Yanmar 9HP noisier than 18HP!?

[Zagato]

Just wondered if the Yanamr 9HP is noisier than the 18HP. Is the 18HP a single cylinder as well. A twin should be quieter I think.

My kids spend a lot of time in the cabin playing but it's deafening with the 9HP Yanmar we used to have and I didn't like to have them in there longer than 15 mins. Cockpit space is tight on a Crabber24, enough room for 4 and that's it and nowhere for the kids to move about a bit! They get restless after 10 mins

 

[Cloven]

I currently have a Yamaha 2.5hp 4 stroke on the tender and its far quieter than the former Johnson 2HP 2 stroke. However, I also regularly use a 30 HP Suzuki 4 stoke on a rescue/teaching boat and it is very quiet indeed - so much so that it is almost inaudible at tickover.

Maybe its something to do with the size of the unit being able to more efficiently damp out any sound.

 

[Lakesailor]

I find it astonishing that boat builders don't address the awful NVH problems of inboard engines.
If car manufacturers can make a powerful diesel engine nearly inaudible just a couple of feet away from you, it can't be impossible on a boat.
Sticking to traditional technology like near rock hard engine mounts and thumping engines derived from plant equipment is not the way forward.

 

[Tranona]

The 18hp is a two cylinder - indeed two of the 9hp ones, but the layout of the crankcase and ancilliaries is different. An 18 is quieter and smoother and many are freshwater cooled which reduces noise somewhat.

The single cylinder 1GM10 is as basic as you can get, sea water cooled and compared with most other diesels light weight. It does bounce around a bit so needs good mounts and preferrably isolated from the shaft with flexible coupling. On my installation I had a Bullflex coupling and a rigid shaft with bearings either end and it was very smooth, but it was in a heavy wooden boat with massive engine beds. Airborne noise is difficult to reduce, partly because there is no silencer on the air intake, but exhaust noise can be reduced by using a big water trap and if room a silencer. However, the problem with boats (unlike cars) is the difficulty of insulating the noise from the rest of the boat. Insulation helps, but there are just too many paths for the noise to escape. Add to that many GRP boats make good sounding boards!

When you move up the size/complexity scale, things do get better. You can hardly hear the 3cyl Volvo running when in the cockpit of my Bavaria, but the noise increases the closer you get and the aft cabins are much noisier. However, it is a "nice" noise and knackered crew have no difficulty in sleeping in the cabins.

 

[Zagato]

Thanks Tranona that's good news Helps a lot with the decision making.

 

[rob2]

I used to get the short end of the stick when testing cars in a wind tunnel - sitting in the tunnel adjusting the test equipment. I can assure you that a BMW which is very quiet in the cabin sounds like a tractor from the outside when doing 60mph! The car body is designed not to drum, no flat panels and lots of sound insulation.

The 1GM engine is likely to be working pretty hard for most of its life and is rather short on balancing devices which multi-cylinder engines don't need so much and, as already said, there's little mass in it to absorb sound. You could certainly devise a silencer for the air intake and quieten the exhaust as much as possible, but the main gain is likely to be from improving the sound insulation on the engine box panels. Unfortunately, whilst soundproofing would be best served by sealing the box, the engine must get combustion air from somewhere and also cooling flow. There was a posting on another thread which suggested that feeding combustion air direct from outside the box is a bad idea - you lose the cooling element and it becomes almost impossible to stop the engine in the event of a runaway.

Rob.

 

[Zagato]

A friend bought a brand new Shrimper last year, (37K ) and his 9HP Yanmar is a lot quieter which I put down to the fact it is contained in a much smaller area which has soundproofing. On a Crabber24 for instance the engine sits forward just behind the steps in a cavenous open area under the cockpit. As it has no insulation and sits behind the steps the noise is almost unbearable in the cabin. It was much the same for my old Folksong - you couldn't stay bleow with the engine running.

How nice it was to go back to a twin cylinder 6HP that you can just hear tick over - bigger boats are not always better, especially when they have cramped cockpits