Near Engine Disaster...Your Vented Loop?

[chuck]

Cruising Bound

Interesting. I have seen so many variations on these raw water systems over the years that I cannot even remember them all. This is one that is yet again a little different. From what I can tell from the pictures, the tee fitting in the line coming from the heat exchanger is there for the purpose of dividing the flow of water from the raw water pump so that all of it does not get injected into the exhaust. The reason for that is so that the engine does not have to push all of the water up the exhaust hose to the stern which will increase back pressure. It does not take the total capacity of the raw water pump to cool the rubber exhaust hose (from the injection elbow to the stern outlet fitting). On our HC 43s if you measure the amount of lift (height) from the waterlift muffler to the top of the loop (or standpipe muffler) in the lazarette it is close to 6 feet. The maximum lift that is regularly recommended is 40” so keeping the discharge water in the exhaust system to a minimum is quite advantageous.

In the systems that I install, there is a valve on the exhaust elbow side of the tee so that it can be choked down to limit the amount of water that gets injected into the exhaust. I usually adjust this to about 40% into the exhaust and 60% discharged overboard. In your case (from what I can tell) they choose to discharge the excess out through the hull on the port side of the boat rather than run it all the way to stern as they do in more cases than not. In most of the HC exhaust systems that we have discussed here on the board that excess cooling water hose runs to the stern and is injected into the stainless standpipe muffler in the lazarette. (Chad I am pretty certain that you will find that this is what the hose that you have running down and aft is). When I rebuilt my exhaust system I chose to discharge the excess out through the hull right next to the exhaust fitting at the stern rather than into the new standpipe so that I could monitor the flow separately from the water that is coming through the exhaust. I have always done it this way so that I can keep a better eye on things. I have always made it a habit to monitor the exhaust water and the excess discharge water from time to time. (Usually every few hours or so if I think about it – kind of like looking at the oil pressure and temperature gauges but not quite as often) If I notice the ratio changes it tells me that there could be a problem at the tee or with the valve at the tee. If the flow becomes noticeably lower in volume I know that either the strainer(s) are getting clogged or possibly I am loosing impeller vanes. This is something I have been taught to do as a kid and have been doing all my life.

As you mention, because the discharge is on the port side of the boat rather than out through the stern (where there is usually a loop all the way to the deck) you had been getting some surges from waves pushing into the exhaust system and being absorbed by the engine. I think it is great that you put the vented loop in that line which will eliminate the problems that could occur if the check valve were to fail to seat each time there was a surge. There still does exist the possibility that if you load the boat down to the point to where the tee that is under the galley countertop gets too low, sea water could in fact come through the intake through hull, through the raw water pump (engine not running) through the heat exchanger, over the top of the tee and flood the exhaust system. On the 43s we have often discussed the galley sink not draining very well due to the fact that the bottom of the sink is not very high above the waterline. If your boat is lightly loaded and you do not have this problem then most likely you will not have a problem with the raw water making it over the top of the tee (which on a lot of HCs also serves as the vented loop). I didn’t have a problem for years until I had managed to load the boat down. Also if the tee is more on the centerline under the galley counter rather than on the port side up against the ice box you will be less likely to have this problem when sailing on starboard tack. If you look into the sink drains, you might be able to get an idea where the water line is. At this point it is just below the bottom of my galley sink which is another whole problem that I have yet to deal with on my boat. We have discussed the problems with the sinks not draining here on the board as well.

Another thing to keep in mind is that more and more people are going to these dripless shaft couplings. Many of these have a water injection fitting on them which is plumbed into the raw water circuit. You have to be careful that the place that you tee into the raw water circuit is on the heat exchanger side of the vented loop and not on the exhaust elbow side of the vented loop. I have seen a case where the engine was flooded through the shaft seal rather than through the raw water pump. I have heard of a second case where the same thing had happened.

I hope that I haven’t rambled on too much here as I have a tendency to do.

[johnrobinson]

Picture of the exhaust pipe reinstalled behind the Cape Horn . . . see last post

Regards, John

[kelpie]

FWIW, in the first 5-10 years we owned our HC 33, we had TWO times where we found cylinder #4 filled with seawater. Each time, a different version of siphon break in line for the cooling water leading to the wet exhaust had failed--closed--leading to water siphoning into the engine. Fortunately, in both situations, we detected the problem almost right away and were able to drain the engine and do several sequential oil changes.

Anyway, for the record, I wanted to mention several aspects regarding these siphon breaks:

1) They tend to be inconsequential in size and easily clogged. Also, cheap and the weakest link in risking back-filling your engine.

2) Even with a larger, more skookum version, we had the problem again--the siphon break valve still clogged on us.

3) Nigel Calder wrote an article where he mentions using vacuum breaker which has an advantage over a typical siphon break:

"A device that may require less attention than a siphon break is a vacuum breaker from the Scot Division of the Ardox Corporation (Fort Lauderdale, Florida). This consists of a plastic poppet valve held against a Delrin seat by an external stainless steel spring. Since the valve is normally closed, neither fluid nor vapor in the system can get past the seat. Salt crystals do not have a chance to form, so there is less likelihood of problems developing than with a traditional vented loop. When the engine stops, the weight of the water in the lines sucks the valve open. admitting air to the system, and breaking any tendency to form a siphon. These valves work well, with just one potential problem: Opening the valve against the spring requires a certain minimum suction pressure. For this suction to develop, the valve must be installed 2' above the waterline at all angles of heel. On many boats, this height may be difficult to achieve."

4) We finally gave up on trusting our siphon break valve and converted over to a simply "T" which is always open and through which water always "leaks" when the engine is open. Tubing from the T (where the siphon valve would normally be) leads up to the box on the inside of the hull where the wet exhaust itself runs just prior to dropping to the exhaust hole which exits the boat. The tubing continually squirts into the top of that box and simply joins the wet exhaust. When the engine turns off, air -- without any questionable, apt-to-fail valve in the way -- is drawn into the tube and breaks the siphon EVERY time. Since this change, we've had no problem for the past 20 years. Also noticed Nigel mention this approach in an article:

"Finally, another approach to siphon prevention is to dispense with the siphon break or vacuum breaker altogether, and replace these devices with a length of hose carried up under the deck and vented overboard; preferably into the cockpit. The hose must be taken high enough that the back pressure from the exhaust system does not cause the vent to weep when the engine is running. In general, as long as the vent is above the highest point in the exhaust hose, there will not be a problem."

[turbin]

[kelpie]