Okay I am working on the tug and a problem has cropped up. We are losing the fresh water as we run at a fast pace. No water is going back into the engine, and it is going out super quick. The exchanger was made by Condenser Service & Engineering Company on 9/19/41, and of course the company is not around any more to offer assistance.

Anyway my thoughts are to strip down the heat exchanger and check for leaks between the fresh and sea water in the exchanger. Now the big problem is I have NEVER attempted this before on anything so large. It is listed as a 12-75 size exchanger, so I am assuming 12 gallons fresh and 75 gallons salt water a minute for cooling. So I am presenting this problem to you. I am thinking about pressurizing each of the tubes and verifying integrity. Is there anything else I need to look for?

I'm only familiar with small heat exchangers on recreational boats. You are saying that the coolant from the closed engine system (with glycol) is disappearing, but no sea water is entering the engine?

Well, a 68 year old HX is pretty old. Are you sure the coolant is leaking through the HX, and not somewhere else like the exhaust manifold (if water jacketed) or (heaven forbid) the head gasket?

I'd start with pressure testing the HX as a whole to verify that it is leaking there. I assume it has some kinds of covers or end plates. I'd check any gaskets there. On the larger HX I've seen, sometimes the whole tube bundle is easily removable for cleaning, inspection, and repairs. It might be easiest to remove the bundle and send it to a HX shop for pressure test and repair. I know of a few on the east coast, there are probably some wherever you are.

Sounds like a good plan. First you need to ascertain the leak is actually in the heat exchanger. If it can be isolated you can pressurize the tube (or shell) side and vent the other side. Any flow naturally indicates a leak in the exchanger. The tube sheet itself may be the culprit though. That should be obvious once you pull the flange(s) off. If the sheet is shot, its time for a total rebuild/replacement. If it's a single pass (as opposed to a U-tube design) now would be a good time to clean the remaining tubes after plugging the offending tube(s). Stainless brushes on rods are generally used on small heat exchangers (similar to a gun cleaning brush). Maximize the flow through the remaining tubes. There comes a point where too many tubes being plugged results in less than adequate heat transfer. Once again rebuild/replace is required. 1941? I'd say someone got their money's worth!

I'm a slow typer. JonL's response wasn't there when I started! Sorry to have repeated his advice.

Just to be sure I'll explain fully... The fresh water is supplied from a 5 gallon day tank and should be augmented about every 8 hours or so. We fill the engine with water at a point on the side of the engine. We aren't seeing any water in the oil (we have a dry sump and can drain the bottom of the tank). No extra water seems to be in the bilges either. Right now the day tank bleeds in about 15 to 30 minutes when we are running.

I have one end off the heat exchanger and it had some old Mississippi mud collected in the bottom of it (boat was in the Mississippi from 1946 until 1977). I am cleaning it right now to see that end's condition. Then I am pulling off the sea water end and then I plan on pressurizing each and every one of the tubes and ascertaining the condition. Is this a good plan?

I did figure the heat exchanger for the culprit simply because we would have seen water in oil or in the bilge. So now the really fun part, cutting off a few rusted/frozen 3/4" bolts.

I'm thinking the "day tank" is actually an expansion tank similar to the plastic coolant overflow tanks/reservoirs in late model cars and trucks. It serves as a surge volume for expansion and should rarely need to be refilled (due to coolant evaporation). The fact you had to augment (refill?) said surge tank every 8 hours prior to your current situation indicates you already had a leak somewhere. The coolant could be leaving via the exhaust in which case it won't be in the oil or bilge. I'm not saying the HX isn't the problem, it may well be. The only certainty is the leak you already had has either grown or been joined by another. It may or may not be the same leak. The river muck smells nice, doesn't it? We used to call it a "clam bake" when we cleaned out the waterboxes. Not a pleasant job. Keep in mind you may have two different problems, don't get tunnel vision.

If your fresh water side is not pressurized (and it sounds like it is not), I'd think that sea water might want to enter the engine rather than fresh water leaving. The seawater pump can generate some pretty significant pressure in the HX. Not sure how much pressure the circulating pump generates. It could go either way....

It should not be designed like that. The leak should always be from fresh to salt to avoid corrrosion, ie fresh water pressure is greater than salt water pressure. Each side has a pump, one simply has more pressure than the other. This is why any leak will lose fresh water to the salt side.

I don't think the systems are designed to leak a certain way, though I could be wrong, especially regarding commercial duty systems. I've designed systems for recreational boats and this was not a consideration. The design was made assuming a non-leaking HX, and that a leak would be detected and corrected before corrosion could become an issue.

The engine needs a certain coolant flow rate to cool it. The circulating pump is sized to provide this flow against the various restrictions in the system -- the coolant passages, the thermostat, and the HX. Usually a system is pressurized to improve pump performance (reduce cavitation) and raise the system boiling point. The pressurization is achieved by the coolant expansion as the engine warms up. There is a pressure regulating cap and a limited airspace above the coolant (in an expansion tank or radiator header) or zero airspace and a coolant recovery bottle to establish the working pressure. This pressure is additive to the pressure the pump creates working against the flow restrictions. The circulating pump is generally a centrifugal pump and the flowrate will go down as the restriction goes up. I don't know how much pressure it can generate.

The HX needs a certain seawater flowrate to perform properly. The seawater pump is generally a pseudo-positive displacement pump that can provide quite a lot of pressure to maintain the desired flowrate even as the HX gets fouled and the restriction goes up. Depending on the system, there can be a lot of restrictions for the seawater pump to work against. Several HXs in series, water jacketed exhaust systems, etc. All prone to significant levels of fouling.

IMO it is not obvious which system has the higher pressure where the leak might occur. It depends on a lot of factors... my gut feel is that the seawater side could be higher especially if there is no pressurization on the freshwater side... and with the amount of make-up required even before the recent increase, it doesn't seem that there's much pressurization.

As I said, this is all speculation. The OP is on the right troubleshooting track.

The system works at 80 psi sea water and 120 fresh, so I think there was a method to the madness. I was so close to getting it apart too it's upsetting. Working today I was cleaning the tubes and found oil residue that made me burn my hands (I am allergic to petrochemicals). It was of all things cosmoline or something similar.

Anyway tomorrow it will be opened and secrets revealed, or not.

Well, those are much higher pressures than I would have thought. I've never seen 120psi on an engine cooling system. What kind of engine is it? It's obviously set up differently from anything I have experience with. My experience goes up through Detroit Diesel two-strokes, about 350hp or so.

Those pressures are higher than I would have expected as well. I am more familiar with large HXs (as in power plant's closed cooling water systems). The DP makes perfect sense for reasons previously stated. I can't imagine why a positive displacement pump would ever be used for this purpose (I have no idea what a pseudo-positive displacement pump is BTW). In any event, it sounds like you're on the right track: dismantle, clean, inspect, repair. Good luck.

The design was made assuming a non-leaking HX, and that a leak would be detected and corrected before corrosion could become an issue.



How would one know there's a leak if the saltwater side had higher pressure? It would not "leak" anywhere, it would just mix.

How would one know there's a leak if the saltwater side had higher pressure? It would not "leak" anywhere, it would just mix.

And...the total water level in the coolant system would increase.

And...the total water level in the coolant system would increase.

Good point.

Owning a boat is such wonderful fun. :sigh:

Those pressures are higher than I would have expected as well. I am more familiar with large HXs (as in power plant's closed cooling water systems). The DP makes perfect sense for reasons previously stated. I can't imagine why a positive displacement pump would ever be used for this purpose (I have no idea what a pseudo-positive displacement pump is BTW). In any event, it sounds like you're on the right track: dismantle, clean, inspect, repair. Good luck.

Pseudo positive displacement is my own term for a rubber impeller pump. They are positive displacement pumps up to a certain pressure where internal leakage will begin to affect flow.

When I was looking for leaks like this I would just put some food color in the fresh water system and run it for a few min with the sea water side off and lines removed easy to tell if it was the manifold or HX.

Pseudo positive displacement is my own term for a rubber impeller pump. They are positive displacement pumps up to a certain pressure where internal leakage will begin to affect flow.

OIC, like an outboard uses. I suppose it's a fitting description.

Owning a boat is such wonderful fun. :sigh:

Like the old saying : A hole in the water into which you pour money. Not unlike a swimming pool. Its far better to have a close friend with one than to have one yourself.

Like the old saying : A hole in the water into which you pour money. Not unlike a swimming pool. Its far better to have a close friend with one than to have one yourself.

That's like saying you want a friend with a Mc 275 and C-22, Hartsfields and a Thorens. You know it's painful occasionally (like when you pay for it) but the bliss endures.