Re: yet another 15hp question

[Don]

I will, once you get it apart and have the new grommets in hand, will be easier to understand that way.
In any event, the problem is that the top of the grommet extends over the top of the water tube, so a bit of the grommet is unsupported allowing salt build up between the grommet and exhaust tube to push the grommet inward closing up the hole. I usually just carefully trim about 1/16" off the top of the grommet to ensure it does not extend beyond the top of the water tube.
Some of the newer water tube replacements must be a little taller, because the grommets don’t extend over the top of the replacement tubes. But, I’m thinking yours is not new enough for that improvement to have been incorporated into the water tube. And, I guess if that improvement had been made to your water tube(s), you wouldn’t be having a grommet problem now!