Need Drilling Advise Please

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Karnic

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Hi Guys
Need some advise and would appreciate your kind and sincere help. Want to drill some holes on my deck to install cleats and deck hinges ( Bimini ). Would use bolt and nuts, with backing plates and washers. As I am not using the self tapping screws, is it ok to drill a slightly ( FEW mms) larger diameter than the bolt or I would have to countersink the hole as well? Also some friend of mine recommended drilling in reverse mode but again as I am using bolts and nuts instead of self tapping I think a slightly larger hole than the bolt would do the job and avoid the cracks in gelcoat.
Thanks for your feedback.
 
Hmm yes I partly agree with what he says but it's aggressively written and a bit confrontational is it not?
I always drill a hole about 1 mm larger than the bolt and heavily countersink both the hole I have drilled AND the underside of the fitting being bolted down if possible. This not only ensures that there is a cavity for the sealant to fill but that the sealant penetrates well into the deck. If it is a high stress fitting I will always use Sikaflex or similar to bed the fitting down on and assuming its a reinforced bolt with a backing plate I would always leave the last turn of the nut until after the Sikaflex has set. There is absolutely no way the bolt can turn when set in Sikaflex as he suggests if this procedure is followed properly. However if fitting lightly stressed fittings to the deck with self tapping screws I would probably not use Sikaflex but a low strength flexible sealant. Still countersink both parts if possible but ensure the screw is completely smothered is sealant before doing up tight wiping away excess sealant before it dries. Simples!
 
I never had any problems drilling holes for all the fittings when I fitted out my boat. Just drilled them ! Just normal clearance sized holes not several mm oversize. I don't remember any issues with the gel coat.

With stainless steel fasteners it is important that they are well sealed or crevice corrosion will occur.

In the few paces I have put self tapping screws into the grp it has been for no-load bearing applications and I have drilled a clearance size hole through the gelcoat then just a little larger than one would normally make a pilot hole into the laminate. I have sealed those in with an epoxy adhesive . It is the only place I ever use a rapid setting epoxy!
 
Thanks All for your help. Really Appreciate. Something that I am not understanding is the COUNTERSINK ISSUE. I am not experienced in this field so it is better I search for your advices. Why should I countersink the hole when im not going to use self tapping screws. Will be using nuts and bolts and also would need to drill holes for cleat and bimini deck hinges. Would use Very few MM oversize drill bit than the bolt, would also require /need to countersink the hole? Would tighten from the nuts under the cleat or deckhinge to install.
 
Karnic said:
Thanks All for your help. Really Appreciate. Something that I am not understanding is the COUNTERSINK ISSUE. I am not experienced in this field so it is better I search for your advices. Why should I countersink the hole when im not going to use self tapping screws. Will be using nuts and bolts and also would need to drill holes for cleat and bimini deck hinges. Would use Very few MM oversize drill bit than the bolt, would also require /need to countersink the hole? Would tighten from the nuts under the cleat or deckhinge to install.
Click to expand...

Because the countersink removes the sharp edge from the gelcoat which stops cracks from propagating and provides a cavity for the sealant around the bolt between the fitting and the deck.
 
Karnic said:
Thanks All for your help. Really Appreciate. Something that I am not understanding is the COUNTERSINK ISSUE. I am not experienced in this field so it is better I search for your advices. Why should I countersink the hole when im not going to use self tapping screws. Will be using nuts and bolts and also would need to drill holes for cleat and bimini deck hinges. Would use Very few MM oversize drill bit than the bolt, would also require /need to countersink the hole? Would tighten from the nuts under the cleat or deckhinge to install.
Click to expand...

If you put in a self tapper without countersink there's a strong chance you will break away a big chunk of gel coat, you really don't want this to happen, if fitting something like a cleat where there's constant stress on it you don't want the edge of the bolt constantly pushing on the edge of the gel coat and causing a stress crack in every direction, countersink also gives better area for sealant to get hold of. All this is basic boat building and should be learnt in early days of apprenticeship.
 
Interesting read, especially the countersink info. That said I have never done that in the past and never had any obvious problems.

It got me thinking about what are the practical consequences of a leak from deck hardware into the (balsa?) core. I have never heard of any boat having a serious problem in that respect, but it seems to me that water getting soaked up in a GRP sandwich would be very bad news. I believe that this is the common method of constructing boats although I am not sure how long it's been done like that.

So what are the consequences of water getting into the core from leaking deck hardware? Is it something that is possibly happening to may of our boats and we just don't know? If so what are the longer term consequences? Just curious!
 
When I was serving my apprenticeship I was always told to drill 2mm bigger than the bolt especially if it is a multiple holes that need to line up.
I would use sealant around the holes to stop any water ingress

jon
 
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