PE (high-density polyethylene) plastic sister frames

Discussion in 'Boat Design' started by rwatson, Nov 28, 2018.

  1. rwatson
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    rwatson Senior Member

    This video caught my attention.
    I am thinking of all the "rotted frame" stories we get. This looks very handy.



    Edit
    Another good one
     
    Last edited: Nov 28, 2018
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Those skinny ribs in the second video wouldn't be doing much, the stuff is way more flexible than timber at similar dimensions, so I'm not sure why it is being applied to that purpose.
     
  3. Ilan Voyager
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    Ilan Voyager Senior Member

    I has been naval carpenter and after naval engineer. I'm very skeptic.
    As carpenter mixing white polyethylene and rotten wood as a way of fixing wooden boats is a shame.
    The boat of the first video is a piece of junk. Sometimes you have to admit that a boat has arrived to the end of its life, and has become a threat for the lives of the owners and passengers. So you decide to throw it or you decide to rebuild it , that depends of its real objective value.
    Not all old boats are worth to be kept. the Miles Smeeton boat (of book Once is enough) is a good example, that was a piece of unsafe sh**, without Guzzwell and his talents the Smeeton were food for the crabs. The Gipsy Moth IV of Chischester is another example, that was a truly bad sailboat. I know there is a trust that keeps it, but that's in memory of Chichester.
    A lot were not good truly seaworthy boats, Those who had sail a RORC boat can tell it. And a lot have been badly built even by high reputation ship yards, often with inferior quality wood, and furthermore badly maintained. The american harbors and marinas had thousands of these boats in the 80s when classical wooden boat building became obsolete. Also in Europe. They were dirt cheap, and 40 years later the survivors are often expensive pieces of junk. A few are beauties.
    .I would not spend hours and bucks on a totally rusted Ford Pinto with a dead engine...It did its job and merits the eternal rest in peace. A lot of old wooden boats are in the worn out Ford Pinto category. You can't even burn them in the chimney because of the toxic fumes.
    When a classic wooden boat appears to be rotting, the disease (the fungus and its trillions of spores) is everywhere, and that ask for very strong measures for killing the fungus and correcting the causes of the rot. So you have to expertise and think over if the boat is worth of being rescued. Rotting is a global problem of the boat
    Often I have seen that it was more clever and finally cheaper to salvage all that could be saved and rebuild a new hull if the boat had a real historical interest, or was a exceptionally good boat of outstanding and/or innovative design unhappily arrived to the normal end of life of a classical wooden boat. Boats are not eternal.
    As engineer I do not see very well the interest of polyethylene for making ribs in a wooden boat as it has a too low rigidity modulus .
    The lone real interest of polyethylene is to keep the planks of the outer skin tightly fit together. The material is neuter, does not corrode, it's easy to work and probably its modulus in tension would be good enough for that, no need of such a thickness 3/4 or 1 inch and 2 to 3 inches wide are largely enough for that purpose and can be hidden under the inside lining. Tight fits are essential in a classical wooden boat rigidity as the planks cannot anymore slide laterally relative to each other. If the ribs are strong enough but are not more able to keep laterally the screws planks (too many holes after a few "re-screwings" and galvanic destruction of the wood) polyethylene is maybe a solution.
    But for rotten or broken ribs the lone solution is to change them.
     
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  4. dinoa
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    dinoa Senior Member

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  5. fallguy
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    fallguy Senior Member

    There is something unsettling to me with all that rot and all that plastic.

    Imagine the pre-departure tour.

    'And here is the main structure of the vessel. Despite all the frames being rotten; the boat is fairly sound with plastic reinforcement. Welcome aboard!

    To wit the 8 year old passenger asks, why didn't you fix the rotten ones?

    Well, sonny, a good shipright is hard to find'

    ?

    A solution seeking a problem and the wrong one, imo.

    I can't top the Pinto reply, but I will share the source of my bias.

    I worked for a plastics company years ago as a financial analyst. They hired a marketing director. We were in a meeting and the hardworking sales VP was there when this young blonde marketing director says her market research shows our business has a 5 billion dollar market potential. Since we were struggling to make 15 million a year let alone 5 later; he turned the darkest shade of red I have ever seen a human. He took off his glasses and his pen hit the table hard enough to shatter. It was at this moment, he decided to speak. He said, I have been working constantly for 5 years developing relationships and selling products. You are going to have to tell me how to do it then because it simply is not there. And, they promoted her to VP. A year or two later, we hadn't created enough new business and I was involved in the determination to cut headcount by 120 after we lost a multi-million dollar contract. The point is...finally, these plastics outfits are trying to develop keys that fit in holes.

    Frankly; it seems foolish to me; especially as marketed with all the rot.
     
  6. Tad
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    Tad Boat Designer

    My thoughts on the subject from a while back.

    Professional Boatbuilder Letter

    To the Editor,

    Wooden boat building, like all construction methods, continues to evolve with availability of skills and materials.

    But I have to come down against the wholesale replacement of bent oak ribs with UHMW-PE. As a semi-temporary sister frame repair in isolated cases, the use of UHMW-PE makes perfect sense. It’s easy to install and holds fasteners well, providing plenty of strength to hold the planking in place. I’ve used it myself to sister one frame in the tight bends of a double-ended fishboat stern.

    I see a couple of problems with UHMW-PE. The first is not all UHMW-PE is created equal (kind of like wood), its mechanical properties vary with the resin used and the consolidation process. These variations will not be obvious to the eye. The tensile yield and ultimate strength of UHMW-PE is about half that of reasonably dry oak. This is probably not a huge issue as traditional wooden boats are overbuilt. The major issue I see is that UHMW-PE is roughly 1/8th to 1/10th as stiff (E modulus) as dry oak. This of course makes the UHMW-PE easy to use with light use of a heat gun, no steam required. Traditionally oak is steamed and bent to the sectional shape of a boat, as it cools the wood holds that shape really long term. UHMW-PE does not do this; it can be bent and re-bent. So the ribs of UHMW-PE are not going to hold the boat’s shape as well (lacking the stiffness) as steam bent oak.

    A boat framed completely with UHMW-PE will need to rely (at least somewhat) on other structural members to maintain her shape. These would normally be the bilge stringer (or stringers), if any, clamp/shelf, and the planking itself. This changes the stresses on various hull parts. Probably the use of multiple bilge stringers (as advocated by Bill Garden among others) supported by a number of stiff bulkheads will adequately support the UHMW-PE ribs. That’s a slightly different solution than just substituting plastic for wood.

    All the best,

    Tad Roberts
     
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  7. Barry
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    Barry Senior Member

    I would be concerned about creep. The boat builder pretty much shows the ability of the poly to creep under low loading. Ie when he bends it easily by hand and it does not return to its exact shape. So even if the hand bent piece is captured at various locations along its length, if there is a load on it, it can easily permanently deform, and not provide much in the way
    of support or strength to the structure. And this characteristic exhibits at reasonably normal temperatures as per the pdf referenced below

    Additionally, there will be a difference of thermal expansion between the two materials, so the plastic will be forced out of position repetitively over time at every temperature change.

    I have not done any gluing of PE, but in a thread a couple of years ago the discussion came up and I don't think that there was a glue solution that would glue PE to either PE or another substrate to attain the same strength as the PE but I am relying on memory here.

    I have a couple of cheap PE kayaks and find that if they are out in the sun, they oil can when they warm up. Inside there are a couple of storage bulkheads with foam and the bulkhead holds the hull ridged so the expansion presents itself.

    I don't know why the boat builder does just not use thin strips of wood which can be bent to match the curve, say 3/16 inch strips, several on top of each other, glued and brad nailed between layers and when the whole process is finished, use some long screws to attach it to the frame. With this method, many of the shortfalls as listed above and the many that other responders added, you would have better chance of
    adding strength to the frames.

    The boat builder states that they have done this on some big boats. I don't see the advantage of plastic over a wood fix


    HDPE - MDPI



    3.1. Creep Strain

    The vulnerability of polymer materials to creep and fatigue phenomenon are widely known, and high density polyethylene is no exception. Unlike metallic materials, the variation in creep strain of polymers under tensile and compressive load is distinct. Furthermore, the HDPE material is suspected to be prone to thermal ratcheting damage, due to inherent low melting temperature. Therefore, a quantitative assessment of thermal ratcheting behavior, coupled with compressive creep of HDPE material, is essential. The experimental creep test results highlight the importance of both applied compressive load and temperature on the creep strain of HDPE. Influence of applied load on creep strain is shown in Figure 3, where the magnitude of induced creep strain increases with increase in the value of applied compressive stress. Importance of applied load on the transition time from primary to secondary creep stage is evident, as the specimen demonstrates different time periods to reach the secondary phase. Yet, all HDPE samples demonstrate secondary creep, mostly within the first few hours of test. At ambient temperature, the creep strain of HDPE at 14 MPa of stress grew by six times the value of creep strain at 7 MPa of compressive stress. Whereas, on comparing the creep strain of HDPE at 14 and 21 MPa of compressive stress, the sample demonstrates a growth of 4.7 times the creep strain value at the lower load. The jump in magnitude of creep strain between 7 and 21 MPa of load is quite significant, which is nearly 28 times the former. The magnitude of the primary creep strain tends to increase with increase in applied load, causing subtle variations in the transition from primary to secondary creep for all the tested HDPE polymer samples. Alike to applied load, on varying the sample temperature under the same load (Figure 4), the creep strain tends to rise with intensification of temperature. Roughly, there is 20 percent increase in the creep strain with 10 C escalation of temperature for HDPE material under the tested load. Even though the material’s response to creep under the two types of loading is similar, their magnitude of deformation is straight out different. The magnitude of primary creep phase amplifies with increase in the sample temperature under the same compressive creep load. Analogous to compressive stress, the magnitude of primary creep strain is proportional to applied sample temperature. A compressive creep test at a sample temperature of 70 C was performed to understand if there is any drastic change in the material response, as the particular temperature is above the standard maximum operating temperature. The consequent reaction of HDPE sample is consistent with rest of the test results.
     
    Last edited: Dec 1, 2018
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  8. Ilan Voyager
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    Ilan Voyager Senior Member

    It seems there is a consensus about this plastic.
    Traditional wood is labor intensive to build and even more labor intensive to repair. When arrived at the end of its life span. as the base material , pieces of dead trees willing to return to its primary natural state: soil, degrades more and more, a wooden boat becomes impossible to fix, no miracle plastic rib or snake oil "penetrating epoxy" will save it. You have to rebuild it or to throw it.
    Rotting is a very bad sign of ageing. When the rot becomes visible in one piece, it's better to examine closely all the boat specially in all those hidden places...There are often very bad surprises. It's like rust in a car or termites in a house...
    I remember that the old boat builders like Alexandre Tertu (a guy able to design and build 120 feet 40 meters wooden fishing boats, called the "Mauritaniens") were ready to fix a broken rib, to replace a few planks or remake a worn deck, brief to do that they considered as maintenance. But they never tried to patch a rotting boat. "La maladie est dedans, il est foutu (the disease is inside, it's fuc**)". "Not worth, a loss of time and money. it's just prolonging the agony. Salvage all you can and let's build a new hull"
     
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