how much water can a ski hold?

I mean the inside, the hull, and not free water, but trapped water.  My bow of old glass XT is heavy, heavier than it should be.  Center of weight is more forward than it should be.  The boat has taken a beating in the surf over the years, been unprofessionally patched, leaks on rough days, dry on flat days, cannot figure out where the last leak is.  I empty water out of boat after each paddle, but clearly some is stuck up front.  So, I hang the boat at an angle (bow 3' off ground, stern on ground), top up, plug always in so no rain gets in.  I turn the boat upside down (still at angle), take out stern plug and about a cup of water would come out.  I started this 3-4 months ago, usually emptying every 1-2 days.  I got the most out at first, naturally, but air was hot, so maybe that contributed.  Now, after much water has been drained, I do not get as much out, though the air has cooled, so that may have something to do with less yields.  But every day some water comes out!  How is this possible?  I can see water getting trapped in something in bow, but eventually it should drip out of wherever it is trapped, if held at angle, right?  I am to the point where I think some sort of condensation is happening at night and this will go on forever if I keep at it.  I have not paddled it in the 3-4 months I have been doing this.  My question:  can a boat form some water (~1/4 cup) via condensation?  If so, I will cease this emptying process.  If no, how long could it take to drain all the water out?  Thanks.

Any salt crust or slurry inside the hull will absorb more water (vapour) from the air over time. Try tasting the next lot you collect. If it's salty you will need to thoroughly rinse then dry / ventilate inside of hull. Pretty sure there are some posts on this site about how to do.

Could be that there are some internal bulkheads, and the hole through the bulkhead is part way up. As you move and rotate the ski about, a bit of water makes its way through the hole. 

Very common problem. I have had new skis that have had water ingress into the hull and it can take weeks to dry full if at all, it depends on the make of ski and its construction.
Firstly there is no water caught at the front of your ski, they are not sealed front to back.
Most of the water left inside your ski is caught in the laminate and foam stringers. The laminate is very porous and the polystyrene also soaks up water. Being an old ski it WILL take weeks to somewhat dry out. You need to leave in the sun for the water to evaporate with the heat and then condense when cool in the evening. The Fenn has a drain in the middle as do most so it requires see-sawing daily to drain, or you can drill a 2mm hole up front and leave nose down to drain.
However your main problem is the water getting in initially? A few places with a Fenn.

1. Rudder cable tubes could have come adrift, normally at the back, this will let tons of water in especially as it gets rougher. Or the cable has cut a groove in the plastic tube where the cable comes out at the front, you cannot see this as it is inside.
2. Scuppers can also leak, well actually the bond between the hull and the footwell, this will leak in calm and rough conditions.
3. Split seam, sometimes hard to notice but can also let a lot of water in, more as it gets rougher.
4. Footplate worn through the footwell, hard to spot as the footplate covers the offending hole, this will let in a lot of water with a very small hole in any condition.
5. Rudder post coming adrift, not common on Fenn but can happen if rudder gets hit hard enough, both top and bottom, usually top cracking more common.
6. Obviously holes in the hull or deck, or pinholes, but they are unlikely due to the weight of your ski.

This is what I have learnt from working on them for many years.
Repair? Go through and look at what I have suggested. Tape up what you can see and then do a soapy water check. Mix up a strong solution of soapy water, blow gently into the bung hole and go over the ski with a soapy water brush, bubbles should appear at the offending hole. To check the tubes, remove the rudder and tape up the hole in the hull and fill the rudder well with soapy water and then blow gently, if the tubes are loose it will not fill up, can do the same with the footwell, tape up the scuppers and fill footwell with soapy water.
Hope this helps.

Ranga you are a great source of info on boat repair.  Thank you!

Thank you all!  Great intelligence.  I knew there was no loose water trapped, sloshing around, so now I know some of the material absorbs and holds water.  I would have assumed glue, resin, laminate, etc, would not absorb water, but that was just an assumption.  Same for foam and other stringer material.  Cheap styro-foam boogey boards, for instance, do not absorb water, but maybe they do if left sitting in water for too long.  Seems to me whatever you put inside a boat hull should not absorb water because you know some water is coming in.  That was ultimately my question:  how much water can these absorbing materials hold?  Now I know.  Thank you!  As for fixing leak, the tubes look firmly attached in back, I packed the tops and bottoms of scupper tubes with JB Weld, let dry, drilled slightly smaller holes to re-open.  I assume steel tube does not crack, just may come loose from fiberglass—rudder and scupper tubes.  The footrest would be easy to spot and fix (have not thought of that).  It very well may be a seam as a few areas on seam have busted in some of my wipeouts in surf break.  I’ve patched where obvious.  The soap test is difficult to me.  So I paint or spray soapy water on seams, then go back (the bung is in very back, behind rudder) and blow in gently one full breath.  I cannot see the seams when blowing.  Maybe the bubbles form and stay.  I will continue searching.  Very interesting about internal materials absorbing water.  Maybe one day there will be a ski that cannot possibly absorb any water internally.  Thank you all SO much for this invaluable assistance, very eye opening.

mickeyA wrote: The soap test is difficult to me.  So I paint or spray soapy water on seams, then go back (the bung is in very back, behind rudder) and blow in gently one full breath.  I cannot see the seams when blowing.  Maybe the bubbles form and stay.  I will continue searching. 

   

I tried to pressurize a ski with my lungs and it was not easy.  Ranga obviously does it with compressed air in his shop.    I'm a woodworker by trade, so I had a good head start on the equipment.  I got the tricks from a pro like Ranga.  And it's all about the tricks...  A one man setup is easy and safe.  It's the only way to track down pesky leaks.     

Blowing in with your lungs should work, but holy crap that's a lot of work.  Definitely a two man operation.    

edit:  Quick google search on how much pressure the lungs can exert.  Atmosphere is 14.7, the differential we produce is 2 or so, maybe up to 3 or 4.  I shoot in a bit more than that with my compressor, but not much.  Be sure to chat with a pro for the complete setup.  

Just a few things.
Be very very careful blowing up the ski. That is the fastest way to destroy your ski, as for how much pressure you can blow is immaterial, it's about the surface area you will be worried about. Blow too hard, especially with mechanical means you can seperate the stringer from the laminate, after that the bin is the preferred option. Two people would be the better option or even a hot day, seal up the ski as best as possible then put in the sun and do the soapy water test, you will get quite a lot of air pressure that way.
As for what is inside the ski, not all skis are made alike. Your styrofoam board is strong enough to hold a person on top of it, high density and much heavier than what's inside any ski so that analogy is not relevant. Some makes of ski will be the same weight after 10 years of use and others 2-3kg heavier after just a few years. If you save on the resin and don't use enough you will have a very porous laminate and water loves its natural ability to get EVERYWHERE, capillary action takes water up trees a very long way so a few mm into a laminate is a doddle, now try getting that water out! They have dehumidifiers to get water out of wood and it takes a long time. Some manufacturers use foaming urethane glue to bond stringers into the ski, very bad idea! it expands and goes every where and that is very porous and is literally a sponge, even looks like a sponge!
Some use wood as part of their construction, and would you believe?, they don't spend days sealing the wood and use it as plain milled timber?
So there are many ways to gain weight.

Sure would be interesting to have a full expose on the strengths and weaknesses of the major brands by Ranga but I suspect he has relationships with manufacturers that he'd rather not mess with.

I've owned a Fenn XT and Swordfish S. Both leak like hell, and I've made valiant attempts to plug them. On my SF-S I've found no less than 3 pinhole leaks - 1 around the rudder tube attachment, 1 at the top of the rudder post, and 1 where the scupper goes through the hull.

I love Fenn. Im a huge Fanboy. (or would the be fennboy?). But dammit Fenn, i really really hope their new Fennix boats have less issues with leaks than the old ones.

As I was trying to locate the current leak last night I was shining a light in the hull and I think fenn uses the expanding urethane foam glue that Ranga mentions above (but am not 100% sure).