Fit a NASA log skin fitting safely

I am a huge fan of NASA. They are a small, British company, who make cracking equipment and an affordable rate from an industrial estate in Stevenage. They always provide very good, pleasant and common sense customer service. My boat, Triola, a 30 foot 1970s Albin Ballad, is crammed with NASAs electronics, including their Duet (Log and Depth), their GPS repeater and their gas alarm, all of which have worked faultlessly, at least, until now.

Triola had been out of the water for a year whilst she had a bit of a refit (including fixing her mast truss and a crack in the front of her keel), finally the day came to launch her at Chatham marina. Once she was in the drink, after joking with the marina staff I had better check she was not sinking, I did the usual frantic leap around below decks to check if any water was making its way in. I noted a little from the stern gland which was fine, however, also some coming from forward! I tasted the water and it was salty. Cursing my luck, I traced the water seeping in to the back of the plastic through hull that was supplied with my NASA log.

I considered my options at this point. I could try and do the skin fitting nut up some more potentially sealing any leak… however tempting, it was too dangerous without checking the outside of the fitting. I pointed out to the marina staff, we were, in fact, sinking (if only a little bit) and would the staff mind awfully whipping her back out again.

Once she was back out again, I tugged on the skin fitting inside a little, and to my alarm, it came away from the hull in my hand!

Broken NASA skin fitting

Broken NASA skin fitting

The outside flange of the plastic fitting had snapped off it would appear some time ago, all that was holding it in place was the bead of Sikaflex, and I had managed to miss it on my usual checks. I thanked my lucky stars it had gone now and not when I was at sea.

I retired to the local cafe for some lunch and some teeth sucking at the removed through hull of the log. I called up NASA and asked if I could purchase the skin fitting from a local chandlers, and that surely there must be a bronze version you could buy at a premium. As ever, the NASA staff were very helpful and suggested the best place to buy it was direct from NASA (for the princely sum of £10).

New NASA log skin fitting

New NASA log skin fitting

Now, the Swedes that built Triola in the 1970s had made her of strong stuff, 2cm layup all around, with good quality integral through hulls that are just about bullet proof and will last until the day of judgement. Said Swede must be turning in his grave that these crazy British yachtsmen are drilling holes in their creations and putting in skin fittings made of cheese. I pointed this out to the helpful chap at NASA who said they had been made this way for the last 25 years.

They had, previously, had so many complaints of these skin fittings failing in this way that they had changed the fitting instructions just in case said skin fitting (made of cheese) decided to part company with its flange.

So what is the correct way of fitting these?

My old busted skin fitting had been bedded on Sikaflex 292, which is actually polyurethane based. The suggestion from NASA was that this could have helped expedite the failure of the skin fitting, as their instructions clearly state to use a Silicon based sealant. This is presumably because the polyurethane will weaken the plastic in the same way a sticker on a motorcyclists helmet can leech the strength from that. When I popped into the Pirates Cave chandlers and purchased said Silicon sealant, Nathan, my man in the know there, pointed out it wasn’t really designed for below the waterline.

Silicon sealant

Silicon sealant

I shrugged and referred him to NASAs fitting instructions.

So I got to work tidying up the hole, removing the last of the Sikaflex. I abraded the inside and outside surfaces and washed them carefully with Acetone (don’t ever use Aceton directly on Silicon, as it will dissolve it).

Outside of the hole tidied up and abraided

Outside of the hole tidied up and abraded

Inside of the hole tidied up

Inside of the hole tidied up

The chaps at NASA suggest only doing the nut up finger tight, as over tightening with tools can place undue pressure on that flange that tried to sink us. A satisfying scludge of silicon squeezed out around the edges as I did the skin fitting up on the inside. I tidied up the inside and outside as best I could.

Liberal amounts of silicon added

Liberal amounts of silicon added to the flange

Through hull inserted

Through hull inserted

Nut done up finger tight

Nut done up finger tight

Silicon tidied up on the outside

Silicon tidied up on the outside

The final stage of the process is to fibreglass in the inside of the skin fitting in, so that if the eminently thinkable happens, and that outside flange does come off, we will not be taking tea with the fishes. I did this by employing my vat of WEST SYSTEM epoxy resin (it has better secondary bonding properties than polyester resin to ‘glue’ itself to the old polyester hull). First of all I wetted the area out in un-thickened epoxy to ensure no dry patches. I next then thickened the epoxy with 406 filler (Colloidal Silica) and added that to create a fillet.

Epoxy fillet created

Epoxy fillet created

I made up another batch of un-thickened epoxy and added a couple of layers of very fine (300gm) biaxial fibreglass cloth creating a little glass fibre volcano in my bilge.

Fibreglass added

Fibreglass added

I’m not desperately pleased with this fudge as when that flange on the outside does give way, it means grinding all this GRP out, and we all know how much of a fun, tidy and non itchy job that is.

Is this a fuss about nothing?

If you do a little digging around the PBO reader to reader forums you will find many horror stories of these through hulls failing at sea. I find it vexing that a great marine electronics company like NASA would not offer a bronze or a modern composite equivalent. I judge myself lucky it went as we were lifted in, it might have been a far more exciting failure had it happened at sea…