When Old Duck came into my possession she was not in the best shape. The first thing on my ‘To-Do’ list was to fix all the boat leaks – if any.

Many will agree that there’s no such thing as a leak-proof small yacht, but each of my previous boats had interiors dry as a bone and my intention was that Old Duck should follow that trend.

While spending one afternoon sipping coffee inside the small cabin as a thunderstorm raged outside, I was more than pleased to discover that Old Duck did not leak like a sieve.

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However, there were few minor leaks on two of the fixed ports and a very annoying drip from both mushroom vents located on the cabin top.

The leaks from fixed ports were quickly solved. I was already planning to replace them with opening ports in a near future, so instead of removing and re-bedding them I temporary eliminated the leaks by applying new caulking from outside. Two sailing seasons later, the ‘temporary’ caulking is still holding well.

As for the two leaking mushroom vents, my plan was to replace them by building and installing two dorade boxes over their location. However, building a pair of dorade boxes cannot be done in an afternoon so I was pressed to find some kind of short-term solution.

The original plastic mushroom vents were causing some annoying drips into the cabin, and were beyond restoration

This needed to eliminate the leaks and provide ventilation while I managed to design and make my dorade boxes.

Going through my stash of new and used boat parts, I located two brand new mushroom vents of the same dimensions.

They were made from plastic and not really of high quality, but I figured they’d do for a few sailing seasons.

Also, I wanted to test an idea I had for some time – how to create a leak-proof mushroom vent. The idea was to avoid any screws or bolts going through the cabin roof as I consider that to be the source of 99% of leaks. My solution was to use a teak ring on which to mount the vent, and a powerful adhesive sealant to hold the ring to the deck.

Removing the old mushroom vents

From a piece of teak plank about 1in thick I fabricated two rings with inside diameters of 4in, which was the inside diameter of vents. The outside diameters were made a bit larger than the diameter of the vent bases.

Then, one sunny afternoon, I prepared my tools and on the way to the yacht club I stopped in at the chandlery and got some white Sikaflex 291.

With the old vent removed the hole needed cleaning up and old bolt holes would have to be filled

Removing the old vents was not too difficult. There was no rust, but the old plastic material was pitted and cracked. This meant I had to work carefully as I didn’t want to break them.

I could see when originally installed they were of good quality – all the fastenings were bronze. After removing the two of them I had the more difficult task of scraping off the old caulking and then cleaning the surface with acetone.

Each vent was fastened to the cabin top with six machine screws – which of course meant 12 holes through the deck.

So I used some duct tape stuck to the roof inside the cabin to close off the bottoms of the screw holes, then mixed up some thick, quick-setting epoxy and filled all 12 holes.

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Measuring the teak

Each vent received a teak ‘collar’ which had an internal diameter for a snug fit of the airway, and a slightly larger diameter than the outside flange.

Sikaflex 291 (shown green for clarity, but available in white, grey or black)

You can see in the diagram (above) how the fixing screws only go into the teak, and not into the cabin top itself. The Sikaflex also takes up any irregularities in the deck, and is particularly useful on curved surfaces.

While I was waiting for epoxy to cure, I inserted the body of the mushroom vent into the teak ring and using a pencil lightly scribed the contour of the vent base onto the teak ring.

Vent base – without screws – glued in place with Sikaflex

I did that for both rings and vents. I used a painter’s masking tape to protect the top surface of the teak rings which would fall outside of the vent base, then applied generous amounts of Sikaflex to the mating surfaces of vent base and then mounted the teak ring to the vent base.

When fixing the vent to the ring I made sure there was a layer of Sikaflex left between the vent and the ring. In other words, you should not press or screw down so hard as to drive the sealant completely out.

Now was the perfect time for my afternoon coffee, and an hour later I positioned the teak rings complete with vents over the cabin top vent holes and then lightly scribed the outside contours with a pencil.

The cabin top outside the scribed circles was then protected with masking tape. I applied Sikaflex to the bottom of the teak ring and carefully placed the ring/vent assembly in place, again not pushing too hard. The Sikaflex takes up any irregularities in the deck, and is particularly useful on curved deck surfaces like this.

After repeating the same procedure for both vents, I went inside the cabin and cleaned off the excess sealant which had been squeezed into the cabin round the edges of the vent holes.

Ban the bug

The mushroom vents I used came with a screen against bugs, but the holes were too big to stop mosquitoes. So I installed a piece of mosquito netting inside each vent in order to reduce the number of thirsty little beasts invading the cabin.

The next day I installed the vent top parts and screws used to operate the mushroom. The Sikaflex is holding very well and is now in its second season.

I was planning to use small brass screws to fix the vents onto the teak rings, but so far I haven’t actually done so. Everything has bonded tightly together and, of course, there are no leaks!

Holes in the bug screen were too large to keep mosquitos out, so Zoran added a second, finer mesh layer

The Sikaflex I used is offered in a choice black, white or grey colours to suit your boat, but you could also use some other types of sealant-adhesive such as 3M’s 5200.

Considering that I’ll eventually be removing those vents again, I opted for Sikaflex 291 mainly because in the past I’ve had a much easier time removing 291 than 5200.

Also, you must consider the type of materials you are trying to bond together – but that’s a subject on its own and it has been written about many times by people who know far more than I do.

So here you have it – a fairly simple way to install a mushroom vent and be sure there’ll be no leaks!

As a final tip, don’t forget to remove the masking tape that caught the excess sealant while the sealant is still wet. It makes a powerful bond when cured.

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This feature appeared in the January 2022 edition of Practical Boat Owner. For more articles like this, including DIY, money-saving advice, great boat projects, expert tips and ways to improve your boat’s performance, take out a magazine subscription to Britain’s best-selling boating magazine.

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