I'm not trying to be sarcastic here or question your knowledge, but
this seems to contradict your comment about a decrease in speed.
Comments? I'm just trying to understand.

Thanks,
Bob


Bob:

Assumptions include:

1. You are not retuning your engine for the high altitude operation, and will
be running a mixture somewhat lean on oxygen.

2. That you have two different size props for a valid reason, i.e. your boat
needs the decreased resistance of the smaller diameter prop to achieve proper
RPM at the higher elevation.

If your boat runs the same at the higher elevation with the larger prop, why
the H would you bother to screw around changing it?

Without resorting to charts, graphs, theories, and etc:

*If* your engine had been achieving proper RPM at the high altitude only
because you had previously been swapping out the props........(please read the
first paragraph of my response more carefully), running the larger prop could
create an overpropped condition and the results would have been as I described.