I hope TwistedTree finds this - when he built is Nordhavn 68 a few years ago, he went against Nordhavn orthodoxy and opted for wet exhaust. He can go into details, but in short, servicing wet exhaust can be done within the boat. Keel coolers and such need to be serviced outside the boat. They also take quite a bit of interior space.
I'll add that while there may not be a raw water pump, they frequently require massive blowers that will cause about as much trouble if they fail as a raw water pump.
Peter
Peter summed it up pretty well, but I'll admit that it's largely a toss up between the two which is why the debate never ends. It ultimately is picking which poison you like better.
For us, dry exhaust was one of the things we really didn't like about our Nordhavn 60. Here's why.
- It's dirty, and in all the wrong places. We didn't get greasy soot particles raining down on the boat like some people, but we did accumulate a haze over all the upper mast instruments. And washing it off was a royal PITA. You had to climb the stack, hold on with one hand, and scrub everything with the other hand. And you are scrubbing oddly shaped and often delicate devices. All the black goo would end up all over you, and wash down all over the boat. So cleaning the stack actually meant a top to bottom wash of the whole boat. In contrast, any haze left by wet exhaust accumulates on the side or transom of the boat where it's easy to wash off.
- The dry exhaust, even with a "hospital grade" muffler, was loud. Not straight pipe, DD loud, but significantly louder than a well muffled wet exhaust. I found it very uncomfortable to even be on the fly bridge when underway. At the pilot house helm, the N60 was 63db which by most measures is pretty good. But my 68 with wet exhaust is 52db. The flybridge was 80db, and the boat deck (loudest place) was 85db. The loudest place on the 68 is the cockpit, and that's 75db/ Every other place on the boat is in the 60s or lower.
- I hate changing impellers. But my keel cooler required just a frequent maintenance for cleaning and/or zinc replacement. And as Peter mentioned, the impellers you can change from inside the boat where the keep cooler work is done from in the water. If you are always in warm water and/or regularly have your bottom cleaned that might be just fine, but for us in cold water where you don't get the bottom cleaned, it was a pain.
- A dry exhaust will last a long time, but when it's time to replace the muffler, it's an ugly project. I have followed a couple of them and it's a big job. Other than impellers, a wet exhaust is largely maintenance free. You just need to keep an eye on the injection elbow or ring.
- Heat exchanger and/or aftercooler maintenance. Yes, both suck with a wet exhaust, and most dry exahaust engines. Yup, it sucks.
- I think the whole "fewer thruhulls" thing is blown way out of proportion. You will have one less thruhull. It will vary by boat, but I still have thruhulls for the wing engine, generator 1, generator 2, watermaker 1, watermaker 2, HVAC, anchor wash, and hydraulic cooling. And if I didn't have a thruhull for the main engine, which in turn feeds my dripless shaft seal, I'd have a thruhull dedicated to the dripless shaft seal. You're gonna have raw water running through your boat. Just accept it and do a good job on it.
- Cleaning strainers sucks. With dry exhaust you won't have to do that for your main engine, but you will still have to do it for everything else.
- Another gotcha if you are looking at a new build is that some engines now require a raw water loop for the aftercooler regardless of how you cool the rest of the engine. I think it has to do with maintaining different temperatures for emissions, but I'm not sure.
So you can see there is good, bad, and indifferent for both. I prefer the wet poison over the dry poison, but I question it every time I have to change impellers, all of which are due shortly....