Dave The later model yams have check valves that can jam  which possibly could cause it to load up. Mine did it when it was new but it caused the motor to gradually drop while either on the water or on the trailer. Bleeding the tilt hydraulics fixed it

Thanks for the replies guys,
I was wondering about the relief valve possibility. As we trimmed it all the way up this afternoon and it did the same when it ran out of travel, motor immediately laboured like hell, and the current doubled to 60 amps.
I even jammed a wood block in the way to stop the motor hitting the trim pins, and in damn near crushed the bit of 3x2 and loaded up which also suggested a relief valve issue.
Trimming up from right down, through the trim and tilt stages, it runs at 28 to 30 amps. This sounds reasonable when you look at the wiring and relay sizing.
I often hear a noise like a valve bleeding off when I have trimmed other motors down in the trim stage. I'm confident the motor is fine...for now..
It is normal for a motor of this type to draw large currents when loaded up. If it was faulty, it would stall.I have dealt with electric motors, ac and DC for most of my working life. But, it dosent mean I know everything..

D.C armatures particularly in very small motors such as this usually wont develop enough torque to drive a load if there is a fault in the armature, shorts in the comm or shorted turns often result in the motor cogging, stalling and normally arcing at the brushes. Ive dealt with all kinds of motors over the last 40 years from tiny to the size of your house and if you get an electrical fault you also get a distinct burning smell, only electricians or armature windings with motor experience will know what im talking about. If its not old then worn brushes are unlikely and if its developing enough power to squeeze a block its unlikely to be the motor as you said Dave. But even when you have been in the game that long your not always right.