Thanks for that post Dave,
I think that I understand most of that. It does seem that a flush mounted thru-hull would tend to push deeper into the hull if forced than a mushroom flange. Mine have mushroom flanges. The hull thickness there is about 7/8 inch. I am still weighing the issues in this decision. If I understand you, the three thru-bolts that are inserted in three new holes to be drilled parallel to the thru-hull shaft and sixty degrees from one another would mechanically secure the backing block and seacock flange to the hull to prevent the thru-hull from being shoved from the outside through the hull and prevent it from pushing the block and sea cock back away from the hull with the thru-hull.
However, that requires three additional holes, all of whoich woudl tend to weaken thehulla t that location, and any one of which might leak; and if those bolts are stainless, then don't they become mini-anodes and aren't they vulnerable to corrosion, or should one use use bronze bolts on a bronze sea cock flange? I wonder whether or not bronze bolts would be strong enough for this application.
This straight threaded (NPS) joint (seacock and thru-hull threads) appears to be mostly held together by the bedding compound (my yard uses 4200). The thru-bolts merely connect the seacock flange and backing block to the hull and do not lock the thru hull in place or result in compression of the enagaged thru-hull-seacock threads in any way that I can visualize. So the three thru-bolts would not seem to have any function with the exception of keeping the sea cock from turning (loosening) unless there was a collision or impact from outside as you outlined. Or are you also saying that the thru-bolted option also protects the sea cock-thru-hull joint from an inside foce such as a flying tool box to greater extent than the lag screw option. Do not the lag screws effectively prevent seacock rotation, while the bedded baking block, thru-hull shaft, and the thru hull flange on the outside of the thull insure against such internal impact damage?
By the way, the Wilcox Crittenden sea cock flanges on the head thru-hulls on my boat appeared to have been original 1965. They still had spray paint on them from the original interior hull splatter-paint. They were neither bolted nor screwed into anything. Indeed there were no baking plates for those sea cocks. Someone much have removed and re-bedded them at some point, because they were bedded solely with silicon.
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