Hull ply planking, leave screws in or remove?

Discussion in 'Boat Design' started by abosely, Jun 23, 2015.

  1. abosely
    Joined: Mar 2015
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    abosely Senior Member

    The Wharram Tangaroa has 1"x3" stringers planked with 9mm, 7 ply, Hydrotek Meranti plywood.

    Some use Silicon Bronze Ring Shank Nails, but need to use bucking bar from inside hull to set them. It's slow and is difficult to get consistent, even pressure.
    Some use large head screws and then remove them after epoxy sets then fill screw holes. I will use screws.

    What about using Silicon Bronze Screws, counter sunk slightly to be flush and leaving them in, filling the small bit over screw head? The outside of hulls are sheathed in fiberglass.

    Maybe this is done & I haven't seen it. But is there any reason not to use Bronze screws and leave them in? Am I just missing something?

    Cheers, Allen
  2. sdowney717
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    sdowney717 Senior Member

    I would leave them in, they add some reserve strength to the glues.
  3. philSweet
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    philSweet Senior Member

    You need to be very careful to get the countersink perfect. In good material such as you describe, you can do that. The screw head must compress, but not pass by, ANY of the fibers in the ply. Get the holes perfectly square to the surface and use the correct countersink for the screw and use a torque + turn system to set the screw (hand torque the last 1/8 turn.) Use a putty knife to check every screw is drawn down.

    This gets you a screw that you can remove without trouble, and you can fair the head with putty after filling the slot with a nonsetting material. Silicone bronze will start to show staining after twenty years or so, so better for painted surfaces than bright ones. You want to use tapered bits with stops, and a countersink with a stop, and the correct frearson driver, of course.

    It's no less work than pulling a screw, shooting in some goo, and jamming a splinter in the hole. But you can build the entire boat in wood, then disassemble, and do all the gluing in one go. That saves as much money in time and supplies as it costs in screws, not to mention being able to take things apart and make adjustments before you start sticking stuff together.

    I'd also recommend using a decently thick skin for the hull. I've used 4 and 6 oz deck glass and deck epoxy over silicon bronze screwed 4mm deck, but it was just on a dink. For a hull, I would want two 9 or 10 oz layers of cloth over #8 screws if I wanted to be absolutely sure no defects would show up down the road.
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  4. motorbike
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    motorbike Senior Member

    You dont need screws. If you're using epoxy, use clamps or drywall screws which you can remove later and fill. Screws are heavy and blunt your tools when working on the panel in the future.
  5. philSweet
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    philSweet Senior Member

    Don't use drywall screws on 9mm ply, they won't draw the ply in. You need a screw with a shank, and you need to drill both the ply and the stringer. If bronze, it is common to run a lubed steel screw most of the way in first to set the threads, followed by the bronze screw.

    You don't want to be working on a ply panel, either. The veneer is less than .5mm. If you do have to, pull the screw, which is why you must not let the head suck in under the top veneer.

    For a skinned plyplank on frame and stringer build, I see no reason to avoid screws. If they help, use 'em. If you want to leave them in, just make sure the are very precisely installed. If you are torturing the ply to any degree, you may need a bunch of them to get the bend radius down to the limits of the panel.
  6. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    If you don't need the option of taking the fastener out, but you don't like the idea of subsequently hitting the fastener with an edge tool, you might consider using plastic nails or staples such as Not as strong as steel or bronze screws, but will never rust, and are fast to set with a nail gun (probably best to buy their recommended brand, which pushes the price up).
  7. gonzo
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    gonzo Senior Member

    I've been using this type for years. The hex head takes more torque than a phillips and the washer head spreads out the load, so they don't sink into the wood. This is only if you plan on taking the screws off.

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  8. PAR
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    PAR Yacht Designer/Builder

    I use coated deck screws and button head "tech" screws for temporary fasteners. Drywall screws aren't very strong, tend to break off in the work and rust at the mere mentioning of moisture.

    Actually counter sunk screws apply more force than dome headed (pan, button, tech, etc.) although they do bury in the work. Deck screws are bugle headed, which makes them dig in quickly and I'll often use a 2" square of 1/4" plywood, between it and the work, to keep divots to a minimum. I both like and hate Phillip head fasteners. They cam out too easily, but if you have to drill one out, they're self centering. Hex does take more torque, though this is relative to fastener size. In smaller sizes, you can easily tear off the head of a fastener in hardwoods, regardless of head type.

    Stainless can be buried with no worries, assuming the piece is encapsulated and the fastener head properly covered. Bronze, copper and monel have no concerns, regardless of being covered or encapsulated. Any other metal needs to be removed.
  9. Jamie Kennedy
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    Jamie Kennedy Senior Member

    How do you guys feel about Robertson heads eh? :)
    Also, how do you choose the right sized screw and how many to use?
    I've always had trouble choosing the right size and number of wood screws.
  10. rasorinc
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    rasorinc Senior Member

    If you buy plans from a reputable company company they list out all your questions. Check out Glen-L marine site. All their plans are done by a NAs.
  11. PAR
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    PAR Yacht Designer/Builder

    There are basic standards for using the appropriate size fastener, based on what you're attaching. In most cases the builder just selects a 6, 8, 10 or a #12 screw or machine bolt. It becomes fairly obvious pretty quickly, usually after you snap a few fasteners. Try to get at least as much fastener as the piece you attempting to hold down, in the stock you're fastening to. For example, if you're trying to hold down a 1x2 on the flat, this means the fastener should be a minimum of 1.5" long. If it's decorative, a #6 will do, though a #8 or #10 will have much better holding power on this hypothetical 1x2.

    Square drive screws are handy and torque down better than Phillips, but you need to be "over it" or you can round them out. They drill out good, if you have to remove a stripped one, but you usually have to pay for the square drive feature.
  12. SukiSolo
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    SukiSolo Senior Member

    Don't use traditional tapered wood screws, self tappers are way better in wood.
    You can find some data on their superior holding power if you go looking. Then just use the grade S/S and shear loading to assess the 'solid' part ie core to see how many or what size relative to job ie size/span etc. FWIW the very small properly rolled (almost roll forging) S/S micro screws are approx 0.9 X 8.8 High Tensile steel, whereas a lot of the more common S/S bolts are 6.6 grade. There is significant variation in quality depending on country of manufacture....;)
  13. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Thanks guys. Wealth of knowledge.
  14. PAR
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    PAR Yacht Designer/Builder

    In the USA real stainless wood screws are difficult to find at the big box stores and almost always are sheet metal screws, which work about as well. Sheet metal screws are sharper and enter wood better, though hardwoods and well loaded parts need clearance and pilot holes slightly bigger than normal. Self tapping screws, particularly in short lengths can sacrifice a lot of their gripping power, to the self tapping portion of the tip. I don't use self tappers much in wood, preferring to drill the appropriate clearance and pilots, which gives the most bite length.

    For what it's worth, testing has shown that machine screws hold better than many wood screws, especially the fine thread versions, in hardwoods.

  15. Steve W
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    Steve W Senior Member

    First off, you are building a classic Wharram so I would expect that it takes very little effort to draw the plywood to the stringers. I too use coated deck screws and remove them, after the epoxy has set up fasteners add nothing to the strength and are very quick to remove. Don't use sheetrock screw, epoxy will bond them in if you don't get to removing well before it reaches full cure and as Par said they break too easily, I used to use them many years ago but kept them in a jar of liquid floor wax so they wouldn't bond in.
    The coated deck screws don't bond in and are not as brittle. It still pays to remove any temporary fasteners the next day if you can. If you are using any type of sheet metal screw that is parallel sided and fully threaded you do need to clearance hole the part you are trying to draw in. I see people all the time bitching because they cant draw 2 pieces together and they are camming out the heads when they are just driving in a fully threaded ss sheet metal screw even though they have drilled a pilot hole, but they havnt drilled a clearance hole in the part to be drawn in. As much as I like bronze as a fastener material im not keen on it for screws as, like ss it cams out too easily when power driven, I liked them in the old days when we used yankee screwdrivers and brace and bits.

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