The one disappointment I have with my 2005 Mk11, with a Charleston Furling mast, is the constant water in the bilge. I have read numerous theories in this forum on possible causes and solutions.
If you have a look at the cross section of the extrusion (in the Catalinas 36 Furling Mast Manual available from Charleston Spars) you will see it comprises two cavities.
A relatively small amount of water can enter the forward cavity from halyard access points however the main entry point is the aft cavity through the sail furling slot which runs the length of the mast above the furling screw.
I guess a small amout may enter at the masthead where there is a cover plate held by 6 screws.

Anyway the advice I received from Jim Kulibert at Charleston is to install a water dam in both cavities below the furling screw as follows:

[QUOTE][I]As you know there are two places for a water dam; forward and aft cavities. The easiest is to pull the mast and insert from the bottom. What you are doing is creating a water dam and drilling a limber hole just above it. We use two pieces of ¼” sheet PVC plastic about 1.5 inches apart that loosely fit the inside of the cavities and fill the center space with expanding foam. You will have to use bolts to hold them spaced apart and rigid.

If you don’t want to pull the mast, you can drill a 3/8” or ½” hole and 6 – 8 inches above the other one, push a balloon in the holes, inflate the balloons and with another hole in the center squirt the foam. A limber hole will also have to be drilled above the foam. It is sometimes difficult to know where the foam has stopped[/I][/QUOTE]
My comment on this approach is that it is rather difficult to do with the mast in the boat and somewhat imprecise using two balloons to act as stoppers for foam injected into holes drilled into the mast as Jim has pointred out, Needless to say I havent done this.The option of putting dams in place with the mast out of the boat seems reasonable.

I might also add here that some time back - John Reiman posted his approach to solving the the problem as follows:

[QUOTE][I]I am the one who installed a dam. To do this, you have to remove the furling drum. This is an easy operation and is pretty intuitive. One detail though: There's one part that you have to make SURE doesn't fall down when the drum is removed. I have a few photos that I'll be happy to send to anybody who feels they can use them. (Anyway, it's good to know how to remove the drum to service once a year or so.)

With the drum removed, just rough cut a piece of styrofoam and push in down below the drum. I then cut a piece of plastic to lay on top of the foam and caulked around the edges. I think a small hand-held tube of caulking would be better - easier to reach in there with. Also, put the foam in on an angle - downward slope towards wherever you plan to drill the weep hole. Then drill a little weep hole just above the lowest part of the dam.

This helped, but it didn't completely solve my problem because the channel in the front of the mast through which the wires ran also send water down into the bilge.

I just had my mast replaced and the new one comes with a dam in both parts of it. They also have a clever way of running the wire with an "s" curve to prevent water from running down the wire.[/I][/QUOTE]As John notes and as shown in the Charleston Manual - there is a dam now installed in the newer mast sections.

Hope this helps.

PS. Can someone help me - I am not sure how to get the furling screw (drum) out of the mast. Has any one done this?