One of the obvious concerns with any sailboat is what is going to happen if a wave will roll over it, and possibly flood it, or what will happen if the sailboat is wiped out to a side or turtles.
Standard Catalina is a disaster in this sense! There are at least half a dozen design mistakes in the hull which will make any attempt at serious sailing deadly.
The very first and possibly largest blunder is the pop-top. Some out-of-mind Catalina designers thought that a pop-top roof was a plus! When I sailed my Catalina for the first time on a lake, I really felt uneasy all the time… Imagine that at sea! So that was the first major project, to glue (with fiberglass and polyester resin) the pop-up part of the roof to the deck.
Then there was the ventilation opening in the bow. I decided this one also had to go. My philosophy was this: I need a dry place in the boat to sleep, and that will be the V-berth. Yes, the V-berth is not very pleasant for sleeping when sailing in waves, but one would usually sleep there when on anchor, so that it would be OK as the main sleeping area. However, any small chance of getting water on the main bed out-weighted the need for ventilation. And I never regretted that decision. So I eliminated the opening in the bow.
Next, there were the two ventilation openings for the gasoline storage which are required by law for venting potential gasoline fumes… However, when the boat is lying flat on port side, they are the best way to get water inside! They had obviously to go as well. (Yes, they had some pipes which in theory would work like siphons preventing the entry of water, but would you trust them? I personally would not!). And I would never store gasoline inside the hull anyway!
The next thing was the issue of the lazarette hatches. The starboard side had no use at all, since the area is easily accessible from the inside, therefore I sealed the hatch by means of two stainless steel plates. But the port-side lazarette hatch is the main access to the interior of the hull so I had to leave it open. I had to replace the rubber sealing on both lazarette hatches and when navigating, the port lazarette remains closed and locked with a carbine hook.
The bottom of the hull had one hole for the speed log. I was never able to make it function, and dumped it, but the hole had to be sealed. And since the dial was installed in the wall of the cabin, after removing the dial, that part had also to be repaired.
Next big project was the main hatch. Designed for fat, blubbery beings who are unable to even raise their foot to step over the sill, it is 70 cm wide in the upper part and is 70 cm deep, an invitation for water to flood the boat. Most probably this was again a tribute of Catalina designers to the American reality, if you exclude the fat customer, you lose the profit.
There are three washboard, and I decided to eliminate one, that raised the hatch by more than 20 cm.! The sides were also reduced by 12 cm., so that vertical opening became 50 x 50 cm. I cut out wooden inserts and then glued everything with fiberglass and polyester resin. I had also to reduce the vertical part of the hatch, which was also about 70 x 70 to 50 x 50 cm. For the time being I decided to leave the sliding cover.
The boat was always taking some water, and for a long time I could not find the places where the leakage occurred. It was not much, maybe half a liter per hour, but it was unpleasant. When I was unable to locate the leaks when on water, I simply filled the boat with water when it was on trailer. I poured about 600 liters and raised the water level by some 15 cm. inside the boat, and when I looked below, five leak points were clearly visible. After pumping out the water and patching up the leak points (some of which were difficult to access and the floor liner had to be cut open to gain access), the leakage became almost non-existent.