The C-Brats Forum Index
HomeForumsMy TopicsCalendarEvent SignupsMemberlistOur C-DorysThe Brat MapPhotos

Twin Engine Harmonics

 
Post new topic   Reply to topic    The C-Brats Forum Index -> Propellers
View previous topic :: View next topic  
Author Message
browntdb



Joined: 05 Sep 2012
Posts: 96
City/Region: Salem
State or Province: OR
Photos: Tortuga Del Mar
PostPosted: Fri Jul 12, 2013 9:51 pm    Post subject: Twin Engine Harmonics Reply with quote

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
Back to top
View user's profile Send private message
thataway



Joined: 02 Nov 2003
Posts: 20778
City/Region: Pensacola
State or Province: FL
C-Dory Year: 2007
C-Dory Model: 25 Cruiser
Vessel Name: thataway
Photos: Thataway
PostPosted: Fri Jul 12, 2013 10:17 pm    Post subject: Reply with quote

Harmonics are generally due to slightly different RPM on the two engines. Many inboards have synchronizer.
_________________
Bob Austin
Thataway
Thataway (Ex Seaweed) 2007 25 C Dory May 2018 to Oct. 2021
Thisaway 2006 22' CDory November 2011 to May 2018
Caracal 18 140 Suzuki 2007 to present
Thataway TomCat 255 150 Suzukis June 2006 thru August 2011
C Pelican; 1992, 22 Cruiser, 2002 thru 2006
Frequent Sea; 2003 C D 25, 2007 thru 2009
KA6PKB
Home port: Pensacola FL
Back to top
View user's profile Send private message Send e-mail
hardee



Joined: 30 Oct 2006
Posts: 12632
City/Region: Sequim
State or Province: WA
C-Dory Year: 2005
C-Dory Model: 22 Cruiser
Vessel Name: Sleepy-C
Photos: SleepyC
PostPosted: Sat Jul 13, 2013 1:30 am    Post subject: Reply with quote

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
SleepyCMoon

_________________
Though in our sleep we are not conscious of our activity or surroundings, we should not, in our wakefulness, be unconscious of our sleep.
Back to top
View user's profile Send private message
chimoii



Joined: 18 Feb 2009
Posts: 271

State or Province: BC
C-Dory Year: 2017
C-Dory Model: R-25 Tug
Vessel Name: Chimo
Photos: Chimo
PostPosted: Sat Jul 13, 2013 10:55 am    Post subject: Reply with quote

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.
_________________
Chimo: a word of greeting, farewell, and toast before drinking once widely spoken in the Inuktitut language in northern Canada.
Back to top
View user's profile Send private message Send e-mail
Hunkydory



Joined: 28 Mar 2005
Posts: 2652
City/Region: Cokeville, Wyoming
State or Province: WY
C-Dory Year: 2000
C-Dory Model: 22 Cruiser
Vessel Name: Hunkydory
Photos: Hunkydory-Jay-and-Jolee
PostPosted: Sat Jul 13, 2013 11:05 am    Post subject: Reply with quote

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

_________________
Jay and Jolee 2000 22 CD cruiser Hunkydory
I will not waste my days in trying to prolong them------Jack London
https://share.delorme.com/JuliusByers
Back to top
View user's profile Send private message Send e-mail
Sea Angel



Joined: 29 Dec 2003
Posts: 734
City/Region: Virginia Beach, VA
State or Province: VA
Photos: Sea Angel
PostPosted: Sat Jul 13, 2013 12:06 pm    Post subject: Reply with quote

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

_________________
Art ka1rx
CBRAT #208
2005 CD25 #075 SEA ANGEL (SOLD)
USCGAUX (RET), USN(RET)
Broadcast TV ENG(RET)
ka1rx@verizon.net
SKYPE: Art.Bartlett4
Back to top
View user's profile Send private message Send e-mail
browntdb



Joined: 05 Sep 2012
Posts: 96
City/Region: Salem
State or Province: OR
Photos: Tortuga Del Mar
PostPosted: Mon Jul 15, 2013 1:10 am    Post subject: Reply with quote

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
Back to top
View user's profile Send private message
thataway



Joined: 02 Nov 2003
Posts: 20778
City/Region: Pensacola
State or Province: FL
C-Dory Year: 2007
C-Dory Model: 25 Cruiser
Vessel Name: thataway
Photos: Thataway
PostPosted: Mon Jul 15, 2013 11:55 am    Post subject: Reply with quote

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.
Back to top
View user's profile Send private message Send e-mail
Display posts from previous:   
Post new topic   Reply to topic    The C-Brats Forum Index -> Propellers All times are GMT - 5 Hours
Page 1 of 1

 
     Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You cannot download files in this forum



Page generation time: 0.0413s (PHP: 62% - SQL: 38%) - SQL queries: 26 - GZIP disabled - Debug on