Reducing engine noise

[John Matthews]

Has anyone successfully reduced engine noise heard from the Wheelhouse in a NC33? Mine is a 1980 NC33 with a Ford Lehman 90HP engine with a wooden engine box lined with a 3mm dense polymer sheet. The noise level in the wheelhouse was 66dB at 800rpm and 74dB at 1500rpm. I have lined the engine box with 45mm ‘QuietLife’ sound insulation as sold by ASAP supplies (their highest spec product recommended for older Diesel engines) but the noise reduction has been minimal – now 65dB at 800rpm and 71dB at 1500rpm.
Is this sound level to be expected or have newer boats had improvements that have reduced the noise. The silencer is in good condition but can it be upgraded? Will replacing engine mounts make a difference?

[39Schuss]

I’ve recently been doing some engine noise reduction efforts. The N42 has space for an extra engine cover which we did not get with the boat.

There’s plenty of science online behind it but my understanding of the general principals are to reduce vibration and noise absorption..

Vibration
For the vibration it’s likely that there are areas past the engine itself, ie gearbox, shaft that are exposed to the floor panels. You could look to add the material you’ve used or something like silent coat (https://www.amazon.co.uk/Silent-Coat-187x250mm-Deadening-Proofing/dp/B00NVF9OIA) which is designed for cars.

Things that are likely to vibrate like inspection hatches in the floor we have 4mm one sided foam tape on which stops them going crazy. I think you can get thinner but it’s what was on hand. Something like the above will help reduce the amount of sound that can start from vibration.

Noise absorption
My basic understanding is that you need something soft, then something hard, then something medium hard. You can layer this approach to enhance.

We added 12mm plywood with a heat/sound absorption barrier on the bottom similar to the one you have described and then on top placed heavy rubber mat similar but a little cheaper than the ‘high mass polymer barrier’ sold by asap. There is then another layer of absorption and whatever ply is on the top.

So I’d say the answer isn’t one thing it’s multiple layers, ideally with something relatively solid like 6-12mm ply in-between.

Where next
We did the first round recently and it’s made a decent improvement. I don’t have a DB meter and though we tested with a phone it , I now need to locate the remaining ‘opportunities’ where the engine is basically vibrating straight through into the aft cabin which will require likely more time of me upside down in the engine bay.

I’m not sure that changing the shock mounts will help that much though they do often corrode out. Can’t talk to the exhaust though the elbow on the Yanmars is recommended to be changed every 2 years.

Not sure if that’s helpful…

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[David Babsky]

Our NC33 from, er, 1982?, with a Ford Lehman 90HP engine, has two removable wheelhouse floor sections with a one inch(?) or thicker sound-absorbent fibreglass wool (?) mat held in place by sheets of perforated (what looks like) thin galvanised steel. (I cut through some of this sound insulation from the saloon into the engine compartment, to make a small hatch through which to tighten – or remove – the engine drive belts, and it took A LOT of cutting to get through the really thick sound absorbent section with a mini angle-grinder!)

Our plywood engine box beneath the wheelhouse floor doesn’t seem to have any lining at all. Nevertheless – although I haven’t measured it (sorry!) – our engine noise and vibration seem reasonable at all engine speeds ..and a LOT less noisy than the deafening high-revving Volvo in Tim Spall’s motor barge the Princess Matilda, judging by his TV programmes! Our engine noise and vibration seem similar to Skyler’s (is it?) on which Nauticat we went out to see the Tall Ships when the NA had that Nauticat event near Belfast a few years ago.

From many trips on many different kinds of large engine boats, I’d say that the NC33 noise is average to lower than many – or most – other boats I’ve navigated/driven/steered/skippered.

There are many reverberating wood-on-wood junctions where the floors and engine-box panels meet: I would apply self-adhesive rubber strips along all wood-on-wood mating surfaces – e.g; where the floor panels rest on their supports, ditto the engine-box panels – to absorb the ‘rattle’ of vibrating wooden panels. That may remove some low frequency annoying ‘throb’ or jitter.

You say “..The silencer is in good condition but can it be upgraded?..” We don’t have any kind of ‘silencer’ on our engine, just a steel elbow (which I replaced, as it had rusted) which conveys cooling water into the exhaust system, and then along a re-inforced rubber hose beneath the aft-cabin bed and out through a skin fitting. This is normal for many, or most, boats, so there’s no exhaust ‘baffle’ box to quieten the engine exhaust. Ours doesn’t seem noisy, but just burbles along as the exhaust noise is absorbed by the injected water. You could – theoretically – insert an exhaust ‘baffle’ or ‘muffler’ box, which cars and lorries have, but I doubt that it’d make much difference. (I’ve ridden in several oldish Rolls-Royces, and they are really pretty noisy compared with, say, modern Audis.)

I’d say seek out VIBRATION, such as where wood panels meet each other, including steps down to the aft cabin, door to aft toilet compartment, etc, and reduce vibration there by using rubber padding between all those surfaces.

[Roger Sturge]

I have a 1977 NC33 Mk 2 with Ford Lehmann 90hp engine. Like David Babsky I have not fond engine noise unreasonable – maybe I will now and will go in search of a dB meter! The engne compartment covers appear very heavily insulated , the plywood box around the engine less so, but I haven’t investigated their construction.

[Roger Sturge]

Access to the front of the engine in a NC33 is a pain. I was very glad of a more agile crew member when my fan belt went approaching the Caledonian Canal at Inverness. I’d be glad of advice from David Babsky or anyone else as to how to create access from the Saloon.

[David Babsky]

Tricky, isn’t it, Roger? ..I was fed up with leaning deep across the engine to try to adjust fan belt tension, or replace belts (..and I snapped a tendon unscrewing the face-plate of the cooling water pump!..) so decided to cut an access panel below the saloon seat to get straight to the fan belt. At first I wondered if this would spoil a water-tight bulkhead, but then realised that limber holes in bilge take for’ard shower water below saloon floor and into engine-compartment bilge, so realised that it’s NOT a water-tight bulkhead!

(I’ll try to find, and attach, at least one picture of this little hatchway.)

The bulkhead thickness is almost two inches, including metal-faced noise-dampening or absorbent material, so I bought a small, specialist angle-grinder (..from Robert Dyas ironmongers – but can’t now find it on their, or any other, online store..) as I realised I could never SAW through the bulkhead, and thought I’d just SLICE thro’ by pushing the grinder wheel against it. I did, but it took lots of DE-EEP cutting to get through the plywood and the sound insulation AND the metal mesh on the engine side of the bulkhead!

I wanted the cut-out panel to look neat and presentable, so asked the local ship-wright (Steve Parrish) at Ramsgate to make it look nice, and he put some simple 2-inch over-lapping (red wood) beading around the little panel I’d cut out, and with magnetic clips it just plops back into place, and the sound level in the saloon is no louder; the engine noise baffling has not been compromised, and nor has the bulkhead integrity. (I can’t think why the Nauticat factory didn’t do this as standard!)

I carefully measured before cutting, to be sure that the little hatch would be at exactly the right point in the engine compartment ..and I’ve now found the picture and attached it. Enter the saloon down the steps from the wheelhouse, turn left to face seat, lift off seat, and there’s the little hatch below the seat.

I hope this helps! ..Yours, David.

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[John Matthews]

Thanks Duncan, very helpful. I’ll definitely extend the sound insulation

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