How to Fabricate a Deck Filler Cap

Summary: Dave constructs a deck filler cap from a Plaster of Paris mold and epoxy resin aboard Al Hoceima - a French-built steel Petit Prince ketch for which replacements can be hard to find. 


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“Al Hoceima” has two large water tanks, one to port and the other to starboard. Each has its own stainless steel deck fitting to fill, and both can be filled from either if the shutoff valves between the tanks are opened.

While filling the starboard tank a few years ago, the deck cap for the starboard water tank’s deck fitting was lost overboard and couldn’t be retrieved.

I searched high and low for a replacement deck cap, but to no avail – we just couldn’t source one that fit. And the thought of ripping apart all the interior cabinetry to get at the deck fitting to replace it with a new one was extremely unappealing.

I decided to take a shot at building my own, using the port deck cap as a mode. The idea was to use Plaster of Paris to make a mold of the old one, and then use thickened epoxy to make the copy.

To make the mold, I coated the port deck cap with oil and then some white lithium grease in hopes that it would release from the plaster. Then I filled a bowl with the Plaster of Paris mixture and set the deck cap into it so that the top of the cap was just level with plaster.

After it hardened, I was able to unscrew the cap very carefully from the plaster (the grease and oil worked!). I made three or four castings this way, and chose the two with the cleanest threads for the next stage.

I then mixed some WEST System epoxy (the fast setting type), and after coating the molds with the unthickened stuff, I added the thickening agent to the epoxy and topped the molds up to the top.

After letting them set up over night, it was time to remove the new parts from the molds. I figured I could just crack the plaster off and be done, but it was far from being that simple. It turned out that the epoxy had penetrated the plaster, making the threads (the most important part) a solid mass of partially epoxy-impregnated plaster.



Top of the new part - epoxy/plaster still attached.

Bottom of the new part - epoxy/plaster still attached.

I picked the better looking of the two molds and after a few hours of cleaning up the threads with a knife and a Dremel rotary tool, I got all the plaster removed from the threads, and now had the new deck cap fully extricated. And it actually fit! Very nice.

All that was left to do was to drill two holes (not all the way through it though!) for the deck key to fit it, paint the top of the cap white to match the deck, and attach a lanyard to it so it wouldn’t go overboard again.

The new deck cap has been working nicely for several seasons now.

If I were to do it again, I’d try coating the plaster mold with a thin coat of varnish or maybe nail polish and letting it dry before adding the epoxy. Maybe that would prevent the epoxy from penetrating the plaster and causing problems in extracting the new part.


The new part after a few years' use.



© Copyright 2008 David S. Malar and Angelika Jardine. All rights reserved.
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