why did exhaust hose submerge?

[alman]

Here's the scenario........boat just launched after winter on land and motoring back to our marina.
After about 2 hours we hear a very loud engine noise, almost like a speedboat, but no other boats around. We realize it is coming from our boat.
Checked the raw water exhaust hose that normally exits the stern about 6 inches above the water line. It is submerged!!! Then the engine alarm gives loud signal, red oil light comes on and engine quits.

Oil check reveals that is not the problem. Then we notice smaller belt is broken. We were able to replace that and be on our way. But very confused as to why the stern went down allowing the exhaust hose to go under water and wondering if it could have caused any damage?

The boat is a 1982 Allmand 31. The engine a Yanmar 3 (something?) 30. It is probably about 15 years old.

[mitiempo]

All boats squat at the stern when powering. The only other reason can think of is that you took on water.

[RobGallagher]

I have a few guesses:
Fuel tank full?
Water tanks empty?
Holding tank dry?
Dock lines, fenders, emergency anchor, etc. are in the lazarettes?
Bar not stocked?
All that crap you took out of the cabin last fall has not been loaded up yet?

The boat might be a little heavy in the ass end....err.... stern.

As far as the broken belt, maybe the alarm had something to do with that? Sometimes two issues are not always directly related to each other.

[celenoglu]

Check your exhaust, while you are cruising. Most probably it is in the water normally while you are on the way. 6 inches is far too less to position the exit. The extra noise of the exhaust without water passage caused you to check your exhaust.

[Omatako]

OK it's always a little hard giving cast-iron advice on a boat you've never seen but here is my take on some of the questions.

Firstly the oil light came on, the engine quit but the oil level was OK when it was checked. I don't think there is too much wrong there, AFAIK some Yanmar panels use common warning lights and buzzers for heat and oil alarms.

Broken "small" belt is probably the water pump drive which would have caused the alarms to go off.

What bothers me is that "the engine quits". AFAIK Yanmar does not have a cut-out system on their engines. I hope it didn't partially seize from an overheat and then free up once it cooled. That could have long term downsides.

I can't explain the exhaust going under water when it's normally not but suffice to say that will not cause a problem. The exhaust outlet on my boat is always underwater.

[centaursailor]

I have a Yanmar GM 30F, sea water cooled engine on my Centaur. The small belt refered to is the water intake pump and if this fails it would set off the overheat alarm. Cutting out is not automatic so there could be good reason to wonder if the engine siezed as suggested.
It could also mean sea water got into the engine cylinders as it was not getting away freely with the exhaust port below the water.
Having enough water aft to lower the exhaust outlet beneath the the level of the sea would worry me most. The exhaust gas pushes out the cooling water and wouldnt work well, if at all, in this senario.
Is it possible the exhaust somehow discharged cooling water into the boat rather than into the sea?
This could also explain the belt breaking, pumping against a fully charged engine with little or no water escaping the system once the exit port droped below sea level?
Just my thoughts.

Safe sailing.

[deniseO30]

You may have been motoring at hull speed and with the engine at full bore. as the engine tries to push it beyond hull speed what really happens is the boat actually slows down and the wake starts to back wash to the stern.

[svHyLyte]

Originally Posted by alman

Here's the scenario........boat just launched after winter on land and motoring back to our marina.
After about 2 hours we hear a very loud engine noise, almost like a speedboat, but no other boats around. We realize it is coming from our boat.
Checked the raw water exhaust hose that normally exits the stern about 6 inches above the water line. It is submerged!!! Then the engine alarm gives loud signal, red oil light comes on and engine quits.

Oil check reveals that is not the problem. Then we notice smaller belt is broken. We were able to replace that and be on our way. But very confused as to why the stern went down allowing the exhaust hose to go under water and wondering if it could have caused any damage?

The boat is a 1982 Allmand 31. The engine a Yanmar 3 (something?) 30. It is probably about 15 years old.

The belt failure and the submersion of your exhaust exit were coincident but unrelated. At speed one develops a bow wave/stern wave with a "hollow" between the two amid-ships with more of the yacht's weight/displacement supported at the bow and stern hence the greater immersion at the yacht's ends. (It is the length between the bow/stern waves that dictates yacht's limiting speed through the water.) Viewed from above at the cockpit, the yacht appears to "squat". Viewed from the side from some distance, most sailing yachts power pretty much on their lines.

FWIW...

[alman]

Thanks for all the input. Putting together the thoughts from a couple of you, this is what we think happened. Belt broke (and yes, it is the raw water pump, hubby was asleep when I posted last evening). No water coming out of exhaust allowed us to hear the very loud sound of the engine. No water to cool the engine, caused it to overheat and seize, setting off the alarm.
Of course, this does not explain the exhaust port below water.
We have checked that at various times over the years. for various reasons...and it has never been underwater before. And the bilge did not fill, so I don't think we were taking on any water.

I guess we'll find out this weekend if there are still problems. We were just 5 minutes away from the marina when it happened and we just sort of slowly made our way to it after we replaced the belt. That took about 45 minutes, finding belt and right size tools, etc., giving engine time to cool.

I'm trying not to think about any long-term serious problems related to it.

[alman]

Originally Posted by Omatako

What bothers me is that "the engine quits". AFAIK Yanmar does not have a cut-out system on their engines. I hope it didn't partially seize from an overheat and then free up once it cooled. That could have long term downsides.
.

I probably don't want to know, but what are those long term downsides you mention if an engine partially seizes?