We have discussed the merits and demerits of wet and dry exhausts in the past. After much study, and dialog with commercial and pleasure users, we opted for a wet system on Wind Horse and saw no reason to change with the FPB 64s. Recently we were challenged on this subject, and after answering, challenged again, with the point (amongst others) that Nordhavn has been successful marketing dry exhaust systems. Which brings us an axiom we learned long ago. To wit, successful marketing and good results in the real world of long distance cruising are rarely synonymous. Rather, it is better to execute based on first principles and sound logic.
Since you already know which side we come down on, we thought it would interesting to share the opinions of two former Nordhavn owners, now FPB clients, who have lived with dry exhausts,
Let’s start with the comments of John Henrichs, who, with his wife Sandy, used to own a Nordhavn 64.
Don’t even get me started on this one. We had a dry stack on the Nordhavn. I won’t spend a lot of time on it, but here are the good points and bad points.
1. No raw water pump.
2. A little less back pressure on the exhaust system.
3. Maybe a little more efficient for the engine.
1. Soot. No way to prevent it. Each time you start the engine, there will be soot. Even in electronic engines as the soot is a by product of combustion. Each day we had to run the engine at max power for 10 minutes to clean out the stack. After docking we had to wait for the stack to cool and then put a cover over it as if rain or moisture gets into the stack you will get a lot of soot on your next start up.
2. Keel coolers. In warm water they need to be cleaned about every 2 weeks. If not, engine can over heat. Keel coolers need zincs which in warm water need to be replaced about every 2 to 3 months.
3. Cost. A good dry exhaust system is more expensive to build.
4. Exhaust leaks in the system. Each system will have several flanges with around 8 bolts and a thick gasket. After time the gaskets get brittle with heat and need to be replaced or they will start leaking.
5. Heat. There is a lot of heat in the dry stack and you will need a good supply of air to cool the stack. We had a 2000 cu ft fan to suck the air out of the exhaust and had two 2000 cu ft intake fans.
I could go on longer, but I wouldn’t ever do it again. Many of my Nordhavn buddies feel the same.
I forgot to mention that Bruce Kessler went around the world in a Delta 70 with a wet exhaust and never had an impeller failure.
Don Stabbert has been across the Pacific and Atlantic on his Northern Marine Starr and hasn’t had a impeller problem.
I have over 25,000 miles on wet exhaust boats and haven’t ever had a impeller or raw water pump failure.
In the diesel class I just attended, the instructor spent time on the subject and explained the dry start (no water on the blades) is what causes impeller failures and once running the impeller runs on a thin film of water for lubrication. He also said failures once running are rare.
Next, lets hear from Pete Rossin who prior to converting to an FPB 64 cruised with his wife Debbie on a Nordhavn 50.
I did have dry on my Nordhavn. Here were the issues – some of which were bad design, material selection, etc. Some are the nature of the beast.
First, my muffler was carbon steel and rusted out in 5 years. Due to the design of the boat, I had to remove the entire mast/stack assembly to get it out – a big deal. Switched to a Hapco stainless muffler and no rust issues after that, but as the muffler was wrapped in insulation, the only way I caught the fact that it was rotten was a slight rattling sound I was hearing in the stack chamber. Had it gone much further, there could have been a fire. The need to remove the stack to change it was s design issue – a mistake you wouldn’t make.
Issue 2 was that it was dirty on start up and the exhaust would occasionally belch black clinkers that would make a big mess. You need to be dead certain that rain and spray can’t get into the exhaust when the boat sits idle – it compounds the problem. The exhaust also has to exit high enough over the boat that it blows cleanly away – this may result in a boat with a higher clearance then you would like.
The lower exhaust sections were double walled with an annulus that was connected to two air injection fans for cooling. They chose cheapy Dayton fans which constantly failed – again, a mistake you wouldn’t make.
The related issue was keel cooling – I don’t like it. Keeping the keel cooler clean was a major hassle and there was an instance where one owner hit a reef, destroyed his keel cooler and wrecked his engine. Having a keel cooler out there is a big vulnerability, especially with your bullet proof hulls this is the achille’s heel.
You already know the primary argument for dry – no salt water, but if you need salt water for your shaft log, watemaker and genset and have a bullet proof system like the 83 and 64 – what’s the big deal? Particulalry in the case of the shaft log, going dry would mean a separate pump to provide the cooling water for the shaft seal – just another thing to go wrong.
You pick your poison, but I believe wet is a better choice.
We rest our case (if you have real world experience to bring to the discussion we’d like to hear from you).
Post script: use the SetSail search function to dig up more on wet and dry exhausts.