best layup method if glass is to be tapped with threads?

I've got a water tank being built into the keel of a glass hull, so the top will be fiberglass. The most practical way to do my fittings is to tap pipe threads into the top of the tank (which will be 1/2, 1 inch, and 1.5 inch NPT). I'm wondering if there is any layup better suited for holding threads? Or also if there is any process that will help make the threads better (for instance drilling and tapping with green resin or very well cured resin is easier) ?

I want to stick w/ polyester or vinylester products like the rest of the boat, to permit consistency in the finish w/ gelcoat and such.

Materials on hand are ortho polyester, iso polyester, vinylester resins; hull & deck, poly fair fillers; chopped glass, milled glass, and cabosil additives; mat, 6 ounce cloth, 1708 fiberglass.

My initial thoughts are make a resin rich layup of about 1/2 to 3/4 inch thick of mat with the vinylester, however also thought maybe adding milled glass to the mix may help give the resin some better tapping consistency - no idea really.

Jon

 

Most of our (semi permanent) fittings are assembled with self tapping screws on standard layup meaning a mixture of WR and mat. The trick is to use a none standard drill size. Too small a hole would make the screw hard to drive, stripping the head, too large and the screw pulls out easily. For larger diameter 1/4 to 5/16", we use a special insert that looks like a tapping die in the thread and a flat head that is driven by a hex drive. I add epoxy for insurance. The hole comes threaded either in metric or english. For larger bolts, we use threaded flat inserts with grips on flange, inserted from the reverse side and epoxied.

If it is water tanks, we Tig weld ears into acorn nuts and glass it in from the reverse side. The bolt has to be of exact size otherwise it pushes into the nut breaking the watertightness and leak starts.

 

Can you use tapping plates made from 316 stainless and glass over these?

 

We use that in engine beds but use a lot of compliant resin around the plate. One to absorb the the difference in CTE and two, to ease off the sharp edge.

 

Does the drill size end up being larger or smaller with the fiberglass?

Here's a question I didn't think to ask. When I actually put the fitting onto the tank, I have the option of using plastic bushings versus metal fittings if I want (ie say if I had a 1/2 brass fitting I could simply tap a 3/4 hole and use a plastic 3/4 X 1/2 bushing between the brass and fiberglass). On one hand a plastic part just isn't going to be as strong as say going brass or bronze right into the fiberglass, but on the other hand it might be of benefit getting a seal without stressing my tapped threads since the fiberglass is likely harder than the non-reinforced plastic part.

Jon

 

Drill size is sized according to the tapping screw to be used. You have to do some trials as the ease of screwing will depend on the thickness and the size of the drill bit.

The second part of your post I don't quite understand what you want to achieve. Sorry could not comment.

 

Thanks - yes I will test this out before I go drilling my tank.

The second part put more simply is do you figure a plastic fitting or a metal fitting is better when screwing something into the fiberglass threads?

What I was explaining is for instance, my pickup assemblies are 3/4 NPT and made of brass. I could tap 1 inch NPT in the tank instead of 3/4, and then use a plastic reducer that adapts 3/4 inch to 1 inch. If I did that the plastic threads might seal against the fiberglass better and/or create some cushion, however the downside is with some loss of strength since technically brass and/or FRP are both stronger than regular plastic.

Jon

 

Plastic or metal is better in shear as compared to FRP. Once you insert a thread into FRP, it is shearing the holes and there will be broken bits of glass that will allow fluid to pass thru. It is not a perfect fit.

However, there is a better method. For fittings, even as small as 1/4". we weld it to a metal flange, then drill hole in the perimeter of the flange. The area around the FRP opening is reinforced, as thick as possible so the screw does not penetrate and cause leaks. Use cork gasket on top for water, neoprene for fuel oil. Since there will be regular servicing required, we group the fittings, the sensor, and everything else in the flange. Just like the fuel service flange in an automobile tank. I use machine screws and acorn nut as described in previous post.

Need any drawings?

 

Agreed, you're best off making a flange and attaching or welding the fitting to it, which then gets screwed down or bolted. You can also embed the flange in the laminate, using waxed dowels to keep the holes clear during layup. Naturally as previously suggested, you'll want the supporting laminate hole perimeter thickened to take the strain of the parts, weight, maybe fastener lengths can be sealed up too. Lastly, it wouldn't be the first time I tacked some nuts to sheet stainless, drilled some holes around the perimeter of the sheet stainless, then entombed this in goo and fabric, for a metal bolt down receiver.

 

Hmmm - OK so in short maybe my gut feeling about tapping fiberglass was right, convinced myself otherwise. Need a new plan.

How about this solution? - make a flange for the fittings and bolt it on: I pick a metal which is similar enough to the fittings I would be using (which in my case is probably brass because there happens to be a fitting I will be using which I can only get in brass).. I get a hunk of it that is 1/2 or 3/4 thick, ie for this keel water tank it would end up being about 2 inches wide and a foot long to comfortably hold the 4 fittings I have (2 pickups, vent, fill). I tap my NPT threads/fittings into that and cut holes to match the fittings in the top of the tank. I then bed the flange in 5200 and through bolt it such to make it a permanent part of the tank - will be a nice wide flange so should seal well, threads will be unquestionable since they are metal.

Thanks in advance!

Jon

 

I get what you are doing and thank you very much for this info! I think I'd be tempted to encapsulate a piece of metal into the laminate that covers the span of the flange - putting some thickness of glass under it in the laminate, then drilling and tapping the threads, then sealing them off from the bottom, so you would have from top to bottom - fiberglass, metal with good threads, a buffer of fiberglass giving the end of the bolts something to protrude into, then a cap on the bottom for waterproofing - does that make sense?

I think though for what I'm doing I can as easily just make my flange and bolt it though - interested in what feedback I get there. The reason I want permanent NPT threads on the tank is I'm putting in 2 sets of fittings because I'm not sure which is going to workout better later, the ones I don't use I will put an NPT plug in. Also am making the pickup tubes removable.

One thing to point out is this water tank is in my keel - attaching a quick picture of what the plan sorta looks like - circles drawn on my little glass pieces are where fittings might go. I'm working aft forward as I close it off. Stopped to figure this out. I have perfect access under the top of the tank until I start moving along and close it off - then the only access will be in the front to allow spraying or scrubbing the inside out.

Hopefully this somehow makes a little sense.

Jon

 

I was thinking about this a little more, the simple thing if I'm going to use a flange bolted on might just be use an aluminum and go up one size with a plastic reducer in between. That would eliminate any concern of dis-similar metals or fittings jamming in place.

Jon

 

Is this tank for drinking water? Polyester resin may make it not potable.

 

No it's just for washing down fishing reels and an occasional shower.

Jon

 

Although worth mentioning - that grey stuff in the picture, which the inside of the keel is well coated with as I move along, is VE barrier coat (or thick VE gelcoat whichever way you look at it).

Jon