I'm not sure I understand the concern about getting water
inside the rudder.
This is no more a concern that worrying about getting water inside the hull
The welds should make it airtight, after which it will get slathered in several coats of paint
, sealing any possible micro hole anywhere.
I removed my steel rudder after about 10 years in use. I could detect no water inside. None...zip...nada..I flopped the rudder over this way and that to see if I could hear or feel any water...nothing !
Here's a thing about rust....speaking as a retired structural engineer
, I can tell you that a piece of steel plate can throw of a tremendous amount of rust. Visually, it can look alarming, but still structurally sound.
It's easy to measure. You can buy a metal thickness gauge on Amazon, there is a huge variety to pick from. $2-300 will get you a good one. You just press the little foot against the steel, and it will read out the thickness....fractions of an inch....mm....
You can spot check an entire steel hull
in a day.
The big concern is always rusting from the interior
where you can't see it. Here is where the gauge comes handy. If the gauge shows a suspect area on the outside, you can " sound" the area every few inches to determine the extent....if any...it can also direct you where to look on the inside.
Finally, with today's modern coatings, visible rust is easy to fix, repair, repaint, etc.
Interestingly enough, after I sand-blasted and painted the interior
of my boat, I discovered a place I had missed. probably about 12" wide x 3' long, under a cockpit
seat. I did not have any more paint
, so I slathered it with West System Epoxy
and it worked as good as the coal-tar epoxy
. never a single
rusting issue there.