Buoyancy Test


When measuring a boat and testing the buoyancy, do the tanks have to be sealed from each other?

Have tested buoyancy on my boat by blowing air into tank and waiting. Pressure drops after half a minute. I can't hear any air coming outside the hull or in the cockpit so think it is leaking into next tank. Would this pass a buoyancy test?

Many thanks

soapysails's picture

Jon, frankly I'm of no help, but noticed your post over the past few days. I'm sure Gunnter or one of the others will get back to you soon.

If it's leaking into another tank, you should find an adjacent tank which behaves the same way. If neither of them have the same issue, the leak must be direct to the outside and should be easier to fix. If it turns out that two tanks are affected, you'll then know roughly where the leak is, and you may be able to fix it by tipping the boat to a variety of different angles and pouring small amounts of epoxy in, allowing it to gather along a corner-edge where it will solidify (assuming you can't reach it easily via an inspection hatch - if you can, you may be able to put water in and find the actual spot where the hole is).

sail_and_oar's picture

I've never done a bouyancy test on my boat but from time to time I find water in the tanks which shouldn't have been able to get there. On one occasion I found a white liquid running out of the aft bouyancy tank into the cockpit. It turned out to be a milk carton I'd stowed there which had leaked.

1 The chines

The part most vulnerable to damage is where the chine rests on the ground when the boat is beached and heeled over. I have also had leaks from the area below the rudder pintle (by the skeg) and about 18 inches back from the transom bow. All these showed very obvious cracking along the paint covering the fibreglass tape. A good rake out with a hot air gun and scraper, some careful sanding and a localised retape and paint sorted the problem.

2 Decking tops

The side decks take a terrible beating from backsides landing on them every time you tack. Remember these are only taped on the outside as the deck is the last piece of the box to go on and there is no (straightforward) way to tape the inside. It's a bit tricky taping these where the fibreglass has to fit around the shroud blocks. This situation shows up when a boat which is usually watertight mysteriously gets water in the side tank when sailing in the rain or when there is a lot of spray flying around.

3 Cockpit well sides

Trailers do more damage than any other single cause. The area immediately above where the boat bears on the trailer is vulnerable to damage but any point where the cockpit well sides meet the floor could leak. A bit of probing with a soft scraper (corner of a credit card etc) may help identify the iffy bits.

Water with a little washing up liquid is really great for finding leaks. Put it in a trigger spray and get a friend to blow into a hose stuck in the bung hole. spray every seam you can get to. Any leak should blow bubbles.

If after all this you still havn't found any leaks it may be leaking into the next tank. One way to identify this would be to connect a pressure gauge to the bung hole of the adjacent tank. The sort of gauge I have in mind is the very simple "water in a pipe" type as used by plumbers for testing drains. Here is a picture of one


It is a very simple plastic tube which you half fill with water. If there is a leak between buoyancy tanks it will blow the water out of the tube with mouth pressure. If you are feeling creative you could make one out of a piece of clear plastic pipe.

Let us know how you get on. If you are unsure about repair methods there are guys here who have been there. We can tell you what we did and how well it worked.


PuffinInTegel's picture

buoyancy test ...
Buoyancy test
I think the picture was taken during a Mirror World championship event in Monnikendam, Netherlands many moons ago.