Just put on a new Vernatone MK2 round 2 chamber muffler on my B series Cummins and the results were enormous. I have a 5.9 M3 330hp cummins and before I was running a Costa Rica engineered muffler system which just had some bends in the exhaust tube and was bad. Here's the pros and cons I found
We'll start with the negative as I don't want to end with it
This was hard to install. Vernatone recommends you install this thing above the waterline (which in my case is the floor) so i had to install it above the engine. Had to build a riser off the turbo so the muffler sits right above the engine. Then I had to buy a ceramic insulation mat to guard against heat from the riser (it is dry until right before it hits the muffler) and wrap the riser in Fiberglass tape. I am going to cover that in Aluminum but haven't got to that part yet. I then had to sand and add on arms to the versatone so that it is removable but well installed when I am in rough water. I built four tabs and put them around the motor in the side walls of the box, and the muffler has four arms that sit on those tabs with a big stainless bolt in each. Lots of work, but the muffler is very secure, you can sit on it, and it comes out easily. This might be easier on a different boat, but overall the installation was well worth it! The only other con would be cruising. It reduced the noise cruising a ton, but I thought it would knock 100 percent of it out. I'd say it got rid of about half the cruising noise. About 1200 rpm you can hear it throwing the water out of the muffler (a cool bubbling noise), and then about 1500 you start to hear exhaust. At 2000 you can really hear the exhaust, but not as bad as before.
Pros: It is dead silent trolling. I mean nothing coming out of the pipe at all at 1000 rpms. When we first started the boat, we all said "wow", nobody thought it would be dead silent. You could here a little noise coming out of the motor box, but nothing out of the pipe. I instantly heard one of the valves clicking out of adjustment, which I couldn't hear over the exhaust. It reminds me of when I had a race car and we would put mufflers on it to hear if the motor sounded ok and then take them off to race. You really can't hear what's going on with all that exhaust noise drowning out everything.
When we went out trolling, I never realized how noisy the wake is. I couldn't hear any of that before, but now all we hear is wake while trolling. The radio can be on low and heard very well as well. My particular boat was a little poorly designed and the exhaust comes out right at the water line (I think the exhaust noise mostly matters when the exhaust comes out at the water line, otherwise you are scaring birds and not fish:) ). The floor prevented me from raising the exhaust up, so I had to kill the noise instead of moving it. Mission accomplished.
Since installing the muffler, we are in the middle of the lowest season for billfish here, and have gotten 5, 4, 2, 2, 4 to the boat on the last 5 trips. Typical this time of year is 1 or 2, and I think this is mostly from the muffler. The two days with 2 I was trying a smaller prop, and I switched it back out and got the last 4. Good fishing technique is vital, but it's nice not to have noise working against me.
Before, we never had fish hit on the muffler side teaser, now both sides are very active. First day out, I yelled to mike, short teaser and instinctively he ran to the non muffler side. I had to yell, the other teaser, and he was like huh? That almost never happened before. I also used to not be able to make the lap and pick up billfish that bite but didn't get hooked as they would run off, but now I make the lap and they are still there. Another interesting story, we had a sailfish on yesterday, and in the past they always ran straight from the boat after they were hooked. Yesterday, this fish swam right up to the boat, and about 5 feet away, the boat startled him and he took off the other direction. He had no idea where we were at. I've only done 5 trips with the muffler, but I can see a huge difference in the behavior of the fish. And there are more of them.
So overall, if you have the time to install it, or better yet, the money to pay someone else to do it, I think this is on par with the Barry control motor mounts I installed a while back. Helps a ton with the fatigue, raises fish(which hang around longer as well), and definitely lets you hear the radio better.
And on a side note, I am in no way connected or affiliated to Vernatone, just a paying satisfied customer.