soundproofing outboards

[catalac08]

Hi
finally got the optimum size prop for my Tohatsu 25hp 2 stroke outboard and now getting sensible max (8.2K) and cruising speed that you would expect from 25hp and 3tonne 8m catamaran combination.

my problem is that at cruising speed 5k/2000 rpm the noise level is ok, 6.3k/3000 rpm is becoming loud and any faster the noise is unbearable except for short periods.

I have some sound proofing sheet glued to the inside of the outboard housing but this gets wet in any sort of a seaway. To completely cover the inside of the housing would probably take another 3/4 sheets 3ftx2ft. Is sound insulation closed or open cell?

Bearing in mind that there is clearance around the base of the outboard to enable it to swivel to steer am I wasting my time and money sound proofing the enclosure if I cannot contain all the sound and what about the vibration transmitted through the hull.

Any ideas or experience of this problem (please no "helpful" comments that i should have bought a 4 stroke, I realize that now but a 4 stroke with an ultra long shaft is not an option even taking the resale value of my 2 stroke (4 years old) into account now.

Also any experience of mounting the outboard through rubber pads to reduce hull noise, this cannot be only my problem.

 

[VicS]

Several aspects of soundproofing to consider.

There are materials that absorb the sound and there are those that prevent the transmission of the sound. The latter are the dense lead like materials.
You'll probably need a combination.

Also barriers can deflect the sound away from you.

Your outboard it seems has rather poor mountings if such a serious amount of vibration is transmitted to the hull.


Soundproofing and vibration dampening is a highly specialised subject and you need the advice of a expert in the field.

 

[tobble]

VicS is quite right about the different types of materials - hence the popular falacy that egg boxes will give you sound insulation.

One approach that might work for you is to build an inner box inside the cover of your ourboard well and put it on flexible mounts. I'm imagining somehting in ply covred in fibreglass, and then glued in on a series of soft rubber pads. tis could probably be done in such a way that you loose less space than just putting sheets of fluff in.

but I suspect structural vibration is probbly a bigger part of the problem. as an experiment you could try putting a sheet of rubber or similar between the outboard clamps and the bracket. make sure there's rubber between all of the parts where the outboard touches the bracket. you might have problems with this in terms of the engine being able to move too much, so make sure it's not going to fall off before you shut the lid. Maybe secure it on a lanyard too...

it might be possible to design a new braket to give better isolation, but as Vic suggests that'd be complicated (but I can see a gap in the market!)

Another alternative might be vibration absorbers fixed on the bracket. these do exist but I haven't seen them for this application. These are tuned to absorb a particular frequency but *could* potentially exacerbate problems at other frequencies. Not terribly helpful I know but maybe a path you could investigate.

hope that helps

 

[Davy_S]

Why not try it from a different approach! I have a 25hp Honda, its a cracking motor and not noisy but I wanted it quieter still. What I did was buy a half inch thick foam beach type roll up mat, the sort that tourists buy to lay on the beach. I stuck this inside my cowling with carpet glue, it worked a treat! I cannot hear the engine unless its on full chat, cheap,cheerfull and it works.

 

[Gordonmc]

To answer your question about sound insulation being closed or open cell, Davy S is on the button with his idea for bed-roll which is closed cell.
You may have a problem effectively sound proofing the motor without starving it of oxygen as the effectiveness of sound transfer deadening is to close all gaps. I had this problem with an outboard well with a 9 hp 2-stroke which would not run with the lid down for quietness.
I eventually half solved the problem by venting the exhaust through the transom through a pipe and leaving a small hatch open at cockpit sole level.
The next owner of the boat put a 4-stroke on a transom bracket to use the well for storage.