Exhaust route on LM28 questions

[johnc]

I am still finding potential issues with the LM28 I have recently purchased.
The engine is a Bukh 36HP saildrive unit. My worry is the way in which the exhaust hose is run.
The hose exits the engine enclosure on the port side and runs for about 1.5 meters towards the bow and then bends back towards the stern through a Vetus type silencer. From the Vetus silencer it runs back towards the stern and connects to the hull exhaust fitting.
This seems a very torturous route and is probally about 5 metres long. My other concern is that the route does not seem to have a anti-siphon unit or even a upward bend.
Now I think this is a recent fittment of the Vetus unit as I found the original large rubber exhaust unit in a locker. It may be that the system was modified not long before the previous owner died and has perhaps never been tried in real conditions with following seas.

Can anyone advise as I do not think this system is safe.

[petedg]

Hi John, we have a 36hp Bukh, 1984 vintage on our LM30. The 50mm ID hose is attached to a similar Vetus type muffler on the starboard side and then continues to a ‘T’ piece fitting astern about 50cm above the level of the muffler. The hose from muffler to stern ‘T’ piece I would estimate is about 2 metres in length and held up with some very large cable ties. There is another piece of 50mm ID hose on the other side of the ‘T’ piece which goes down to the hull exhaust fitting which is close to the water level. The large difference in height between the muffler and ‘T’ piece would certainly prevent any water going back the wrong way. It has worked without problems for the last 5 years, although during the coming winter I will have to replace the hose because it has sprung a small leak close to the muffler, although a temporary repair with self amalgamating tape should see us through till the end of the season. I would suggest that you need an upward bend at the stern.

[Sula]

Jul 26, 2019 17:50:12 GMT petedg said:

 I would suggest that you need an upward bend at the stern.

I agree: the exhaust needs to rise as high as practicable (in our sort of boat) before falling again to the exhaust outlet, in order to prevent water being washed back into the engine.  This can be achieved either by routing the hose so, or by means of a proprietary gooseneck fitting.

You mention a 'muffler', but a water-trap would, I think, be more common.   The distance between the water-trap and the top of the gooseneck/upward rise should not be so long as to result in the water-trap over-filling when the water before the top of the rise falls back when the engine stops.  

My understanding is that an anti-syphon valve is not (usually) required in the exhaust itself as there is enough gas with the water to break the syphon if you have the gooseneck rise under discussion.    An anti-syphon is generally required in the sea-water cooling circuit before it enters the exhaust injection elbow.

Vetus provides some good guidance and diagrams in their catalogue which can be viewed here. viewer.zmags.com/publication/39b85263#/39b85263/1 See especially pages 80-81, and 84-85, including the calculation of the capacity of the water-trap compared to the length before the gooseneck.     

[johnc]

Thank you Petedg and Sula. You have confirmed my concerns that the installation of the exhaust system is far from satisfactory and does not comply with the manner set out in the Vetus catalogue. I have also noted that there is no anti syphon loop in the water connection to the exhaust bend. The device I described as a silencer is in fact a "waterlock" What is more concerning is that I have not had the oppertunity to test the engine. This will now become a priority with a complete rework of the exhaust system.

Many thanks for your help

[tonyp]

In case it helps at all the pic below show the anti-siphon system on our DV24. The pipe with writing on it comes from the thermostat and goes right aft under the poop deck where it rises up above water level, with an anti siphon at its apex. The pipe then descends the loop and comes all the way back to enter the exhaust elbow. It seems a long way for the pipe to go but you somehow have to get the hose above water level to eliminate the risk of siphoning. I suppose you could go under the cockpit seats and up the side of the hull and back again, but this system seems to work fine.

Tonyp

[Brian & Glenda]

A loop rising above waterline serves no purpose if it does not have a one way anti-siphon air inlet device at the top of the loop. The exhaust in our LM32 with Bukh DV36SME rises in the stern, behind the rudder post and has a vent loop at that high point. There are good illustrations of what is required in the Bukh workshop manual, but I am far from home at this time, so can't post that illustration.

[Sula]

Aug 5, 2019 21:46:26 GMT Brian & Glenda said:

A loop rising above waterline serves no purpose if it does not have a one way anti-siphon air inlet device at the top of the loop. The exhaust in our LM32 with Bukh DV36SME rises in the stern, behind the rudder post and has a vent loop at that high point. There are good illustrations of what is required in the Bukh workshop manual, but I am far from home at this time, so can't post that illustration.

I believe you are mistaken.  I can find no such illustrations in the DV36 Works Manual, Operators Manual or Parts List, and have just trawled through a Google image search for boat wet exhaust systems and found dozens without any anti-syphon in the exhaust, and only one with (that relates to an installation where the whole engine is below the waterline, and even then not is not clear to me what it could actually achieve).  

The only purpose of the loop above the waterline ('gooseneck') is to stop water from outside the boat being washed into the exhaust system (e.g. by waves approaching from the stern, or the stern squatting lower in the water at high speed or with a heavy aft load).  There is no need for an anti-syphon in the gooseneck because the content of the exhaust system is mainly exhaust gas, not water.  
 
When the engine stops any water on the engine side of the gooseneck falls back to the water trap, while any water on the outlet side simply drains down out of the exhaust outlet to the sea.  The gooseneck will then be filled only with exhaust gas and air, and there is no continuous water which could create a syphon.  (In fact, some systems have the air inlet for an anti-syphon valve elsewhere in the cooling system plumbed into the gooseneck.)

No boat I have owned has ever had an anti-syphon valve in the gooseneck, and if I had one I would be concerned about the potential for exhaust leakage into the boat.

You do need a water trap below the level of the engine of sufficient capacity to ensure the cooling water already in the exhaust system cannot reach back up to the engine (it also acts as a something of a silencer).  Vetus provides a formula for use to calculate the water trap capacity required, given the length and diameter of exhaust hose, reflecting that the water will be only a minor proportion of the content of the hose: it is mostly exhaust gas.  The Vetus formula assumes that water forms only 25% of the content of the exhaust hose, and no doubt that is informed by extensive research/experience, and includes a generous uplift for safety.   

Additionally, if the cooling water injection point into the exhaust elbow is close to the waterline (Vetus suggests if less than 15cm (6") above waterline (n.b. at any angle of heel) an anti-syphon valve needs to be fitted well above the waterline (usually between the water outlet from the engine and the injection point into the exhaust elbow, can also be between cooling water inlet and engine).    This anti-syphon valve is to ensure that once the engine is stopped additional water cannot syphon into the exhaust system from the cooling water intake, via the water pump and engine.  An anti-syphon valve in the exhaust gooseneck will not achieve that. 

Excellent expert advice, diagrams and the formula mentioned can be found in the Vetus catalogue here viewer.zmags.com/publication/39b85263#/39b85263/80
and here viewer.zmags.com/publication/39b85263#/39b85263/84 (all of which is relevant whether or not you use Vetus parts).