Sea water in bilge, dark particulate settlement, Cummins QSB 6.7

TMorgan 400 Fly

New Member
Jun 2, 2021
24
Cape Coral, FL
Boat Info
2018 Sea Ray 400 Fly
Engines
2x Cummins QSB 6.7 V drive
I purchased the 2018 Sea Ray 400 Fly (Sundancer) last May. At that time and ever since seawater collects in the bilge when underway. In addition to simple seawater, I get a fine black particulate that settles out. I vacuum it dry after use, but any use fills the bilge. In the last year, I (meaning MarineMax) replaced propeller shaft seals, cutlass, swim platform hydraulics, trim tab mounts, and underwater lights, and checked all obvious signs of intrusion.

No avail, water still builds and led (I hypothesize) to the failure of my stern thruster from submersion. (So I replaced that, too.)

Before I go to water pump replacement (no harm) and find that's not the issue, or haul out to replace all the through-hulls (which don't leak when not underway, or show evidence of leaking), I have been asking questions of anyone who would answer someone suggested that the source may be exhaust and seawater mix.

In the 400, the exhaust is above the waterline to prevent backflow. But it's not at all obvious to me how I would check this.

Thoughts?
 
"Black particulate" sounds like the scaling that develops on interior cooling passages in the engine/exhaust. Is it gritty and when you rub it between your fingers it smears kind of like carbon? It sounds like it's leaking decent enough that you should be able to observe the leak while it's happening? Will it produce a leak if you idle in your slip for a while? Get everything really dry and clean and then observe closely/often. You should be able to start narrowing down the point where it is emanating from.
 
"Black particulate" sounds like the scaling that develops on interior cooling passages in the engine/exhaust. Is it gritty and when you rub it between your fingers it smears kind of like carbon? It sounds like it's leaking decent enough that you should be able to observe the leak while it's happening? Will it produce a leak if you idle in your slip for a while? Get everything really dry and clean and then observe closely/often. You should be able to start narrowing down the point where it is emanating from.
I had a 52DB that took two years to get dry. In the end it was the hull to deck joint. Hard to believe - but a good amount of water came in while underway.
 
Get a few Blink cameras and put them in various locations in the engine room and watch them on an iPad while underway.
With the black grit I would focus on the exhaust for the leak
 
Is it a single issue, or two different issues that happen to be occurring at the same time.

Can you determine what the black particulate is? I agree with @Lazy Daze, if you rub it and it smears that sounds like carbon which would take you down one path.

I had black particulate one time, that wasn't carbon, rubbery instead. Turned out to be the dampener throwing out the dust.

Finding where water is coming from can be a real challenge sometimes. I don't know the arrangement of stringers and cross pieces which can control where water can flow once it gets in the bilge. Are you sure its sea water? If so then have you eliminated all the easy things - hoses and connections which have sea water?

Then you move to any place there is a hull opening.

Sounds like it only collects when the boat is running? Is that correct?
 
@Lazy Daze I'm trying your suggestion. Vacuumed the bilge dry and am letting it sit overnight. I will open it up in the a.m. and try idling.

@NorCalRobert I have some GoPros which I will station once I have a better idea of where the general source is. That's a much better idea than having a tech sit below decks when I run!

@FootballFan I will do that next leak again. I did have rubbery, small chunks early on. I hypothesized that was part of the cutlass bearing which was very chewed up when Marine Max replaced it two weeks ago. This is definitely small, dusty-smearing particulate. And, of course, I have now eliminated the leak emanating from my shaft seals. I am sure it's sea water because I found that the freshwater usually generates a soapy bubble when pumping or vacuuming out - e.g., shower sump.

Thanks all for these posts - definitely feel like you will help me diagnose and fix this, though I hope it doesn't take 2 years, like @BeeBoating
 

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