The "Tinny" sound?

[Fisherman]

Some aluminum boats give off a tinny vibration sound, especially at certain RPM's. Why? What is making this noise?

[BroadCove]

Most folks that are concerned about a "tinny" noise with aluminum boats are referring to the noise generated by the hull moving through the water (which can be modulated/reduced through many different avenues). You mention vibration and rpms, so I'm guessing that you're not referring to that sound. The short answer to your question is that the "period" of the engine vibration at a specific rpm (think of it just like the period of ocean waves) is the same as the natural period of the material, in this case, the aluminum in your hull. That generates harmonic resonance, which is briefly explained, like everything else it seems, on wikipedia:

http://en.wikipedia.org/wiki/Resonance

FRP boats are made of different types of materials and are no where near as rigid as aluminum, and thus they don't exhibit the same tendency to vibrate. Just like tuning forks for musical instruments are made of metal, not FRP!

Hope this helps.

[kmorin]

fisherman, there are a couple of events going on there.

I'll 'resonate' BroadCove's post.

One is resonance a sort of 'internal echo' of the boat. Another is the harmonics, they're kind of related in some ways but in general they're different 'animals'.

Depending on the boat, engine and drive you can experience resonance when you slowly advance the throttle and find one RPM setting where the noise/hum/tinny vibration is loudest, that is a resonant frequency. In some boats this is the RPM where the engine is least in balance; 1450r's for a Merc 4 cyl I/O for example.

In other circumstances, like running very hard into a wave at good speed, you can get a very loud humming sound and a leg shocking vibration from hitting waves. This would be the harmonic of the hull's resonant frequency, but this is much less common than the resonance of the engine and drive to the hull and that is most common with inboards compared to outboards.

The metal is 'ringing' or vibrating in many places, and that metal is aluminum and the aluminum is in sheets or thin plates and the sound of that material in thin plates is 'tinny'; in fact the name tinny sounding comes for the sound of sheet metal vibrating!

Engines at different RPM, props at different RPM's will produce a vibration in "that" hull which 're-sounds' or builds and is louder than the other vibrations caused at higher or lower RPM's. This fact (in practice) has vibrated off hand rails welded on decks, loosed engine mount lock nuts, and shaken loose countless fittings in metal boats.

In some cases, especially yachts or fully finished power boats, the amount of insulation, joinery and other 'finish' is enough to eliminate these sounds by dampening the resonance in the metal structure. In a bare boat, welded with power and no cabin or hull ceiling, they're more common but they're just a function of the entire boat being one thin (relative to all other materials) piece of metal; once the boat is welded it's 'one' piece.

I built one 40' power boat with stiffeners in the stern over the prop that cracked out! I was surely confuse why the hull would crack with all the nicely even spacers I'd welded to the bottom? An older hand at the cannery I was working near asked me if I'd considered making the spacing 'uneven'? I hadn't but I did. The boat has fished for nearly 30 years since without any problems after I 'mis-adjusted' the layout of the hull longs over the wheel!

I don't pretend to understand (exactly) why this worked but I do know that the same size stiffeners, welded to the same schedule by the same welder, with the same alloy wire and power supply - lasted when the uniform pattern of the layout was broken up; but would crack with all the same factors evenly spaced. I will assign this to resonance and harmonics as I don't know what else to attribute the 'fix' to? I never did have this happen to a half dozen other power boats of this class.

You could compare a well built welded alloy boat to a tuning fork in this sense- it's all one piece and resonates when the right pitch vibrates that structure or it resonates in harmonics when the entire structure is 'hit' with enough force like slamming a wave face.

Cheers,
Kevin Morin

[Chaps]

Along the lines being discussed here . . .

For the last couple of weeks my boat and I have provided a platform for sea mammal observers in an area around a pile driving project. I noticed that anytime I was just drifting within about 300 yards of the pile driver my hull would pick up the sound of the impact which produced an amplified echo of the activity throughout the boat. It was like standing in the middle of a big speaker. The pile driver would hit the pile and a few milliseconds later the hull would repeat the sound as if someone was in the water hitting the bottom of the boat with a hammer.

Thought it was kind of cool.

[Fisherman]

Along the lines being discussed here . . .

For the last couple of weeks my boat and I have provided a platform for sea mammal observers in an area around a pile driving project. I noticed that anytime I was just drifting within about 300 yards of the pile driver my hull would pick up the sound of the impact which produced an amplified echo of the activity throughout the boat. It was like standing in the middle of a big speaker. The pile driver would hit the pile and a few milliseconds later the hull would repeat the sound as if someone was in the water hitting the bottom of the boat with a hammer.

Thought it was kind of cool.

When I'm sitting at anchor with my motor off I can pick up other boats running around me over 1/4 mile away. I don't pick up all boats though.

[JETTYWOLF]

when I drop my anchor straight down under my boat on "hard bottom", be it 10 feet or 30 feet, I can hear the chain hit metal anchor.

Can't do that in your synthetic boats.

C'C'mon

[dave]

I love when the water slaps the hull. It is music to my ears. Everything has a resonance. Adjust your RPMs a bit one way or the other.

[speedboats]

When I'm sitting at anchor with my motor off I can pick up other boats running around me over 1/4 mile away. I don't pick up all boats though.

This is exactly what passive sonar relies on. The speed of sound in water is something like 4800ft/s at 60 degrees F, so fast enough to consider the information given to be 'real time'.

The crackng panel with evenly space supports can be explained with standing wave forms. Ever got a rope, tied one end off so it remains stationary, straightened it out on the ground, then proceeded to shake one end from side to side. As you slowly shake faster and faster you will eventually find a speed where you'll have many waves and what appear to be stationary places between them. Now if you measured the distance between the stationary points, and the number of cycles per second, you would have the resonant frequency of that rope for that length. The height of the wave is the amount of energy you have put in. You should also notice that the space between the the stationary points is exactly the same along the whole length of the rope.

Longer thinner panels are easier to resonate, as there is more area to absorb energy and less stiffness to resist vibrating. Longer panels will generally have a lower resonant frequency, while shorter panels have a higher one.

Resonant frequencies also work in multiples. Ever spun a lathe under a single flourescant light? (not OSHA recommended). AC current (depending on where you are) has a frequency of 60Hz, which means the bulb flashes at 120 times / second. Now under this 'strobe' light, the object on the lathe will appear stationary a rpm's of 120, 240, 360, 480... 1200...2400...etc.

Now, by shortening a longer panel into evenly spaced short ones you have essentially set up the panel to behave like the experiment on the rope. The supporting structure becomes the stationary points along the wave, while the whole length of the panel acts like a large panel absorbing all the energy, so now the panel will vibrate at certain multiples of the original lengths resonant frequency.

By stagering the supporting structure unevenly, each wave between the stationary points is a different length, so they cannot act together along the panel as they'll vibrate at different frequencies.

We overcome alot of this problem with what we put into our boats, particularly non structural parts. Things like side shelves or non supporting panels are stitched in place and then urethaned in place. Urethane is great as it stays flexable, therefore absorbing vibration energy, as does hull liner, carpet, adhesive non-skid pads, etc.

[pjay9]

All this PHYSICS talk...so cool...takes me back to college classes and engineering...wished I had pursued marine engineering...oh well! I do have to say that I really enjoy these types of discussions and when there are drawings it is even more intriguing and holds my attention! Keep it coming!

BTW, my 90Yamaha has a vibration around 1200 which decreased when I installed a Yamaha prop...not saying it is the prop as there are many factors..pitch/diameter/material...plus much more external factors. Probaly even has something to do with the pilot...I know Jetty would tell me that!

So keep these discussion going! THX everyone. Capt PJ

BTW, out last evening with 25-30 knots of SW (prevailing) wind on the lakes...exciting! Had a wedding party.