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post #1 of 8 Old 09-04-2004 Thread Starter
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Underwater Exhaust

I just finished a 2 week cruise on my 1988 Catalina 34, my second year with the boat. For the first time, I left with full water and fuel tanks plus gear for our family of 5. I quickly noticed that while motoring at about 7.5 knots, the stern sank to the point that the exhaust port was underwater as were the automatic and manual bilge pump exit ports. All were above water while sailing and when anchored. I realized that I was creating several new through hulls which were not as sturdy or as protected as the normal through hulls. Is this normal and expected or am I playing with a dangerous situation if a hose should fail?
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post #2 of 8 Old 09-05-2004
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Underwater Exhaust

In following seas it''s not unusual for the exhaust on my C270 to be covered for various periods, and perhaps the others momentarily, but never for long. Catalina Direct has a flapper valve available for exhaust outlet but installation is difficult. Not really sure if it would be required except in extreme conditions with water lift exhaust. I''ve never motored anywhere near that speed, the outlet for manual pump is pretty high the stern must have been down quite a bit if it was always underwater.
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post #3 of 8 Old 09-05-2004
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Underwater Exhaust

My friend has an ''87 C34, and we typically motor just over 6kts. with the Kubuta diesel. What horsepower does your plant put out, and how hard are you driving it when traveling under power?
What happens when you reduce speed: at what point do those ports clear the water?
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post #4 of 8 Old 09-06-2004 Thread Starter
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Underwater Exhaust

My 1988 Catalina 34 has a Universal 25 HP diesel and I was motoring at about 2400 rpm, not max speed but close to it in order to try to shorten an 11 hour trip. This is my first boat with an inboard and I haven''t come upon any guidance regarding speed/rpm''s when motoring. If I''m pushing the boat too hard, someone let me know. Thanks.
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post #5 of 8 Old 09-07-2004
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Underwater Exhaust

The rule of thumb is 80% to 85% of maximin RMP is the prefered cuising speed. Check your manual. You should find the maximun horsepower rated a given RPM. IF you run 80% or so of that you will be fine.
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post #6 of 8 Old 09-13-2004
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Underwater Exhaust

diesels like to be run hard so dont be shy about running her at 80%.
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post #7 of 8 Old 09-14-2004
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Underwater Exhaust

On my Oday 31 my exhaust goes under the water when motoring at a fair rate of speed. Usually when we are in a hurry, following seas, everyone is in cockpit. So far it has not been a problem. I hear the bubbles and it shoot out the water when it comes up. The exhaust hose or pipe goes up inside the boat to help eliminate the sea water from siphoning into the engine. Hope it never fails to do that.
I have noticed that when I do push the boat the exhaust water seems to be a little black.
Not sure what causes that.
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post #8 of 8 Old 09-16-2004
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Underwater Exhaust

Same on the Catalina 42, both bilge pumps and exhaust are below sealevel at times. Thought about it, don''t worry about it too much, all hoses have loops in them, so siphoning is unlikely (unless the bilge pump runs....and then the back flow starts a syphon untill it runs again....and again....) I plan on putting in a buzzer for when the pump runs.

If any of these hoses fail, stopping a leak is as simple as stopping the boat.....much easier to deal with than a hole well below the water line.

Oscar.
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