Drilling holes in grp

[waynes world]

My god all this talk over drilling holes in GRP. All i have ever done is mark out, small pilot drill mark wit say a 6mm bit, drill to 14mm for a 12.7mm bolt. counter sink from the top for sealant spread. then use a large SS backing plate, if flat above then a good few inch larger than the fitting. All holes drilled using an HSS bit or a deep hole saw. Puraflex 40 is good aswell as Sikaflex 291i. When bolting done dont squeeze all gunk out, one way to help getg an even gasket is to use penny washers if you can.

 

[Neeves]

My god all this talk over drilling holes in GRP. All i have ever done is mark out, small pilot drill mark wit say a 6mm bit, drill to 14mm for a 12.7mm bolt. counter sink from the top for sealant spread. then use a large SS backing plate, if flat above then a good few inch larger than the fitting. All holes drilled using an HSS bit or a deep hole saw. Puraflex 40 is good aswell as Sikaflex 291i. When bolting done dont squeeze all gunk out, one way to help getg an even gasket is to use penny washers if you can.

But maybe you were not planning to take part in a single handed, round the world yacht race in a vessel called (something like) Jester.

I don't know but I look at worst case scenario.

Jonathan

 

[Neeves]

These are the hole saws I use for high tensile steels and aluminium. The one top right is 27mm,the middle one 12mm and the larger one 50mm. The restriction is they drill 'shallow' and you might need to drill from both sides. The two on the right I used to drill 800MPa steel for my bridle plates, see below. I've drilled upto 13mm plate with the two on the right, I have only just bought the bigger one, part of a set, and have only used a 20mm dia. for drilling mild steel.

My application is drilling my bridle plate to accept 2 part, low friction, threaded rings or a slightly simpler chain hook to accept a similar LFR for a mono hull snubber.

You can buy the hole saws fairly easily from Aliexpress or Temu. They are not expensive. To drill HT steels I have a big bench drill press but for mild steel, I was impressed, my battery B&D worked well.

Jonathan

The bridle plate in use, 2 part 316 threaded LFRs (you can buy similar LFRs or deck bushes from Allan Bros in Essex and other sources.


The unassembled bridle plate.

A snubber chain hook using the same ideas as the bridle plate


The drill bits that made the threaded LFRs ... work

 

[Neeves]

I now note that you did define the thickness of layup and you should be able to source hole saws, like the ones I have, above, capable of drilling in one pass and certainly from both sides.

Jonathan

 

[zoidberg]

Like Poey50 I've researched this to death. I've looked hard through Don Jordan's notes and estimate ( 'calculate' infers too much precision ) that, with a laden Design Load of 6500lb, the maximum load on each bridle leg would be of the order of 4500lb. I judge the arrangement I've chosen will be well capable of managing that.

It is, in my view, important to note that Don Jordan was concerned to 'prove the concept' - which he did. He was content to make sound suggestions regarding what should suffice - and leave it to others to refine details of materials. For example, the ropes he had available to him more than 40 years ago for rodes and bridles were far inferior to those available to us today. He had good reasons to suggest 'stepping down' the diameters of the several sections of rode in his JSD - bulk, weight, handling and storage considerations. There is also the significant fact that stretchy nylon laid rope absorbs about half its weight in water, while losing perhaps 20% of strength. We have far better options available today.

As an aircraft engineer, he would have been very aware that the Max All Up Weight was the figure with which to conjure, and not the builders' Light Ship ( empty ) weight.
He would have thought it self-evident that we prospective users should have MAUW in mind when selecting a rope rode which will carry the largest-ever loads to be inflicted on the boat and gear.

It is almost inconceivable that commercial pilots would attempt a takeoff with ice accretion still on the wings and control surfaces - but they do. It is almost inconceivable that deep-ocean sailors will select a JSD rode that is too weak for the actual laden displacement of the boat - but they do.

There are several areas of some uncertainty in this drogue business, and we can only make the best informed judgements we can. I'm aware that some ( many? ) just bolt on a couple of s/s straps because 'that looks about right' and give it no more thought. However, rather more thought is needed. The mechanics of bolted fasteners swiftly becomes complex. My research - which includes asking some very switched-on engineers - indicates that 'bolting steel sheets' together is fairly well understood, both in theory and practice. It swiftly becomes complex when multiple bolt patterns are considered, and very much more complex when dissimilar materials and 'sandwiches' - such as titanium deckeye/grp layup/woven carbonfibre layup/large steel backing plate - are introduced.

That is the realm of 'Finite Element Analysis', which needs specialist software, powerful-computer time, expert data input, and interpretation of output. In that process there would a long list of 'reasonable assumptions' in the various steps which may make the results rather dubious. These would still need the seasoned interpretation of a top professional to say 'Do this and not that'.

I've gone straight to those top guys and listened carefully to their 'Do this and not that'. It should be sufficient. And it's entirely my responsibility.

 

[thinwater]

^^ Yup, that about sums it up.

Are the holes 12.7 mm, or since Harken is a US company, are they actually 1/2-inch? They do make 1/2-inch holes saws, but I'm guessing a twist drill after a pilot will be more accurate.

Because of the limited bearing surface of the fitting, you may need a plate on the outside as well. The GRP technically has enough compressive strength, but I'd expect the gel coat will crack and things might work loose if you don't. There is a real stress riser at the edge.

The pretension (which I don't recall anyone addressing) is debated a lot. It must exceed the maximum expected tensile load on the bolt or the joint will actually be loose under load and will move. Very bad. Some will say as much as 70% of the min tensile strength of the fastener, but in this case about 5000 pounds each, or about 40 ft-pounds lubed (you NEVER torque SS dry). However, except for steel-against -steel, there is always question as to whether the pretension will stay (creep). Also, your loads are not this high (remember that just two bolts may carry the load). This is also why we use adhesive sealants to help reduce movement.

I would think about 60% of this is about right (24 ft-pounds).

All of this assumes solid glass, no core, no voids, and backing plates on both sides, larger on the inside. If fender/penny washers are used directly against glass, even extra layup, they MUST be double thickness. Standard washers will just bend under high load.

US Bolts - Tensile Strength and Proof Loads
https://sabreindustrial.com/content/Tech PDFS/003 Bolt Clamp Load Chart.pdf

 

[fredrussell]

Just as an aside, is it not the done thing to connect JSD mounting points to aluminium toe rails, should a boat have them? Very strong part of boat, no?

 

[zoidberg]

Now THAT ^^ is helpful. Thanks, Drew.

The Harken holes ARE ~1/2", of course. The bolts/nuts I've acquired are made by Bumax of Sweden. I'm guided by their UK tech rep. I'll cross-reference to the charts ^^^

I'll take on board the suggestion of an outside plate..... and of adhesive sealants. An epoxy 'for metal' might be suitable.

I'm aware of the issue of pretension - or 'preload' - and hadn't gleaned an opinion, so ~40 ft-lbs 'lubed' is the target.

Considering the circumstances in which this assembly would be put to the test - Max Wave Strike - I'll be well pleased if the only damage is confined to cracked gelcoat. It is also quite unlikely I'll put myself into places where this sort of event could become a regular occurrence. In the words of Miles Smeeton - 'Once Is Enough'.

 

[Stemar]

'Once Is Enough once too many'.

Fixed that for you. The idea of needing such a thing reminds me why my idea of blue water sailing is adventuring out of Lake Solent to (gasp) Weymouth

As an aside, I thought the idea of the series drogue was to ensure that large shock loads weren't transmitted to the hull. Otherwise you could just use a big builders' bag. I'll never use one, but I'm still curious

 

[zoidberg]

Fixed that for you.
As an aside, I thought the idea of the series drogue was to ensure that large shock loads weren't transmitted to the hull. Otherwise you could just use a big builders' bag.

There are occasions when it is desirable to simply 'put the handbrake on'..... or park the boat. In many circumstances, it is enough to 'heave-to' for a while, or simply 'lie ahull'. I've done the former while bumbling across to Baltimore a few years back in a little Hurley, while a F6 - or was it F7? - came through in the night. Don't know for sure, for visibility was about 10 metres, and I snoozed the time away.

On wilder occasions, one wants to keep the boat 'end-on' to prevent being rolled by a big breaking sea - and that's what the JSD was designed to do.

The smaller the boat, the smaller the 'big breaking sea' required to do the dirty deed..... and there are plenty of those to be found in the Western Approaches and Celtic Sea!

 

[B27]

My feeling is that the fastenings on those bits of bling are so close together, they are not going to distribute any force into the GRP effectively.
The GRP between the bolts isn't big enough to add much strength.
More so after it's been mauled about by drills or hole saws.
You might do just as well with a single M16 bolt and eye nut or similar.
A long stainless chain plate would be far more effective at distributing the load into the GRP.

If I was seriously involved, I'd look at the material properties and maybe ask a mech eng person to do a proper analysis, in the absence of that, I'd follow convention and use a shroud plate similar to how 99% of boats spread the rig load into the GRP.

 

[zoidberg]

There are differing opinions, B27, on many things.... and this is one of them. I did ask a competent 'mech eng person' - two, actually - and I'm following the advice that resulted. And I bear in mind that Harken made the things - and still sell them, as do others - for the specific purpose of carrying substantial loads into GRP.

I have looked in detail at Load Rated Lifting Eye Nuts. Have you? If not, try Here
The WLL is a nominal 700kg. and for a vertical pull/lift. That does not satisfy the requirement.....

 

[thinwater]

The "interesting" thing about chain plates is that they are designed to take the load in ONE direction. Thus, a plain chain plate probably is not optimum either. Pretty sure about that. but it is also going to be boat-specific.

This was just to prompt discussion. The angles are exaggerated to make a point.
JSD chain plate design.

Another thing to consider is that if the wave hits from the side or the boat broaches, the load can be from leeward (single leg loading) and there could be a chafe problem if the fitting is not on or very, very near the transom. don't think the mounting location was defined. This is why most mounting mount to the side but extend beyond.

If they are to be on the transom, then some sort of bracket is probably needed on the inside to transfer the load to the sides. Could be laminate, but complicated.

 

[zoidberg]

This was just to prompt discussion. The angles are exaggerated to make a point.
JSD chain plate design.

Another thing to consider is that if the wave hits from the side or the boat broaches, the load can be from leeward (single leg loading) and there could be a chafe problem......

Thanks yet again. I'll look to incorporate some of that thinking.

 

[B27]

There are differing opinions, B27, on many things.... and this is one of them. I did ask a competent 'mech eng person' - two, actually - and I'm following the advice that resulted. And I bear in mind that Harken made the things - and still sell them, as do others - for the specific purpose of carrying substantial loads into GRP.

I have looked in detail at Load Rated Lifting Eye Nuts. Have you? If not, try Here
The WLL is a nominal 700kg. and for a vertical pull/lift. That does not satisfy the requirement.....

I thought you said Harken have no record of these, so how do you know what purpose they are sold for and how they are supposed to be used?

 

[zoidberg]

I thought you said Harken have no record of these, so how do you know what purpose they are sold for and how they are supposed to be used?

I made contact and asked them. So could you....

 

[zoidberg]

This pic is of the fastener layout for the shroud chainplate on the new Pogo 36.

I'm not the only one....

 

[penberth3]

This pic is of the fastener layout for the shroud chainplate on the new Pogo 36.

I'm not the only one....

Refer back to Post #31 from @B27. Those holes are too close together, it's effectively the "stitch-drilling you might do to form a large opening. It will pull out like the perforations on a postage stamp.

 

[justanothersailboat]

Pogo may know something I don't, and their engineers are better than me, but it still looks wrong!

 

[thinwater]

Pogo may know something I don't, and their engineers are better than me, but it still looks wrong!

Not that I know, but here are pics.
a. The bolts are smaller than the heads.
b. We don't know the layup. It is probably quite stout in that area, spreading the load in a semi-circle.

Lagoon uses a very similar pattern.

It is common on composite chainplates for the spread to be in all directions, not just down. Bolts all in a row is not optimum either.

And look at this Lagoon.