Screws with glue

[Pipefitter]

In structural situations in boat building,Is it better to use screws with glue when possible? Even if epoxy is strong enough by itself,I still like to know that there is both.Kind of one of those holding hands situations it seems.I know in some racing situations they might omit screws to save weight but in situations where it is something you would like to last,wouldn't screws be kind of taking the one straw off that breaks the camels back?

I find that the addition of screws act as instant clamps for the glue joints beyond where clamps may even be able to reach? Any thoughts from some of you pros out there?

[Barry Pyeatt]

Several viewpoints on this with a lot of varying opinions.

Anytime you penetrate the structure of wood you create potential for future maintenance issues.  Sure, encapsulate the wood with epoxy is the answer.  Fine until the stresses and movement of the wood itself cause cracks to develop that may go totally undetected in out of the way places.  Invariably, that will occur where you can't get to it to see it.  Then we cover over it with opaque finishes so any discoloration of the wood below the epoxy is hidden further by the finishes over it.

Epoxy on its own is, according to many, the end all for boat construction  the absolutely only way to build a boat.  Some have eliminated all metal fasteners from their construction so that there isn't any danger of metal deterioration or wicking around the fasteners.  STitch and glue is a prime example with total encapsulation of the materials in epoxy in many cases to hopefully prevent any incrusion of moisture from ever occuring.

In some cases it works and seems to work well.   If you have a good bond with epoxy, then why would you need to depend upon anything else to help to hold the parts together?   Well, in traditional construction with mortice and tenon construction and well fitted parts that interlock there is a lot of strength in the interlocking parts themselves.   If well fitted and protected, there shouldn't be any additional fasteners required to hold them other than pegs and perhaps clinch nails or riviting.  But lots of caulking and sealing.  

Fortunately we've progressed to the point where technology has helped us in a lot of areas.  Adhesives are one example.  But each of them has their positive points and their negatives too.  Some are not compatable with all woods.  Some are not compatable with other adhesives.   So do we come back to epoxy again?

Not necessarily, there places and uses where older, tried and true is still a better way to go.   A lot of new builders think that galvanized fasteners are as simple as getting zinc coated screws or bolts.  The don't realize that galavanized parts are not just zinc coated.  Lot of builders still feel that the ultimate would be bronze fittings and fasteners, the unless they get a particular broze formulation, the bronze fittings and fasteners are not the way to go, they are too brittle.  So Brass, corrodes, steel corrodes, stainess steel will as well under the right conditions.  Just what is a builder to do?

A lot of quality builders will drill oversized holes first and then plug them with epoxy thereby sealing the wood from moisture incursion around the fasteners.  Lots of additional effort, but it works.  For a home built knockaround boat?  Not likely for most.   Would they do it for thru bolts?  Not many.  Too much effort and trouble to go to.  

So...attach it with screws and epoxy for adhesive, then once cured back out the screws and fasteners and fill the holes with epoxy.   Should work and generally will unless the joint is starved because there isn't enough epoxy there to form a good bond and it was clamped or pulled too tight with the fasteners.    This is probably a good place to use other adhesives.

Leave the screws, nails, etc. inplace and seal over them?  Sure if they are not going to somehow work their way loose and corrode or allow moisture to get into the area.  Plug them and they are finished over.  Glass over them and they are out of sight and out of mind.  Just epoxy over  them and paint?  I don't buy it.  Not on a wooden boat anyway for a lot of uses.

Unfortunately epoxy isn't a complete answer to all the issues, but when used in conjunction with good building and finishing practices and proper techniques you end up with a good sound project.   Even with fasteners inplace if done correctly.  

Build with resin glues or polyurethanes and/or some of the other new wonder glues and they will often do a better job with less maintenace issues down the road.    Oddly enough some of them won't even stick to themselves to make repairs and you have to go to epoxy to make the bond.  But they work well in their intended uses.  

Biggest danger?  The novice that thinks that Polyester resins are adhesives and suitable for wooden boat building.  Or tries to use general construction products for marine use and gets away with it for awhile.

Enough of a rant that wasn't intended to go that way at all.  

Oh, personally I am allergic to epoxy and can't use it at all exept on spot repair work with protection so I don't come into contact with it or any of its dust created by sanding.     So I have to use other products that work.  Wasn't always that way but we all get older and wiser eventually.

[wkisting]

Pipefitter,

I'm no pro, but I'll tell you what some experienced folks have told me in the past. Others can chime in to de-bunk any myths that may have passed on to me through bad advice... never can be sure what to trust...

1. structurally, a well-constructed epoxy fillet and taped seam is supposedly much stronger than the surrounding wood, so even if screws technically "increase" the holding power, they don't actually lend any practical advantage since the surrounding wood will still likely be damaged or buckle before a well-constructed non-screw joint would fail. Not sure if that's true, but from the S&G boats I've seen, I'd be willing to believe it and say fasteners really aren't necessary.

2. screws can make a convenient way to join surfaces when, say, an alternative clamping method can't be devised or used effectively. Use them as temporary clamps, if necessary, but it's supposedly better to remove them from the final boat because they can (so I have been told) either (a) work their way loose/out to the point of causing a potential break/hole through epoxied surfaces, or (b) corrode. In the latter case, even stainless will corrode (theoretically) because stainless steel needs exposure to air to maintain its corrosion-resistance, which it doesn't get if sealed in epoxy. Then again, it might never get exposed to water either, so I'm not sure if that's a valid concern or not. If you use permanent screws, some folks claim silicon bronze is the only 'safe' way to go. Not sure if that's true. Personally, I've been known to 'glass in a few galvanized fasteners if I know the area is not likely to ever be disturbed. Never seen any troubles yet.

3. from experience, I will say that fairing can turn into a MUCH bigger hassle when fasteners are left in, unless a good measure of care is exercised. It's a fairly easy process to fill an empty screw hole, but if you leave the fasteners in, you'll need to take care all screws/fasteners are well counter-sunk. Even then, if you have to do any shaping or planing in the area of fasteners, you may find yourself cursing as your tools suddenly run aground against the heads of protruding fasteners or worse, end up printing through the fiberglass/paint.

4. theoretically, some kinds of repairs may be more difficult to make if the area contains fasteners whose exact placement is unknown, or may interfere with cutting tools, etc. while making the repair. I think that would only be an issue if, say, you had a bad-but-isolated case of wood-rot and needed to cut out and replace a section of the wood. The presence of fasteners would complicate matters, probably. Perhaps not.

Again, not sure if all that advice is accurate, but it's what I've heard "on the street".  

[Edit to add: Arrgh! Barry slipped in his much-more-informative post while I was typing mine! ]

[Barry Pyeatt]

Hey, I was just ranting.  

[wkisting]

LOL, rant away, Barry! I was just teasing you.

Am I just imagining this: Didn't there used to be a "wink" smiley when you typed a semicolon followed by a right-parenthesis?

Now, it makes this symbol:  

[Edit to add: On my screen, it's a lightbulb. Does everyone else see that, or is it just my computer?]

[Charlie Jones]

Wes- there's a fallacy in your point number two. Stainless desn't corrode because of absence of oxy. It corrodes (called Crevice Corrosion) because of absence of oyx IN THE PRESENCE of water. The water, particularly salt water, that doesn't get oxygen is the cause, NOT the absence of oxygen.

Where this is often seen is the interface between two pieces of wood, screwed together with stainless, that leak a bit of water, so the fastener is wet, but no air can get to it.

Properly encapsuled screws won't have the problem.

Personally I use screws where they are needed for additional support and remove or never add them  where it isn't. For example- on the Princess up on the stem there are a couple of screws called for in the plans, to tie large sections together. Those I wouldn't remove.  But the temp screws holded the chines together while you fillet- those come out- totally un-necessary once the fillets are there.

[Pipefitter]

I know there is some instances where the cumulative glue surface in some situations is so great that it is way beyond the strength needed to hold parts together. But take planks where they intersect hardwood frames for one. It doesn't seem that those points would be enough for just glue. I do see how stitch and tape would be well strong enough but that isn't just glue only. Another instance might be where planking was tortured somewhat into place. That stress is always pretty much going to be there even after the plank gets used to it's new shape. Structural instances such as a thwart in a frameless boat, that is used in some certain designs that becomes the stiffener for a hull. Before the thwart is added,you can twist the hull. Now add a rower adding flexure to this thwart while rowing,hull trying to flex going over waves or whatever. This one structural member is subduing the stresses on a hull fore to aft and is reliant on 2 glue joints even if there is a riser under the thwart. It just seems one gets the feel for what needs both screws and glue,thru-bolts, nails even. I can see the woes of leaving exposed fasteners in some construction or when below waterline. Trailered boats see oscillatory stresses not found on still boats that never see anything other than from shore to water. It just seems that at the very least,these joints would atleast need tape to keep the glue from fatigue or stress cracks over time.

I know I wouldn't like to use screws in plywood endgrains. The other day I tried the PL Premium urethane glue with small screws in plywood on a corner where screws were going thru the face end on one piece of ply into the end grain on the other. I also just glued the joint together on some other pieces set up the same way. I could break the joint apart and it did actually take endgrain wood with it but I could break it. The screwed pieces (this is 3/4 DF marine ply btw) with glue are taking alot more to break and it has to destroy the whole joint before it does.

Omit the glue from the screws only joint and it will pull the screws from their holes. In this instance,it seems that one compliments the other.

[wkisting]

Charlie,

That's what I was trying to say about stainless, but you exlained it much more clearly than I. That sentence right after the oxygen part ("Then again, it might never get exposed to water either...") was intended to mean that water was a necessary element. I should have worded it better.

Another common place that stainless corrosion is seen is on vinyl-coated stainless lifelines... the vinyl coat/tubing admits a bit of water over time, but traps the moisture in place without admitting much oxygen... over time, the lines can break very easily (from crevice corrosion, as you say).

In any case, what I have been told about how this applies to screws is that if you encapsulate a screw, you need to make sure it's in a "low-wear" area (a spot where it's never likely to get exposed to water due to running aground, or banging against a dock, or whatever else might chip through the encapsulation) or it will turn into a hassle later due to the crevice corrosion thing. That was what I've heard, but of course, I've encapsulated many screws myself with no adverse effects so far, so it's probably more a concern than a real danger if reasonable prudence is exercised in the placement and encapsulation of each fastener.

[PAR]

In tape and seam or stitch and glue construction, the fillet of epoxy and fabric creates a flange (custom thermoset plastic angle iron so to speak) which greatly increases the load carrying ability of the joined pieces. It's this engineering technique that permits lightly framed (if any at all) boats to be built and survive the marine environment. In most cases it's a"custom" flange with rolling bevels and no stress risers at intersections with other structural elements (like there are with other engineering techniques). The ability to make a custom molded, perfect fitting, stronger then the substrate structural bond/element insure it's continued use. There are several of these "newer" concepts used in these building methods, most have stood the test of time, especially combined with older, well proven engineering.

That said ". . .if it needs to be strong, then through bolt it. If it can't be, then use bigger fasteners and through bolt it . . ." Most of us have difficulty understand all the concepts and principles involved in some of the engineering that may go into a boat, so the "belt and suspenders" approach, using fasteners and goo, but in most cases one or the other is fine, both usually being stronger then the survivability of the substrate under max load. Over building is a common trait in homebuilt vessels. It's also a common trait in working craft or heavy service commercial vessels, so you're not alone.

Going a crazy with fasteners and goo probably will not hurt you any, though increasing scantlings (dimensional sizes of materials) surly can.

Bonding fasteners, by filling over size holes with epoxy then drilling for the fastener size is cheap insurance and most quality builders use this technique. Embalming the wood with penetrating epoxy or coating with unthickened laminating epoxy can hold off moisture gain (if the coatings are maintained), though neglected epoxy coatings will kill you come repair time.

In the end I try to look at reparability. Some pieces will have to come off from time to time to have bedding renewed or be repaired or replaced. I think about how I want to remove that piece, before I install it. This may mean no epoxy coatings, just a bedding compound, but having bonded epoxy fastener holes. It may mean, I intentionally leave the amine blush and epoxy over it (with through fasteners bearing the load) so I can have the abrasion resistance of fabric in epoxy, but the removable piece, because the bond is weak. It also could mean no epoxy at all, using more traditional methods and techniques to good effect, such as sacrificial rubs with an oil based finish and bedding compound. Epoxy is costly, messy and I try to avoid it, if I can use something else that works as well.

Stainless steel is pacified, ferrous steel alloy. It's like most steel alloys, but has usually over 10% chromium added, which creates a passivation coating of chromium oxide when exposed to free oxygen in the air. This oxide layer reforms (like aluminum) and keeps it free from corrosion when it's scratched. It can and does corrode when it gets involved with other things or combination of things. Drag some carbon across it's surface, galvanic reaction, sulfide cracking, stress pitting and crevice corrosion, plus a few others. Pit and crevice corrosion are typically seen in marine applications.

Pit/crevice corrosion basically boils down to the hard oxide layer not being able to form, because of a lack of free oxygen or another material competes for it's electrons (like the captured oxygen in water or chloride in sea water). Without it's protective oxide coating, corrosion starts in microscopic imperfections on the surface of the metal, then begins to grow. In the event of a crack or scratch, they get widened from the growth of the corrosion, which weakens the metal. In the event of a pit, the underlying material will corrode with the surface showing little to no damage, until sudden collapse or failure of the area.

[Pipefitter]

There is alot of things that perpetuate an anerobic environment for stainless steel. The one step wash/wax stuff people use on their boats is one. Armor all is another as does corrosion block spray. I have gotten in boats where people slather this stuff allover everything(hazardous, slippery boat) and see surface rust and green colored SS screws where it wouldn't have had more than waterspots if they had just cleaned it with soap and water.

Fillet and glass works alot on the same principle as welding steel. Take a T joint in mild steel plate and add three fillet passes on either side of the joint. You now have a small gusset beyond the bonding of the metals.

[Brian R Walters]

Gotta say right off the bat, these posts have a LOT of great info. What I've seen on our workboats, water taxis and logboats that come in for service agrees with this. We tend to overbuild our cruising boats as well(aluminum power boats to 42') as though they are workboats. As far as our woodbonding goes, the basic rule of thumb is that your adhesive and/or mechanical bond should be stronger than the material itself. We do a lot of failure testing with new products that we might want to use. It's kinda fun actually, to laminate a piece and see what it takes to pull it apart, and (most importantly !) where it fails. It's rarely the joint if the joint was sound. With the type of loads inflicted on boats relevant to this board, almost any appropriate adhesive is great. Hell, we use construction subfloor adhesive (PL Premium) to glue our 3/4" ply floors to the 2X2 aluminum angle floor framing ! Does a great job. If we have to pull a floor to make a change, say seating layout, the bottom ply has to be belt-sanded off the framing because the failure was between the plies !
 Also, kudos to Paul for bringing up the service/replacement issue. We see most of our boats for service work, so we know personally the guys who will be dealing with our decision after a season or after a few years. In the back of my mind anytime I'm installing a part : what about the guy who has to deal with this down the road.
 As builders, whether pro or for fun, we have a lot of technology available to us to get the job done. One of the reasons I like this board so much is the amount of knowledge and experience here.

[Howard]

PAR:

You mentioned the term "pacified". As I understand it, the term applies to cleaning up the surface of stainless to remove any trace of carbon steel particles that might break through the surface and start the rusting process.

I've heard that after welding up some fittings or something, the piece should be "pacified". How does one go about doing that?

[Pipefitter]

Usually it happens to stainless because it wasn't cleaned right away. Particles from the machining process can oxidize and attach themselves to the surface and go undetected. Nitric acid will "pacify" SS. but it shouldn't be necessary if it is cleaned well.

Improper grinding stones or finishing grits will cause SS to surface rust. We had to use certain cutoff wheels and abrasives for finishing SS.

I should also add that there is pickling paste which if I recall correctly is oxalic acid that we used to clean welded areas with,iron out which is really just citric acid that would clean surface rust off of SS. Been awhile since I welded any stainless but I remember using these cleaners on it. On some stainless manifolds we once built for a pharmeceutical manufacturer,they required everything to be pacified whether it needed it or not. It is more of a concern when using low quality imported SS.