Our original seal did not have the vent tube. We stay in the water year round but I would burp it before heading out on a long trip. We encountered some rough, confused seas off the mouth of the Columbia River and about an hour later a horrible moaning sound came from the engine room. It took a while to realize the seal had gone dry and was "chattering". A reburp took care of it. Evidently bubbles under the boat from some pounding found their way into the shaft log. We were travelling with a buddy boat, another Catalina 42, who reported a transmission problem about 3 hours later. Told them our story and they reburped theirs to solve the problem. A call to PSS and they happily sent us each new seals at no cost with the necessary vent. We just ran it above the waterline.

I doubt the connection to the thru-hull alone would provide any cooling flow but don't see how it could hurt. Just make certain that the seal is not a high spot so any air would be trapped. The thru-hull needs to be higher than the seal with no high spots in the hose.

Larry, would you mind posting the link to the SS barb you found? The original nylon one has always made me a bit nervous. Thanks.