Sealing Thru-hull fasterners in fiberglass over wood

W

Wood Zeppelin

New member
This is not a C-dory question, but it is a "dory" question.

I have a 14.5' river dory / drift boat. It's a few layers of epoxy based fiberglass over marine plywood. People sometimes attach these UHWM plastic skid sheets on the bottom (makes the boat slide right over river rocks). Silicon-bronze screws are used and go thru the hull into the ribs on the inside. The problem is (a) any screw through the outer layer of fiberglass could let some water in (even though the screws are fully embedded in the ribs and (b) thermal expansion and contraction of the UHWM is significantly more that the fiberglass over wood. This puts sheer pressure on the screws and tends to wiggle them side to side over time. It took many years, but one of the screw holes has now turned into a leak.

Idea: Drill the holes a little oversized. Fill them with cast Polyurethane of the right durometer (flexibility) (or maybe 4200/5200). The drill into that and put screws back in. My thinking is (a) the Poly or 5200/4200 creates a watertight sleeve around the screws. water can not get into the wood or fiberglass end grain and (b) this will allow the screw to wiggle a bit side to side with flexibility without opening up a bigger hole over time.

Am I missing anything with this plan? Am I over-engineering it? (my specialty). Should I be afraid of drilling bigger holes? See anything wrong with this idea?
 
Unfortunately the 4200/5200 seals are not 100% perfect--and this is why the epoxy plug, and then reset the screw or penetration fitting in thru the epoxy plug. You can certainly try and 5200 and see if it works. There are also flexible epoxy resins.

The expansion/contraction issue is also likely to allow leakage thru the single part polyurethane sealant.

I dealt with this type of issue on my Cal 46, when during the restoration/rebuild, I put polycarbonate windows. (The windows in the Pilot house of the Cal 46 are large. The Lexan material have a far greater expansion contraction than the underlying fiberglass. We put 1/8" spacers (compound cork and synthetic rubber) to keep the plastic off the fiberglass and fill all of that material with structural silicone adhesive. This is what the mega yachts do with the "black" windows. The material is actually held in place by the silicone adhesive and the holes for the screws are oversized so the window material can "move" without stressing the screw. This stuff is so adherent and resistant, that a "dab" which accidentally was placed on a SS 1/19 wire 5/16 diameter, was still there 6 years later, despite heavy use and abrasion at that point.


Here is an article on bonding PTFE.. This addresses the issues with bonding of this material.


Are you using 1/4" or 3/8" PTFE sheets? If 3/8, you could use pan head screws, maybe with a washer--and make the "hole" or slot large enough to allow for the expansion/contraction. (you would have to router cut enough around the slot that the screw head is below the surface of the PTFE.

Another thought is to penetrate the hull fiberglass/ply, put in the epoxy plug, and put a bolt thru this, instead of screwing into a rib. Again, it would be a good idea to slot or over size the hole for the PTFE.
 
I don't think the 3M 4000/5200 is suitable for filling oversized holes with screws in them. While a good adhesive for sticking things together, they are not really that strong by themselves. I feel that a screw imbedded in one of those products would be relatively easy to pull out. Hard epoxy is a better choice.

If you want to see, lay out a bead of 4000/5200 and let it cure. See how much give it has and how hard it is to pull it apart.
 
Thanks guys!

Based on your responses I will not use 5200 or 4200. Epoxy plug is an option. I used to work with casting 2-part polyurethanes. The reason I'm interest in this material is that you can vary the "durometer" which is essentially the hardness vs flexibility. My experience with epoxy is that it doesn't have that some kind of flexibility. The cast polyurethane is super tough and super durable even in flexing conditions. I'm wondering if anyone has experience with using 2-part polyurethane in this way (thru hull and boding to epoxy/wood). ?

Thataway - yes I also thought about that idea. I could counter-bore into the plastic sheet with a pan head screw and oversized or slotted holes. Currently the screws are countersunk which allows for probably zero movement!

Possibly I will combine both ideas.!

I ran into another issue after removing the plastic sheet. The boat has wooden chines that allow serves as "side-bumbers" attach the the outside of hull, at the bottom. the plastic sheet was laying right over the bottom of these chines. Trapped water will sit in the tiny gap for a long time before drying, and even with protective epoxy/varnish over the wood, eventually rot the wood. The best idea I have so far is to bevel or otherwise remove some of the plastic to at least create a tiny air gap so it can dry out. I don;t want to cut the plastic away entirely because it does protect the chines from impact with rocks... Any other ideas are welcome!
 
The counter sunk screws are definitely an issue. Just dong the slot and allowing movement should take care of the problem. I would coat I probably would use G flex epoxy on the screw shaft when running them in. 'Also seal the wood with the G flex. (where it goes thru the fiberglasss/ply bottom.
 
I have built a couple of drift boats. The first was a Don Hill 16' McKenzie river double ender with transom. The second was a 16' McKenzie river Square Ender AKA (Rapid Robert). The later can be seen in Roger Fletcher's book "Drift Boats and River Dories".
My advise would be to get rid of the UHMW bottom. You will never be able to seal the holes. don't even try, it will just be a mess. Since you already have epoxy fiberglass bottom. I would seal the holes with epoxy ( after letting the wood dry for a long while) and then if you dont think the glass is thick enough put on another layer of triaxial glass and finish coat it with graphite epoxy mixture. over time as the epoxy graphite wears off, sand it down and apply some more. This is how Ray Heater of Ray's River dories builds his and they are some of the finest boats made. I used his methods using 20oz triaxial fiberglass and the boats were very durable. Most of the guys that run UHMW bottoms screw them to bare or painted plywood bottom. That way at least the wood can get a chance to dry out when not in use. With the fiberglass bottom water will be trapped in the plywood and it will eventually rot.
 
I've had several wood drift boats, some with UHMW bottoms, some without. There are advantages and disadvantages to UHMW but here is a trick that can help: As you mentioned UHMW expands and contracts with temperature to a much greater degree than the plywood and the differential expansion can rack the screws back and forth. The biggest problem is actually caused when the UHMW shrinks in cold weather or cold water. This put tremendous strain on the screws. One way to overcome this is to screw on the UHMW on the coldest day of winter. The UHMW will be contracted as much as it ever will. What will happen on hot days is the the UHMW will expand but can bow away from the bottom and not put strain on the screws. When you slip the boat into our cold NW rivers, the bottom will contract and be flush with the plywood. You do need to seal the plywood well with good paint so any water retained between the UHMW and plywood cannot soak in.
I used this technique on a old plywood boat built in the early '50s that I refurbished. I ran that boat hard for 20 years (up to 100 days a year) and had no problems -- finally sold it to a fellow who wanted it as a collectors item.
 
Thank you all for your input, wisdom, experience and expertise!

Guess what!? I am happy to report that after removing the UHMW there are no obvious signs of water intrusion into the fiberglass/wood (except for maybe the one screw that somehow caused a leak). I did I find some of the screws to have "wiggled loose" and backed out a bit.

And - I had pre-drilled the holes and coated all the screws with a good sealant before screwing them in originally. Which probably protected the holes from too much water.

AND, as it turns out, I haven't been using the boat in winter. Sure the air temps drop in winter, but I wonder if that's less of a strain than dipping it into an ice cold winter river. Like a sudden thermal shock. Also, I live in Western Washington where it doesn't get as cold as some other places.

I'll have more to say later...
 
thataway":68o6bp51 said:
The worry about winter is the freeze thaw cycle with moisture, expanding and contracting of the water intrusion, if present.
Click to expand...

Good point!

Now that I think more about it, the cold water isn't any different than the air (well, it will remove heat faster from a warm body). But if the air and boat are already at say 35deg and so is the water, then no difference to the materials...
 
ssobol":12hrwmq5 said:
Use a flat washer at each screw to provide the gap between the hull and the plastic.
Click to expand...

Yeah, I always wondered about the water sitting in the. The original mounting instruction did not call for this. I think they assumed the UHMW was going onto a wood-only hull, so no water-intrusion caused wood/fiberglass delamination to worry about. But still, the wood would have to be varnished or sealed pretty good !!?
 
drbridge":2iiamw9g said:
I have built a couple of drift boats. The first was a Don Hill 16' McKenzie river double ender with transom. The second was a 16' McKenzie river Square Ender AKA (Rapid Robert). The later can be seen in Roger Fletcher's book "Drift Boats and River Dories".
My advise would be to get rid of the UHMW bottom. You will never be able to seal the holes. don't even try, it will just be a mess. Since you already have epoxy fiberglass bottom. I would seal the holes with epoxy ( after letting the wood dry for a long while) and then if you dont think the glass is thick enough put on another layer of triaxial glass and finish coat it with graphite epoxy mixture. over time as the epoxy graphite wears off, sand it down and apply some more. This is how Ray Heater of Ray's River dories builds his and they are some of the finest boats made. I used his methods using 20oz triaxial fiberglass and the boats were very durable. Most of the guys that run UHMW bottoms screw them to bare or painted plywood bottom. That way at least the wood can get a chance to dry out when not in use. With the fiberglass bottom water will be trapped in the plywood and it will eventually rot.
Click to expand...

Thanks for this. Yes, I have heard of the graphite method. Since I've actually had good luck with the UHMW so far, I'm going to give it another go. Just one screw hole leaked after 10+ years, and it seems to be some kind of anomaly, like maybe it smacked a rock directly and racked the screw to the side. AND it's also true that I've only had the boat in Western Washington, which has mild temperatures. I think in a more extreme climate I'd switch to graphite at this point (or maybe next time around!)

Thanks!
 
ssobol":2vuziq56 said:
Use a flat washer at each screw to provide the gap between the hull and the plastic.
Click to expand...

Does this apply to boat painted wood hulls and well as glass-over-wood hulls?

Would you be worried about water getting into the tiny void space created by the washer's hole sandwiched between to layers of material. And since this is right where the fastener goes into the hull, it's the last place you'd want to trap water...?
 
Wood Zeppelin":30hqmbpd said:
ssobol":30hqmbpd said:
Use a flat washer at each screw to provide the gap between the hull and the plastic.
Click to expand...

Does this apply to boat painted wood hulls and well as glass-over-wood hulls?

Would you be worried about water getting into the tiny void space created by the washer's hole sandwiched between to layers of material. And since this is right where the fastener goes into the hull, it's the last place you'd want to trap water...?
Click to expand...

My idea of using the washer as a spacer does expect that the screws into the wood would be well sealed. Also, SS or bronze screws and washers. There may eventually be some rust or corrosion, but that's what periodic inspection and maintenance are for.
 
Epoxy Plugs:

Hey everyone-

I like the idea of the screws going into epoxy (or two-part polyurethane) plugs. This way, no screw penetrates into wood, and the interface between wood and fiberglass would be sealed off as well.

The previous screws were #10 x 3/4" long. #10 screws are approximately 1/8" in diameter. So I figured I'd drill 3/8" holes to have 1/8" of "meat" all around the screws. And, I'm concerned about drilling these holes too big...

MAIN QUESTION:

- There are a lot of these screws (about every 6") going into the underside of the chines. Would drilling all these holes create structural weakness?

Other questions:

- Does 3/8" sound good? What about 1/2". Should I keep them as small as possible or not worry about a larger hole?

- Since now screwing into tough epoxy/poly rather than wood, could the screws be shorter (and therefor the holes less deep!?)

Thanks in advance for all and any input!
 
- There are a lot of these screws (about every 6") going into the underside of the chines. Would drilling all these holes create structural weakness?
Click to expand...

Why in the World would anyone put screws into the boat's bottom core, underside of the chines every 6"?

Please post photos of where the screws are located.

You want to removed a thin rim of the core material, between the inner and outer fiberglass layers. You drill the holes big enough to get whatever you are using to remove the core with. I use a Dremel Tool, with about 1/4" bit, so a hole of 1/4 or slightly larger would be proper. Do not put in any larger hole than necessary.
 
This is a wood drift boat, with a fiberglass exterior layer only. AKA "fiberglass over wood". So there is no "core" or inner layer of glass.

I'll take some pics
 
OK--I understand that it is a wooden boat with fiberglass over the bottom. So how are the screws now gripping the bottom wood? If you drill out the hole, then there no longer is the bevel for the flat head screw to hold the bottom in place.

So are you basically depending on the epoxy plug to hold the screw in the bottom planking (I assume plywood).

Are you going to glass over the screw head after it is in place?

I have never seen epoxy plugs used in this type of application. I would be reluctant to depend on just the epoxy plug in the bottom planking being good enough in adherence to hold the bottom screws in place. (hold the bottom in place.)

I looked at several of the drift boat builds using epoxy and glass--basically they were built as "stitch and glue", or glued bottom and then covered over with the glass--but no screws. if Screws were used they were counter sunk and then the hole plugged and sanded smooth before the glassing.

I guess if you were building a new bottom, you could do the epoxy plugs in the chine log--and then have a template that would allow placing the screws thru the bottom ply, counter sinking and then glassing over.

I have made a number of small boats--not drift boats specifically, but using the same techniques as used in drift boat building using epoxy resins, and then bottom glass covering.
 
thataway":3gg1j6wh said:
OK--I understand that it is a wooden boat with fiberglass over the bottom. So how are the screws now gripping the bottom wood? If you drill out the hole, then there no longer is the bevel for the flat head screw to hold the bottom in place.

So are you basically depending on the epoxy plug to hold the screw in the bottom planking (I assume plywood).
Click to expand...

Hi Bob,

No, not screwing on the bottom of the hull. This is for adding UHWM. Refer back to the original post for a full description.

The UHMW plastic sheet method is debated among drift boat people. On the one hand, it is superior abrasion resistance. On the other hand, attaching in to the bottom means screwing it in (adhesive bonding is to complex for me to get into, and has marginal results I hear.) If screwing it in to just a wooden hull, this may be simpler. Mine is glass over wood. I used sealant on the screws (which are countersunk into the UHMW and recessed a bit to avoid hitting rocks), worked well for many years.

UHWM has a higher rate of thermal expansion than the wood/glass combo. (Again, see the original post). This puts a lot of pressure on the screws. Eventually, one of the screws started to leak.

I have taken the UHMW sheet off, and I'm actually impressed! The one screw that leaked seems to have a widened screw hole, but all of the rest were fine! No sign of water intrusion where the glass/wood interface is cut by the screw hole. (I think I used 4200 sealant, or maybe Sikaflex). I don't know why the one screw leaked.

Instead of the UHWM I could add a layer of graphite impregnated glass or maybe "kelvar", but for various reasons I'm opting to stick with the UHMW for now.

With all that said, here's my question: The existing screw holes are about 1/8" (#8 screws were used). I could drill the over to say 3/8" and fill with epoxy or a 2-part polyurethane. I'm leaning towards Poly, because the right blend (durometer) can have enough flex the allow the screws some lateral movement, and enough toughness to hold the screws really well. I'm not sure if epoxy has as much flex and toughness as the Poly would(?) In addition, plugging the holes would seal off the wood, as well as the wood-glass interface in the hull bottom. Even if the screws wiggled loose, the water would not get past the plug that the screw was embedded in.

Although the plugs may allow some lateral movement to the screws, I don't want to use countersunk heads anymore. Instead, I'd (1) counterBORE the holes in the UHMW (2) drill the holes oversize and (3) and use truss-head screws. This way, I'd have "clearance holes" that would allow the UHMW to move under the wider screw heads..

I'm curious if:

- Anyone has experience with 2-parts poly vs epoxy in an application like this? Would it bond the both the wood and the epoxy-glass layer that's over the wood just as well?

- I assume the chine logs are not really a "Structural" member, and the bottom of the ribs are not critical (the top is), so drilling some 3/8" holes about 1/2" deep would not "weaken" anything structurally?

- does 3/8" seem like a reasonable size for a plug for a #8 (~1/8" major diameter) screw?

- The old screws were 5/8" long. I'm thinking a screw into the new plugs (rather than the wood) could be a little shorter, like 1/2"?

This may seems like "over-engineering", but I want to do it once and not again for as long as possible!

Any other thoughts anyone?
 
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