Mercury/Tohatsu 4hp 4 stroke drive shaft sheared.

jamie N

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As the title. The engine's a 2010 model, in relatively good nick and has been serviced correctly I believe. The engine was running at 3750 RPM, I fitted a tacho for RPM & service life, and it's probably had between 1000 & 1500 hours of operation. At the time of the shaft shearing, the motor had run for 7 hours that day, being used intermittently on a trip from Inverness to Stromness.
The shear is directly at the point where the key for the water pump is, and the shear is 'perfect' and perpendicular to the shaft; as if it was manufactured to be that length.
As the shaft broke, the only indication was an increase in RPM and noise, no evidence of anything impacting the prop. I should say that this happened at the southern end of Swoma in the Pentland Firth, with a variable 3kts of wind, so it did grab my attention for the next few hours until we tied up.
They've made oodles of these engines, and I wonder if it's a fault that others may have had?
 
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