I'm not an expert in this area, but I had, or I should say "still
have" a similar problem intermittently. I think the advice on the
locking pawl may not apply to your power trim/tilt system. The pawl
is something I've seen on smaller manual tilt systems. If your '73
115 Johnson is like my '89 225 Johnson there is a normally open
hydraulic circuit leading to the top of the tilt cylinder that allows
the motor to tilt up if you hit something. In that circuit is a
reverse lock valve. When you shift into reverse the valve is
electrically closed, keeping the engine down. Check all your wiring
connections, but it could be the valve itself. The valve is available
and resonably priced, but you have to pull the motor off the bracket
and get the valve body out of the (corroded)housing to change it.

In the meantime, try giving it a little trim down before you back into
the slip. I got into the habit of trimming back down after I come off
plane anyway and I rarely see the problem.

Good Luck

FushFan