Prop unbalance

[sailfreak]

A friend of mine owns a cat with twin engines, each equipped with a 3 bladed folding prop. Both props develop an unbalance already at low speed (1000 revs approx.) and not disappearing at higher revs. Being a mechanic himself my friend claims both props are fitted correctly and open and close ok. Does anyone have an idea on how to correct this?

<hr width=100% size=1>sailfreak

 

[boatless]

Bear in mind that it may not be a weight imbalance, or even dissimilar bades. If the prop 'disc' is receiving water at varying speeds, the effect is that the prop is working at different speeds in different parts of it's rotation. This means that the prop is flexing itself and the shaft. No prop works in perfectly clear water (unless it's mounted as a 'puller') so all suffer to a smaller or larger extent.

What's it like in reverse? This test won't really be conclusive if a) the rudders are now in the inflow to the prop, or b) if it's not safe to try it at speed - likelihood of damage to rudders/mechanisms.

<hr width=100% size=1>my opinion is complete rubbish, probably.

 

[Sunnyseeker]

Obvious I know...but has he got scissor type rope cutters they often cause vibration. Some builders wont fit them because of the vibration...

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[oldsaltoz]

G'day Sailfreak,

Also check the prop shafts are not bent.......

Avagoodweekend.



<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.

 

[Benbow]

Do each show the same 'unbalance' when run alone ?

If not could it be 'beat', ie interference between the vibrations of the two causing rythmic waves of reinforced noise/vibration ? (Thats why a twin prop plane often has a characteristic rythmic droning sound)

I should say that I have absolutely no experience of twin engine cats, this is pure speculation.

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