I replaced the 327Q engines in my 38’ aft cabin Chris Connie with a pair of counter rotating Marine Power 350 Sport Pac engines with Velvet Drive 2.57:1 gears thru existing Monel shafts and 20x20 props. (BTW, Marine Power gets 5 stars from me for the quality of their product and for their after-sale support.)

On run up, the engines sounded normal and what one would expect from a pair of 350’s. That is, until I hit 1,800 RPMs. When both engines hit exactly 1,800 RPMs, the engine sounds changed. Both engines emitted what I can only describe as a surging, pulsating sound in harmony and unison with each over. It was as if the two engines were acting as one.

I advanced one engine about 25 RPMs and the engines sounded normal again. The strange sounds only occurred when both engines were exactly at 1,800 RPMs.

What was causing this? Has anyone else experienced strange engine sound?

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I'm going to take a shot at this.  As you engine runs faster and faster it produces a higher pitch.  When two instruments are just slightly out of tune with each other you will get a wa, wa, wa sound.  The slower the interval between the wa wa (s), the closer the pitch (RPM).

See more info here:    -- From :  https://www.xaprb.com/blog/2014/01/18/how-to-tune-guitar/

Beats are fluctuations in volume created by two notes that are almost the same frequency.

When notes are not quite the same frequency, they’ll reinforce each other when the peaks occur together, and cancel each other out when the peaks are misaligned. Here’s a diagram of two sine waves of slightly different frequencies, and the sum of the two (in red).

Beats

Your ear will not hear two distinct notes if they’re close together. It’ll hear the sum.

Notice how the summed wave (the red wave) fluctuates in magnitude. To the human ear, this sounds like a note going “wow, wow, wow, wow.” The frequency of this fluctuation is the difference between the frequencies of the notes.

My guess is that the engines were running at slightly different RPMs and producing a slightly different pitch.  Hence the wa wa wa sound.  You can use this sound to sync your engines.  Move one of the throttles up or down very slightly and see if the sound speeds up or slows down.  When it is very slow and then stops your engines are running the same speed.  This was very easy to hear on our 31 with original exhausts.

I suspect some of the club members are laughing at this idea and think I'm nuts (Dick?).  Hopefully Jay can report back in with his results and well see if I am crazy or not.

-Darin

WOW Darin, this is deep and I will have to go over it a few times to understand it. No way do I think you are nuts. I checked out the how to tune a guitar and will have to go over it when I have some time. This takes me out of my comfort zone as I do not have a musical background. The only instrument I play is the radio. And yes, it is the "wow, wow, wow, wow" sound.

Thanks Darin

No Darin, I don't think you are nuts. When I read this I thought immediately that at 1800 or so the boat is struggling to get up on plane and the engines are under a good load, and, slightly out of sync, thus making that weird wa-wa sound. Advancing the throttle a bit probably got the engines closer to sync and the sound goes away. An experienced boater with a good ear usually doesn't need a tach to sync. You can do it by ear. Don't trust 50 year old tachs to be dead accurate. I have been lied to by tachs many times over the years. Take the boat up to around 3,000 rpm and sych it by ear and then see what the tachs say. I'll bet the boat at ear synced 3000 or so is smooth & free of weird noises

If these are the original mechanical tachs the have a point of accuracy, probably around 3000 rpm.  Below this I think they are under and above they will over report.  Low hours on the tach may actually be low by as much as 10% if the owner cruises like I do around 1500 to 2000 rpm.  Your ears will work well, as confirmed by the original synch light.  Incidentally like Dick states, it is a bear to get the engines to stay synched when I am running around 1500 or below.

When I repowered, I replaced all the instruments with brand new electronic units. I guess that an electronic tac is more accurate than a mechanical unit. Even so, I take the instrument readings with a grain of salt.

And Dick, you are correct. At 1800 RPMs the boat was struggling to get up on plane. The stern was down and it was kicking up quite a wake. It was the least economical speed and I did not want to operate at that RPMs.

I usually cruised at between 2900 and 3100 RPMs. and the engines sounded great.

I was never overly concerned with getting the  engines in synch, I often ran with the engines at different RPMs. For example, if I was cruising SW with  NE wind I would advance the starboard engine by 50 or 100 RPMs to counter the wind trying to push the bow to starboard,

Perhaps I should have paid more attention to engine synch. Instead, I focused on how the boat handled and felt.

Jay, nothing good happens at speed when engines are run out of sync. Harmonics and various other vibrations that you may not even feel or hear can cause undue wear and tear. Once you advance one engine over the other it forces the higher rpm engine to carry a disproportionate share of the load while the other engine though helping to push can more or less just be along for the ride. Trim tabs and rudders should be used to compensate for sea conditions.At speed the boat should be pushed equally by both engines. The only time I use my tachs is to get close to the rpm's I want to run at, beyond that Dick and others are correct, set your RPM's by ear and you will get used to hearing them "sync" up. Your boat will run better, you will use less fuel and the wear and tear on your engines will be spread evenly. Grab a hand held rpm meter and take some readings on your engines at different speeds(with a second person aboard of course). You will be shocked at how much a tach be it new, old, electric or mechanical can be off.

Not sure if this is relevant to the topic, but I do believe both propellers will sing at  a lower rpm range if they are balanced and matched well.  This sound will occur at a certain speed and after throttling up a bit stop.  Basically like taking a wine glass and using your finger to create the sound.

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