Hunter 30G Broken prop shaft strut

[Ken Cross]

I just found it interesting that 2 1989 30G boats that I know of (ours and one other, both located on the same dock) have within a year both had broken prop shaft struts. Both had no indication of an impact. Does anyone know how this happens and how to prevent it?

We both noticed no excess vibration, just a sudden clunking noise as the shaft began hitting on the stern tube during turns.

Ken

 

[kloudie1]

Electrolysis. Look at fracture face of old parts to confirm electrolytic corrosion.

 

[Jackdaw]

Everything designed and made/constructed by the mind and hand of man has a design life-span, measured in time or duty cycles. Sometimes this is a designed date, sometimes its just how it works out. Nothing is built to last forever, and nothing does. Maybe the parts and/or the build worked out to 30 years. That's a pretty good run.

But you're right, it is a coincidence.

 

[jssailem]

What is the strut there to do?
My guess. Provide a stiffy for the prop shaft. Damper the harmonics of the shaft as it spins from still to 3000 plus RPM's.
Over time that is a lot of vibration. It is placed on a sailboat. If we put a big stout stiff 6x6 in the water to hold the shaft in place it's going to slow the boat. So we shape it. Configure it to have minimum drag in the water stream. And it needs to be light weight. Then there is cost. We can not use high tech metals the market will never pay the price.

A boat is a compromise from the bow to the stern. In time all things have a limited service life span. They just do not announce that in the sales brochure. The forum is a good place to share such experience.

So happy it was not a greater danger to you guys Ken.

 

[LeslieTroyer]

Damper the harmonics of the shaft as it spins from still to 3000 plus RPM's.
n.

John I think your a bit high with the 3k number. Most transmissions have a 2:1 or better gear ratio so more like 1500RPM. Unless of course your driving a hydro-plane.

Les

 

[jssailem]

Ok.. I was giving Ken too much credit (3000 RPM prop). Yes the transmission reduces the high revving Yanmar to a rotation rate that the prop can be used to push the boat through the water. 2:1 2.5:1 3:1 all relevant.
I was thinking about the metal harmonics that occur when the prop is spinning. All of this is transmitted to the strut. Through the years there have been plane crashes that have been attributed to metal fatigue due to harmonic vibration. Seems one can extend that vibration fatigue idea to the design of the strut on Ken's boat.

His inspection says no evidence of striking anything. Two boats of similar design and age/use. Not definitive but worthy of consideration as a possible issue.

 

[sailor832]

A bent shaft between the strut and the prop will put a lot of strain on the strut but you will not feel to much vibration until failure.

 

[Ken Cross]

A bent shaft between the strut and the prop will put a lot of strain on the strut but you will not feel to much vibration until failure.

It could be something like that, but I won't know until I get it on the hard in a couple of weeks. I need to wait a bit due to the Admiral's upcoming knee replacement.

My plan is to temporarily mount the new strut and check run-out at the prop taper and mid shaft.

Ken