so my twin engine Fountain has external mufflers that were made by Fountain, they are pretty big each one is about the length of the swim platform, for a couple of reasons I do not want to remove them and install straight tips but the boat is too quite,,so I was thinking of drilling a couple of 1/2 " holes on the rear/bottom of each
one, has anybody done that? any pros/cons?

The only "con" i can see by doing this would be possibly less water going through the tips for cooling....if they are covered by the platform and people wont potentially be touching them after a hard run then happy drilling!
Can you instead drill out the insides of the mufflers from the back opening? Let the exhaust go out the tips like it normally does but will less in the way??

Remove and sell the current ones and buy straight through with flappers. I know you said that you didn't want to remove them but I would. I think that the exhaust holes are sometimes an issue getting sealed correctly at the factory. Great time to make sure that they are sealed.

If it sounds anything like a car with a rotted out muffler (hole in outer case) I wouldn't do it. Additionally, there may be the possibility of losing the packing material through the hole you drill and/or introducing water into the system where it doesn't belong.

Here is a link to Fountain muffler patent, which includes a schematic of the internals. I'm not sure if drilling holes will accomplish what you are looking to do. I would probably see if you can open them up and remove baffles. Then you are just running a straight pipe out the back.

https://encrypted.google.com/patents/US6443255

exactly why I am asking,,LOL

Thanks a lot,,,the power of OSO

thanks for the reply,if I had corsa mufflers I would ,but if you look at the attachment on Techmans reply you can see the Fountain mufflers are a lot different,,the rim angle is not a 90 degree and the muffler itself is about 7-8" round which most tip rims are 5/6" round,,so I am trying to avoid getting custom angle tips and having fiberglass work done ,,,and then after all that I will most likely get a ticket for loud exhaust the 1st time I go out on the chain LOL,,(thats where I boat)

My factory mufflers were installed with Y-Pipes, so as I come up on plane things get a lot louder with the exhaust also exiting through the drive exhaust bellows. Since Fountain's have such a high X-dimension this is just above the water line. So, I get a nice rumble at idle without that harsh bark if I didn't have mufflers.. But as I plane out, the boat has a nice roar. Kind of like having a auto-switchable exhaust.

Plus, I love the look of the exhaust tips even with the end of the swim platform while running. They spit water several feet and just enough sound back there to get looks, but not too much negative attention.

Yep ,I have the same set up,,,just thought about making it just a bit louder at idle,,