultra-mass-production of small, simple shapes. With modern rotomoulding processes it is feasible even for 10' dinghies. But it's just not a good material for a one-off or for a boat of significant size.

[gabdab]

So is it going to be more flexible ?
From wikipedia :

Stiffness
A high modulus of elasticity is sought when deflections are undesirable, while a low modulus of elasticity is required when flexibility is needed.

I need to make a search on less stiff boats structures then .
Are inflatables stiff ?

[TeddyDiver]

Originally Posted by gabdab

.
Are inflatables stiff ?

Nope.. they (generally) need separate stiffeners or rigid bottom etc..

[PAR]

Look, with just the basic numbers in hand, it becomes clearly obvious that this isn't a structural material. It's fine if you need a cutting board to clean fish, or an overly heavy center console podium for your helm, but it's got so many issues working against it in structural applications, that it's not viable in anything, other then very light weight injection or roto molded applications, where the weight of the material can be some what accommodated for with form.

In other words, cut a piece of 1/2" thick 3.5" wide, 8' long HDPE and clamp it on edge in a vice. Under it's own weight it will sag grossly and flop around like a noodle. Now take the same dimension in plywood and note the differences. The plywood will sag, maybe a 1/16" and will not willingly flop around without some force. The same would be true of solid wood, a 'glass laminate, metals. The HDPE will flop around when the wind blows gently.

When working with these types of materials, it's extremely important to fully understand the dynamics and principles of the engineering in the structure you're attempting to employ a new material into. Of course this assumes an equal understanding of the new material as well.

I'm not trying to dissuade you, nor insult you, but plastics in particular are generally highly engineered products, who's inventors understand the concepts of the physics, chemistry and implied engineering, necessary to make reasonably safe structures.

In other words, if you're having to look up the definition of "modulus of elasticity", then you're not ready to attempt this type of engineering endeavor, by quite a bit.

[marshmat]

plastics in particular are generally highly engineered products, who's inventors understand the concepts of the physics, chemistry and implied engineering, necessary to make reasonably safe structures.

In the case of plastic boats, I would certainly hope so!
But have you seen the injection-moulded junk being sold as household gadgets lately? Sometimes it seems a lot of consumer goods fabricators just go straight from marketing designer to tool/die maker, skipping the engineer altogether.... and then the thing breaks 4 weeks later from some stupid stress concentration that any 1st-year engineering student could immediately tell you is wrong. Swiffers, cheese graters, vacuum cleaners, you name it- anyone with a bit of money can and will injection-mould any old piece of junk, engineering not required.

< end rant >

if you're having to look up the definition of "modulus of elasticity", then you're not ready to attempt this type of engineering endeavor, by quite a bit.

Agreed.

HDPE to me is a milk jug. Rotomoulded HDPE is a cheap dock bench / storage box. Ultra-stiff-super-duper-engineered-boat-structure-grade-HDPE is the stuff of a plastic company exec's fantasy.

[gabdab]

thank you for the explanation, very concerned I appreciate.
Maybe then I should look into GRP(glass reinforced plastic).
Maybe that is working as paneled structure .

[PAR]

GRP can be done as sheet goods, though it's not the most economical use of the material, in regard to shape limitations, unnecessary waste and additional labor during multiple lay-ups.

If using 'glass as a hull material, you have many options with method, application and engineering solutions. I can think of at least a dozen different 'glass methods to produce a basic hull shell. This would run the gauntlet from preformed sheets, over a frame work making a chine hull through quite elaborate cored composite structures.

To me a home built 'glass hull falls into just a few categories, single skin over a male mold or one of the simple cored builds, again over a male mold.

If it was me and I wanted to build a 'glass hull I'd make a male plug out of plywood station molds and 1x2 cheap stringers, cover this with plastic sheeting, then layup a single skin of suitable scantlings to address anticipated loading. Of course the laminate schedule and scantlings would need to be engineered by a designer, but all else could be done by the home builder.

I prefer this method because you only have to layup the hull once, unlike a cored structure which requires both sides of the core be laminated.

Matt, don't be so harsh on HDPE. I love the stuff and use it for many things. My current build has rotating masts and an HDPE bushing is used at the steps of each stick and at the partners. The centerboard has a thin sheet of HDPE between it and the case, ditto the rudder blade and the rudder head cheeks. The tiller has a pivot bushing of this material and a couple of home made (weird shaped) custom blind sheaves are also HDPE.

There are lots of uses for this stuff, but as a structural element it has it's limitations. I'm sure your wife thinks you are a wonderful man, but would hesitate in recommending a career change to male prostitution because of physical limitations ( ). HDPE has similar issues . . .

[Cheesy]

Originally Posted by marshmat

In the case of plastic boats, I would certainly hope so!
But have you seen the injection-moulded junk being sold as household gadgets lately? Sometimes it seems a lot of consumer goods fabricators just go straight from marketing designer to tool/die maker, skipping the engineer altogether.... and then the thing breaks 4 weeks later from some stupid stress concentration that any 1st-year engineering student could immediately tell you is wrong. Swiffers, cheese graters, vacuum cleaners, you name it- anyone with a bit of money can and will injection-mould any old piece of junk, engineering not required.

< end rant >


Agreed.

HDPE to me is a milk jug. Rotomoulded HDPE is a cheap dock bench / storage box. Ultra-stiff-super-duper-engineered-boat-structure-grade-HDPE is the stuff of a plastic company exec's fantasy.

Industrial designers make every engineer’s job a nightmare! Here is another little tidbit for your rant, most plastics that can be molded with any sort of aesthetically pleasing finish are very susceptible to ESC. Take something made from ABS for example and put a few drops of cooking oil (most household cleaners will work to) on it, if it has any complexity to its shape the residual stress from the molding process will enough to generate cracks, in fact the required stresses are so low that it is very likely the part will literally disintegrate!

[kach22i]

Example:
http://www.hovpod.com/workpod/workpo...teristics.html