Caution - this is just a write up of my experiment, it is not intended to show how to modify the water ballast. Use any ideas here completely at your own risk!
Note: On my 1990 26S, even under fairly light loads, the water line inside the tank comes up to close to the vent hole opening (before I modified it). Weight of the boat especially to the rear affect this and I had about a 100 pound outboard on the transom and in the Laz area, several anchors, a bunch of chain and rope, assorted other junk. On one of the forums, another guy with a slightly older D model notes that his water level inside the ballast tank always remains a fair amount below the vent hole. So apparently either some slight changes were made to the design over the years which could influence this issue or it could be just the way the boats are loaded and how much they are loaded. In general, if you keep the boat light weight for racing, maybe this is never an issue. But if you load the boat much especially to the rear, this will drop the boat with respect to the outside water level and since the ballast tank water level is exactly the same as the outside water level, eventually the water level in the ballast tank can come up to or exceed the level of the vent.
On the Macgregor 26 foot classics built from about 1988 to 1995, if you heavily load the aft end of the boat, it can put the water ballast tank valve below the water line. If you follow the safety notice posted at the ballast valve to only operate the boat with the valve completely closed and the access port also closed, this is normally no problem. Under this heavy load condition, if both of these valves are tight and only slightly leak, just a little water drains to under the starboard seat bilge area.
However, if both the ballast valve and access hole are left open and the boat is heavily loaded especially to the rear, this result in a much heavier water flow with the potential to swamp the boat. I know this from personal experience, one time I had asked the crew to close the valve and access ports and they did not (crew in this case happened to be my early teenage kids - whom I should have been more responsible for). Fortunately I noticed the valves open very early into the trip but still had a fair amount of water come into the boat.
The Macgregor 26M model has a round white containment around the ballast tank access port and this will significantly raise the water line required to swamp the boat. I copied this idea from the 26M after I had my experience and built a "Pool" around the ballast tank valve and access port. I also raised the ballast tank access port. The idea behind this and why it is a safety issue is illustrated in the bottom figure on this page.
If you always securely close the ballast tank valve and access port, you don't need to worry about this mod. In my case, these were accidentally left open with a heavy load. Since I screw up something about half the time I take the boat out, I wanted this particular issue simply no longer possible under any situation so I added the pool.
The pool and raised vent are shown in the picture below. To make the pool, I first used a can of expansive foam to make the backing structure for fiberglass. The foam was sanded into shape after it had hardened and then I used glass tape and epoxy to build a water tight structure as shown..
(note - the 12V low pressure air pump in the first picture below is not related to this mod, this was just the picture I had available)
Below - sailing mode with vent access tightly closed. JB weld epoxy is used to glue the PVC pipe in place along with some thread of the PVC into the fiberglass
Below: Low pressure / high volume air pump attachment
Below: Larger hold access to the tank
Below picture. This picture shows the construction of the pool.. Without the pool, any water that comes out from either the valve or the access hole would drain towards the left in the picture and end up in the starboard bilge area below the seat. The access hole valve shown in the below picture worked well but I changed it what is previously shown for several reasons. One reason was that I needed the vent hole access larger so that the ballast tank could be decontaminated for Zebra mussel water (they fill the whole ballast tank with 140 degree water - YIKES). The second reason was that I wanted the low pressure air pump fitting for blowing the ballast while the boat was on the water.
The diagram below illustrates the potential issue and why I did this modification. It is simply because on my 1990 26S, I can get the water ballast access port and valve below the outside water line if I heavily load the boat especially at the aft end. The valve is just under the companionway stairs - also to the aft end of the boat).
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