Hospital Silencers & Prop position.

[leechy]

First things first thanks to all the replies on my new battery bank.

Now down to buisness.As i said before i am currently replacing my old engine (Perkins D3.152) with a new Beta 43 Greenline,The primary reason was noise reduction so the the next hot topic is fit a hospital silencer or not ,are they any good,ive priced one from Midland Chandlers Online (£180.00).My mate is having a brand new boat built by a well known Stoke-on-Trent firm and they advise to fit 2 ordinary silencers at £20-£30 each instead as this works just as well at half the cost.Right thats the first one out of the way ,here's the next.I'm fitting an aquadrive CVB10.10 and dont know how far forward or back my prop should sit from the outer bearing.Is there a set distance? My shaft used to sit about 1.5 inches before but i've asked other people and opinions differ from between 1 to 4 inches. The total shaft movement is about 9.25 inches untill it touches the tiller. Ive put the thrust plate in for the Aquadrive today and if the shaft clamp is tightened in this position the prop will sit about 2.75 inches away from the outer bearing ,so basically is this OK or do i need to cut the shaft to get it down to about an inch ?

 

Thanks Again

Leechy.

[alan_fincher]

First things first thanks to all the replies on my new battery bank.

Now down to buisness.As i said before i am currently replacing my old engine (Perkins D3.152) with a new Beta 43 Greenline,The primary reason was noise reduction so the the next hot topic is fit a hospital silencer or not ,are they any good,ive priced one from Midland Chandlers Online (£180.00).My mate is having a brand new boat built by a well known Stoke-on-Trent firm and they advise to fit 2 ordinary silencers at £20-£30 each instead as this works just as well at half the cost.Right thats the first one out of the way ,here's the next.I'm fitting an aquadrive CVB10.10 and dont know how far forward or back my prop should sit from the outer bearing.Is there a set distance? My shaft used to sit about 1.5 inches before but i've asked other people and opinions differ from between 1 to 4 inches. The total shaft movement is about 9.25 inches untill it touches the tiller. Ive put the thrust plate in for the Aquadrive today and if the shaft clamp is tightened in this position the prop will sit about 2.75 inches away from the outer bearing ,so basically is this OK or do i need to cut the shaft to get it down to about an inch ?

 

Thanks Again

Leechy.

Just an oppnion, doubtless there will be others, but this one at least came from a professional surveyor....

 

When surveyed our boat was found to have about 120mm of shaft showing between the stern tube and the back of the prop boss.

 

The surveyor recommended halving this to around 60mm, (which isn't far off the 70mm implied by your 2.75" measurement).

 

Not sure why he didn't recommend less, but assume it has something to do with large debris not being able to get jammed between the propellor tips and the end of the swim, either above or below the stern tube. It's generally recommended that there is a couple of inches between prop and counter, and prop and skeg, for this reason, so this margin of clearance may be best maintained between prop and any other underwater part of the boat ?

 

Alan

[Cafnod]

I really would not like to say on the aquadrive, really you should follow either their recomendations or those of an engineer who regularly installs them. were there no instructions with the coupling?

So called hospital silencers I agree seem unecessarily expensive, also I am not sure they even exist. My father in law installed stand by generator sytems in Hospitals and other public buildings all other Canada and he has never heard of a special hospital generator silencer.

Even a very small hospital has a much bigger engine than a narrowboat

They are housed in sound insulated seperate buildings too.

I would probably fit a straight through silencer followed by a baffled silencer.

The more modern engine you have fitted will have much quieter mechanical noise levels than the Perkins.

Depending on your engine placement you may be able to insulate the engine enclosure, worked wonders on our boat.

Hope this might be of some help.

 

Richard

[John Orentas]

The 'Hospital Silencer' title has nothing to do with hospitals, they are expensive but they tend to be good quality and 'big'. In silencers big tends to be efficient so 'you pays your money', personally with my thrifty instincts called in at a wholesale exhaust and silencer stockist and bought the biggest unit I could fit into the available space, quality is not much of an issue with diesel silencers as they don't suffer from corrosion.

 

I carried out something of an experiment which worked in the event, the outlet is placed seven or eight feet forward of the steerer's position, this gives a considerable reduction in noise and more importantly puts a lot of space between the driver and the exhaust gasses.

 

As for the 'free prop shaft length' the best rule is simply position the propeller in a central position up and down and for and aft, two to four inches of visible shaft is OK. The fact it has an Aquadrive is immaterial.

[charles123]

The hospital silencer is very big and will take up a lot of space but the kind of upgrades you are talking about will give you a one off opportunity in inproving these things, therefore if you have the space I would do it. While expensive it is a one off cost and you know you have fitted the best.

Secondly the majority of noise in the inside of the boat comes from the engine itself not the silencer so thats the other area you could consider improving. I used low cost insulation that works to a degree but I cant claim its really quiet (see my build blog) The only way in my opinion to really reduce noise is to have a cocoon and the only one I know of is Beta, but since you have already specified your engine that does not apply

 

Charles

[Guest]

I once read that the prop should be positioned at an imaginary point where the swim was projected to a point; I am not altogether convinced. Sorry have never had a "hospital" silencer.

[dor]

As Charles says, much of the percieved noice comes through the engine mounts. The Aquadrive should allow you to have softer engine mounts which will reduce noise transmission.

 

When I fitted a Python Drive (similar idea to the Aquadrive) it reduced the noise on the stern considerably.

[Canal Shop Man]

The name 'Hospital Silencer' DOES have to do with hospitals. It was originally designed for use on hospital generators which needed to be 'silent' so as not to disturb the patients. The applications are now widespread, but the name has stuck.

 

I have been on a customers boat which had one of ours fitted and the effect was quite dramatic. He was about to fit 32mm thick sound deadening panels around the engine bay. After that I think he would need a warning light to show him the engine was still running!!!

 

Whilst they are expensive (we supply them for £145) they last for ages, being of very strong construction. We get them made to measure in a few days so you have the advantage of siting the inlet and exhaust wherever you want (within reason) which can make it a lot easier to fit.

 

With regard to the gap between the prop and the hull, I think too big a gap will allow big logs or other debris to get jammed, but I reckon 2 3/4" shouldn't be a problem.

[Cafnod]

A friend of mine is time served as an electrical and mechanical engineer with the NHS and has done a lot with standby generator systems for both the NHS and the MOD. He does not know what a hospital generator silencer is. Any hospital generators are a hell of a lot bigger than you can drive with a narrowboat engine.

My father in law spent his working life in Canada installing and maintaining standby generator systems in public buildings and has never come across the term hospital silencer. I served my apprenticeship as a fairground engineer and have had to do a lot with silent running systems in recent years, never heard the term there either. Suspect it is some sort of marketing name that has become embedded in the (often highly inaccurate) canal mythology.

[Guest]

I've used hospital saddles.

[elessina]

fit a hospital silencer or not ,are they any good,ive priced one from Midland Chandlers Online (£180.00).My mate is having a brand new boat built by a well known Stoke-on-Trent firm and they advise to fit 2 ordinary silencers at £20-£30 each instead as this works just as well at half the cost.Leechy.

 

Leechy,

 

Our new Perkins 92hp with hospital type silencer on our Replica Dutch barge is much quieter than our Beta 20hp engine on our narrowboat. I put this down to the hospital type silencer, not the engine, so I would recommend the hospital silencer, if you have the space to fit it.

 

The alternative of two silencers is interesting. Probably take up smaller space. Must be better than a single one. Assuming the boat builder has fitted them before he will know how quiet they are, and has little to gain either way, so may be worth trying.

 

I believe the term 'hospital' silencer does indeed derive from silencers fitted to hospital generators some many years ago, in the same way as hospital radiators take their name - common in hospitals.

 

Ian Petchey

DB Elessina

[leechy]

Thanks again folks all this advice is great and also gives both points of view.My new engine does have flexi mounts so that should help with stopping the vibration .After reading all the views on the prop position i think i will leave it at 2.75 inches and give it a test then if it does need trimming i can soon take out the Aquadrive and trim a bit off the shaft.I have started to insulate the engine bay with fire proof,foil faced foam with an extra internal plastic membrane ,total thickness 32mm (midland chandlers again im afraid £20 for 900 x 600).The trouble is i dont know how far to go.I've done under the counter all round the back of the diesel tank,dont know wether to do the weed hatch,then i'm going to make a removable ply cocoon,with foam, for the engine ( joiner by trade so that shouldn't be a problem). should i do the 200mm under the engine supports and the metal engine baulk head.lastly any ideas on ventilation methods for the engine "cocoon" and how would you make aventilation baffle ?

[charles123]

Thanks again folks all this advice is great and also gives both points of view.My new engine does have flexi mounts so that should help with stopping the vibration .After reading all the views on the prop position i think i will leave it at 2.75 inches and give it a test then if it does need trimming i can soon take out the Aquadrive and trim a bit off the shaft.I have started to insulate the engine bay with fire proof,foil faced foam with an extra internal plastic membrane ,total thickness 32mm (midland chandlers again im afraid £20 for 900 x 600).The trouble is i dont know how far to go.I've done under the counter all round the back of the diesel tank,dont know wether to do the weed hatch,then i'm going to make a removable ply cocoon,with foam, for the engine ( joiner by trade so that shouldn't be a problem). should i do the 200mm under the engine supports and the metal engine baulk head.lastly any ideas on ventilation methods for the engine "cocoon" and how would you make aventilation baffle ?

 

could you take some photos and post them?, silencing an engine is a topic very interesting to me and I am sure many would be interested in how you go on

 

Charles

[Clarissa]

Probably good idea to find out the back pressure of silencers in series or hospital type, and ask Beta if suitable. I guess each extra silencer adds x back pressure.

Propellor needs to be positioned in ideal place for water flow, ie. tips not too close to hull and rudder when hard over. I cut an inch from the shaft by lining bilge and gearbox with newspaper, wetting with hose and using grinder.

Propellor doesn't want to be too close to exit as you may want to fit a rope cutter one day and would not want to replace shaft with longer. My propellor is about 35mm back from the hull exit (cutlass bearing).

Another point - in full ahead the engine leans forward slightly, pushed by propellor, on flexible mounts and thus the propellor shaft must be slightly drawn in and so the space reduced (not by much).