cold molding construction

Discussion in 'Boat Design' started by adriano, Sep 29, 2013.

  1. adriano
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    adriano Senior Member

    Hi
    I would appreciate if anyone could give some feed-back on drawbacks when using marine plywood (90° to planking) both for permanent frames and keel let's say for motor boats up to 23'.
    I suppose plywood should be avoided for these members!?
    Thanks
    Adriano;)
     
  2. FAST FRED
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    FAST FRED Senior Member

    Makes a great boat , but is labor intensive.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Hi Adriano. Plywood has good and bad things to consider. First is it's not as stiff nor as strong longitudinally as solid lumber, which is why you usually see solid wood frames in plank over frame boats. This said, you can use homogenous and/or monocoque build techniques to compensate for this a bit, such as taped seam or Lindsay Lord construction methods. It's important to understand the difference from an engineering point of view, if making changes of build method conversions.

    Plywood doesn't have to be avoided, but it does have to be sized, placed and attached properly.
     
  4. adriano
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    adriano Senior Member

    Thanks Paul,

    We can say that, using plywood frames (90° to planking) versus solid wood frames, two times the thickness of the solid wood would have to be employed?! (advantage: only time saving by CNC cutting!)

    My feeling is that Plywood keel (90°/vertical to planking) should be avoided,
    the mechanical properties of this member would be too weak!!??
    Definitey far better alternative is orizontal laminated solid wood planks keel!!
    Is this correct?
    Thanks
    Adriano;)
     
  5. SukiSolo
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    SukiSolo Senior Member

    I have built cold moulded rowing racing shells and double chine racing dinghies so have some experience of 'plywood' construction. Cold moulding is brilliant but you need a very solid mould to build up the veneer on. Sheet marine plywood is great for planking but slightly more limited for vessel shape. My own personal preference is to use traditional stringers (sometimes laminated to achieve low stress and complex curve) and a solid hog (inner keel). For one off's and limited number builds, timber is still OK if a little expensive. As Fast Fred says it is more time consuming.

    For a bulkhead across the boat (transverse), ply is also OK but I recommend either using epoxy fillets or local edge thickening in solid timber to get a wider bond and spread stress. Make sure you have a drain hole through it or limber hole if it is not a buoyancy tank. Very important to let water out and stop rot starting inside any hidden corners. Microballoons are excellent for creating a smooth path for any internal water to escape.

    For use in a power boat I personally would use at least a hog. Stringers can be replaced with epoxy fillets if they are big enough. If you have an inboard power unit you will need to use bearers and these should definitely be of solid timber although you can let in local high density timber for engine mounting if you want to keep overall weight low. If it is an outboard you have a lot of weight at the stern and you need to transfer that properly and the thrust forces. This is where stringers and a substantial inwhale can be very useful, even if they are relatively local.

    If you have CNC facilities, you will be able to cut solid timber as well as plywood. Most CNC routers have at least 80-90mm Z axis travel, so pretty substantial sections can be machined.
     
  6. adriano
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    adriano Senior Member

    Thanks SukiSolo,
    Here am thinking of 7 mt. wood veneer cold moleded sport fishing boat with outboarder.
    I agree on bulkhead filletting/epoxy/fiberglass reinforcmenet, no question!Limber holes includede.
    You would suggest to use stringers ruther than frames, which is fine but in case there are need of intermadiate frames I would use 2 vertical laminated solid wood planks of each 10 mm. ruther then plywood!.Guess this is the most common way!
    Regarding keel I am also referring to hog, definitely I would use, if not solid, laminated orizontal solid plank of approx. 10 mm. each ruther then vertical plywood!
    It's more work involved but I suppose this is by far the safest way by any point of view, am I correct?
    Thanks
    Adriano
     
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  7. PAR
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    PAR Yacht Designer/Builder

    The physical attributes of plywood, in it's various grades and species choices are fairly well known. I don't understand the issue you are faced with. Properly sized, plywood is fine, but so is solid wood in both frame or keel applications. Typically, plywood will be heavier for a similar application than solid wood, but only in certain building methods. In fact, if using the Lord method, the resulting hull structure (shell and it's supporting elements) will be lighter than any other, including molded solid wood structures. Of course, there are limitations and issues with this type of build method, but it's certainly valid.

    It would help if you could be more descriptive about what you're trying to do. For example a molded hull may not need any frames at all, with just athwart stiffness gained from furniture and other internal structure (cockpit seats, cabinets, etc.). The Lord method provides this as well, though typically it employs a keel (often of plywood) and again athwart ring frames or furniture partitions.

    If you're looking for a light weight build, selecting materials should be based on the best weight/stiffness/strength ratios appropriate for the SOR and budget. In other words, a light racer, might well do with just a molded hull shell and a few well placed ring frames, where necessary, while a wholesome cruiser might be able to tolerate, a full set of solid or plywood frames on fairly close centers, without a big weight penalty subtracting from it's abilities underway.

    Simply put, if we knew what the application was, some suggestions could be offered for a build approach, appropriate for this particular project, but general and broad statements about suitability of individual build methods, without an idea about it's application just isn't reasonable.
     
  8. PAR
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    PAR Yacht Designer/Builder

    You bet me to the post button.

    How fast will this 23' fishing boat go? What's it's weight, with a full load? Where will it operate (general sea state)?

    Framing systems have good and bad things to consider. Molding over stringers permits thinner planking, as some of the longitudinal stiffness is borne by the stringers. Again, if properly sized, no additional internal stiffeners are required other than existing partitions.

    Traditional frames and ring frames provide more athwart stiffness, but usually with some additional weight, some of which can be subtracted from the planking. You also lose some internal volume, because of their depth into the hull.

    A combination of longitudinal stringers and ring frames are typically employed in commercial craft, so you can gain as much hold capacity as practical, while using the easier to install ring frames in locations their intrusion isn't as significant.

    Without knowing the basics of the SOR, addressing you structure choices here is limited. You have all the options available, but are they appropriate for the SOR and budget?

    A typical 23' fishing boat, say capable of low 40's MPH at WOT isn't a very tough set of equations, unless it must tolerate severe service in rough water. Maybe you'd be best advised to get Dave Geer's book "Elements of Boat Strength" and run through the numbers on various build types, just to see where you stand in the SOR and budget.

    A CNC machine doesn't care what it cuts.
     
  9. adriano
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    adriano Senior Member

    Thanks Paul,
    I understand the complexity of the issue and each and every boat needs different approch.
    My basic question, regardless what wheather condition the boat will be exposed, in general for 6- mt. motor boat with outborder let's'say up to 70-100 hp, using plywood especially for Hog is not a great idea! That's what simply wanted try to get confirmed.
    Thanks
    Adriano;)
     
  10. PAR
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    PAR Yacht Designer/Builder

    I'll assume you mean a keel with the term hog. You can use plywood for the keel and is the usual recommendation on Lord style builds. It wouldn't be my first choice, unless going for a Lord build, but without a expected service duty, it's just guessing. Two identical 6 meter boats could fair quite differently, given different service requirements. One may be quite suitable, while the other is bashed to bits in short order regardless of the material choices in the keel.

    Generally plywood is used to save costs and for it's dimensional stability. A well engineered cold molded boat wouldn't necessarily need a keel, just appropriately size planking on the centerline and sufficient athwart stiffness, possibly from other elements.
     
  11. adriano
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    adriano Senior Member

    Thanks again Paul,
    Actually "hog" might be not the best member name, I was meaning hog/keel(one piece)
    i.e. hog which has an extention (30-40 mm.) out of the planking.
    Also under this circumstances vertical plywood( keel )can be recommended??
    I would doubt!
    Rgds
    Adriano
     
  12. PAR
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    PAR Yacht Designer/Builder

    I could design the keel/keel batten assembly either way, but generally I wouldn't use plywood, unless I was employing a specific method. A vertical plywood sandwich will work, but solid wood will be a better choice in most situations.
     
  13. adriano
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    adriano Senior Member

    Yes, I can understand that marine plywood in several cases can be a good alternative
    in boat building but might have its limits.Not being a real expert I assume that, in hog/keel construction, plywood would be a cheaper option and less durable versus solid wood or even better laminated orizontal solid wood planks! Also keeping in mind that optimal wood mechanical property is longitudinal ruther than tangential to wood fibers
    Thanks
    Adriano
     
  14. PAR
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    PAR Yacht Designer/Builder

    A laminated solid wood assembly does eliminate the usual issues associated with solid wood members. Keep the lamination layers 1" (25 mm) or less in thickness and you'll be fine. Typically, depth of the element is key.
     

  15. adriano
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    adriano Senior Member

    Thanks Paul,
    Anyhow it wasn't matter of particular hull in this case, somehow it was matter of basic principals of plywood use vs. laminated/solid wood in boat structures.
    Adriano
     
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