Is it not correct to use silicon bronze all thread 1/2" for keel bolts?

The effective strength of cut threads for a nut since the rod is in tension is also threaded has a smaller root diameter just as an all thread rod has.
I don't think you can make a case there for solid rod versus threaded. Leak passage is resolvable, got to seal ends no matter what.
I suppose threads might work against wood in a hole. Being a power boat, maybe not like could happen in a sailboat with boat heeling over.

But yeah, solid round bar is better than all thread. I found a supplier who said $86 for silicon bronze 6 foot 1/2" rod which is good price. Online Metals

I am putting back in the original bolts after JB weld coating them and they were in good shape after 54 years. They also came out easily with light taps.
I was thinking of adding some few keel bolts.

To seal them I used T plus 2 rectorseal, teflon white pipe dope meant to seal for water.
I marked with tape the square notches orientation on bolt head
I greased up both bolt ends with that sealer and wrapped 7 inchs of knitting yarn around base of bolt, lined up with the keel hole pushed them in and tapped in with a 1/2 inch socket extension till seated into the wood. Then inside the washer and nut. Went together well. The bottom bolt end is recessed into oak keel so will also get some goo to fill the cavity.

A lot of bolts being different lengths came up with idea if using large copper wire with lock wrench snapped on. Drop wire down bolt hole and under boat marked with painters tape the wire. Then you know what length bolt is needed.

 

Idea of adding bolts was from another discussion on here where I have pulled out iron spikes, used to hold floors to inner keel. Inner keel is bronze bolted to skeg keel. The iron spikes were wasted to nothing, which means could drill through floor, inner keel, skeg and use a bronze bolt to secure all 3 together, versus lousy rusted away oak eating steel spike nails used 54 years ago which have done nothing for a long time anyway.

The inner keel has socketed frame pockets let into the white oak, so in order for it to detach itself, all of those frame heel ends would have to snap off, unlikely since I replaced most of them 20 years ago and they are in excellent shape. And the frame spacing is around 7 to 8 inches apart edge to edge