R21-EC Motor Box Insulation Upgrade

[SGIDAVE]

I have completed the rehab of the insulation lining the motor box. It was coming loose along the aft end of the box. I did some research and decided to use a product called Soundown. They make many products but the two most appropriate for this project were the "composite insulation vinyl foam sheets" and "acoustic absorption foam." I used the composite product which looks like this:

This is the 1 1/2" thick material which is closer to 1 3/8" in actual thickness. Note the vinyl decoupling layer in the middle. It has a mylar/foil layer on one side.

Getting started, I brought the motor box into the shop

Then the work began. Pulling out the original insulation was easy. The insulation along the sides toward the front was still good. The aft sections where damp to saturated with water the closer I went aft. The water inside the motor box came from the unsealed screws holding the teak cupholder

Removing the adhesive from inside the box was the worst part of the job. I started out trying Simple Green (wishful thinking, I know...) which didn't touch the adhesive. I had some acetone so I tried that. It worked fairly well. I needed to scrape the adhesive with putty knife once softened with solvent. Wiping with rags just smeared it around. I ran out of acetone and moved onto Lacquer Thinner which is basically methyl ethyl ketone (MEK) which, being a ketone, like acetone, works fairly well to dissolve the adhesive. Please note, however, that the Lacquer Thinner is HIGHLY TOXIC so you must wear a respirator (fitted with organic vapor filter cartridges) and wear chemical resistant gloves (not nitrile or latex which dissolve in MEK. Good ventilation helps too and both MEK and acetone are highly flammable so no ignition sources nearby, etc.

Anyway, I would recommend getting at least a quart of Acetone for the job; in my area a gallon costs slightly less than what 2 qts cost so I buy by the gallon.

I dry fitted the Soundown pieces. It cuts easily with boxcutter and scissors. Then I glued the Soundown fasteners with marine epoxy thickened with woodflour to ketchup consistency.

I used the Soundown brand spray adhesive acording to their directions; spray onto the motorbox and the foam, let set untill both are tacky then put in place; it bonds instantly, so the is no wiggle room or adjusting once the foam touches the box. The aluminum spikes push up through the foam, I pushed the large grommet down over the spike, then trim it to 1/4" then push the 'button' over the tip. Very neat.

Soundown's installation instructions tell you to tape all edges with mylar tape. Then, all joints are taped with more mylar. That's expensive tape.

This was after getting the first piece glued in place

I then prepared to re-attach the drink holder. Here is one of the screw holes as I found them

A good practice is to 'chamfer' the holes in gelcoat before applying bedding compound. It makes a place for the sealant to live and do it's thing. It also prevents further gelcoat cracking. It this was a cleat or something under stress, the gelcoat can crack further out alway from the initial hole. Here is the chamfered hole

Now the sealant can nestle in around the fastener and really work. As the fastener goes through the hole the gelcoat is cut back by the chamfer, so no cracking! That's a good thing...

Here is the drink holder remounted

And here is the completed box

Materials: (1) 32" x 54" x 1 1/2" thick Soundown Composite sheet ($95.76)
(1) 32" x 54" x 1" thick Soundown Composite sheet ($73.08)
(2) Soundown Aluminum fastener kits (for a total of 20 spikes) ($35.98)
(1) Soundown spray adhesive, ($18.99)I ran out and used 3M Spray 77 adhesive to finish (cheap).
Mylar tape: I had some from another project but expect to spend $40 for a roll; I used maybe half a roll.

I bought my Soundown from LowCostBoatingStore (http://www.lowcostboatingstore.com/sear ... &search=GO). Note they sell the plain foam and the composite which they call 'Barrier' on their website. They offered favorable shipping AND pricing compared to Jamestown Distributors on the foam. They shipped very promptly. I bought the fastener kits and spray adhesive from Defender though. You can view the Soundown installation manual on the Jamestown Dist website. VEry helpful!

I hope to get the boat out on the water this weekend. Maybe this will make a difference in the sound.

Fair Winds,

/david

 

[tuggertoy]

Very nice Dave. Let us know how much difference it makes. I have a 21 Classic and a much larger hood so I would not want to spend the money unless it makes a major difference in the sound level.

Eric
Tugger Toy
2006 R-21 Classic

 

[Pearl]

Great article Dave, the insulation upgrade is on my to-do this for this winter. I see that you used both 1 1/2" and 1" sheets of the material. Where did you place the 1 1/2" sheet and the 1" sheet? I spoke with Soundown and they said that it is important to have a good seal around the bottom and to add a hold down latch to the motor box. Thank you

Eric
"Pearl"

 

[SGIDAVE]

Great article Dave, the insulation upgrade is on my to-do this for this winter. I see that you used both 1 1/2" and 1" sheets of the material. Where did you place the 1 1/2" sheet and the 1" sheet? I spoke with Soundown and they said that it is important to have a good seal around the bottom and to add a hold down latch to the motor box. Thank you

Eric
"Pearl"

The 1.5" material went on the top and forward ("front") of the engine box. There was a small piece left which I used to cover about half of the aft end along with a piece of 1" material. The sides got 1" material.

My bottom seal looked a little ragged along the "front" so I've ordered more for the factory. A hold down latch is a good idea.

I used the boat yesterday for the first time since the upgrade. I took my sound meter. Inside the pilothouse there is more noise than in the cockpit. My meter, from the helm seat facing back toward the motor, gave readings of 102-103 db at WOT. In the cockpit from the aft bench seat, with the meter's sensor facing the motor box, the readings where 98-100. The noise inthe cockpit is not a problem. The noise in the pilot house becomes a problem when running at 3000 rpm or more, for me anyway. It's like the pilothouse is a reverberation chamber. Anyway at lower RPMs is fine. The boat does nicely at 2200 RPM as a combination of speed and reduced noise. I think insulating the hull, where the reverberations occur, would be most useful.

In all, I would evaluate the "effectiveness" of this upgrade as reducing the sound "a little bit." My engine box insulation was falling down onto the engine, so I needed to do something. Sealing around the cup holder is the key there to prevent further insulation adhesion failure. Bottom line, a diesel engine place inside a small fiberglass hull, is somewaht noisy at WOT. The hull acts like an echo chamber. This is not a 45" trawler.

I still love the boat!

Fair Winds,

/david

 

[tuggertoy]

Thanks for the update. Sounds like (literally) the moderate improvement in sound attenuation may not be worth the expense. As my motor box insulation is hanging in there (literally) I probably won't change anything until I have to.

Thanks again,

Eric
Tugger Toy
2006 R-21 Classic

 

[SGIDAVE]

Thanks for the update. Sounds like (literally) the moderate improvement in sound attenuation may not be worth the expense. As my motor box insulation is hanging in there (literally) I probably won't change anything until I have to.

Thanks again,

Eric
Tugger Toy
2006 R-21 Classic

I agree. There is NO direct correlation in terms of sound reduction such as doubling the motor box insulation reducing the motor noise by 50%...for instance, my motor box had 1/2" insulation and I tripled that (to 1.5") in parts of the box and doubled it (1") in the rest which resulted in slight overall improvement (aside from the immediate benefit of the insulation not resting on the mixing elbow). That's why I say the hull itself generally serves as an echo chamber. It's a good thing it's a slow speed boat. If it were a planing boat, yikes, it would be a really loud most of the time.

It's still a GREAT boat thought!

/david