Gearbox failure on yanmar 54hp

I have a new Beneteau 43 (

I have a new Beneteau 43 (approx. 18months). The boat has a Yanmar 54hp diesel and a 3 blade Gori folding prop. My trouble started approx. 3 months after purchase when I discovered the gearbox had had a catastrophic failure. The casing looked like a whole had been pushed through it. Yanmar replaced this gearbox without question but could not explain what had happened. Three months later the same issue. This time I got a diver to have a look at the prop to discover it had seized in a semi open position. Yanmar replaced the gearbox and prop and explained that the seizure of the prop had caused both gear box failures.
Since this time the engine has only been used to get in and out of the pen for racing and has had very low usage. I have a diver check out under the boat each month specifically the prop. All is fine. Last night I looked in the engine well to discover yet another gearbox has failed. Yanmar are now suggesting it is user error. I cannot believe this to be the case as I do not slam it into and out of gear and since the last gear box was replaced the engine would have been lucky to do more that 2000 rpm for a total of 20 hours. Surely no manufacturer would make something so delicate.
Does any on have any idea what is going on?

"Hi Shaun,
What is the exact

"Hi Shaun,
What is the exact model of the engine and the model and reduction of the gear box?
Thanks, Ron"

Hi Ron
The model of the engin

Hi Ron
The model of the engine is a Yanmar 4JH4AE – 54 hp. Not sure about detail of the gearbox but assume it is standard spec for that motor.

"Hi Shaun, please advise lates

"Hi Shaun, please advise latest on your gearbox problem, I am having severe "clunking" when I shift into forward after the engine has warmed up.Engine hours 212.Same engine."

I have commented on this before. I would make extra sure the damper plate has not worn the springs causing excessive stress on the gear. You do not need to change the damper when changing gears, but it is highly recomended.

Prop issue?
I've had my share of experience with Gori 3-blade folding on my Jeanneau37.2007, and never again a Gori!!
That said, they may have improved.
The main advantage of the Gori is supposed to be the 'overdrive function'.
To get that to work, boat has to move in reverse (like reversing out of the dock), the quickly put in forward.
If you reverse out of the dock, goes to forward and throttle up, you are in OVERDRIVE!!
Your engine and not forget your gearbox gets totally overloaded if you try to go above idle which you most likely do.
In order not to kill the gearbox, this is what you may have to do.
1: Reverse out of the dock.
2: Shift into neutral -forward to stop the boat and start slowly move forward, but NEVER above idle!!!!!!
3: When the boat has started to gain forward movement, shift into neutral so the prop may fold, then shift back into forward, and you may increase throttle.

If you manage all of this in a crowded marina and perhaps with some current, you deserve a gold medal!

I see only two options:
1: Always dock 'stern to'. Just be sure you dive under the boat before and check that the prop is not in 'reverse' pos from you last reversed into the dock!

2: Throw away the Gori and get a decent prop!

I went for flex-O-fold which has worked for my purpose very well!