Broken drive shaft, change more than the shaft?

[masthugg]

Hi,

I bought an old Yamaha outboard (5C -87) that I use to power my sailboat. Recently the drive shaft came right off at the impeller drive pin. Didn't hit anything in the water. I found a used lower unit with attached shaft. I mounted it and it works.

I am however a bit concerned that there might be an underlying reason to the drive shaft braking. Judging from what I can find online, breaking the drive shaft on small outboards seems very rare.

I did notice that the thing that holds the shaft halfway up the shaft (long shafts have these) seems broken, and it is an old damage, not from the drive shaft breaking this time. The holder seems to be reasonably in place since I can still fit the lower unit without force.

Do you think I should

a) be happy with everything as it is
b) remove the holder, since maybe it broke the old shaft and will break the new one
c) replace the holder (seems impossible)
d) check or fix anything else to avoid breaking the new shaft ?

[DougR]

Your suspicion that the broken support bracket might have had a hand in breaking the shaft is probably spot on, particularly since only long shaft engines are equipped with the support bracket.

Most likely the long drive shaft has a " critical speed" somewhere in the RPM range of the engine and without the support bracket the shaft begins to whip. If the shaft whip is unrestrained it can begin to fatigue the shaft and cause the shaft to fail at its weakest point, which is the impeller key way.

Better replace the damaged support bracket before another shaft breaks.

DougR