Mystery Leak

[JimCt]

Somewhere I have a leak in my 170 hull. Leak is signifigant; about 1 gal per hour.  Have determined the deck isn't leaking.  On the trailer I hosed in about 15 gallons of water in the inspection port and shifted the angle of the trailer fore & aft and tilted it side to side some on an embankment and found no drips.  THe bottom has paint on it and visually I've found no suspicious areas.

Thinking about it, if I could somehow pressurize the area between the deck and the hull and get under the hull with a spray bottle of detergent and spray around I might be able to see where the problem is.  To pressurize the area I'd get air down the inspection plate hole with a leaf blower.  I'd seal the blower output nozzle in the inspection port with rags or something.  My question is, can I expect the deck & inner liner to be tight enough to be able to get even a modest pressure built up inside?  The boat is stock and has not had any modifications.  The fuel tank is mounted on deck so there isn't a fuel line opening through the deck.

Does this plan sound reasonable or is there some better way?

[scott_gunn]

Sounds reasonable to me.  Blowers can put out a lot of air!

My guess is going to be the drain plug area.  What other thru-hulls do you have?

[JimCt]

That's the mystery... there are no other thru-hull fittings, not even a bilge pump discharge. Stern drain plug for the bilge is water-tight and I've rebedded the fitting in 5200.  I've filled the bilge area with 15 gallons of water and no drips there.  There are the two stern deck drain scuppers but if they were leaking, they'd pass water to the deck.  Only thing I can figure is that the bottom paint is masking a bad spot somewhere; it's just a case of finding it.  Probably I should strip all the bottom paint (not looking forward to that job) to see if there is any hull damage which was repaired poorly.

[steved]

Hey JimCT - bummer about the leak. To understand, are you getting the leak with the boat just sitting in the water or only under way?  If it's  when you are just sitting, must be below the water line. Your bow eye isn't below the water line is it?  If not, must be a masked crack as you say.  Look carefully at the chines.

In terms of trying to find the leak with a blower, I don't think you are going to get a sufficient seal between the hull and the deck. There is a lot of flex in the deck where it meets the hull under the rub ring. Just coincidentally, I saw one boat bump an Aquasport on the rub rail just this weekend. The whole deck flexed and the rub rail moved visibly so they can't have been fixed as tightly as you would need to get up sufficient pressure.

How about trying the reverse. That is, leave it in the water for a couple of hours, then pull it out without pumping out the water. Put it up on the trailer and dry the exterior carefully. If it got in somewhere, maybe you'll see it seeping out.

[Seadog]

Jim - I'd check all your below deck hoses and connections first before stripping all the paint off.  You might want to go ahead and replace them all while your at it, just to be sure. I'd strip the bottom paint only once you've ruled everything else out.

A gallon an hour is alot of water.  Seems like if the hull were breached as the cause of the leak it would be pretty apparent once you hauled her out, but it doesn't mean it's not the hull, just more likely to be a hose or connection somewhere.

Steves right - don't think there is any way to sufficiently seal up the deck to pressurize the bilge.

[JimCt]

Thanks for the ideas. A gallon an hour is a lot of water...

There are no hoses or thru-hull fittings anywhere.  Only openings in the the hull are one bilge drain (sealed with a pipe plug) and two deck scuppers.  When I put the hose water in the bilge while on the trailer these areas remained bone dry.  I really didn't want to put too much water aboard because of the added weight on the trailer and on the points of support for the hull.

The bow eye is above the waterline by a fair margin.  I was out fishing for about 5 hours Sat. morning.  Seas were such that the bow eye wasn't dunked much as far as I could tell.  But then again, I can see it while I'm standing at the helm.  Good idea.  I'll re-bed the fitting.

Never occurred to me to see if there is a difference in the leak rate underway vs. moored.  I'll try it.

I agree that the likely suspect areas are the chines.  I've done repairs on a Whaler which had hidden cracks at the chine areas; difficult to see under a layer of bottom paint.

Really appreciate the feedback!

Side item: When out trolling Sat. I used my nifty little eBay 6 hp kicker.  Worked out perfectly.  At WOT, the 6 pushed us along at 2-3 kts trolling 2 wires and umbrellas.  Getting out & back used the Merc.  Total fuel used: 6 gallons.

[JimCt]

OOPS!  Meant to say I can't see the bow eye while underway.

[Seadog]

Do you have a bait well?

[JimCt]

No bait well.

[steved]

Another stupid thought. Check out your fresh water tank.

[JimCt]

No fresh water tank.

This boat is very basic.  No tanks, hoses, bilge pumps, in-deck storage or thru hulls.

[Argo]

Check the bolts holes that mount your motor to the boat.  They could be leaking around them.

[JimCt]

Argo,  I've also had my eye on those bolts.  If they were leaking wouldn't the leak come through to the inboard side of the transom and be visible around the bolt heads/washers?  Anyway, the leak would be above the deck level.  Motor bolts have 5200 excess squeeze material around the washers so it appears that the mounting was done properly.

Since I can't see any obvious damage to the bottom, I'm beginning to come around to steved's thought: small stress cracks in the chine area(s) and possibly around the bottom strakes.

Think maybe I should pick up a can of Peel-Away, strip the paint off in those areas, & see what I can see.

[Bluesbrother]

Just a thought, I haven't seen the inside of a 17 ftr but if the transom os similar to the 19.6, there is the possibility of delamination either inside or outside arround the engine bolts. If it (delam) is far enough to get to the bottom of the hull on the outside, water could get into your bilge underneath your transom since my transom had space between it and the hull laminate. If you have a delam. on the inside bolts water could come in right below your deck. The delam can be near impossible to see, however you should be able to hear it if you scratch the area with a solid object( would sound more hollow). Good luck.

[JimCt]

Thanks for the note Bluesbrother.

I'll get out my sounding mallet tonight and see if I can hear some hollow spots in the transom.  Everything looks good in that area, but looks ain't everything.

Maybe sounding would also work for finding a hull crack?  My guess is that there'd be a change in the hull resonance in the area where it is cracked.  That would be a whole lot easier than stripping the bottom.