Hull ply planking, leave screws in or remove?

It's the best way, minimal marks, no dents, even good pressure. And the old trick of packaging tape works flawlessly. It's worth to prepare them as the system saves a lot of work later while giving excellent results on thin plywoods.

 

Thanks for sharing your expertise. Here is a chart showing the drill sizes for various threads. Note that the recommended hole produces a 70%-75% thread. Perhaps for screwing machine screws into wood a slightly smaller hole would be good.

 

These drill size recommendations above, aren't for using machine screws in wood.

In hardwoods, you can simply use a tap and literally cut machine threads in the wood. I've done this and the above chart is a good guide, though I usually go a size or two smaller than shown above. In softwoods it's some experimentation, but I don't tap them, just drill a pilot hole, slightly deeper than the fastener will penetrate. For example I'll use an 1/8" hole with a #10 machine screw, but on hardwoods, I might make this a fraction bigger to prevent splitting. In the end, if you want it to stay put, squirt some epoxy in the hole and run the fastener home.

 

Somewhat along the line of screws for holding down the plywood when gluing to stringers, what about the 1"x3" stringers being glued to the bulkheads. The bulkheads are 3/8" ply with 1"x3" glued to side presenting the 1" edge to the stringers.

Would there be any benefit to using two bronze screws at the bulkhead/stringer glue joint and leaving them in? Or would that be more likely to cause a stress riser in the joint?

I can use clamps to hold stringers to bulkheads when gluing them and not use screws at all easily enough if that if better.

Making the best glue joints is the main concern, not trying to save a little time of money.

Cheers, Allen

 

If you're using epoxy and appropriate gluing techniques (the right filler matierals), you really don't need the fasteners. If you feel better about having some, use stainless and leave them in, well covered and coated in goo. Temporary fasteners can be pretty much anything, though I religiously use deck screws, because black phosphor drywall screws tend to break off.

 

If using stainless steel screws does the alloy matter? I have read that some SS alloys corrode when sealed off from oxygen.

SS screws left in would be handy as far as assembling.

Cheers, Allen

 

Yes, in a technical sense this is true, but experience with relatively cheap S.S. fasteners (304), buried in transoms, stringers, etc. over the years, I've found they remain unmolested, again assuming good technique. Simply put, if you seal the S.S. from oxygen effectively, you also seal it from moisture, so no corrosion.

 

I do agree. 304SS is largely enough for screws that will be out of moisture and "buried" into the wood and the resin.
The corrosion of SS deprived of oxygen is a special case very rarely encountered on yachts, except in poorly installed keels. All screws, nuts etc... in contact with outside must be 316, better 316LMo (alloy with molybdenum). I have installed hundreds of hardware on decks with epoxy plugs. I always use a sealant or O-rings so no water can ingress between the plug and screw.
SS is protected by the oxidation of the chrome, so while attacked by corrosion oxygen is needed to reform chromium oxide. If you use a SS screw in steel or aluminium where water can migrate into the threads and remained trapped the metal while oxidizing will absorb all the disposable oxygen and deprive the SS which will also oxidise. If you have a high permanent stress things can get worse (crevice corrosion). Add some electrolytic chemistry and you can have a serious problem.
But it's a situation most commonly encountered in marine engineering of barges, tugs, platforms etc...The solution is very simple; use another metal and/or seal. The problem is well known and sealing is part of the routine. Some silicone greases do marvels. The grey aluminium silicones for engines are perfect as the are impossible to see in the joints and are very resistant. Do not use cheap bathroom silicone it will be destroyed.