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Motorbox Insulation


WakeGirl

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Make your direct drive quieter with this mod.

For you direct drive owners out there, do you want a quieter boat? Tired of having to shout to the observer sitting right next to you in the observer's seat? Here's a guide that should help, a quick rundown on how to effectively insulate your motor box against the sound coming from the engine itself. Note: this will do absolutely nothing for the exhaust noise, you need Silent Rider mufflers & possibly the STE Exhaust for that part. Those parts are available through your Malibu dealer.

This could be done by 1 person, but it is easier with 2. If you have someone that is handy with angles/tracings/measurements that would be willing to help then they would make this process easier. For our purposes, this article will refer to the insulation kits available from Overtons, but you can essentially get the same thing in bulk from McMaster-Carr.

Tools and Supplies Needed:

  • Either Overton's noise reduction kit part #25005 (I used just over 2 kits for mine) or insulation & aluminum tape from McMaster.com - you will need part # 5692T35, & roughly 8' of it to complete the project.
  • Sharp scissors
  • Masking supplies (tape, paper)
  • More spray adhesive (maybe 2 extra bottles for entire engine box)
  • Aluminum tape that either comes with the Overton's kit, or readily available at Home Depot or Lowe's in the HVAC department
  • Epoxy or Superglue
  • Drill & bits (if you want to drill the box) & screwdriver
  • Black marker

Estimated Time to Complete: An afternoon
Difficulty Rating: Low

Preparation

The first thing that needs to be done is to remove the motor box from your boat & find a flat space on which to lay it, upside down. To prepare the box, you need to clean the inside thoroughly & finish by wiping all of the gel coat down with either alcohol or mineral spirits. Once finished with that you can begin fitting the insulation to the box.

STEP 1: Measuring & fitting the insulation

In this picture you'll see 1 full roll of the insulation laid out in the motor box.

01.jpg
Measuring & fitting

You can see where it bunches in the corners - this is where you need to put relief cuts as shown in the next 3 pictures. It will be evident when you lay the foam in there where the cuts need to be from the creases that show up as you see in the previous picture, & you can just take a black marker to the creases & make your cuts from that. Better to start with small cuts since it's easier to take more material away then to put it back once it's gone.

02.jpg
Relief cuts


03.jpg
Relief cuts


04.jpg
Relief cuts

Once you get that, do a template for the sides & rear using anything large enough to trace - paper or cardboard will work. Take separate tracings for each side since chances are that they won't match. Trace those out on the foil as shown in the following picture - make sure to get it on the correct side of the foam respective to which side of the motor box it's going on.

05.jpg
Tracing a template

STEP 2: Setting the anchors

The Overton's kit comes with sharp anchors with a flat base that can either be drilled/screwed in or glued. Attach those as shown in the following picture, using whatever you're comfortable with that will hold well. We used epoxy because the box didn't seem thick enough to drill effectively for screws. Put those on & if you use epoxy or heavy-duty glue, let them set up according to the instructions for the epoxy.

06.jpg
Setting the anchors

While they're setting up, tape off the carpeted area as shown in the following picture to protect it from the spray adhesive that you will use in the next step. You can use any kind of tape that will stick, duct tape certainly does.

07.jpg
Taping off the carpet

STEP 3: Using the spray adhesive & laying in the insulation pieces

When the anchors are ready, spray in the spray adhesive. Doing only one section at a time makes it less stressful & easier to get good results. Make sure that you're in a ventilated area when you do this. I'd recommend having extra adhesive around since what they supply you with isn't really enough. Spray both the inside of the box & the backside of the foam with it being careful of over spray, & wait for it to get tacky before laying it in the box.

The following picture shows the foam in the box with the anchors poking through with a retaining clip to hold the insulation snug. You just have to be careful & patient when doing this - it's best to get the entire piece of insulation lined up over the top of the anchors & then press it down rather than laying it in up to the rivet.

08.jpg
Anchor & retaining clip

The length of the anchor just snips off & you push on the finishing cap as shown here.

09.jpg
Finished anchor

STEP 4: Finishing up with tape

Taping the seams, no big secret here. One comment on the tape - it might be better to use aluminum HVAC tape from the start rather than the stuff that comes with the Overton's kit. Our tape did not hold well & we ended up replacing a lot of it.

10.jpg
Taping the seams

These final pictures show the difference between the insulation with & without the tape - big difference visually in my opinion.

11.jpg
No tape on the seams


12.jpg
Seams taped up


13.jpg
Finished!

Conclusion

After living with this for almost 2 seasons, this is definitely something that I would recommend doing. As I mentioned at the beginning of the article, it will not make a difference for noise coming from the exhaust but it will make a noticeable difference for the noise coming from the engine itself, especially for those sitting in the driver's & observer's seats. We also had no heat issues as a result of this modification.

Note to people ordering boats: The best thing to do is to order this option when ordering a boat from the factory unless the price goes up significantly from the $150 upgrade charge that it has been at for a while now. You will not save much money by doing it yourself. Personally, if I were to order another direct drive, I would order every silencing option available, MSST, Silent Riders, Insulated Motor Box & STE Exhaust.

~ WakeGirl

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Oops, I put this data in the wrong thread. Here it is again.

************************************

Last year, a bunch of us insulated our motor boxes. Did it work. Answers were yes, no, maybe, depends. The last answer is the most correct = "depends". It depends on how much of a drop in sound you're looking for... and at what speed. The two double kits from Overton cost about $150 total if I recall.

Here's the post I made on MBO with the dB results "before/after". I'm a geeky guy sometimes. Rather than opinions, I want data. Then the data is what helps form/validate an opinion.

Data: The data says there is an INCREDIBLE difference at slowspeeds. 10-12 dB is a HUGE drop in sound. (Slow = exhaust tips underwater and motor is idling.) At speed, (speed = exhaust tips above water and boat is moving fast enough for wind noise to add to the reacket), there is negligible difference.

Opinion: Worth the effort to install... although a bit of a PITA. If I was ordering a new boat from the factory, I would absolutely have them do the job.

***********************************************

Finally have some BEFORE and AFTER sound measurements. Did the Overton's installation, did NOT cover up the drink holders; left the small gap (for ventilation) in the motor box at the rear.(1998 RLX; 310 carb)

My apologies.... I only took "C" scale ratings BEFORE.

I took "A" (human range) and "C" measurements afterwards.

NET SUMMARY: Makes a TREMENDOUS difference at slow speeds....ie, when the exhaust outlets are UNDER water. At planing speed, not much difference. Am I glad I did the PITA project.... YES! What an incredible difference for slow speed travel with the family. At speed.... who cares... I love that throughty rumble.

The results: A drop of 10-12 DB in the front area at slow speed!!! A slight drop (1-3 DB at 36 mph. At speed, it seems like ALL the noise is coming from the exhaust outlets..... NOT the motor box area.... and wind.

BEFORE: "C" Scale Only;

At Idle: Driver Seat 82-85 DB

Passenger Seat: 82-85 DB

Rear Seat 82 - 85 DB

At 36 MPH: Drivers Seat 98 - 101 DB

Passenger 98 - 101 DB

Rear Seat 103 - 104

AFTER: "C" Scale

At Idle: Driver Seat 71-74 DB

Passenger Seat: 71-73 DB

Rear Seat 72-74 DB

At 36 MPH: Drivers Seat 98 - 102 DB

Passenger Not taken

Rear Seat Not Taken

AFTER: "A" Scale

At Idle: Driver Seat 60-62 DB

Passenger Seat: 58-61 DB

Rear Seat 60-62 DB

At 36 MPH: Drivers Seat 87 - 89 DB

Passenger 87 - 89 DB

Rear Seat 100-102 DB

NET NET NET: YES, it's worth doing this project !!!

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Great Article!!!

Including the McMaster part # 5692T35 would be helpful. It takes 8' of material. Aluminum tape and the spray adhesive are also available at Home Depot.

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I did the insulation on my SLXI last year and it was the best $100 I ever spent on my boat. I noticed a big difference at idle and at approx 30 MPH, my usual cruising speed.

Very easy DIY project too. I didn't worry about cutting every piece perfectly because it's inside the motor box and the tape covers all the seams.

Edited by Dan P
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  • 4 months later...

by the way... I didn't use the anchors in the motorbox and just used the PRE-stickied material from mcmaster and it holds up very well! Much less work and NO MESS

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  • 2 weeks later...

Why is everyone using the foil faced foam? From Mcmaster's site it looks like the skinned version actual has better sound dampening characteristics. I know my old MC 190 had an open faced foam on the inside of the motor cover and it was fairly quiet and it didn't present any other maintenance problems.

Any thoughts?

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  • 2 weeks later...

Used the non-foil faced insulation and it came out great! There is no noise eminating from the cabin now. It's a very different feel/sound now, more deadened. Like when I turn the motor over to get it off the trailer there's only a slight hint from the motor box that anything is going on, even the "malibu reverse whine" is muffled although not gone.

I don't have any numbers for reduction os sound but it's good enough and very noticeable to me and that's the bottom line. It now sounds on par with my buddies SN196. The 196 has the characteristic CC gear whine, the bu has V8 rumble.

I'll post some pics when I get a chance to take some.

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I chose the "skinned" surface on the insluation for the same reason. I thought if it has a higher NR coefficient, why not take advantage of it, even if it is slight. Perception is reality.

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foil faced will reflect more heat from the adhesive. Over time the stick'm will breakdown with continued heat exposure and foil apparently limits this exposure.

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foil faced will reflect more heat from the adhesive.  Over time the stick'm will breakdown with continued heat exposure and foil apparently limits this exposure.

The heat is what it is. There is an amount of heat that is radiated from the engine into it's surroundings- the motor box. Through everything that heat passes the heat will be less. The only difference between the foil faced and the skinned is a minute mass of foil. I would say that the difference in temps that the adhesive sees between the two is a fraction. Where is the heat going to go once it gets reflected? Hot flows to cold so where is it going to go.

MC and CC both use a "skinned" type of insulation and my '94 MC had a open faced foam under the box and never had problems in the ten years I had it.

I think your statement would be true if the temp in the box got higher than the flow temp of the adhesive but, it doesn't.

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  • 2 weeks later...

SkiSix,

Great explanation. I had the same philosophy when I chose the "skinned" foam over the foil faced, but I couldn't have explained it nearly so effectively. Where were you when I was trying to understand the difference between enthalpy and entropy?

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SkiSix,

Great explanation.  I had the same philosophy when I chose the "skinned" foam over the foil faced, but I couldn't have explained it nearly so effectively.  Where were you when I was trying to understand the difference between enthalpy and entropy?

I love it now, it's as quite as anything on the lake now inside the boat. Behind the boat it's still got the rumble. I'll take rumble over CC gear whine everyday.

I hope next year in the heat of summer I'm not eating crow and having to fix it but, so far it's staying up and looking good.

One thing I noticed and I dont' know if it's relevant, is that I covered the slot above the cup holders. Around the same time I noticed that my baselines in PP were delivering slow times and I had to bump it by 35rpm's. At the same time, the hull has not been cleaned all summer and that's makes a big difference. I'm left wondering whether I took a performance hit by covering those slots since that's goingn to be the least resistant path for cold air to get to the air filter.

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SkiSix,

Great explanation.  I had the same philosophy when I chose the "skinned" foam over the foil faced, but I couldn't have explained it nearly so effectively.  Where were you when I was trying to understand the difference between enthalpy and entropy?

I love it now, it's as quite as anything on the lake now inside the boat. Behind the boat it's still got the rumble. I'll take rumble over CC gear whine everyday.

I hope next year in the heat of summer I'm not eating crow and having to fix it but, so far it's staying up and looking good.

One thing I noticed and I dont' know if it's relevant, is that I covered the slot above the cup holders. Around the same time I noticed that my baselines in PP were delivering slow times and I had to bump it by 35rpm's. At the same time, the hull has not been cleaned all summer and that's makes a big difference. I'm left wondering whether I took a performance hit by covering those slots since that's goingn to be the least resistant path for cold air to get to the air filter.

That was the very thing that I wondered about when we covered ours. There wasn't anything discernible at all, but it did make me wonder.

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An easy enough test would be to remove the cup holders and see if the rpms came back. Now, I'm not saying that the boat doesn't perform well enough with the slot closed, it performs wonderfully, and 35 rpms isn't anything I would ever notice if I didn't have PP. Rarely do I do WOT runs so that would be an easy test to- w and w/o cup holders.

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I covered mine too, and didn't notice any loss of power, but I don't have any way to measure that statement. It doesn't "feel" any weaker than before, though.

I thought long and hard about covering the vent hole before I did. I assumed that it would be more of a discharge point than an inlet for fresh air, when the boat is moving. I figured that the velocity head of the air coming into the vents at the front of the windshield would be greater than the pressure of the relatively still air at the rear of the motor cover.

I was more concerned with a possible over heating problem due to a reduced air flow. It seems like Edwin poked a couple of holes in his motor cover on his SLXI because of this reason.

What are ya'lls thoughts?

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I know that the malibu literature says that the "fresh cool air" comes from those vents and gets circulated via the FIBECS thingy-ma-jig, blah,blah,blah. Whatever. Let's think about this practically.

Path one:

There is positive pressure going through the vents at the front, then that air takesa few turns and goes through some dryer hose and down to the hull and through the vents in the stringers and then into the motorbox and then out the stringers to the vents in the back.

Path two:

Ambient air 3", at least on the RLXi, away from the air intake.

I think the path of least resistance is the 3" path. My premise is air supply to the engine. I am not saying that FIBECS does nothing- I'm sure it does something but no other manufacturer has done something similar to accomplish the same thing.

As far as over heating is concerned, there is some benefit to allowing the heat a path to get out of the motor box. Just having holes and no forced air flow doesn't help that much. To exchange heat, there needs to be flow. If you experience a over heating problem, turn on the blower.

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Last night I did some WOT runs to see if there was any difference bewtween slot open and slot closed. I simulated slot open by removing the cup holders in the motor box.

Run 1 up and back: Slot closed

rpm headed North: 4810 PP speed indication 43.2mph

rpm headed South 4830 PP speed indication 43.4mph

Run 2 up and back: Slot open

rpm headed North: 4730 PP speed indication 42.9 mph

rpm headed South 4770 PP speed indication 43.0 mph

I think the above data says that having the slot covered doesn't hurt. I don't know about 0-36MPH times other than to say that "it felt the same". There was a good cross wind so I thought that and average in both direction would be more fair. The air was about 54 degrees. I know that PP speeds are not calibrated that high and it is probably a fair bit off but the rpms tell the story anyway.

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Kinda strange, don't you think? Particularly if you believe the air intake benefits in some way by being that close to an air source.

What prop are you running on that boat?

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I would have too, I found it strange enough that I thought I should do it again but, alas it was time to ski and with $3.25/gal gas I don't want to waste any of the precious stuff.

The one thing that I can think of that would explain the difference is IF the whole FIBECs thing actually pressurizes the motorbox. That seems highly unlikely becuase it's all open and it's a significant volume.

Keep in mind that they were somewhat unscientific readings and I think that the difference of the two falls within the repeatabliity of the test- it was gusting winds.

The other thing is that I didn't notice any sound difference when the cup holders were removed. That too surprised me.

I put the cupholders back and I'll leave it as it is. I do hope that next year during the heat there are no problems.

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