Drilling holes in fiberglass

[j1r11]

Hi all, i need to drill some small holes for my transducer and some gunwales rod holders, but i am afraid of the screws not holding. Do i drill a hole first then use any screw or is there a special screw?

Am i supposed to put a metal plate on the backside for strength?

 

[TruckDrivingFool]

Re: Drilling holes in fiberglass

Different situations require different solutions,

For the transducer holes it'll depend on where you drill them. Most of time there will be the transom wood there to back it up so you can just drill and screw. Make you seal them good with 3m 5200 or 4200 to avoid water intrusion into your transom wood though to avoid leading to a rotted transom. Many say the best bet is to drill your holes over size, fill them with resin, then do your mount holes. this gets you transom wood sealed with resin and in theory avoids all chances of rot. Many do the first option.

For the gunwales your best option is to use a machine screw,washer, and nut with a metal backer plate to spread the load out. another option is to use screws with a wood backer but once again you'll need to deal with sealing it up good or sooner or later you'll be redoing it due to rot.

 

[jjkc]

Re: Drilling holes in fiberglass

Different situations require different solutions,

For the transducer holes it'll depend on where you drill them. Most of time there will be the transom wood there to back it up so you can just drill and screw. Make you seal them good with 3m 5200 or 4200 to avoid water intrusion into your transom wood though to avoid leading to a rotted transom. Many say the best bet is to drill your holes over size, fill them with resin, then do your mount holes. this gets you transom wood sealed with resin and in theory avoids all chances of rot. Many do the first option.

For the gunwales your best option is to use a machine screw,washer, and nut with a metal backer plate to spread the load out. another option is to use screws with a wood backer but once again you'll need to deal with sealing it up good or sooner or later you'll be redoing it due to rot.

Well said not much to add there.

 

[smokeonthewater]

Re: Drilling holes in fiberglass

A screw into fiberglass over 1/4" thick is quite secure. simply drill the hole slightly smaller than the STAINLESS steel screw, redrill/countersink through the gelcoat ONLY slightly bigger than the screw to prevent cracks in the gelcoat, dab the holes with 5200 or 4200 NOT silicone and screw em in.

If you are attaching to an unusually thin panel or feel that more strength is needed for some reason, use stainless bolts, washers and nylock nuts with a treated plywood backer and 5200 to seal the holes which should be a snug fit for the bolts.

 

[j1r11]

Re: Drilling holes in fiberglass

Different situations require different solutions,

For the transducer holes it'll depend on where you drill them. Most of time there will be the transom wood there to back it up so you can just drill and screw. Make you seal them good with 3m 5200 or 4200 to avoid water intrusion into your transom wood though to avoid leading to a rotted transom. Many say the best bet is to drill your holes over size, fill them with resin, then do your mount holes. this gets you transom wood sealed with resin and in theory avoids all chances of rot. Many do the first option.

For the gunwales your best option is to use a machine screw,washer, and nut with a metal backer plate to spread the load out. another option is to use screws with a wood backer but once again you'll need to deal with sealing it up good or sooner or later you'll be redoing it due to rot.

I plan on keeping the boat at least 7 years, so i def. want to avoid any rot and extra work. I am going to take some pics later and post them showing the area i want to drill.

 

[j1r11]

Re: Drilling holes in fiberglass

A screw into fiberglass over 1/4" thick is quite secure. simply drill the hole slightly smaller than the STAINLESS steel screw, redrill/countersink through the gelcoat ONLY slightly bigger than the screw to prevent cracks in the gelcoat, dab the holes with 5200 or 4200 NOT silicone and screw em in.

If you are attaching to an unusually thin panel or feel that more strength is needed for some reason, use stainless bolts, washers and nylock nuts with a treated plywood backer and 5200 to seal the holes which should be a snug fit for the bolts.

Am i countersinking the head of the screw to be flush with the surface, or just a touch bigger of the actual thread diameter? When you say "bolts" do you mean screws? My transducer bracket holes are not very big. I believe the screws that came with the transducer are only about 1/2"-3/4" long, would that be long enough?

Should i use course of fine thread?
Thanks

 

[smokeonthewater]

Re: Drilling holes in fiberglass

Just a touch bigger than the threads... Bolts use nuts.... Screws do not
I'd use coarse because they are less susceptible to corrosion but either will work.

After all all we're doing here is holding a few ounces of plastic (n stuff) to the back of a boat.


Edit: I had said fine but meant coarse.

 

[joewithaboat]

Re: Drilling holes in fiberglass

Am i countersinking the head of the screw to be flush with the surface, or just a touch bigger of the actual thread diameter? When you say "bolts" do you mean screws? My transducer bracket holes are not very big. I believe the screws that came with the transducer are only about 1/2"-3/4" long, would that be long enough?

Should i use course of fine thread?
Thanks

Bolts and nuts if you go all the way through...

If using screws, this is when counter sinking the gel coat is important. Its only to keep the gel coat from cracking when you cinch the screw tight. It is not to provide a recess pocket for the screw head as it will be under the flange/plate.

Things that usually mount on the transom often come with short coarse thread screws to provide enough grab but not drill all the way through the transom and penetrate the hull. The key is to use the proper drill size (just bigger than the shank of screw but smaller than the o.d. of the threads) and to use a hand screw driver or a screw gun with the clutch properly set.

Don't for get the 3m 4200 or 5200. I buy small tubes and they keep well in the fridge after opening. Walmart sells it. They also will have good marine stainless screws. Academy has a better selection if you have one.

 

[jigngrub]

Re: Drilling holes in fiberglass

The transom solutions are good ones here.

The problem with the rod holder solutions is getting behind the fiberglass to install the wood or aluminum backer plate, on most fiberglass boats that isn't possible without removing the cap.

Another solution is to use riv-nuts:

http://www.enfasco.com/Frame4.htm

Another solution is to use the plasctic drywall anchors.
Mark and drill your holes.
Coat the outside of the drywall anchor with 5200 and insert it in the hole
Fill the hole in the anchor with 5200
Cover the screw threads with 5200 and mount your bracket.
Let the 5200 setup.

 

[j1r11]

Re: Drilling holes in fiberglass

The transom solutions are good ones here.

The problem with the rod holder solutions is getting behind the fiberglass to install the wood or aluminum backer plate, on most fiberglass boats that isn't possible without removing the cap.

Another solution is to use riv-nuts:

http://www.enfasco.com/Frame4.htm

Another solution is to use the plasctic drywall anchors.
Mark and drill your holes.
Coat the outside of the drywall anchor with 5200 and insert it in the hole
Fill the hole in the anchor with 5200
Cover the screw threads with 5200 and mount your bracket.
Let the 5200 setup.

I can get behind my gunwales, no problem for a backing plate. It does not look 1/4" thick so i would def. put a plate.

 

[j1r11]

Re: Drilling holes in fiberglass

Dumb question, but would the transducer go on the inner cavity just to the left of the unit or the area about a foot below my tie down loop?

 

[joewithaboat]

Re: Drilling holes in fiberglass

I can get behind my gunwales, no problem for a backing plate. It does not look 1/4" thick so i would def. put a plate.

You should be good to go with through bolts and backer of some sort... plywood, aluminum, or big fender washers. You should not need any sealant to seal it but it might help with strength.
You talking about rod holders for storage or trolling?

 

[j1r11]

Re: Drilling holes in fiberglass

You should be good to go with through bolts and backer of some sort... plywood, aluminum, or big fender washers. You should not need any sealant to seal it but it might help with strength.
You talking about rod holders for storage or trolling?

They are rod holders that you can angle for trolling , or just holding the rod upright. I may eventually want real downriggers if thats possible to mount on my boat.

 

[joewithaboat]

Re: Drilling holes in fiberglass

They are rod holders that you can angle for trolling , or just holding the rod upright. I may eventually want real downriggers if thats possible to mount on my boat.

I that case, do your best to mount something substantial to the back side. Plywood probably the best. Sometimes a bit tricky with the contoured fiberglass. You don't want to crush the curve with a piece that is too big.

 

[j1r11]

Re: Drilling holes in fiberglass

I that case, do your best to mount something substantial to the back side. Plywood probably the best. Sometimes a bit tricky with the contoured fiberglass. You don't want to crush the curve with a piece that is too big.

Yea, i wish it was just flat, but of course that would be too easy.

 

[River_Lizard]

Re: Drilling holes in fiberglass

Most good depth finders/fish finders come with instructions on where to mount them. If you get too close to your out drive your transducer will not work properly, if you mount it too far to the outside then when you're up on plane it won't read correctly due to air bubble etc.. Sometimes you just don't have a choice in the matter and you're depth finder just won't work when you're going 25 mph. I always try to mount transducers at around 50-55% the distance between the outside corner of the boat to the center of the outdrive/lower unit. But never closer than 45% to the drive unit. I'd say right by that last step down under the tie-down eyelet. But again, read the instructions, they will tell you how much the of transducer needs to below the bottom of your hull.

 

[smokeonthewater]

Re: Drilling holes in fiberglass

I've never had any problem at all mounting 6" away from the drive.

 

[oops!]

Re: Drilling holes in fiberglass

installing backing plates on a boat can some times be a real pain....

i do it for towers all the time.

you need to get your arm way up there, and sometimes remove the upholstered panel.

but for cleats and down riggers, you should use backing plates

 

[j1r11]

Re: Drilling holes in fiberglass

I think i am liking the idea of starboard and adhesive.

 

[smokeonthewater]

Re: Drilling holes in fiberglass

my .02: just relax and drill a couple holes

It REALLY IS no big deal.