Twin Engine Harmonics

browntdb · Posted: Fri Jul 12, 2013 9:51 pm Post subject: Twin Engine Harmonics

We just got Tortuga Del Mar to Puget Sound and were able to do some initial cruising in the South Sound. It was a wonderful experience to circumnavigate Anderson Island, where our vacation home is, and to go to some neighboring bays.

I noticed that there is a harmonic deep frequency droning at most any RPM unless I am going very slow 4 mph or extremely fast, 23 mph. I have twin Yamaha Hi Thrust 50 HP engines. I have 13.5x15K props on each. The top RPM I can get at full throttle is about 5200 RPMs. I am wondering if the harmonic droning noise is due to being slightly over propped.

I would welcome any thoughts from other CD22 owners with twin engines. Also, any thoughts to a different prop would be welcome.

Thanks,

Terry

thataway · Posted: Fri Jul 12, 2013 10:17 pm Post subject:

Harmonics are generally due to slightly different RPM on the two engines. Many inboards have synchronizer.

hardee · Posted: Sat Jul 13, 2013 1:30 am Post subject:

I have twin 40 Yami's. Have learned that it takes some practice to match the rpm on both engines, but it will come. I can't say I have had the harmonics problem. Do you get electronic out put to your plotter from your twins? Might check there and see if the rpms are matching up.

Enjoy.

Harvey
SleepyC Moon

chimoii · Posted: Sat Jul 13, 2013 10:55 am Post subject:

I run twin 60 Yamahas. Each has its own tach. So synching is easy but I often do it just by ear. Just tweak the theottles until you get harmony.

Hunkydory · Posted: Sat Jul 13, 2013 11:05 am Post subject:

Like Chimoii, the gauges for my twin 40 hp Honda's show whether their in sync or not & without looking at them it's still pretty easy to sync them up by listening to the motors & adjusting rpm to stop the harmonic droning you mentioned. I have run many different sizes & types of props with no effect on harmonic sound balance. The same no effect in place with varying boat speed & motor rpm if both motors stayed balanced at equal rpm.

Jay

Sea Angel · Posted: Sat Jul 13, 2013 12:06 pm Post subject:

Just a silly little note on twin sync'. -

I have found that the tacks (RPM) on the each engine MAY not be so accurate to use for a firm sync setting, but still as a good quick reference point. By this I mean to use either as a reference engine, particuliarly when one of the throttles may not hold its setting with time.

As an example:
I have one (port) engine that will slowly decrease RPM over abt 15 minutes at cruise, but the other's RPM (starboard) remains solid. With this in mind I'll set my speed/PRM to the desired settings and ride the port engine throttle by setting a "zero" beat by listening to their sounds.

Hope this isn't too formal, but with practice these engines will sound like only one when in sync. An old friend of mine taught and scolded me to "TUNE THOSE ENGINES!" So I share this, I hope, in a more gentle way.

Art

browntdb · Posted: Mon Jul 15, 2013 1:10 am Post subject:

Thanks for the comments. I appreciate it. When I was getting the harmonic droning, I had each tach set to the same RPM on its digital gage, does that mean I need to change the RPM's so they don't match? Sounds like I need to just experiment some with the two engines.

Terry

thataway · Posted: Mon Jul 15, 2013 11:55 am Post subject:

The most accurate way to sync the engines is with a tuning fork. Since I had several low frequency forks left over from my medical practice, I would usually use these to get the sync exact. Tachs are definately off--but do give a good basis.

The boats where I have had built in synchronizers would ignore the tack and tune by the harmonic or vibration. In most cases--depending on the individual--the best way is by your feeling of the "beat" of the sound.