We have 20 thru hulls...
... Do you close your thru hulls when you leave your boat?
Mother of God. 20 through hulls Really? Maybe it's what's making your hull stronger....
This reminds me of an old Boeing joke. You might appreciate this.
So with the 787 being built of new fancy composite material Boeing was learning the hard way with some of the structural parts. Most embarrassing of these was that they were having a hell of a time keeping the wings on.
They kept adding more and more reinforcement to the composite and every time they would mount the wings they would fall off.
It got to the point where the engineers were so embarrassed that they started mounting the new wing-box at night, so that if
if fell off again nobody would see.
Well it did fall off again and the only person to see it happen was the janitor. He laughed at the engineers each time it would happen without holding back. One night, after the wings fell off again one engineer, in a fit of rage asked the janitor "What? You got a better idea??"
The janitor said, "yeah I do. But you'd never listen."
"Oh yeah, so what's your idea?"
The janitor wiped tears from his eyes and said, "drill a bunch of 1/2 inch holes, one inch apart right along this line where the wing joins the fuselage."
Needless to say, the engineers laughed him out of the hangar and banned him from working there at nights where they were testing the wing-box.
After failure and failure and failure again, one engineer finally suggested not increasing the size of the composite attachment and suggested trying the engineer's idea. They were desperate, and tried it in secret and low and behold it held! They tried every stress test imaginable but the wings would not come off!
Finally they sheepishly went back to the janitor and asked him how he knew that drilling a line of holes through the most important part of the structure would hold????
He said, "You guys and all your education never pay attention to anything. I don't know anything about composites, but I can tell you from working with toilet paper for the last 40 years that NOTHING will tear along a perforated line!"
Now, in answer to your question. No, I never close my through hulls when leaving the boat unless it's for more than a month. Properly maintained, they don't leak. I maintain them. They stay open. They're more likely to come undone underway with stresses of the hull and floating debris to hit right? Why would they suddenly fail at the dock?
By the way, I found a picture of the original hull mold for the Jenneau 54. I've attached it here: