Engine RPM: 4800
Gear Ratio: 2.15
Prop Pitch: 15 or 17
Prop Slip: 15 (I've heard that is a good average to use)

The speed I get with a 15 pitch prop is 27, which is about what I get
with that prop at 4500' elevation.

If I put in 17 for the prop pitch value and leave all else the same, I
get 30.5 mph.


Of course you would get a higher speed using the 17" pitch prop in the
calculator versus the 15" pitch prop, leaving, as you say, all else the
same. The problem is that outside of the calculator all else is not the
same. When you tow your boat to the higher elevations, the simple fact is
that your engine will not produce as much horsepower as it will at sea
level. This is due to lower air density at the higher elevations. This is
why your dad wisely compensated for the loss of power at high elevations by
dropping to the 15" pitch prop. It is somewhat analogous with dropping a
gear in your tow vehicle when climbing a hill.

So, if your boat is properly propped with the 17" pitch prop at sea level,
meaning that the engine reaches its proper maximum RPM at wide open throttle
(WOT), it is perfectly reasonable to expect that you will need to compensate
for the loss of power at high altitudes by changing to a lower pitch prop.
This will allow the engine to continue to reach proper maximum RPM at WOT,
because you essentially have reduced the gearing in your drivetrain (see
hill climbing analogy above). This will also probably result is a lower top
speed than at sea level, but you will have that with the 17" prop at high
altitudes as well.

The consequences of not performing the prop switch could range from the
inconvenience of reduced performance of the boat to potentially damaging the
engine. Much depends on the current health of the engine, and how well the
low altitude prop is dialed in for the boat at sea level. Personally, if
the boat runs correctly at seal level with the 17" pitch prop, I would most
definitely swap to the 15" prop for the high altitude trip. You're only
working with a four cylinder engine with a modest amount of power and
torque, so putting the extra workload on the engine at high altitude would
be ill-advised, in my opinion. Changing a prop is not all that time
consuming, and doing so will make the boat more responsive and possibly
prevent running your engine outside of its recommended operating range,
which is not a good thing to do. This a clearly a case of Father Knows
Best. You would be wise to follow his footsteps in this situation.