TOPIC: Slipping gear or Prop

I recently took my project boat out for the first run. Motor started up, reverse worked, forward worked at low idle. Once past the 5 mph buoy I slowly increased the throttle although it accelerated for a few seconds then it lost power and made a grinding noise. I immediately returned to neutral, tried to accelerate slower again - same result.
Researching the problem and hoping it is not the gears I discovered that the thrust washer was missing from the prop. I purchased a thrust washer and have installed it (see attached drawing). Is it possible that the missing thrust washer caused the problem? I will need to test it in a trash can and I am not sure what would happen when I accelerate. I can take it to a lake but it is about an hour away and I didn't want to drag it out without first getting some advice.
What do you think?

What’s it sound and feel like when you spin prop by hand. I would think if gears are damage you may feel or hear something? Maybe clutch dog is rounded off? I don’t see attachment or know the motor type so just guess. Assuming you made sure shifter was fully engaged so shift rod was all the way down/up..

Sorry I left out that info. Motor is 1964 Evinrude 60HP. I tried to spin the prop by hand in both forward and reverse but I couldn't do it without using what I think would be excessive force. Spins easily in neutral naturally. I did check the shifter and it is fully engaging in both directions.

I think you will find the shift clutch & forward gear with rounded off clutch faces.

Right off the bat pull the lower drain plug on the gearcase and see if you have bright metal shavings coming out with the oil. That will tell us pretty quickly.
Secondly, just because the control goes fully into reverse and forwarrd does not mean the gearcase is. There are also adjustmments in that area and also the shift rod connector may not be properly installed.
Cbeck the gear oil first before we get into anything else. That's easy.

I checked and changed the gear oil, nothing unusual in the oil. I feel like I need a "Mechanic" buddy to help we work through stuff on the boat. Has anyone have any idea how to find someone with old motor experience? To make it more interesting I live in the Palm Springs area with a lake one hour away. I wish I could afford to just swap out the engine but that is not an option.

A stupid mistake I made a few summers ago which still haunts me.
Same issue you had but without the grinding. Put the boat in the water and it would back away from the dock without any issue. Shift forward and slow not a problem. Start to speed up and was ok and then the motor would rev without gaining any speed. Took the boat out of the water and thought I spun the prop. Took everything apart and found nothing unusual.
Turns out the motor was mounted too high on the transom and the prop would cavitate as soon as enough speed kept water away from the "bite" area. Really dumb move on my part but just to eliminate the possibility make sure your prop is in the water.

If you have a spare propeller, bring it to the lake.
Also, invest in a service manual for the engine. These are pretty simple engines to work on. As I mentioned before there are shift adjustments that need to be verified and adjusted if necessary.