Scantlings for welded HDPE boat construction

I have searched the web for scantlings for HDPE construction but am drawing a blank. Anyone know where I can find this info? There are companies building workboats with this material. I am assuming that they are framed similar to hulls built with other sheet goods such as plywood, aluminum or steel but have no idea of appropriate scantlings.

 

Can you find anyone selling plans for such construction ? What sort of boat do you have in mind using HDPE ? I'd imagine a closely spaced, interlaced web of longitudinal and transverse frames would be called for, given this material is not very stiff. What that all adds up to in weight, and labour, could become an issue.

 

This is just an opinion, taken on the fly, without thinking much, and I would not wonder that some expert in this forum would disagree with it.
If you use, for example, ISO 12215, for the scanting of your boat, you can use the methodology of the metal hulls (homogeneous material in its thickness), defining a new "metallic" material, HDPE, and using its mechanical properties .
You could use any other regulation but ISO 12215 is the most used, and most appropriate, for small boats.

 

Interestingly, HDPE boats do not hold marine growth, ( or paint, reliably, for that matter) an advantage for some applications. But it is such a flexible material, that moulded boats that have a lot of "shape", indentations and corrugations etc, are most appropriate.

 

No ,I do not know of anyone selling plans for such construction. I did find a video of one of those large workboats that look like a RIB, like the Coast Guard here uses no instead of RIBs which was built by a UK company. All the bottom framing was pretty much what you would expect for a boat built from any kind of sheet goods, no mystery there although I would expect you would need closer spacing due to the lack of panel stiffness. I am a boatbuilder by trade and am well aware of the cons of the material but there are also pros. I don't have any particular build in mind, I am just intrigued by different technologies and never want to stop learning. As I mentioned there are very nice work boats being built in the Netherlands out of HDPE and obviously someone is building the boats the Coasties use here. I would love to just spend an hour aboard one. Lack of panel stiffness and weight are the biggest drawbacks as I see it, oh, and then there is the cost of the extrusion welder of course which would make it difficult for a one off build.

 

Is it possible to put the sheet through a swaging process, to get some nice deep pressings ? I'd assume after being softened by heating. That could solve a lot of the longitudinal framing problem. Obviously this has been a common practice with alloy boats, the pressings allowing a relatively thin material to be used successfully. You might even be able to use another alloy boat "trick", slotting the panels into suitable extrusions, for easier and stronger joins.

 

The lack of stiffness is a problem. Some manufacturers fill the boat with foam to stiffen it up.
Triumph 186 Sportsman: Roplene Plastic Fantastic - boats.com http://www.boats.com/reviews/triumph-186-sportsman-roplene-plastic-fantastic/#.WVpsDITytEY

 

Scanting rules for HDPE can be found in DNV-2-21 Craft Standard (it was recently updated as DNV-GL document).

 

I have seen some claims from makers of some Polyethylene boats that the flexibility of the material results in a better ride in rough water. That has to be regarded with scepticism. imo.

 

I'd say it's fairly accurate, the flex softens that hard impact, just the opposite of aluminum or carbon. Not saying it's a good or bad thing, just depends on what the desired result is.

 

A better ride is always a desirable result. You don't get it from spongier, more flexible boats, all else being equal, from my observations. It certainly does not help inflatables. A planing hull that dishes under load, is a dynamic hook that increases vertical accelerations. The impact might be slightly delayed, but when it kicks in........

 

Then you have not searched very far...

Rhino Marine, the reference in HDPE boats http://www.rhinomarineboats.com/
Arya Shipyard - Workboats, New Build, Steel Workboats, Aluminum Workboats, HDPE Workboats http://www.aryashipyard.com/patrolboat.php
Demo HDPE Vessel - Tideman Boats BV http://tidemanboats.com/demo-hope-vessel/

and so on...

 

I didn't say it would handle well, just that the ride would be softer, at some speeds it may be fine, others not so good. But the advertising claim would be fairly accurate.

 

I remember people would get rid of old plywood planing hulls when the bottom lost some of the original stiffness, after repeated use in rough water. The claim was that the ride quality had gone downhill. If that is true, then flex does not equal improved ride. It is sometimes difficult to gauge what is the enemy in a flexible hull, though, it could be as much the lighter weight overall, the extra noise and vibration transmission, as much as anything else. Polyethylene is a very different type of material , and it being deformed under load no doubt does not have the same implications as for glass, or alloy, where cracking is a likely outcome. PE may be more inclined to stretch, rather than fracture. But it does seem to have found success with boats, though I doubt sheet material is the most likely method.

 

I am well aware of companies such as Tideman Boats but not the others mentioned so thanks Ad Hoc. Not seeing any indication of scantlings though, maybe they could be hired to work them out if one had a particular a particular design in mind. I am also well aware of the lack of stiffness but usually this can be addressed with closer spacing of framing to reduce the unsupported panel area with any material. There are some photos on some sites that show framing but its hard to imagine stringer dimensions for example when the material is also rather flexible. Perhaps one could use a foam/glass structural grid with bonding flanges just like we do in composite boats, there are methacrylate(sp) adhesives specifically formulated for bonding dissimilar materials to unprepared low surface energy plastics such as HDPE although I am skeptical of how well the bond strength actually would be. We have done repairs to such a grid in a carbon hull where the plexus bond had failed. ME, all the boats in the links Ad Hoc supplied are built out of extrusion welded HDPE sheet material, roto molding is just for small boats like kayaks.