"Milton Waddams" wrote in message
...
. Maybe
someone here can explain why the dynamic balance showed what static
balancing couldn't.

Milton

Consider a crankshaft with two crankpins at 180 degrees to each other, and
without balance weights.
It will be in perfect static balance since the two crankpins are on opposite
sides and therefore cancel each other out.
Now rotate it. Although the two crankpins are opposite to each other they
are in different positions along the shaft and since each one is trying to
fly away due to centrifugal force there will be a 'couple' (of magnitude
force x distance apart) trying to turn the crankshaft end over end. So to
achieve dynamic as well as static balance you must add weights on the
opposite side of each crankpin. That is the difference in principle between
static and dynamic balance.
But even if your shaft and propeller assembly were in dynamic as well as
static balance you would still have got vibration from excessive clearance
in the outer bearing since the side forces generated by a propeller in the
water are not constant because the top of the prop is nearer to the surface
than the bottom and every time blade passes close to the hull there is a
change in side load so the prop needs a decently fitting bearing to stop it
whirling about and causing vibration.