Hull ply planking, leave screws in or remove?

Oh, and don't use a ridiculously heavy glass sheathing as I think someone suggested just in an attempt to hide fastener head print through, its a lot extra weight that no multihull needs. I don't know what Wharram specifies but one layer of 4 - 6 oz or dynel is all you need with extra layers along the keel of course.

Steve.

 

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.

Steve.

 

Well, I commented to leave them in IF you went to the expense of using bronze screws. Otherwise why use bronze screws in the hull?
Bronze is soft metal, so I dont see how it can mess up your tools comments when someone else works on the hull.
They must add some insurance to the hull splitting open from a failed glue joint, they are not a worthless addition, although that most likely is not going to happen. Appearance wise above the waterline, they might print through eventually, as in see the heads.

If you use sheet rock screws and some get stuck in epoxy, heat up a bit real hot, and put in the screw slot, the heat will soften the bond.

I drove 4 thousand bronze number 12 1.5 inch long square drive screws from McFeeleys, but they had #2 bit drive and very few camed out as in under 10, this due to the much larger number 2 bit drive.

 

Thanks everyone. I'm going to predrill the plywood panels anyway, partly to have uniform spacing and as mentioned to be sure get good and uniform pressure on glue joint.

Since there isn't really going to be any real advantage to leaving the screws in, I'll remove them after epoxy sets a little.

Was planning on using Kreg Maxi-loc screws since they have a flat washer head to spread pressure out and don't have to worry about accidentally burying screw heads unless coated deck screws would be better.

Cheers, Allen

 

Kreg screws are nothing more than way over priced button heads or tech screws. Do your budget a favor and look around, as a box of truss, button or tech screws will serve the same, for much less. Lastly, even these broad headed screws will leave a bigass (technical term) dent in the panels, which is why I use a little square of plywood under the head of screws. This permits me to use cheap and strong deck screws, yet not leave a big depression, when it sucks down to the work. I literally have boxes of 1", 1 1/4", 1 1/2" deck screws, most with a little hunk of plywood or 1/4" thick chunk of oak, pre-drilled and countersunk for the screw. They apply more pressure over a much larger area and if you cover these pads with some packaging tape (yep, plain old box tape), epoxy oozing through the fastener hole will not stick the block to the work.

 

Coated deck screws are a good idea, much better than sheet rock screws.
You could get use a flat washer under the heads to limit them making a divot in the planks. Better yet are like the ones Gonzo shows which are flat. Having those threaded all the way, if the planks are real thin less likely to be a problem drawing it tight. But if thicker planks, then screws with rolled threads only part way up are better. Then if you use a stepped drill, not a problem at all using screws threaded all the way to their head.

 

Ah cool PAR, I'll use deck screws & scrap ply. Make up a bunch ahead it time.

Cheers, Allen

 

About 30 years ago we made up boards to surround the hockey rink at a local rec center out of 1/4" hdpe and have been using the offcuts for pads for temporary screws ever since, releases quite well from epoxy.

Steve.

 

I always use appropriate pilot holes for self tappers, usually in the size selected for 'thick' ie 3mm (1/8") sheet aluminium. Data sheets for this guidance is available. This works very well for most decent timbers, though worth upping a wee bit if very dense timber. TBH not using any appropriate pilot with self tappers is asking for splitting and other nasties you definitely can do without....

I once had to supply evidence of the superior holding power of let's say parallel screws ie self tap and machine screws, over tapered traditional wood screws for rail carriage internal structures. The inspectors were satisfied and those cars continue to journey on the UK rail network. We had specified fasteners that industry was not familiar with! at least we brought them a bit more up to date.

We also have some fun in the Imperial/Metric area over this side of the Pond, with screw sizes but I'll go no further....

 

Have to agree with SukiSolo on the machine thread holding power VS sheet-metal threads.
I worked for years as a welder/machinist in cotton gins and got permission to use machine thread self-tapping screws to mount saws to cylinders and we went from 30% failure to about 1% failure. They pulled down tighter because of the thread pitch, plus there were three threads in contact with the cylinder wall instead of one (two, at most).
LF

 

Again, the size of the contact ring on the work depends on how much crush it'll receive. A pan head, button, truss, tech or other, including a flat washer under a pan, will only offer a fraction of an inch of contact area, under the fastener head, so a dent or divot should be expected, with all but the lightest amount of pressure. Now, a #10 bugle head with a 2" square hunk of 1/4" plywood under it's head offers 4 square inches of surface area to spread these compression loads, so the dent is made in the plywood pad, not the work under the fastener head. I just finished a lapstrake, with a brightly finished sheer strake. This strake had 4 or 5 dozen fasteners along it's lap and sheer clamp contact points. Any type of screw, regardless of head shape would have left huge dents and divots, but my plywood pad (actually I use oak pads, just because I had a bunch), under plain old deck screws, left only a small fastener shank hole and no dents.

The image below is this boat and the tape covers the upper set of screw holes, but the lap is clearly visible. Try to find a fastener hole or dent from a screw head. The holes where drilled out and 1/8" dowels used to fill them. They're on about 4 - 6" centers along the 3/4" tall lap joint. It depends on what you want, a guarantee of no dents or a fairly significant possibility of dents. On a bright finish, you'll want as much insurance as you can get, because you can't go back and fix it. If you look closely, the deck does have filler to cover the dents made holding down the decking. I used tech screws for this, knowing the deck would be sheathed, textured and painted, I didn't care so much.

 

I built a 28' OB boat using boat nails or sometimes called ring nails.
Are they still available?

It was a plywood boat and no sheathing was used.

 

Yes, Jamestown Distributors carries bronze ring shank nails. Duckworks might also, not sure about them tho.

Cheers, Allen

 

Paul, what size hole do you drill when using machine screws? The standard sized drill for a tapped hole in steel? Does it matter if the material you are setting the screws in is a softwood, a hardwood, or an epoxy plug? Do you ever grind a little notch in the end of the screw so that it sort of cuts its own thread?

 

The hole should be the size of the shank, which on small machine screws, is hard to check. These screws are "rolled" so the threads stand proud of the shank. I tend to just make a guess, but there are exceptions. Hardwood need a bigger hole, while softwoods prefer a smaller one. If drilling into an epoxy plug, you're "bonding" so the hole should be the size of the threads or slightly bigger. This is because the hole is getting more epoxy, when the fastener is inserted.

The tests I was refering to, are for machine screws and threaded fasteners, used in wood, compared to wood screw threaded fasteners. Bonded fasteners are a different animal. I think West System did a set of tests not long ago, so these will be available on their site.

Ring shanks work well, but I just don't like nails, as they can't be easily removed and on some materials can follow grain lines, can cause splits, etc.