Mysterious Large Bubbles

[Vincentc]

While running the engines at the dock I notices large bubbles coming out from under the port side of the boat every few seconds.
This is located approximately 2 feet aft of the aft engine room bulkhead and only appear on the port side of the boat.
Since I first noticed this, it occurs every time I run the engines at the dock. It stops if I put the engines but resumes when back to neutral.
The bubbles are very large, I would guess about a pint of air.
The boat is a 43DC with 671's.
I would appreciate any insight about what is happening.
Thanks

 

[Play'N Hooky Too]

Sounds like you may have air being forced back through your raw water intake. Can you see bubbles in the intake sea strainer while this is going on?

When was the last time you changed the impellers in the raw water pumps?

 

[SEVEN]

To be clear, the bubbles are coming from under the boat not under the motor correct?

 

[racclarkson@gmail.com]

Do you have dripless shaft logs?

 

[Pascal]

2 feet aft of your ER aft bulkhead? That s near the shaft seals. Do you have dripless with water cooling ? Not sure how air would be forced out that way.... it s usually coming off the raw water pump. Do you have enough raw water out of the exhaust ?

 

[Captain Ralph]

Give us an idea (pictures) where the shaft water injection hose is connected to your engine.

 

[KJScott]

Logic tells me that the first place I would look would be the raw water exhaust. Not familiar with your model but on mine, the exhaust comes out under the boat several feet forward of the transomwhen idling at the dock. Sounds like air is getting in there somehow.

 

[Pascal]

Logic tells me that the first place I would look would be the raw water exhaust. Not familiar with your model but on mine, the exhaust comes out under the boat several feet forward of the transomwhen idling at the dock. Sounds like air is getting in there somehow.

AFAIK Hatteras never used underwater exhaust on older boats.

 

[Captain Ralph]

Again Hatteras never used underwater exhaust on older boats.

Hatteras did use propeller air injectors on some performance models.
Air was naturally drawn when underway on some, exhaust was diverted on others.
I don't think at idle air(exhaust) would blow under water.

I'm still thinking the raw water side has an air leak before the raw pump.
Strainer lid, hose or bad pump fitting?
671, Jabsco pump, low side zinc fitting?
A top mounted exit fitting to the shaft log injection hose could blow air that way.

I'm guessing out loud here but a picture of the injector hose installation would sure help.

 

[Vincentc]

Thanks for your good comments.
I do have dripless stuffing boxes and there is water injected. There is good water flow out of the exhaust pipes at the transom. The only underwater thru hulls in the area are the engine raw water intake and the propeller shaft. Both the shaft log and raw water intake are just forward oh the ER bulkhead and the RW intake is forward of the shaft log which is at the bulkhead.
The bubbles flow up and around the port side chine roughly 6 feet forward of the transom in the same spot. The will be a single large bubble every few seconds. I would guess the location corresponds on a forward aft plane to a point a couple of feet aft of where the port shaft enters the hull.

Both raw water pumps were rebuilt within the last 5 years. It’s been a couple of years since the port impeller was changed. Hard to envision how air would be pumped out of a water pump intake or exhaust. That brings to mind the fact that water in injected into the exhaust.
I have pictures on my phone of the bubbles if that would help.

 

[Vincentc]

Regarding the dripless shaft, I’ll need to look and see where the water injection hose is connected at the engine

 

[Pascal]

I would check the shaft seal water injection. Sometimes there is a cross over so the seal gets cooled down if running on one engine. I would try disconnecting the hose at the seal and fire up the engine to check for flow

It’s the only thing in the area that could let air out

 

[racclarkson@gmail.com]

The reason I asked whether you had dripless seals was something I randomly came across years ago. I Googled "air bubbles in dripless seals" and found it straightaway.

https://southernboating.com/engine-upkeep/maintenance/shaft-seal-maintenance/

From that article: Overheating the housing can be trouble. In most cases, this is caused when air is allowed to accumulate inside due to most inboard engine drive shafts being installed on an angle, thus creating bubbles that can travel up the shaft. Dripless systems use the pressurized lubrication water to force any air out.

You didn't say the brand seal you have. I have Tides and am, therefore, familiar. Were it me, I might check the water flow with everything off. You should have a generous stream coming in with the hoses off the seal. I'd cap those hoses and then check the raw water flow (as others suggested) by running the engines. My thinking is poor to no flow might be the source of your issue. Just a guess, but would you have PSS seals? Tides would have a cross feed and the odds of a double failure are longer.

You mentioned two years on the impellers. Would it be fair to assume that you've not de-scaled the raw water circuits? I'm speculating the 1/4" to 3/8" spur lines and hoses would be good candidates for clogs.

It'd be great to hear your findings. Best of luck.

 

[MadHatter53]

Sounds like the guys here got this one. I wonder if you can isolate it to one side(eng) or the other.
it seems odd that with the angle one would assume the air would be trapped at the high side and stay there until bled out. I have standard packing so I’m not familiar with the dripless.

 

[bostonhatteras]

Do you have dripless shaft logs?

you win the prize! i agree. just because a hose is attached to some other raw water hose doesn't mean the take off fitting is immersed in the used raw water at all times. it all depends on flow and placement of the take off fitting. Absolutely the first thing i would look at. in gear it might just be washing the bubbles into the prop wash and they cant be seen. at higher rpm the take off is likely immersed and the dripless fittings don't burn up.

 

[Vincentc]

They are PSS dripless. I installed new bellows a few months ago. Based on the location of the bubbles the shaft log seems the only logical source. However, I’m seeing bubbles containing a whole lot of air. I estimate each bubble has a pint or more of air and a bubble comes out at least every 5 seconds. It’s about like when you invert a 16 oz cup, push it under water then turn it right side up.

When I find time I’ll go down to the boat and study connections and perhaps check the flow of water to the seal. Will let you know what I find

 

[sgharford]

I know you said you had plenty of water coming out exhaust, so this probably not helpful at all, but I had same symptoms when I replaced the impellers on raw water pump for mains and the bronze impeller housing came out. I accidently put it in upside down and the intake became the out take - blowing a large amount of air out the intake thru hull and bubbling up side like you described. Curious to see what's causing your issue....

 

[racclarkson@gmail.com]

I know you said you had plenty of water coming out exhaust, so this probably not helpful at all, but I had same symptoms when I replaced the impellers on raw water pump for mains and the bronze impeller housing came out. I accidently put it in upside down and the intake became the out take - blowing a large amount of air out the intake thru hull and bubbling up side like you described. Curious to see what's causing your issue....

That reminds me of the Hatt that used to be next to me. The mate rebuilt the raw water pump on one of his generators 18 mos. back, 30KW I think. He installed the pump backwards and didn't bother to fully test it. Docking him from a charter one night, I called his attention to the smoke billowing from the exhaust of that genny. It must have been minutes from catching fire when he shut it down. It never ran again--seized. It probably blew some bubbles, before the impeller cooked itself.

My current trawler neighbor had trouble cranking his generator. He cranked and cranked. When it got sluggish, he thought his battery was bad. Then it was his starter; he rebuilt it. Two or three months in, he asked my advice. I had the sad job of telling him he had hydro-locked the motor.

Just shows you how easy it can be to do thousands of dollars in damage through ignorance and/or carelessness.

 

[Pascal]

That genny should have shut down long before seizing... surprise nobody realized how loud it must have been without water

 

[racclarkson@gmail.com]

Well little on that Inspected Passenger Vessel worked. I know it was common to disregard the high water alarms.