Alumacraft 1436 Engine Harmonic Resonance at 3/4 throttle

[fwman1]

I'm breaking in my 5HP Tohatsu and found that around 4000 rpm there is a significant vibration near the transom. This 2011 Alumacraft 1436 is the first jon boat I've owned and am not sure if this is common, or if it is a potential problem I should research further. The engine is firmly secured on the transom, and that transom shows no sign of prior damage.

 

[poorthang]

engine not broken in. engine not being run at WOT as designed. 4 stroke cam tuning? damaged prop?
turn the throttle to full open, run to 6k rpm and see if she gets smooth.

 

[Weldorthemagnificent]

It is a single cylinder engine and won't be as smooth as a twin. I'm not familiar with the break in procedure but I'm assuming you haven't gone wot yet. Without seeing it, I'm guessing at 4000 it will be on the verge of trying to go on plane and the loading/unloading cycle in the hull is the cause of your shake. Be interesting to see how it acts when you push it further. You'll probably never run it at that rpm as you'll either be in the high end of the range or putting around. A friend has the 6 HP tohatsu and it's a good motor, just a little lumpy and can't really be compared to the old johnnyrude 5-6 twins.

Sent from my CLT-L04 using Tapatalk

 

[DaleH]

The harmonic is typically set at an RPM that the engine doesn't run at, but just transitions through. On larger OBs it's usually in the low 2K range as boats typically start planning or stay on plane from 2,500 to above 2,800 or so. At least that's where it was when I was in the engine bizness ...

Not sure 2-strokes use dynamic harmonic balancers, but maybe that's used on newer 4-stroke technology.

 

[fwman1]

I didn't realize this was a thing. The engine is a 4 stroke, brand new and I'm just doing the break-in process.
You are correct that when you pass through this rpm range it does smooth out.
I appreciate the experience shared. Thank you.

 

[poorthang]

The harmonic is typically set at an RPM that the engine doesn't run at, but just transitions through. On larger OBs it's usually in the low 2K range as boats typically start planning or stay on plane from 2,500 to above 2,800 or so. At least that's where it was when I was in the engine bizness ...

Not sure 2-strokes use dynamic harmonic balancers, but maybe that's used on newer 4-stroke technology.

lots of folks dont know that ALL engines have an rpm range that goes through an imbalance. usually at the very bottom or very top of the scale. size of balancer can narrow that range quiet a bit but there is a negative to making balancers larger. most two strokes have the imbalance at the bottom of the rpm, and most 4 strokes will have it somewhere in the lower range. i guess that with a small outboard, which may be used for trolling, and is most efficient at peak of torque curve will have that imbalance somewhere between the most used rpms just as DaleH said above.