How can I make my boat engine quieter?

[dearboat]

I am looking for a soundproof an inboard motor compartment to install a high-quality soundproofing foam barrier to lose up to 35 dB of noise.

[WillieC]

Welcome, dearboat!

There are a number of products out there. I was just perusing the Hamilton Marine catalog yesterday and recall seeing the good stuff with the lead or lead-like liner that is part of the sandwich construction which also includes foam and reinforced foil. It is about an inch thick. Google boat engine insulation.

This stuff was used on my A25 to try to soothe the inner savage beast 3 cylinder Volvo Penta. Nice try. Definite improvement over the original oil impregnated fiberglass shard remnants that clung to the lower walls of the enclosure. I ripped that out and left it alone. (Still dealing with the oil. It IS a Volvo.)

The 35 db attenuation sounds optimistic. The success with any material will depend on a good installation and realistic expectations. The ancient VP emits such a range of low frequency protestations that can only be addressed with a new Beta engine.

Keep us posted!

[DesertAlbin736]

Never did get a chance to be aboard WillieC when the engine was running. Have said this before, but the Yanmar 3GM30F in our boat is much quieter & way less vibration than the VP MD17C that Stephan Duggan had in his A25 DeLuxe "Manatee" which we got a ride in up in Ontario's Thousand Islands region. How much of that difference is due to the basic difference between the two engines (24 HP YM v 36 HP VP) or the AquaDrive CV joint, /thrust nearing & soft motor mounts we have I can't say. Pretty sure the foam engine box insulation we have is OEM.

[dkirsop]

The key to sound reduction in a boat is to first decide how you are going to enclose your engine. It is important that all potential openings through which sound can be transmitted be sealed. In most boats, with the engine bay open to the hull, this is almost impossible to achieve. Especially if the intake air is not ducted into an engine box.

Cable entries, exhaust ducting, coolant hoses and other openings also need to be sealed to prevent sound leakage. You can not do much about the propeller shaft, but only a minimal opening should be used to exit the engine box.

Note the term "engine box". Absent a sound proof engine room you will need one to achieve any appreciable sound reduction. The A25 was designed with sound reduction in mind and provides all of the above along with a lining of sound absorbing material. Based on memory of previous tests the sound level at the wheel is 78Db. Acceptable but not quiet.

In two previous boats, where the engine bay was simply a compartmentalized portion of the hull, installation of sound absorbing material provided very little sound attenuation. I did not measure it but I would estimate only 5Db reduction was achieved. Conversation at 3/4 throttle was impossible.

I wish you success on your venture and encourage you to first think about how you will contain sound transmission and then consider what sound absorbent material you will use to finish the job.

[WillieC]

Resurrecting an old thread with an excellent title...

The Starfleet Commander just handed the prime directive...we need a quieter boat. She's ready to have me install a modern engine in our A25, damn the expense.

I, the broken down, crusty curmudgeon of what lamely passes for The Boat Guy, am NOT ready to replace this Volvo Penta beauty. It ran nearly flawlessly on our recent Princess Louisa excursion. Daily engine checks revealed known oil leaks, drips, occasional spurts and belches, but nothing alarming and easily dealt with.

So, I need to get serious about finishing the sound insulation. (I should note that the real offensive noise is the low frequency stuff. This constant drone seriously impedes verbal communication so we have resorted to more primitive forms of sign language that readily convey frustration and general disapproval but are limited in conveying nuance.) The dog house cover is well (enough) insulated with the one-inch multilayer sandwich material. Previously I removed the lower engine box sidewall insulation from inside the engine compartment as it was in bad shape as noted elsewhere. A real mess of oil-sodden shredded fiberglass capable of lacerating human flesh just by looking at it.

I am thinking of using the same sandwich material only installing it on the outsides of the lower engine box. I need all the room I can get inside the box just to maintain the old green monster and I like the hard fiberglass surface since it is easy to clean up, not that it needs it.

Is it acceptable, meaning effective, to install the insulation on the outside of where the noise is coming from? The way I see it is the whole boat is a reverb chamber to amplify the percussion of the slapping pistons, whirring gears, bending pushrods, etc. Applying insulation to any and all of these surfaces ought to dampen the resonant frequencies and hopefully save the SC's vocal chords from continued damage from issuing instructions to the hapless crew. Whether or not the crew is listening is not part of this discussion.

I understand Mr. K's point about sealing up openings. Fortunately there are few in the A25.I think I'll start with finishing the lower box on the outside. Any opinion on which way it should face? I am thinking shiny side out, away from the engine, with the foam side glued lightly to the fiberglass enclosure. It will end up mostly wedged between the box and the fuel tank, portside, and between the box and the battery boxes on the starboard side. I hope I can stuff one inch thick material therein.

Would any benefit result from installing this insulation on the underside of cabin sole surfaces?. There might be room between the fuel tank and its cover/sole. The starboard side floor structure would be fairly easy to insulate with the proper pins.

Moving aft, the removable floor section above the shaft compartment which allows access to the shaft seal, muffler, bilge pump, etc. would also be easy to insulate. Again, here is another resonating section of bare fiberglass hull sections that could benefit from damping, including the side and rear vertical components. Does this make any sense? the hunk of 3/4" serious marine grade plywood that serves as the access cover takes up a whole symphony of classic rock tempos throughout the entire rev range. You should have witnessed it before we ever knew shaft alignment was a thing. We currently have installed one inch thick rubber interlocking exercise mat, stylish gray to match the new sunbrella and bilge paint gunnel stripe. This stuff works great for helping dampen a lot of noise and is easily removed for inspection and cleaning.

Comments and suggestions are often appreciated. I'll try to remember to snap some pics today while the fiberglass person is repairing the mishap in Friday Harbor. No film at eleven until the check clears.

[WillieC]

In talking with a few other A25 owners, well Andy and Mindy, I was reminded that most (many, some?) A 25s have the one piece engine box with half lid. To access the whole engine you gotta lift the THE WHOLE BOX out! Dumb. I kinda like my split box with the sliding half cover.

Anyhoo, I did put sound insulation on the outside of the lower box half. I think it is quieter. Or my hearing loss has progressed…. The left over scraps went on the deep bilge cover panel lid. I figured that whole portion of the sub-sole enclosure was just a bass drum amplifying whatever low frequencies reverberated therein. I’ll say it is quieter. I’ll let you know after the South Sound Voyage later this month.