Changing Thru Hull Fitting

When doing this what type of sealer should be used between the fitting and ball valve, Teflon tape, form a gasket?????

Is this below the waterline? If so the ball valve should not be separate from the fitting, it should be a single unit, a proper seacock. The tailpiece that connects the seacock to the hose should in my opinion be sealed with a more permanent sealer like 3M 4200 (semi permanent) or 5200 (pretty permanent). This is not a place you want leaks, which could sink the boat and kill everyone onboard. If well above the waterline, then use normal plumbing techniques.

Groco offers a bronze base which accepts a bronze ball valve. The base has NPT threads which match a typical ball valve. Both Groco and Buck Algonquin offer suitable BRONZE ball valves. Do not use a brass ball valve though.

Action's post/link shows the base.

THread sealers can be many products such as Form A Gasket, Rectorseal #5, Loctite 567. TFE tape can also be used but NOT the typical cheap, thin stuff in the blue roll cover package. I use the thicker "gas use approved" tape which usually comes in an orange cover and shows the gas approval. Any decent plumbing outlet will know the type. I will admit that I tend to use one of the paste type sealers more these days and the TFE tape less.

As for a sealer between a tailpiece and the hose connected to it the 3M products can be used but so can other sealers such as Rectorseal #5. It's not a glue like the 3M , it's also messy but no where near as bad as the FOrm A Gasket.
The hose barb should have ridges cast into it and when clamped properly will not allow the hose to come off. Use good clamps.
The sealer need only act to prevent weeps as sometimes the rough surface of the barb can allow a slow weep of water. Not likely but can happen.
I use Rectorseal #5 on mine and when I want the hose off, and I do when winterizing the engine, it is removable but it is still a fight.
Other wise the hose will freeze to the barb needing it to be cut off.

C lectric and Action’s advice is spot on. I used the Groco flanged adapters and would just add that Groco also makes a composite backing plate that you can epoxy to the inner hull. The flange base bolts to the backing plate from the underside and makes an extremely strong connection without having to bore three bolt holes per through hull into your boat.

Follow Rod’s how-to on this—and on anything else boat-related—and you won’t go wrong.

https://marinehowto.com/replacing-thru-hulls-and-seacocks/

If desired a thru hull in a genuine sea cock could be replaced while the boat is in the water.