Drilling hole in GRP

[fullerjohn]

Hi there. This is my first post. I have a Ring16 Ski Boat and have bought a boarding ladder to fit onto the rear transom.
I have probably a very silly question but here goes -
I need to attach the ladder to the transom by 4 x 4.5mm self-tapping stainless steel screws. I know to surround the proposed hole positions with masking tape to prevent chipping/cracking to the GRP. ... BUT, what size drill bit do I use to drill the four holes - smaller than 4.5m yes? If so, use a 4mm drill bit???
Hope this is not too basic for any replies!
Thanks
John
Scotland

 

[sailorman]

Hi & welcome
i would always use a smaller size drill bit & either use a "tap" to form the thread or turn in the machine screw.all bedded in sealant on final assembly
& add backing plates on the inside to spread the loads

 

[old_salt]

I would not use self-tapping screws on a boarding ladder, their will be a risk of them puling out. If you can get to the inside get yourself 4.5 mm, better 5 mm machine screws or bolts and large washers for the inside.
I hope this helps.

 

[Phoenix of Hamble]

If you do need to use screws, then a 4mm drill bit is somewhat on the large size..... I'd go for 3mm...

But agree, bolts/machine screws are a far better idea....

 

[fullerjohn]

Many thanks for the replies chaps. I can get to the back and will therefore use bolts & machine screws. The very quick replies are much appreciated.
Cheers
John

 

[guernseyman]

Definitely bolts + backing plate. Think of overweight man overboard, clothes sopping wet, trying to get back on board from a rough sea.

By the way, the hole needs to be the size of the solid central part of the bolt. Align the bolt and the drill and hold them up to the light. The thread can cut into the GRP. But, if you are using a backing plate, you can drill a hole the same size as the bolt OD, so long as you seal it.

 

[SvenglishTommy]

I agree with everyone on the bolts. I've had no luck at all with self tapping screws into grp and always end up going for bolts. I've had more trouble with the hole being too small than too big and find that GRP doesn't take to self tapping screws well at all.

 

[Guest]

Couple of points here ....

a) Self-tappers are not advsied with GRP ... they will hold for short while and then tear out later.
b) Holes should be drilled slowly and correct size for bolt.
c) Always when fitting anything to GRP like this have a backing plate.

For this I would advise a section of 3/8 or 1/2" marine ply ... well soaked in varnish or other preservative. You can resin / tack glue it to the inside of the hull making sure it is fixed sufficiently to take pressure of drilling through etc.
Then drill through from outside into the wood suitable to either take long stainless bolts (best) or long stainless screws. The holes in the GRP should have a smear of epoxy or resin to seal the cut edge ...
Smear silicon sealant on the ladder fixing feet that will mate with boat surface ... now bolt / screw up. If using bolts - make sure you have large "Penny-Washers" or metal plates to spread the load on the timber.

With that fixing - you will have confidence that you will take a persons weight on it ...

 

[fullerjohn]

Many thanks for this - just the detail I am needing. I will certainly use the tip of plyood backing. I am a fairly competent DIYer but have never worked with fibreglass before!
Cheers
John

 

[colvic]

If you can get underneath to use backing plates then ALWAYS use nuts and bolts as these are so much stronger.

Phil

 

[Talbot]

If you are putting in self tappers, it is always a good idea to drill the gelcoat only out to a slightly larger size as that will reduce the chance of the gel coat chipping

 

[pW2]

Use a good polyurethane mastic to seal the bolts and ladder. Epoxy cracks and silcone can fail and then nothing will work as a sealer ever after. Use a drill the same size as the bolts.

 

[oldsaltoz]

G'day John, and welcome to the WBW forums, a mine of information.

Most Ski boats have a ply transom with a fibreglass cover; just drilling a hole and passing bolts thru' may lead to problems down the track as water may get into the ply.

Consider drilling the holes out at 8 mm and filling them with resin that has been mixed with Micro-fibres, a little bit of tape will prevent it dribbling out till it cures. Then drill the holes to the required size and back them with some aluminium or stainless steel backing plates.

I have replaced a lot of ply transoms over the years, all of them rotted out due to water ingress.

Avagoodweekend......

 

[brianrunyard]

Bed the fixing with a suitable sealer. Put plenty of sealer round the bolts and in the hole, to keep any water out of the transom.