Yanmar 30 - Rumbling

[Ron Sabadash]

On my OD37 the engine seems to be "Hunting" or groaning between 2000 & 2500 RPM every couple seconds. I thought it was cavitation as it set up a vibration in the boat, however, it does this at the dock also when the shaft is in neutral. Above 2500 the rumbling goes away. Any ideas?........ possibly dirty injectors?Also, with the 30hp yanmar and a 2-bladed prop, what is the nominal revs for cruising speed.......... god help us....... only when there is NO wind? I hear it's around 1800RPM.Ron

 

[Dave]

Rumbling

Mechanical systems like rotating engine components have what are known as critical frequencies. These are frequencies where the components will resonate and cause serious problems. I don't know for sure if this is the case for your problem but one trick is not to operate the engine in a critical frequency. In other words if it resonates at 2500 don't run it at that speed. Most systems are designed to have these frequencies outside of the operating range so that the motor will pass thru these speeds rapidly while it is first starting up for instance, or the frequency is beyond the speed of operation. My Universal 25M does the same thing at certain speeds but otherwise seems fine so I avoid those ranges.Dave

 

[Howard Levine]

No Tach but,

Hello Ron, I have noticed on my boat a cavitation/vibration but its at all rpm's probably a new cutlass bearing or shaft/prop alignment problem. Since my boat doesnt have a tach i cant say for sure but 2000 rpm should be the crusing speed. Our boat doesnt gain much more speed at full throtle than it has at cruising speed.  I'd just stay away from that rpm that gives you the sound at least until you find out more. Hard to tell without hearing the sound, any smoke along with it?howie