The Noise Letter
Coming Soon to a Boat Near You–We Hope!
On a trip last year, I had a fellow say
something like “I’m sure glad I got to take the trip this way.” Not being
sure what he meant by “this way,” I asked. His response surprised me. What he
said was, “I mean on a motor boat. I understand that in a few years, they aren’t
going to allow motors anymore and I don’t think I want to go in one of those little
boats.” Today I wonder if he may have been closer to right than I knew at the time.
When I first came to Grand Canyon in 1980, there was real tension between the
motor and oar guides. Sometimes I’d drive past a row trip and the boatman
wouldn’t even look at me, let alone say “Hi.” At the time, being new to
Grand Canyon and all, I just went along with it. But I never understood why things had to
be that way. Since then I’ve heard more and more about the “motor versus row
issue” of the late seventies. Now I’m starting to see how things got out of
hand. I’m also taking a look at what’s coming next.
Today we are in the middle of another issue that has everybody
talking—Overflights of the Grand Canyon. This is one of those issues that has two
very opposing sides with no real simple answer. But the issue isn’t overflights; no
one wants to deprive people from seeing the Grand Canyon. The issue is noise. One of the
key things for all to remember, be it from the river or in the air, is that we work in the
largest echo chamber in the world.
The next “noise issue” will be the upcoming River Management Plan
and the wilderness designation for the whole Park. Both due to be on paper by 1996.
That’s coming real fast, maybe a little too fast for some of us. The primary season
is here for ’94—which means it’s almost over. Then it’ll be ’95
and, boom, ’96 will be here before we know it..
That means its time to get our engines in order to start making a real effort
to reduce motor noise. It can be done, and river guides need to take the lead on this one.
It’s the right thing to do for our piece of mind, for our way of life, for Grand
Canyon, and for providing the best possible river experience. So let’s get on with
it.
After the season, GCRG will host the first, and hopefully, only Grand Canyon
Noise Conference. Stay tuned on the when and where. The idea will be to bring people,
boats and motors together to exchange ideas and see what’s really working. We’d
also like to do some noise level testing and document our improvements.
This “Noise letter” is not a how-to manual on noise reduction, but
more intended to get us thinking about noise, or better yet, less noise. It covers some of
the very basics of noise itself, but more importantly goes over some of the things that
have been done that seem to work. If it does it’s job, it will generate some new
thinking on the subject and some new ideas.
WHAT IS NOISE?
Before we can talk about noise reduction we need to talk a little about
noise itself. The rumble you hear from a motor rig is, for the most part, a result of the
vibrations (or sound waves) generated by the engine. Some of the vibrations emanate
directly from the engine as noise, while some of it is transmitted to the boat frame. The
frame can act just like a big speaker, converting vibrations into sound.
The problem is, the noise radiated from the overall rig is very complex. For
instance, the noise coming directly from the engine is a mixture of sounds from different
sources such as the reed valves, piston slap, carburetor intake, bearings, the exhaust
housing doing it’s thing and the prop turning. Each brand of engine has its own type
and volume of sound due to different design parameters. Therefore, the noise coming from
the boat varies with different engines and is effected by boat design and mounting
systems.
If you want to confuse things even more just ask, “How do you measure
noise?” Their are two significant factors in describing noise: volume and tone.
Volume is measured on a logarithmic scale called decibels (dbA). Cutting the sound in half
drops the noise level by 3 dbA. For example: a reduction from 96 to 93 dbA is a 50%
reduction and 93 to 90 is 50% of the remaining noise. Distance also affects the sound
level because it drops very quickly. Doubling the distance from the noise source drops the
noise level by 6 dbA, resulting in one-forth the noise.
The tone of the noise, or harshness, is also a factor in the irritation
level. Certain frequencies and tones are more annoying than others which makes measuring
noise very subjective.
What does all that really mean? Well, one engine, whether tuned properly or
not, may sound quieter on one boat than another because of the frequency of the vibration
and the resulting tone of the noise. It also means that since you can’t really
control the frequency or tone, we need to manage the vibration, the shake, rattle and roll
made by the engine.
SO WHAT WORKS?
Although we’ll never erase vibrations completely, we do have several
ways to cut it to a minimum. One is to absorb it with something like rubber, foam, fancy
laminates or other space-age materials. Another is to tighten or stiffen things so they
will not vibrate at a frequency that generates noise that bothers people.
A great deal of the noise is generated under the engine cover. So if we can
keep it from getting out we’re headed in the right direction. One approach is to line
the inside of the engine cover with an inch of foam that has a thin sheet of lead
laminated in the middle of it. This seems to work well. In one test, the foam resulted in
a 4 dbA reduction, which is slightly more than half. A further step is to cover the engine
with some sort of sound absorbing material, in this case a closed-cell foam.
Additional improvements can be made by wrapping some closed-cell foam around
the driveshaft housing between the engine base and the lower unit where all the gears are.
This helps absorb noise emanating from the exhaust tube—but be aware that this could
cause overheating in some engines.
MOUNTING SYSTEMS
Most engines are mounted on a “jackass” used to lift the engine
up out of the water when a collision with a rock seems imminent. This is where
“Vibration Management” really begins. For starters, the jackass should be rattle
free—when you shake it without an engine on it, it shouldn’t make any noise. If
it rattles, fix it. This can normally be done with some good quality spring washers on the
pivot bars or a spot weld or two on those things that don’t need to move.
Next, take that old piece of waterlogged wood off the back and replace it
with a 3/4-inch thick piece of gas resistant 60 or 70 durometer rubber which will absorb
some of the vibrations before they get to the frame. Then, when you mount the jackass to
the frame, use an old piece of boat rubber between the frame and the jackass. Even if you
don’t use a jackass, the rubber will help bunches.
FRAMES
After that, the frame has to be stiff. The transom needs to be solid and should have good-sized gussets connecting it to the side of the frame. Do it right—really bolt the thing down. Check it occasionally while you’re on the water. Then put a 1/2-inch thick rubber stress mat (like the ones used in restaurants) or some astroturf on the floor. Even an old Persian rug that your mom wants to throw away might work. A mat helps absorb vibration from the floor of the frame, and the boatman will love it since it will put some spring back into his/her step. Face it: a rug on the floor is comfy.
BOAT DESIGN
Boat design is also a big factor in the noise level produced by a boat.
Not so much for the people on the boat, but more for the people you pass. Big rafts with
open stern designs do not have anything to block the racket thrown aft of the boat.
Depending on the design, a piece of old boat rubber hung vertically behind the engine will
help. The more enclosed it is, the less noise will escape. Makes sense, right?
Boats with enclosed sterns can gain even more absorption by hanging a piece
of rubber inside the tube from the water line to the top of the tube. This will not only
absorb some of the exhaust noise, but it acts as a splash guard, keeping water from
nailing you in the back. Try it. You’ll appreciate it first thing in the morning.
BOAT BALANCE
Boat balance and proper loading can contribute to the noise factor as well. If the boat is so overloaded up front that the bow is “plowing” through the water, it takes more power to push the boat. More power means more RPMs which equals more noise. Too much weight in the stern has the same effect and translates into more drag. A well balanced boat is also easier to drive and uses less gas.
PROPS
The prop is important too. Regardless of the sound treatment in any
installation, the easiest way to reduce the noise is to reduce the RPMs. Full throttle
cruising in flat water may not only be hard on the engine, it burns more fuel and creates
more noise. Running at 3/4 throttle dramatically improves everything with very little loss
of speed. The right size and pitched prop will not only improve torque but will keep the
engine in the proper RPM range.
Once its damaged, change it. A bent prop wobbles and multiplies all those
vibrations we’re trying to quiet. But its even worse than that. A trashed prop, or
one that is mispitched to the job, will quickly rattle a working engine into spare parts.
HIGH TECH
Several people have asked about the new state-of-art Electronic Noise Cancellation technology. Unfortunately, it’s not something that can help us yet. It is being used on some cars, but the noise levels and physical properties of big boats are a long reach from the current technology—and well beyond any of our budgets.
LOW TECH
With all the rubber, foam, gussets, and spring washers, we still need to
ensure that there is a direct link between the hand on the throttle and the brain that
operates the hand. It just takes a little sensitivity to reduce the noise as you pass
other boaters. In the overall scheme of things, backing off the throttle a tad can only
improve the Canyon experience for everyone down there, hikers and river people alike. If
you have to run full throttle all day just to make the schedule—maybe the schedule
needs some work as well.
There are other things that can and have been done to reduce noise levels,
which brings us to the next step. While we’re out there motoring around, let’s
think about the racket we’re making. Try some things. See if they work. Let’s
talk to each other and share ideas on what works and what doesn’t. Let’s make
some noise about it!
I want to thank Lindley Smith, Senior Manager of Technology for OMC (Johnson
& Evinrude), for his help on this subject over the last year or so. His interest comes
not only from his knowledge of the subject, but also from his love for Grand Canyon.
This is a subject we have been batting around for a long time. Round and
round we have gone on it. We don’t have the luxury of just talking anymore—the
River Management Plan is coming at us fast. It’s time to act now.
Tom Vail
