Tuesday, 12 February 2013

NASA Clipper Duet Depth Transducer, Part 2

Hint: top cap screws off (with heat and a hammer).
We ran into a couple of new “knowledge gaps” with the removal of the old transducer and installation of the new one.  A guy from the boatyard told us to dig out the old seals around the through-hull fitting and remove the whole thing.  After puzzling over this for awhile, Patrick called the NASA Marine Instruments guys, who said, “whatever you do, don’t remove the through-hull fitting.”  The fitting is made to go in once, and if you need to change the transducer, you simply unscrew the top black cap and remove the transducer.   

Sounds simple, but the black cap had also been sealed down and was not going to come off without a struggle.  After several muscle-straining attempts, Patrick came up with the brilliant idea of heating the cap with a hairdryer and gently tapping the sides of the cap with a hammer.  That did the trick.

Now the question was how to put it all back together again, where the biggest mystery was what kind of sealant to use.  Boating forum searches and a call to Guru Bob suggested that two types of sealant would be needed: one that makes a water-tight seal but without bonding pieces together for eternity, and one that bonds for eternity or until you have a go at it with an ice-pick.  Those two sealants are, respectively, mastic silicone and sikaflex 291.  The diagram shows a cross-section of the through-hull fitting with the transducer inside (gray) and where we applied the sealants.

Cross-section of transducer in through-hull fitting.

Old transducer out; Clean through-hull cap for re-use.
After the old transducer was removed, all traces of the old sealant were removed from the through-hull fitting and everything was cleaned.  The sides of the new transducer head were then loaded with mastic silicone.  Patrick opted for a tight spiral pattern, squeezing out mastic like toothpaste around and around until the surface was covered.  The transducer was then passed through the through-hull fitting FROM THE OUTSIDE.  There seems to be some debate about this, but we chose this method because it makes more sense to have any excess on the outside (water side) than on the inside.  When you do this, the excess oozes out around the transducer sides, but once it dries, you just remove the excess with a cutter.  This gives a water-tight bond between the transducer head and the through-hull fitting.

New transducer in place and excess mastic silicone cut away.
For good measure, Patrick added a small bead of Sika 291 to bond the top of the transducer head to the bottom through-hull cap.  In principle, this can be removed with a razor or scraper.  If we ever need to change the transducer again, I anticipate lots of swearing and perhaps a bit of blood-loss as a result of this step.

Next, the top cap of the through-hull fitting was screwed down onto the bottom cap.  Patrick chose not to put any mastic silicone on the threads as our predecessors had done, but instead put a layer around the outside (clear) to fill the gap between the top and bottom through-hull caps.  That should be easier to scrape off later if needed.  Finally, a new bead of 291 (white) was run around the bottom of the cap to reinforce the bond between the hull and through-hull fitting.  The final touch (a special feature for spatially-challenged Patrick) was to add a red arrow to indicate the direction for unscrewing.

New transducer in through-hull fitting.
After re-running and connecting the cables, Patrick had the courage to test it again to make sure it was clicking, and proudly reported that it was.  He reported all these steps to me while I was lounging around on a white sand beach in the 84° F / 29° C Florida sun, reminding myself of why I wanted to live on a sailboat in the first place.