Comments from the Yahoo J-27 Group concerning water ending up in the bilge ... where does it come from? How do you find the source for sure?.

There are many sources possible for the intrusion of water into the boat, water which most commonly shows itself in the bilge:
It might come in at the top of the mast. Many (although not all) are open at the top.

There may be a bad seal of the mast at the deck.

Check the cockpit drain hoses. Because of both bad fit on the ends and old, cracked material, they may be leaking.

When the hull is put on land (eg. for winter) it can flex and cause leaks around windows, although stains will usually show up.

Poorly caulked handrails, chain plates, sail tracks and other deck hardware are possible sources.

Transducers mounted through the hull or add-ons such as threaded keel plugs may also leak or seep water.

Icing during winter storage (especially on boats not regularly checked) can cause cracks in scupper hoses, or even blockage of scupper hoses and resultant overflow of the cockpit.

So how does one locate the exact source of water on any given boat?

Probably the best explanation of a means to find the water seeping in was given by Ian (owner of Stranger) who said:

The best way to find leaks is this:

Get a big squirrel cage blower fan, like a 2-3k CFm fan, and mount a
piece of plywood to the fan exhaust that is large enough to cover the
forehatch, with a hole in the center for air flow. Then plug any
large holes in the boat with rags or plastic or something. Get a
spray bottle of 30% dishsoap and 70% water. Put the fan on top of the
forehatch, turn it on, and start spraying suspect areas with the soapy
water solution. You will see air leaking out of the boat creating
soap bubbles wherever there is a leak.

This method works great and is the BEST way to find leaks. I have
done this multiple times and it works great. You'll find every leak
on the deck. It will find leaks in the toerail, deck fittings,
scuppers, whatever. There should be enough air pressurizing the
interior of the boat that when you open the companionway you hear the
air rushing out and can feel the positive air pressure. If you can't
feel the air pressure then you either have a very large leak, too
small a fan, or have not plugged all the large holes.