How to Soundproof a Boat Engine Compartment: 5 Affordable Tips

Many boat owners are looking for a way to soundproof their boat engine and direct injection diesel engine in particular. They can go way over 100 dB, depending on the size and type. And that “knock” that older diesel engines make is really what ticks many people off.

But regardless of how loud the engine is, it’s the material surrounding it that determines how much of that noise escapes from the engine compartment. So that’s where the focus of this article will be. These are the best marine soundproofing tips that you can implement by yourself and for an affordable price.

1. Insulate the inboard engine compartment

This is something you must get right, otherwise you could spend a lot of money without reducing the noise. To insulate a boat engine properly. the material that you use must be able to block low frequency sounds. Because that’s really what any engine does. It produces vibrations and these vibrations resonate on the surrounding material.

You know those acoustic foam panels with those eggshell crater design? Yeah.. that’s not going to do much for you. Those foam panels can only reduce the echo, but they can’t really block the noise to any great degree, especially low frequencies.

Instead, you should get a sound deadening material that is typically used for car engines. These materials are dense and strong enough to block impact noise. They are also heat and liquid resistant.

The one that I personally recommend for projects like this one is FatMat because I’ve seen it in action and it makes a big difference. My friend used it to soundproof his pickup truck. The engine and the cabin and it made a huge difference for traffic noise and engine noise. So I’m confident that it would work well for a boat engine as well. Plus the product information states that it can be used for marine soundproofing.

There are other similar items such as Dynamat and Noico, and there’s also standard Mass Loaded Vinyl. All of these materials are similar and would probably work well.

I’ve used Mass Loaded Vinyl for other projects and it works well. The only downside is that it’s not self-adhesive so you need to use a spray adhesive for it. Still, not a big deal by any means.

But for this project I would stick with FatMat because I’ve seen it work and it’s more affordable. It’s self-adhesive and all you need to do is stick it on the engine compartment.

Make sure that the compartment is properly covered once you’re done. If there’s still noise bothering you after you’ve done this, you can always add more layers to improve the thickness and insulation.

2. Insulate the outboard engine

Insulating the outboard motor is technically easier but the effects will not be as dramatic. Simply because it’s on the surface and therefore the noise is closer to your ears.

The outboard engine is covered with cowling, usually made of fiberglass. This is a thin cover that does block some impact noise. But if you can make it thicker by using dense material it will be even better.

Again I recommend using FatMat for this project because it’s efficient against low frequency vibration noise, it’s affordable and easy to cut to size. You can cover either the inside or the outside of the cowling.

Covering the inside would be better because the engine noise would then hit this softer material first. But this would require removing the cowling with most engines and then placing the material and returning it back on. An easier way to do this is to simply install the FatMat on the outside.

The downside of this method is somewhat reduced effectiveness and most of all it would look pretty ugly. So overall you’ll probably need to get your hands dirty to get the optimum result in this case.

But if you’re only worried about the “flashiness” of the FatMat looking ugly on your motor, you can get a darker insulation material that is not as obvious until you get pretty close to it. One that is similar to FatMat but also looks better is the black Noico sound deadener. It’s self adhesive and the installation process is the same with both. Simply stick it to any surface that you want to insulate and you’re done.

Speaking of which, you’ll want to cover as much of the area as possible. Make sure to leave the air vents and any other necessary openings intact, but cover the rest of the area completely.

3. Cover any gaps and cracks you find

Any gaps between sheets of insulating material should be covered with aluminium foil tape. This tape can also be used to cover any small gaps or cracks in the cowling and the engine compartment.

You don’t want to leave any unnecessary exit areas for sound and that’s really the whole logic behind doing this. I also advise using a few layers on each gap for better insulation.

4. Examine the cowling for large cracks and loose parts

Examine the cowling on the outboard motor. There can be some loose or broken parts. If any are present, they might amplify the vibration noise when the engine is working.

In that case either tighten the loose parts or use the aluminium foil tape to make them immovable. If the cowling is severely cracked and some parts have taken a life of their own, you may have to replace it entirely.

5. Add thickness to the floor

This is not a solution for every boat. It depends on the design. But for example if the boat cabin or starboard is directly above the inboard motor, it would make sense to cover the floor with a sound deadening mat.

It should be at least 1/2 inch thick, waterproof and non-slip. I would personally go with 3/4 inch thick rubber mat. If placed in a fancy cabin it wouldn’t look very nice, but you can always cover it with something nicer like a rug or some type of cover.

This will obviously not block the noise entirely because it can travel through air as well. But it will definitely muffle the noise coming through the floor and that is a noticeable portion of the problem.

To conclude

Marine soundproofing is an area that most soundproof experts avoid. Why? Because it’s impossible to get the noise completely under control. We are perfectionists by nature and want to BLOCK the noise. It’s a professional illness of sorts.

But with engines that’s almost never possible. Engines need to breathe, or they’ll overheat, or they need a lot of space for the working parts. You can’t soundproof anything as long as air can pass through. But sometimes we have to make compromises and focus on things that we can change.

Speaking of which, any structure that is made thicker and denser will by nature be more soundproof. This is common science. So if you can create a thicker and denser barrier between the source of the noise (in this case the engine) and the surrounding environment it WILL make a difference.

So you can focus on surrounding the engine with additional soundproofing material. Or you can try to insulate the floor or the entire boat cabin. Or in an ideal situation you would do boat. Uh. I mean both. Having said that, from a noise reduction perspective it’s always better to tackle the source of the noise as close as possible.

Hopefully this helps! If you have some personal experience with marine soundproofing, feel free to share it in the comment section below.

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