Drilling holes in grp

[zoidberg]

I have a task to drill 8 holes in grp, each to admit 12.7mm machine screws with minimum tolerance. The grp moulding is 10-11mm thick and will be reinforced with ~6mm of fresh carbon fabric layup. These m'screws will secure 2 specialised 95mm OD deck-eyes using suitable locknuts and 120mm OD joint washer plates.

I'll need some specialised guidance on 'pre-load torque' but more urgently, whether to use a 12mm toothed Forstner drill bit, or a diamond-brazed hole saw bit....?

 

[Neeves]

If I have this correct - this is grossly over engineered for your yacht.

How thick is the joint washer plate? If it deforms the holes will lose integrity

But your machine screws are larger than the holes you intend to drill (with the 12mm Forstner drill bit) leaving no room for any sealant - but maybe this does not matter.

Jonathan

 

[Stemar]

12.7mm is 1/2 an inch. I reckon a 1/2" Forstner bit from each side would give the cleanest cut, but what are you going to attach to the bolts that will take that sort of load? For anything anywhere near normal, I'd use a 13 or 13.5mm drill to allow a bit of space for goop to seal the hole because, without it, they will leak.

 

[Boathook]

I counter sink the surface to allow a bead of sealant to form round the 'bolt'.
For hole saws Starrett is my preferred make, and the smallest diameter does seem to be 14mm.

As others have said that is a massive size fixing and using a standard spanner my pre load torque would be as tight as possible.

 

[justanothersailboat]

I've not had much luck with Forstner type bits. In that kind of thickness I seem to be doing very well with a step drill for larger holes, finishing up the diameter from the back. Remember to countersink the top edge to relieve stress on the gelcoat and give the sealant somewhere to be squeezed into rather than out.

To get the most out of fittings that heavy you are going to need a really stout backing plate!

 

[PaulRainbow]

I would use a hole saw, be much neater.

 

[B27]

Is it possible to use a drill stand?
I'm thinking of the sort they use on site for steels, which clamp on magnetically and the drill goes right through, although obviously magnets won't work, you'll need to fix the stand down somehow.
The holes are deep enough that the angle of the drill matters.

I would probably use a pilot hole and open it out.
If you get the pilot hole in the right place, ordinary HSS bits which can be sharpened will do.
If I needed to open up the hole in one direction, I might use a carbide burr.

A 'thinking ahead' kind of bodger might have drilled the holes before adding the carbon reinforcement.

 

[justanothersailboat]

I can back up B27's suggestion of carbide burrs - they are really, really good on fg and don't get wrecked by it

 

[rogerthebodger]

Is it possible to use a drill stand?
I'm thinking of the sort they use on site for steels, which clamp on magnetically and the drill goes right through, although obviously magnets won't work, you'll need to fix the stand down somehow.
The holes are deep enough that the angle of the drill matters.

I would probably use a pilot hole and open it out.
If you get the pilot hole in the right place, ordinary HSS bits which can be sharpened will do.
If I needed to open up the hole in one direction, I might use a carbide burr.

A 'thinking ahead' kind of bodger might have drilled the holes before adding the carbon reinforcement.

When I use my magnetic base drill stand on non magmatic material like stainless steel I G clamp a piece of flat steel in place first the mount the mag drill in place

To drill GRP with a HSS twist drill stick some masking tame over the location which helps to stop chipping any gelcoat.

I generally use a Holesaw but the OP requires a hole that is smaller than the smallest hole saw generally available

 

[zoidberg]

Thanks. Some further explanation....

The 'deck-eyes' to be mounted will be the attachment points for a Jordan Series Drogue. I took best advice available on the structure of the JSD. I also thought hard about the fixings to the boat structure - which many don't - and consulted the best specialists I could find. Given that no-one could reliably define the eng-characteristics of the grp layup where the fixings must be mounted, that advice was to 'wallpaper' on several layers of carbon biaxial cloth..... a 'belt and braces' proper job. There is no merit in placing 'just enough to get by'. Like a Martin-Baker ejection seat, it may never be used but if so, it MUST be right first time and wholly infallible.

The fixings are a pair of Harken 'Specials' in titanium. They're so unusual that Harken/Peewaukee don't have records, but they're 'good for 20,000lbf'. They're probably worth, retail, rather more than my boat - but I just happened to have them!

 

[fredrussell]

Fixing porn!

 

[Fr J Hackett]

You will probably end up tearing a chunk out of your boat if the Jordan rouge ever comes under full loading. It is far more usual to use bigger plates and backing pieces with multiple fixings to speed the load over a muck larger area. As Fred said you have succumbed to fixing porn.

 

[MisterBaxter]

How big is the carbon reinforcement patch, out of interest? I would say - from my position of uninformed armchair ignorance - that if that's big enough, the overall spec looks about right to me.
Re the drilling, I recommended Perma-Grit tools on another thread and they would be worth a look for this too.

 

[zoidberg]

How big is the carbon reinforcement patch, out of interest? I would say - from my position of uninformed armchair ignorance - that if that's big enough, the overall spec looks about right to me.
Re the drilling, I recommended Perma-Grit tools on another thread and they would be worth a look for this too.

Perma-Grit.... "
Unfortunately we do not make any holesaws smaller than 19mm. Generally, any smaller hole is made with a twist drill. Sorry unable to help."

The carbon bi-axial patch should be about 450mm x 250mm ....ish. Larger both ways, if I can manipulate the piece inside the small lazarete. It's cheap enough.
There are certain 'mods' to the boat I don't want to ever wonder about again.

As for 'fixing porn', I do concede those items certainly constitute 'boat bling'. But what on earth would I do with them, if not this?

 

[srm]

I seem to remember that in the original design spec Jordan advised a pair of substantial "chain plate" type hull fittings flush with the hull and similar spec backing plates inside. He advised against any fitting that provided leverage such as winch or cleat.

 

[justanothersailboat]

Seems like quite a few JSDs have now been used successfully, many of them with fittings of lower spec than these and backplates less fancy than a giant slab of carbon fibre. I think the circumstances of retrofitting things to old boats make it hard to get high confidence in engineering calculation... but then people talk themselves into crazy levels of overbuilding. That can cause its own problems when you pass the force, and damage, on to something else.

I think this is all proportionate and will probably be fine.

 

[DinghyMan]

I seem to remember that in the original design spec Jordan advised a pair of substantial "chain plate" type hull fittings flush with the hull and similar spec backing plates inside. He advised against any fitting that provided leverage such as winch or cleat.

Poey50 did a load of research before getting his drogue attachment plates made - see Making drogues

As you say they need to be very substantial - we made them 10mm x 60mm 316 Stainless with four 10mm bolt points and a 75mm x 75mm x 3mm plate backing each bolt

 

[debenriver]

Milwaukee DIAMOND MAX Diamond Grit Hole Saws start at ³⁄₁₆" and go up by 1/16" increments to ⁵⁄₁₆" then by ⅛" increments to 1⅜" – then by ½" increments.

Cheers -- George

 

[Neeves]

When I was installing a new transducer in Josepheline I used a hole saw, as one would use on wood. The transducer was located on the centre line roughly under the mast. The layup was thick and I drilled a pilot hole right through and then used the hole saw from inside and out. I simply used a large AC powered hand drill.

The hole saw coped well, later I used a similar hole saw on stainless steel, a slightly bigger hole, and totally destroyed it.

So the hole saw was good for fibreglass, heavy layup, but no good for stainless steel. My wood hole saw set is called Blue Moll.

But a transducer is bigger than the holes you need and I don't recall the smallest hole saw.

When I was developing my Bridle Plate I had need for drilling HT steel, Bis 80 but I also drilled the 7075 alloy of aluminium. I bought tungsten carbide hole saws from China and they come in a range of sizes, below 1/2", but the holes I was drilling were larger 21mm from memory. I destroyed a few and have bought some replacements since, some bigger still. They were not very expensive.

The problem is that they do not drill deeply, maybe 15mm. You don't mention how thick is the layup. I have never tried them on fibre glass. I have used them on 15mm mild steel with a small B&D battery drill - works a breeze. Again you could drill a pilot hole, right through, and then drill from both sides.

I'll check today what sizes Blue Moll come in and the sizes of the tungsten carbide drills. I'll try to take some pics.

It is time consuming. After you drill you have the waste piece inside the drill bit and you need to take the drill bit apart to take the waste piece out. In your situation/location you need a nice warm, dry day.

I, in common with some others, think your glitzy pad eyes are the wrong devices. They introduce a 'turning' tension. I fully understand why you are using them - but they are not really designed for your applicationYou need chain plates, as used in Lagoon cats and most other cats, at the transom, attached to the hull (their orientation will then be the same as the tension direction). If you worry about this you could double up and have chain plates on the hull and right angled chain plates on the transom such that the 2 chain plates 'overlap'. The hull transom interface must be one of the strongest parts of most yachts. You will still need beefy through bolts, so your original question is still the same. Your carbon fibre reinforcing is a good, excellent, idea - but I'd go over size in area, and have matching chain plates on the interior (matching the external chain plates - a simple, roughly, right angle piece would be ideal, both across the transom and along the hull). Cat chain plates, must depend on the size of the cat, are beefy - for Josepheline, 38', they must have been made from 15mm stainless plate.). I'd assumed the plates were 316 but they might have been 2205 Duplex. Our chain plates were slightly curved to match the curve of the top sides of the hull.

Jonathan

 

[Neeves]

Blue Mol

BLU-MOL – Disston Tools

Freely available in Oz, but maybe difficult to find in the UK.

Jonathan