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Author Topic:   Help Needed for Drilling Into Fiberglass
LakeErieGator posted 03-21-2005 09:00 AM ET (US)   Profile for LakeErieGator   Send Email to LakeErieGator  
I plan on installing a VHF antenna on my 2003 Dauntless 180 by attaching the antenna bracket to the centre console. I have never drilled holes into fiberglass before and would welcome some advice for the preferred method.

I would greatly appreciate if someone could share with me the benefit of their expertise for the best method to drill a clean hole in the centre console that will minimize the chance for spider cracks or other forms of cracks from happening around the holes. Thank you.

jeffs22outrage posted 03-21-2005 10:33 AM ET (US)     Profile for jeffs22outrage  Send Email to jeffs22outrage     
Put masking tape over the area you are drilling. Mark your drill points, start the hole with a smaller bit that what your final hole size will be, then finish with your finish hole size bit.
ratherwhalering posted 03-21-2005 10:57 AM ET (US)     Profile for ratherwhalering  Send Email to ratherwhalering     
Whenever I drill a hole in gel coat, I always feather the gel coat. When a screw is inserted into gel coat, it cracks. There is no flexibility in gel coat at all, whatsoever. It is like trying to screw into glass. For example, when I drilled the holes for mounting blocks, I drilled the holes with a drill bit slightly smaller than the diameter of the threads of the screws. Once I had all the screw holes, I took a large drill bit (there is an actual bit called a "reamer" that is meant for this, but you can use a larger bit to accomplish the same thing) I drilled the top of the hole until the spoils turned from tan to green, This only took 4-5 rotations of the large bit. The gel coat is very, very thin. The object is to slightly broaden the gel coat but not the fiberglass underneath, to which the gel coat is adhered. The screws threads will now bite into the fiberglass and wood underneath the gel coat, but not the gel coat itself. I added a touch of 5200 in the new hole to seal. Even if you are using machine screws, washers and nuts, you still need to feather the gelcoat.
whalerajm posted 03-21-2005 11:46 AM ET (US)     Profile for whalerajm  Send Email to whalerajm     

If you are mounting your antenna to the console make sure you drill into an area with wood backing... if there is no wood backing and you can reach interior of console where mounting screws will penetrate, use a ss machine screw with a washer and nut.

Before you start drilling, have you considered mounting the antenna on the rail around the console? Easy on easy off with no holes.

whalersailer posted 03-21-2005 12:43 PM ET (US)     Profile for whalersailer  Send Email to whalersailer     
A countersink bit also works very well for what Rather describes. Plus, the countersink bit won't bite nearly as much, and is therefore much less likely to go farther into the glass than anticipated.


bsmotril posted 03-21-2005 03:54 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
I disagree with starting with a small bit, and never do that because of the potential for scarring. When you start small, and then go to a larger one, you often chip out the gel coat in a larger area than that of the hole alone when the head of the larger bit gets into the hole and the flutes snag the edge. Instead, I start with a bit one or two sizes bigger, or better yet, a counter sink bit. Use it to fist drill a shallow cone shaped hole just deep enough for the outer diameter of the bit to go through the gel coat layer. Then, use the smaller final sized bit to drill your hole. That way, there is no possible way for the bit to snag and chip our the gelgoat.
I've found that tapine is not a 100% gurantee against chipping also.
Chuck Tribolet posted 03-21-2005 08:20 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Part of getting it right is to have the screws line up
perfectly with the base. To do this, use a "Vix Bit" which
you can get from any large HW store. Drill one hole, bolt
the base through that hole, drill a second hole diagonally
opposite, starting with the Vix bit, bolt the base through the
second hole, then drill the last two, again starting with the
Vix bit, and bolt.

And measure twice, cut once. And check what's behind where
you are drilling about six times.


jimh posted 03-21-2005 11:49 PM ET (US)     Profile for jimh  Send Email to jimh     
Many years ago I was employed by a software company that wrote computer aided manufacturing software (CAM). I spent a week in Cincinnati learning how to program numerically-controlled machine tools (NC). It was there that I learned that the standard practice in machining was to spot pilot holes using a large diameter drill bit, and just penetrate into the work-piece a very short distance. This created a small diameter pilot hole or indentation for guiding a smaller drill which would actually drill the hole of the desired diameter. The reason for this was that a large diameter drill shank would hold much steadier and have no wobble, thus marking the spot for the holes much more precisely than if drilled with a small diameter drill that might wobble or bend under load.

That said, I find it much easier to drill pilot holes with a hand electric drill by using a small diameter bit which is gently and slowly started into the gel coat surface. Just make a shallow hole so the larger drill will start precisely and not go skipping across the gel coat and mar the surface.

Advice given above about enlarging the gelcoat area by hand is very good advice. Also, installing a self-tapping screw into fiberglass laminate works well if you don't make the hole too small or too large. If the hole is way undersize, it will take a lot of pressure to make the threads. If the hole is too large the screw will not obtain good purchase.

For anything like an antenna mount which will have considerable loads, you should through-bolt and use backing plates or fender washers as suggested above.

Backlash posted 03-22-2005 08:33 AM ET (US)     Profile for Backlash  Send Email to Backlash     

Over many years of drilling more holes in fiberglass than I care to remember, this procedure has worked best for me:

1. Drill a small pilot hole.
2. Use a counter-sink bit to counter-sink around the pilot hole.
3. Drill your final hole size.

I normally do not use masking tape as I have found that the pencil mark shows up better on fiberglass, than on the tape, and the tape really doesn't prevent chipping. You may get some minor chipping when drilling the pilot hole, but the counter-sink bit will take care of that.

I have tried using a larger diameter bit (than the pilot hole) to counter-sink the hole, but the larger bit is hard to control with a hand-held drill. It is very easy to plunge the larger bit too far into the fiberglass. The counter-sink bit allows very precise depth adjustments.

As far as mounting the antenna bracket to the console, I would use SS machine screws, washers and nylok-nuts. Regular wood screws will loosen over time. I always use machine screws with nylok-nuts whenever I have access to the back of the mounting panel.


montaukman posted 03-22-2005 09:41 AM ET (US)     Profile for montaukman  Send Email to montaukman     

Everyone is mentioning a counterbore bit. There are two types of counter bores in my mind. One is the tapered and the other is the squared off bore.

Which type of c'bore are we talking about here. I would assume it is the tapered.

Also, if you use the tapered c'bore bit, to get to the shank diameter on the screw, It makes sense that you have to go deeper with the tapered bit, since it gradually tapers to a larger diameter the deeper you drill. Won't this affect the amount of material that is now going to provide purchase to the screw since you are removing a deeper amount of material in order to get to the desired shank outside diameter?

Hope i worded that correctly. Am I possibly confusing counterbore with counter sink?

Just curious. I may stil be doing my own electronics install and need clarification.

All the best,


kingfish posted 03-22-2005 09:46 AM ET (US)     Profile for kingfish  Send Email to kingfish     
Actually what is being talked about here is properly referred to as a "counter-sink", which is designed to countersink the head of a flat or oval head screw so that it is flush with the surrounding field. A "counter-bore" on the other hand is designed to provide the conical seat for a flat or oval head screw but to also bore the that seat down into the material so that the surface of the screw head is *below* the surface of the field around it.

What you want is a counter-sink.

rmart posted 03-22-2005 09:53 AM ET (US)     Profile for rmart  Send Email to rmart     
When drilling into fiberglass, I was taught to tape over the area being drilled and then begin slowly drilling in reverse to "mark" or "scar" the area to be drilled. It is my experience that this helps the drill bit from skipping when you begin to drill in forward gear.
montaukman posted 03-22-2005 10:05 AM ET (US)     Profile for montaukman  Send Email to montaukman     
Hi Kingfisher,

Thanks for the help. I do notice that countersink and counter bore are interchangeable on the internet. I did a search for both bits and sometimes countersink comes up with the conical signature you described while other times, it comes u with the flat signature. I guess the flat is the way to go. can get expensive if you plan on using different sized screws.

Last question on this subject.

How do you ensure the the flat c'sink bit doesn't wander since it is cutting flush to the gelcoat and doesn't provide a conical pre-drill guide. Like in auger bits, they have a screw like extension that guides you to the pre-drilled hole. I haven't seen this on these bits. Am I missing something and sorry for belaboring the topic.

MyOutrage posted 03-22-2005 01:14 PM ET (US)     Profile for MyOutrage  Send Email to MyOutrage     
Once drilled properly, should you dress the hole w/ silicon or some other water tight product?
kingfish posted 03-22-2005 02:16 PM ET (US)     Profile for kingfish  Send Email to kingfish     
I drill the screw hole first, then remove the drill bit and use a countersink bit to enlarge the surface of the hole, through the gelcoat. This not only relieves the stress on the gelcoat so it doesn't split or chip, but it also makes a "well" in which I put a dab of 3M 4200 so the hole is sealed around the screw when it is driven through the caulk. With the screw hole drilled first, the countersink doesn't wander.
LakeErieGator posted 03-22-2005 04:14 PM ET (US)     Profile for LakeErieGator  Send Email to LakeErieGator     
Thank you for offering advice on the best methods to drill a hole into the fiberglass centre console. Sharing knowledge and expertise helps not only promote love for our Whalers, but also helps pass on to others a love for boating in general.
ratherwhalering posted 03-22-2005 11:02 PM ET (US)     Profile for ratherwhalering  Send Email to ratherwhalering     
I understand what you folks are getting at regarding using a pilot hole and/or countersink first, although I have noticed that "new" holes in gelcoat tend to chip much less, if at all, when you start with the small diameter hole, and then hand bore the gelcoat with a countersink/reamer/large bit. While we are on the subject, I use a dremel bit and cordless drill to accomplish this same effect on old holes that have already begun to spider. The bit is a cone shaped grindstone that really allows you to broden an existing hole without chipping the surrounding gelcoat. Much slower than a bit, but it sands the gelcoat instead of cutting it, leading to less chipping.

By the way, Chuck, that is very good advice!

whalerajm posted 03-23-2005 08:16 PM ET (US)     Profile for whalerajm  Send Email to whalerajm     
I have successfully used same approach as ratherwhaling...using a rounded grinding stone bit to get a countersink effect. It's also works for filling old, screw holes in the deck...carefully grind "minicrater" on top of hole, fill with gelcoat patch...I've also sealed screw hole with 5200 in some cases before adding far both techniques have held up very well and are virtually invisible except for lack of nonslip pattern.
Chuck Tribolet posted 03-23-2005 09:09 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
If the hole is just in the fibreglass console, no, I don't
seal it. If it's in wood, YES. Or in the hull, YES.


jimh posted 05-29-2005 07:15 PM ET (US)     Profile for jimh  Send Email to jimh     
I recently removed nine screws from the transom of my boat. None of these had been made by me; all were from previous owners. They had been employed in holding SONAR devices or speedometer pick-up's and associated cables. It was interesting to observe the holes which were left behind.

In the case of four of the holes, ones associated with the original SONAR transducer installed on the boat and probably fitted by the original owner or dealer, the holes had been carefully enlarged with a large-diameter drill bit or countersink so that the gelcoat was not bearing on the shank of the screw. All of these holes were still very nicely circular and had no cracks.

In the case of most of the others, installed later in the boat's life I assume, the holes had not been enlarged in the gelcoat area to a larger diameter. All of these holds had chips of gelcoat missing, and some had radial cracks in the gelcoat emanating from them. The holes were irregular in shape at the surface of the gelcoat, making filling them more difficult

This confirms the advice mentioned above about the importance of relieving the hole diameter for any fastener so that the gelcoat layer is not bearing on the shank of the fastener. To not follow this advice is to almost guarantee a crack or chip when the fastener is installed.

Also, there were six holes in the cabin sole, associated with a Porta-Potti since removed, which were made at the factory. Unfortunately all of these holes were poorly drilled and showed signs of gelcoat cracking and chipping. Can't win them all, I guess.

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