Sueños Mojados

Discussion in 'Boat Design' started by Saqa, Jan 11, 2015.

  1. Saqa
    Joined: Oct 2013
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    Saqa Senior Member

    kero I am used to planing mono hulls at wot so this is new ground for me. Is 13cm of transom immersion still considered dragging deep?

    I am trying to follow the canoe stern style to prevent squat and deep transom immersion with minimum WL beam but at the cost of deep overall draft in this shape
     
  2. Saqa
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    Saqa Senior Member

    G'day fellas. I would really appreciate some thots on this hull form. Some guidance on whether if I am on the right track. I feel this is it if leaning towards the canoe stern form but lack experience to validate it to an informed capacity
     
  3. Saqa
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    Saqa Senior Member

    There are pics doing the rounds on facebook of carnage in a marina caused by cyclone pam. The cat pics are interesting in that what if they were built in HDPE, how much damage would they have suffered and how big the repair job and bill it would be?
     
  4. Saqa
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    Saqa Senior Member

    I have to quote this gem from another thread, there was a fair bit of this in this thread here too

    So I wanted a tough boat made from HDPE my situation/ local that I listed out. Since there was noting like that available I thot to have a go myself and ask for assistance from anyone inclined to help out in any capacity

    I am still slugging away at somehow making this little craft that I can get on the water, moor in a feet or two of water with broken coral and oyster crust rock bottom in the tropics

    A category 5 tropical cyclone just passed over my home, 350kmh winds. I am away from my home in Fiji. Phones are still out and the whole region cut off by flood waters from help and authorities. Some info has come through though and looks like the roof of my flat has blown away and major damage in the rest of the town and a lot of destruction Fiji wide with a lot of boats smashed! Glass, timber, aly as well as pontoons, jetties and such. Apparently 13 more of the buggers might be brewing out there for this cyclone season

    Seriously, is my desire for a simple small cat made from HDPE which not only is bloody tough but also easily repaired by my ability and a welder still an ill conceived poorly thot out idea? Am I really doing myself a disservice by not entertaining anything that deviates like suggestions of glass ply or foam composite construction? When the bloody thing get picked up and thrown on top of the shoals or rocky shore, which do I have a hope of pushing back into the water and maybe welding in a patch or tear and be back in business without losing the financial investment and time and sweat that went into the project?

    Ad Hoc mate, I know you are a very highly accomplished guy but the garbage that you brought into this thread makes my blood boil. I asked for some assistance in helping me steer away from any bad design or build mistakes towards getting the simple tough craft on the water. Beams are something I too would like some direction/ help with as although I am a very good craftsman, I am a poor mathematician
     
  5. groper
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    groper Senior Member

    Hey saqa - seems a long time between drinks :)

    If a Cat 5 cyclone blows over and your not there to look after your boat - it wont matter what its made from, even the big steel ships are smashed to peices on the shore...

    The problem you were facing is that you wanted to power your boat with a small outboard motor and you wanted it to be fast and fuel efficient etc... you cant have all those things unless you keep it lightweight - and the weight figures you were publishing were much higher than what could be acheived with plywood or composite etc...

    Why not build in ply - like you did last time - but run a big *** HDPE or UHMWPE rubbing strip the full length of the keels?

    Designing good performing boats is about good engineering to acheive minimum weight- that is, making efficient use of material properties combined with efficient structure arrangement and geometry. Building the entire thing from HDPE is not very efficient use of material. If you look at polycraft - the entire boat is rotomolded from plastic - they are heavy and guzzle fuel, and if you notice their hulls have many closely spaced chines to improve the stiffness - due to the low modulus of the plastic they have a stiffness problem.

    Hence your quote from adhoc above - if your not prepared to consider deviation from your current line of thinking - then there is no point coming here to ask questions, just go and do it your way and get on with it :) Me and Adhoc didnt gel very well early in the peice either, but as i grew wiser, were good friends now :)
     
  6. Saqa
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    Saqa Senior Member

    Mate I am looking forward to your launch, your boat is looking awesome

    I still cant get my head around one thing? A boat built out of HDPE that projects as being lighter then the Silver 29 and running same or more power as the silver and around the same WL beam and length.... how is that inefficient?

    The weights and power I listed betters that of the Silver, how will that guzzle more fuel then the Silver? Please help me get my head around this as this keeps coming up the entire 8 pages

    There is more to PE boats then Polycrafts. I had a Mac from NZ for many years used around twice weekly. It did not have the steps of the Polycraft but was more rigid and lighter due to curved panels and internal structures. The whole boat is a fender and can be dropped to the ground from a couple of stories without damage

    I also dont get the last paragraph, just because the material choice is unconventional why does that mean that there is no point in coming here asking for some guidance with things like shape/motor relations and such or documenting a project?
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    All the Polyethylene boats I've seen have plenty of "shape" and usually in the form of ridges and corrugations, which suggests without it, it is like Rolf Harris's wobble-board.
     
  8. Saqa
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    Saqa Senior Member

    Does that include the Macs and Triumphs in your viewing list?

    Fact
    I experienced two cyclones in the last four years first hand. Looking out I could see a pair of massive PE tanks getting picked up and bashed into the building next door. Thats around a 15m rolling flying hit into a concrete wall and metal fence and a truck without damage to the tanks
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Took a quick look at the Mac boats, these are , of course, moulded shapes, and with convexity to stiffen it. I would not want to be building with flat or near-flat panels.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    And you'd be right. Despite what others "think" or "feel".

    Numbers don't lie.

    Take a simple 50x6mm FB...the 50mm being the horizontal dimension.

    If you placed this on the edge of a table, clamp it down. Then apply a load at the end. Say the length is 300mm, i.e. like a classical school ruler. Apply a 1kg load, what happens?

    1) The stress is simply 10MPa. So can the HDPE take this?...well look at typical values shown here yes, as its yeild is 29.5MPa.
    What is the defelction?
    It is 8.7mm.

    If the FB is ally, the stress is of course the same, but the proof stress/yield is 125MPa, if 5083 and 240-260MPa if 6082 ally. So easily takes the load.
    Its deflection is a mere 1.5mm.

    If the load is now 3kg, what happens?

    1) HDPE stress is 30MPa....opss...fails and if the load is just a tad more, it shall break. As its breaking strength is 30.5MPa. Thus total catastrophic failure is imminent.
    Its deflection with 3kg is a whopping 260.9mm

    2) If ally, 30MPa is still with its design limits and thus easily absorbed. And the deflection is still a mere 4.3mm.

    Thus, no surprises, all low modulus materials deflect very easily and fail well below conventional materials under an applied laod when sued as a structure. Unless of course the objective is to have the material deflect and not absorb the load. Thus you need to stiffen it up considerably to take any punishment. Like extreme shape, such as a cylinder/barrel etc. It also aids in not having to take any real load since it just 'bounces' about.

    But hey why let facts get in the way of a rant?
     
  11. Saqa
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    Saqa Senior Member

    I guess wheelie bins survive garbage night abuse through some sort of miracle then :/ Have to wonder how much better a foam sandwich wheelie bin would be over the usual HDPE ones? Maybe ply would be better?

    HDPE also has shape memory. A flat panel will take its shape back after deflection and when secured on all edges it makes for a very good panel

    Ad Hoc, I cant get a piece of HDPE sheet material to break. Forget about the tiny 1kg load. I have been bending the 4mm offcut back and forth and she dont break. I also cant put a hole in it with a hammer head. What gives? Why doesnt it break like you say?
     
  12. groper
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    groper Senior Member

    I have zero doubt that you can build your cat in plastic, that it will be tough and that it will go where you tell it to. Personally I wouldn't do it, nor do I think it's a good material to build from. How it turns out- who knows ? All you can do at this point is look at the material properties and make a plan with those properties in mind. That is, it's low stiffness will be the first consideration when you trying to figure out frame and stiffening grid spacing. So how far can you span your chosen plastic sheet material (thickness of hdpe) before it feels too flexible? Obviously the curvature affect this and the sheet thickness and the grid spacing. So you either have to build in a thicker sheet, use tighter grid spacing, more chines and more curvature in order t match the same stiffness as plywood or composite. The density of hdpe is also higher than ply and composite, so no matter which way you cut it- it's impossible to design this boat (or anything else) in hdpe, to be more efficient than what is possible with a lighter and stiffer material... Not that it can't be done, but it will never be as fast or as efficient given the same horsepower...

    I like the idea of a tough keel for beaching and bumping coral- that's why I doubled the glass skin thickness on the bottom of my boat... By your way of thinking on this- should I have build the whole thing in double laminate? So by the same reasoning- why build your whole thing in hdpe and not just Make the bottom tough?

    You can't make comparisons to the sliver 29... You don't know what it weighs- only what they publish. And you don't know what your boat will weigh either, but it MUST be heavier because the material properties dictate that is has inferior mechanical properties compared to other materials...

    Lastly, if your happy to have a flexible boat and keep the weight modest- you will need to consider fatigue and weld cracking. All that movement will cause problems...
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Have you looked at the mechanical properties of typical HDPE ref'd above?

    You need to look at the yield stress 4279 pis or 29.5MPa. Then you need to look at the tensile break, that is 4423psi or 30.5MPa. However the key property is that of elongation at break. It is 1350%. In other words, it will continue to stretch and stretch when a load is applied - just like any typical plastic. Once the elongation has exceed 1350% of its original length, then is shall fail totally. This is the 'plastic' region, and beyond the yield, or proportional limits. So it has failed since the material is simply stretching, it can no longer carry any load. I was using their terminology of 'break' for simplicity, but clearly it just confused matters further, my bad.

    Thus if you apply a load beyond 29.5MPa is fails....it goes into the plastic limit. If you apply a load that produces a stress of 30.5MPa or more if fails and breaks. But it only totally breaks if the amount of elongation or stretch is beyond 1350% of the original length.....if not..it just stretches and doesn't actually 'break' per se. But it has no load carry capacity at all.

    So the bending of back and forth is meaningless in this context since all you are doing is most likely stretching it well pass the yield, but not enough beyond the 1350% elongation to total failure. Doesn't mean it is a great material, just means it stretches a lot under load. That's all.

    The question you need to ask yourself is, would you make an aircraft structure out of this stuff.....and would you make a football out of steel? There is your answer, if you understand the implications of both material properties. If not, then it shall always be beyond you, despite any protestations to the contrary.
     
  14. Saqa
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    Saqa Senior Member

    I am surprised by your very poor example of aircraft. Do you realise we are talking a simple utility craft? Are you familiar with the term "paddock basher", which is the purpose in life for this boat? The wheelie bin is a more accurate analogy for a boat made to take a lifetime of beating and moored in harsh grounds

    Unless you believe that ply or foam composite is better for a wheelie bin type construct then HDPE, lets not dispute the material choice. HDPE is a proven material for utility boats and implementing sufficient rigidity is simple

    I have taken a Mac hull apart and welded it back together when fitting out my old boat as I need to fit captive nuts for fasteners for the floor and handles and elec trolling motor and things. Its not hard and I know what makes them work. Only the gunnels are cylindrical section. The hull is flat panels formed by rotomolding as one seamless piece including the gunnel section. Each skin is done as a whole and welded together. The same applies for the Polycraft but they use corrugations for panel stiffness while the Mac uses internal box section stringers and cored with foam in certain areas

    A panel fitted to a boat is nothing like a panel clamped to a table by one edge. Clamp the other edge to another table then talk about load carrying capacity

    So I am left with three things now that I cant get my head around

    1) Isnt a Mac a tough little water craft? In comparison to a similar size composite or tinnie?

    2)A boat built out of HDPE that projects as being lighter then the Silver 29 and running same or more power as the silver and around the same WL beam and length.... how is that inefficient?

    3) Just because the material choice is unconventional why does that mean that there is no point in coming here asking for some guidance with things like shape/motor relations and such or documenting a project?
     

  15. Saqa
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    Saqa Senior Member

    But I can, that same published displacement figure. I am working to beat that by not carrying all the crap the Silver does that I dont need and twin 40s or 60s

    Everyone just assumes that I do not have either the experience or knowledge to make a workable structure using PE. Thats not the case, I have worked with it previously

    Any way here is a pic of the state of my flat after tropical cyclone winston hit
     

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