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Question on how to do a leak test

Alan Glos

Active Member
I have performed a few leak tests on Sunfish and Lasers by blowing air into the interior of the hull via the drain hole and then covering the hull with soapy water and looking for the telltale bubbles. Last summer I used a shop vac (in blower mode) and was initially alarmed with the hull expanded somewhat (no damage, but I think I put a little too much air in the interior!)

How do some of you do leak tests. What device do you use to blow air into the interior of the hull and what p.s.i. level is safe and effective. I hear horror stories about people blowing the decks off the hulls, and I want to avoid such an outcome.

Also, where do folks find the most leaks? So far I have found leaks in dings that penetraate the skin down to the fiberglass mat, in and ariound the mast hole (especially the bottom of the mast hole where the butt of the mast chafes through the fiberglass tube), the daggerboard trunk near the bottom of the hull and along the hull topside and the deck joint under the metal trim. Are any other areas common leak sites?

Alan Glos
Cazenovia, NY


No real PSI level but we do use a household type canister vacuum after, like you said, a good shop vacuum kind of ballooned up the hull and we were afraid we might break loose some of the interior foam blocks.
Outside of what you listed the only other points we found leaks were obvious damage points on the keel (there's really no support inside the hull for the keel) and at the bailer location.
While a lot of boats have "cracking" around the rear of the cockpit on the deck we found they were always just in the gel coat and never found a leak in that area.
I remember the first time we did a hull check years and years ago as novices and we went crazy as we kept hearing an air leak. But soapy water all over the boat didn't show anything. A more seasoned sailer pointed out the weep hole in the front cockpit wall under the front deck lip that keep the hull from "pressurizing" when left in the sun...LOL


According to the Team Vanguard factory instructions :rolleyes: ( http://www.teamvanguard.com/2007/Boats/Sunfish/Rigging/repairs.htm#rigging2 ) the test is done with no more than 3 oz. of pressure – that’s 0.1875 psi – or about what it takes to lightly inflate a Mylar party balloon or a latex…. glove.

The safest contraption I’ve heard used over the years consisted of a rubber test tube stopper the size of the drain hole – the sort with a hole in the center - with a brass hose fitting stuck in it, a length of hose with a brass “T” fitting somewhere along the length, and a manual tire pump. On the “T” fitting is tied a latex umm… errr… balloon.:eek:

Most of the equipment comes from the pet store or hobby shop cheap.

Pump until the balloon inflates a little, add air if it gets droopy. :p

Common leak points I’ve encountered – lower dagger board slot corners, deck seam, upper and lower rudder plate screws (old style), bailer hole, mast tube top seam. Less common – other hardware screws, mast tube bottom (when a mast has no end cap and does the usual damage), dings that go unrepaired.

Alan, on my old boat I used a piece of electrical or duct tape over the weep hole in the cockpit wall. Use a regular bicycle tire pump with a needle attachment that you would use for a basketball/soccer ball. Push it right through the tape for a good seal.


Where were you Scott all those times we were trying to seal something to the damn deck drain plug...LOL
Great idea.
I tried drying the water out my boat with a leaf blower once. Several pops later and my foam blocks were all disconnected. Since, I have reset or removed/replaced the blocks. Niether job is very fun.

Be carefull too much pressure can really be bad.


Good ideas and warnings.

I've got a couple of "air mattress" cylinder pumps that come with various funnel-shaped adaptors. One is just a small, 8 to 10 inch 2" diameter thing that would take a fortnight to inflate anything larger that a periwinkle's heart (not that I plan on doing that, so no need to drop a dime to PETA).


If the boat is tight you can check by taping off the vent hole and simply blowing into the drain hole with your lungs. It doesn't take much to find a leak.
This week I watched some engineers at work leak test a 1000 gal. fuel tank on a barge and blow it up. The wing-nuts reasoned if the gauge isn't moving you need more pressure. Anyway a few to many psi will bend welded 1/2 inch steel plate and angle. Pretty cool to see. Not much fun to fix. I'm guessing the welds were good to go the first time.


New Member
I haven't tried this, but you could put the bare hull into shallow water without the rigging. While in the water, have somebody sit on the hull while you apply a soap solution with a brush to the suspect areas. The pressure from the displaced water—plus the weight of the helper—might produce sufficient internal pressure to check for leaks.

If testing the daggerboard trunk at the bottom (inverted), remove any port covers.

(Low-tech, my specialty). :D