Wood screws and sheet metal screws in FRP

[fryewe]

All,

My experience with sheet metal screws and wood screws and thick (quarter inch and thicker) fiberglass reinforced plastic, when installing instruments in the cockpit, has given me mixed results. Many times the screw begins to bind before the screw goes in deep enough to provide clamping pressure. Sometimes the head starts stripping; sometimes the screw breaks...even after I have drilled the appropriate sized pilot hole. Repeatedly screwing a screw in the hole will generally "chase" a set of threads in the FRP...threading until binding, then backing the screw out...and gives me better results and prevents breaking screws in most cases, but this is really labor intensive.

I am considering installing a rub rail on my hull, requiring attaching an 18 foot 25mmx25mm (1X1) teak rail on both sides of the hull (half inch to five eighths inch thick) every eight inches (26 1-3/4 inch screws total).

Is there something I am missing? Should I be using a lubricant for the screws? If so, what lubricant is best? Are there dies for cutting screw threads that I don't know about that could help? Or should I not be using wood/sheet metal screws, but using machine screws for this application? If I use machine screws, I need access inboard for the length of the hull, which is covered with ceiling strips and lockers hung from stringers whose fastener heads are plugged. Removing all this inboard interference would be very labor intensive.

Appreciate any advice.

 

[Bill Roosa]

Welcome to the club. I've had similar issues with lextan. Fiberglass is tough stuff. I have had success using the screw anchors for none structural (floor boards) items. If it is going to have to carry any kind of load I ALWAYS through bolt.
Do you have access to the back side of the rail? Clearly you would need the first mate to help in this type of endeavor.

 

[Ray Bowles]

Unless these screws are stainless aren't you asking for trouble at a later date? Bedding the screws with butyl tape offers purchase for the threads when using screws and that allows a larger pilot hole to be drilled. In the end, I second Bill's comment about using bolts. They work for everything and can be removed years later.
Ray

 

[Ross]

This will make how many entry points for water? You could bed a backing strip of wood with 5200 and when that has cured fasten to that. No hull penetrations, no entry points for water.

 

[Tejas]

My limited experience is that the size of the pilot hole is crucial, and I've not been able to find a table for fiber glass. I have had success of first trying various pilot hole sizes for a particular application in a scrap fiberglass.

 

[Noel]

I would use ss sheet metal screws in the bulkhead for the instruments. Make sure you countersink through the gel coat after drilling pilot hole so the surrounding gel coat doesn't crack out. You don't need much meat for the screws to hold, so drill the pilot hole larger than you would think.

 

[Tom Metzger]

I agree with Tejas and I usually start with a drill 1/64" larger for a sheet metal screw in fiberglass than I would use in hardwood.

 

[Don12364]

Pilot hole sizes for self tappers are normally given for screws driven into metal.
Metal has some flow abilities so the pilot drill size can be close to the screw shank diameter.
FRP does not flow and is rigid; so the quoted pilot sizes are a little too small and this causes splitting of the surface and, occasionally broken screws.
Also stainless self tappers are not made of the same hardness steel that non-stainless ones are are so they shear off more easily.

What I do is to measure the root diameter of the screw with callipers, add a few thousandths and try a drill of this diameter. If it is still tight then I go up another drill size until the screw drives correctly.
The holding in FRP will never be as good as into steel anyway but my technique does at least avoid stripping, one can take the screw out and put it in again and I don't get screws pulling out either.
No lube is needed.
Deep holes may need an even larger drill to avoid binding.
Experiment and see.

 

[fryewe]

Thanks, Donalex and others. Much of the advice I am getting suggests oversizing the pilot holes a little bit, including off forum advice from a reputable boat builder in Japan. A similar installation to the rub rail I will be adding used bronze sheet metal screws to fasten through both the wooden rail and a brass rub rail atop it, through the FRP hull. It has been in place for six years with no leaks, so I think I have my answer.

I'll let you know how it turns out when I have it completed.

 

[Cool Beans]

I've had success drilling and taping for machine screws for small items. . .

 

[Ross]

Wood screws work because they dig deeply into solid wood. Sheet metal screws work because they are driven into SHEET METAL. FRP is a lously substrate for wood or sheet metal screws.