Installing a toilet in my trailer sailer today so drilled two holes in the bottom of the boat for the inlet and outlet.

The toilet area has an internal liner so expected to punch through maybe two or three millimeters of that and then after possibly an inch and a half of air hit the hull. Started drilling from the inside as it is hard to tell from the outside exactly where the toilet is.

Big mistake. Turns out the area between the internal liner and the hull in those lower areas is full of filler except for a thin balsa core just before the outer skin. The internal liner is horizontal at that point and I thought the hull would pretty much be horizontal at that point as well. Turns out it's not. I also thought that I would simply overdrill the inside liner afterwards so that I could tighten the nut against the inside of the hull.

Below is a core sample that was drilled out and now I have no way to have the flange of the skin fitting sit flush with the outside of the hull. Furthermore the thickness combined with the angle means that the True Design skin fitting doesn't have any thread left to fit a seacock too even if I could get the angle of the flange right.

Not impressed with myself. To make it worse I used a slightly undersize 32mm hole saw and had to file the hole 5 mil or so wider so the obvious solution of epoxying the core sample back in and re-drilling somewhere else may not be easy either (not that that will look particularly attractive on the outside of the boat or the inside for that matter).

I guess the only plus is that I have discovered that the bottom of the boat is pretty damn solid and I also have confirmation that there is no water in the balsa core. (Not quite sure what the purpose of a 12mm light balsa core is when you are just going to load an additional 40 millimeters on top with heavy filler/resin or fibre rich resin because it wasn't easy to drill through). At least it's only one hole as I learned my lesson by the second.

Would appreciate any bright ideas for rectification. Meanwhile I'll be sleeping on it.

If you make a jig. Block of wood drilled on a drill press a drill the hole at 90? to the outside of the hull, can you get the fitting in?

Can really feel your angst. 12mm thick balsa core in that size trailer sailer seems about right. As you have noted the extra filler to get to the internal liner underside level is due to the difference of the external hull deadrise angle / shape and the need to level the floor surface in the loo.

Assuming the skin fitting is not the recessed type, TrueDesign have a long skin fitting as here - surely this would be long enough?

www.theboatwarehouse.com.au/plumbing-sinks-toilets-pumps/plumbing-fittings/trudesign-long-skin-fitting-white/?sku=SAW-450241&gclid=EAIaIQobChMIytjR4L_hgwMVutQWBR1YyQ2GEAQYBCABEgKFUvD_BwE

Assume you have the installation tool

www.theboatwarehouse.com.au/plumbing-sinks-toilets-pumps/plumbing-fittings/trudesign-skin-fitting-installation-tool/

Two suggestions - firstly check the skin fitting is compatible with epoxy high strength filler by phoning Truedesign

A Accept the hole for what it is, insert the long skin fitting using high strength epoxy filler including it forming the taper under the skin fitting head due to the hull angle difference, and then using it or low strength epoxy filler to make a suitable fairing on the external hull surface around the skin fitting head. So the fairing would be fore and aft and start fwd of the skin fitting head and be wider at the not closer to the hull centreline edge of the skin fitting head - so the more outboard edge, and then fair in fore and aft in an aerofoil shape. Inside nut of the skin fitting bearing against the grp laminate of the inner hull as shown in the photo. Internal liner access hole wider.

B Alternatively (as per Tarquin) you could drill the hole again from outside perpendicular to the hull outer surface in both planes, insert the long skin fitting type and fill the gap created around most of the periphery with high strength epoxy filler, and your away. Have to check the Truedesign skin fitting head has sufficient diameter to cover the extra width of the wider hole - the external laminate angle looks about 20deg different to internal - so about 7% extra diameter. The internal laminate elliptical shape hole created would need to have added s glass and epoxy resin of same thickness as the original to attempt to get the internal laminate structural continuity back to where it was- A sufficient round glass laminate patch say 5 times the outer ellipse perimeter extent.

There might be other plans................

I can't see a major issue, you just need to make a fairing block for the inside and use a long enough through hull. Really no different from mounting a sounder transducer to shoot straight down with the exception of only needing one fairing block, the internal one. You could form it with epoxy thickened with glue fibre or go all out and use milled fibre ( glass powder) you can mix it up so it has zero slump, bedded in like that you wouldn't even need the nut on the fitting but it would look nicer. Or you could make a flange fitting and fix it to the in side of the filler material.

I reckon this is one of the jobs where you get down with yourself after you do it, and then after a day or two you just fix the thing up and try again. I would glue and glass the plug back in (epoxy loves a gap). Then grind away the gel coat in a circle about 150 mm diameter around the hole and then I would glass the outside and interior laminates, but it doesn't need to be as beefy as your laminate looks. My 38ft cat has one layer of 600 gm double bias, so two or three circular patches of 400 or 600 gm double bias would be great outside. As long as it isn't near a trailer roller. Then fair with microballons and then some paint or flowcoat. Then you can start again with a nice new holeThe time taken to worry about it is nothing in the whole scheme of things but I have always found it necessary to leave a moment of stupidity for a couple of days before I go and undo my errors.

Why the outlet underwater? A holding tank is a good thing, as well as being mandatory in many areas. Even just a small one which allows you to ensure that you can dry out for the night and use the dunny (with a bucket of water for the intake) and have people swimming around the boat without any worries. I like having a holding tank above the waterline and the exit is just above the waterline. It reduces the number of immersed through hulls - we have two. But on a mono dropping the waste could be an issue when heeled.

I would just grind out the interior lining so you are left with just the outer skin. Make it a decent sized recess so that you are left with plenty of area for the flange. Dig out some of the balsa as well and fair in the recess with expoxy with Aerosil. Mix it so it's the consistency of toothpaste and aim for a thickness of 5mm or so. This will set extremely hard so if sanding is required do it sooner rather than later and wear a mask. It's pretty much what Kankama suggests but easier.
www.ebay.com.au/itm/131022059016

Caution: Aerosil is dangerous stuff to use, don't breathe in the dust, it's more dangerous than asbestos.

Thank you so much for your replies everyone.

Yes thinking time is essential but this has been speeded up by your ideas. In the end I'm going to plug the hole and redrill in a more appropriate area albeit only about two inches away.

I know the std procedure for glassing over holes in fiberglass boats with champhering and a number of layers of ever decreasing circles but I'm not going to go with that.
In the end it is a small hole with an extremely deep plug to be epoxied back in. The area of epoxy contact is therefore large in relation to the hole to be plugged and hence the risk of the plug being bashed out by epoxy bond failure is much much smaller than if I was epoxying in a disc of say five or ten millimeters thickness. It is also keyed in if you like by having hollowed out a bit of balsa core right at the bottom.I'm talking 60 millimeters thickness here for a hole which is no more than 35 millimeters diameter.

I coated the exposed balsa with epoxy with an artist type paint brush and left it for a couple of hours. I have now just epoxied the plug back in with white base Norfill which I have previously used with success to reform a small chunk out of the bottom of the rudder. I even used a syringe to fill the central pilot hole that the hole saw leaves. I thought the filler was thicker than it was plus the gap wider so when I first did it the plug just dropped out but now it is taped at the outside of the hole and the slump is working in my favour because it fills in any areas around the plug that may otherwise have voids. I played it up and down with the tape on the underside to try and eliminate any voids but there is 60 millimetres of the stuff and it is epoxy so small air pocket voids probably aren't a big deal. As always I will make sure that there is an appropriately sized timber plug tied around the sea cock in the unlikely event that it ever gives way.

The point about the area not resting on a roller is a very good one and fortunately I had thought of that before originally drilling.

Thanks again all.

PS. The natural anxiety before drilling a hole in one's boat is a well founded instinct :(.