Engine Harmonics and Synchronization-gctid814666

[smitty477]

"kev_rm" post=815400 wrote:
Smitty, I know you are are far more experienced with these boats than I am but the reason someone might care is exactly because of harmonics. The entire assembly regardless of the exact parameters of each component can be in resonance, which will increase magnitude of vibration massively, or not if not. People use engine synchronizers to avoid this because getting really close with engine RPM avoids it most of the time. As far as I'm concerned they are nice to have because as long as your ears work and you have enough patience, it can be handled manually.

Hello Kevin,

When referring to these boats and drivetrains produced in the late 70's to mid 90's I have seen a bunch of issues and what leads to those.

These engines sit on a rubber bed of isolators and 'float more that one might think - if you take a slow motion video of the engine in use and while docking it is surprising.

So here are a few thoughts regarding these boats/engines:

- Most owners do not know if there pt and stb tarns have the same ratio's while underway

- Most owners have never had their props balanced

- With the ones that are balanced they do not really know the specs and how far out they might be

- Therefore they might be able to sync the engine rpm but at the same time they are unequally loading the engines

- When these boats are run with active sync (ie - Glendening) I have seen what can happen

So the questions become -

- Do you want to equally load your twin engines at higher cruising speeds

- What will happen with each engines if one is forced to provide 10-25% more power than the other

- What will happen to the overall efficiency of fuel use over ground

- What affect will rpm have on these goal(s)

[kev_rm]

Smitty, I know you are are far more experienced with these boats than I am but the reason someone might care is exactly because of harmonics. The entire assembly regardless of the exact parameters of each component can be in resonance, which will increase magnitude of vibration massively, or not if not. People use engine synchronizers to avoid this because getting really close with engine RPM avoids it most of the time. As far as I'm concerned they are nice to have because as long as your ears work and you have enough patience, it can be handled manually.

[Cool_Beans]

The repairs involved straightening out a strut and having the bent props repaired and a cutlass bearing replaced along with some engine mounts. Not that I don't trust the repairs (I don't though. It's right until I fix it myself) but I planned pull and realigned everything either this fall or probably next spring.

Smitty, I'm not trying necessarily to sync up RPM but the exhaust noise from each engine. I'm assuming that would be load dependent. I'm not sure how a mechanical synchronizer works. . .if it just matches throttle position to each engine, or rpms, or load senses via a governor? I have no idea :S That's why I asked.

I have noticed that throttle position and rpm's vary per engine depending on my speed. It sounds (pun!) like I might just have play/deal with it until I get a chance to get everything tuned and aligned before I try to deal with it.

thanks!

[smitty477]

Shaft alignment and prop balancing are both important.

At the risk of beginning a non productive discussion I will ask this ...Why would anyone want to sync engine speeds?

Background /reasoning - unless both transmissions have the exact same ratio and both props are conditioned to the exact same dimensions (impossible) would it not be best to match the engines loads rather that the engines rpm?

After considering both trans and props we use a number of methods to do that such as : fuel use gages, and boost gages.

Before we had props balanced to S1 specs and with trans at the exact same ratios we found the rpm for load matching to be 70 rpm out of equal at 2,500.

After prop matching the rpm was within 20 or each other but still not at the same rpm for an equal load.

Thoughts - goal to match your engine loads not your rpm.

[floatyerboat]

You said after being fixed, what was fixed?

[Cool_Beans]

"Norton Rider" post=814675 wrote:
Check the shaft alignment and balance the props. My buddy did both of these things to his 3988 and the annoying harmonics almost disappeared.

Both of those are things I plan on doing at some point, probably fall haul out. The shaft alignment I'm going to assume is probably out a little anyways. Some of the engine mounts look to be untouched and there are shaft savers on each engine.

Do either of you know how the synchronizer works?

[Norton_Rider]

Check the shaft alignment and balance the props. My buddy did both of these things to his 3988 and the annoying harmonics almost disappeared.

[diveguy]

I have a synchronizer on the FB that works great. On the lower helm I have to go by ear but can usually get in sync by playing with the throttles which is easier at the higher rpms. Not sure what else you can do but interested in what others say.

[Cool_Beans]

My engines are not synchronized. I can't quite get the throttles correct to balance the harmonics out. The engines have a horrible drone that is annoying the crap out of me! But I can tweak the starboards throttle to the point the harmonics are sooooooooooooooooooo close to being in sync that a hair more in either direction throws it back out of balance and I have to start over. It's sucking the joy out of boating inch:

Not really sure where to begin with this. My original trial runs with the boat a year ago, the PO could sync them up ez and it was so quiet on the flybridge. After being fixed, I can't do it. . .

Any info, suggestions, fixes, etc. are much appreciated!