The drawing here: https://sailboatdata.com/sailboat/ds-20/ makes it look like the bilge is not going to drain anywhere. It will need be pumped overboard. Same for the cockpit. Both are below the waterline. Your description of the setup seems wrong. If" the cockpit drains into the bilge, and the bilge outlet drains into the cockpit near on the port side near the stern" you have created a loop that doesn't drain anything, with no mention of the pump(s) needed to get anything over the side.

For the compression post question it might be a good idea to look for pictures of DS20's that are for sale online. They will show pictures of the cabin so you can see what may be supposed to be there. If none of them have a post, you have an answer, though a compression post usually makes sense.

 

I was thinking the same thing about the loop. I was unclear in my description - the cockpit drains to the large bilge area below, which is separated from the cabin by a plywood bulkhead. This has a drainage hole at its base, so it drains into the bulkhead beneath the companionway step/sink. This area has the non functioning bilge pump, whose hose routes to the cockpit outlet near the stern. It also has a hole at its base, which allows water to drain to the cabin sole if the pump doesn’t work. This seems like a poor layout and has led to soggy bulkheads when rainwater collects in the cockpit and inevitably drains below. When installing a new bilge pump I wonder if I should locate it in the main bulge area and close up the drain holes, or keep it beneath the companionway. It is more accessible there, but necessitates the connection between bilge and cabin. In any case, I am thinking that moving the bilge pump outlet to the outboard well would make more sense. This is the companionway bulkhead with the old bilge pump:

Great idea about searching for other DS20s for reference photos. I was able to find two, one with a compression post and one without!

I took some closer pics of the area beneath the mast step. I can’t see any evidence of a post having been there, other than what could be a single bolt hole through the reinforced gusset area, which is about 6 inches forwards of the mast plate bolts. The circle and other small screw holes are from the old lights I removed, which were directly below the mast.

The boat was sailed this way by the previous owner. Even if it the original design doesn’t use the compression post, I wonder if adding one, or a nice arch like AllThumbs, would be a worthwhile addition.
Today I have stripped out the interior for a coat of paint and sanded down the tiller handle for a new finish. Thanks!

 

A compression post is not a bad idea. The original Mariners used an arch, later variants use a compression post. Rot in either the arch or at the bottom of the compression post is always a not-fun repair. Something also to note is whether or not you have a transverse bulkhead at the chainplates (can't tell from your photo). The Mariner has swept back shrouds, and there is no transverse bulkhead at the chainplates. As a result, in heavy winds, the boat actually compresses slightly, leaving the lee shroud looser than I would like. The racers found this out years ago, and inserted a compression rod across the cabin between the chain plates to hold shroud tensions correct.

I hate learning things the hard way.
Fred W
Stuart Mariner 19 #4133 Sweet P

 

I think I will put one in and try fastening to the hole in the arch. I have a nice piece of solid oak I can make work for the purpose.
Before that, I have to deal with a soggy bulkhead - the one separating the bilge from the fuel compartment and engine well. Also, the plywood core of transom of the engine well is rotted and will need to be replaced. I have dug out most of the rot from below and could insert a new pre-epoxied piece from below, but the edge is irregular and inaccessible. I suppose the other option is to cut open an access from the exterior. It will be a bit longer before launch…
Hole cut in bulkhead, looking into fuel locker and stern:

Opening beneath engine well where old transom disintegrated:

 

Looks like it should keep you busy for a while. Good luck!

 

In working on the bulkhead repair I got a better sense of the cockpit drain situation. I was indeed wrong - it appears to drain into the waste tank through a T at the sink drain. This seems even worse, as the cockpit volume is easily greater than the waste tank. My thinking now is that it would be better for the cockpit to drain to a sump bucket in the bilge. This can have a manual pump or a second switched bulged pump. The main bilge pump will sit in the bilge rather than under the sink in the cabin beneath the companionway. I will then close up

all of the bulkhead drain holes when I re-do the tabbing so that the cabin is isolated from the bilge.

 

Cockpit drains most often gravity drain to the outside.

 

Assumes the cockpit floor is several inches above water line - tain't necessarily so in small boats. I won't buy a small sailboat that doesn't have cockpit drains above water line, but it means a shallower cockpit than boats that don't self drain. FWIW, Mariner version 1 did not have a self-bailing cockpit - and they are loved for their deeper, more comfortable cockpits.

Fred W
Stuart Mariner #4133 Sweet P

 

Yes, the Siren 17 I had before was self-draining. The DS20 definitely drains to the bilge, and in this case into the sink drain. I am further confused because I don’t know where the waste holding tank is. The head that came with it is the portable type and not connected to the pump out. Tomorrow I will take apart the sink drain and try to figure out where it goes. I guess the upside of having to do all this work is that I will know the boat inside and out…