Is it possible? Stumped on this one.

So we finally had a chance between life plans, break downs and so on to take out boat for a small trip last night. I had a load of half fuel, full water, 6 people and normal overnight gear on board. We pulled out of the marina and took off east running between 3800-4000 rpm and cruising at 32mph for about 30minutes when the speed started dropping off mph by mph until I reached 17mph. At this point the RPMs had not changed, the engine had no alarms there were no odd sounds and so forth. I throttled back and brought the boat to a stop, opened hatch, all looked, smelled and sounded good. All gauges were also at normal ranges. There was no wind and hardly in current or waves in the lake. After about 20 minutes of checking around, I throttled back up and got back to full 32 mph before pulling into a local marina for dinner. The trip home at times seem like it was also trying to fall of at times however a storm started heading in, waves and wind were way up so the drive was completely different. So back to my original question? Is it possible for something to have happened that would allow rpm's to stay the same yet the boat speed cut in half??
2003 Rinker 342 Fiesta Vee, 350 Mag MPI, Bravo 3 2.20:1 w/ 22p 4 x4 props
Comments
Dream 'Inn III -- 2008 400 Express
If you just reduce power, without adjusting tabs and drive position, yes it would come off plane at 17 mph.
PC BYC, Holland, MI
We definitely dropped to 17, we were starting to fall off plane as the rear was settling in. I am taking a 60 mi one way trip this weekend, I will closely observe and try to capture any other information if it happens again. The only thing i noticed was my gps was reading high 11V low 12's when this happened. Could this have been the result of something going awry with my voltage? We have been noticing lower than normal operating voltages. Any ideas where to start diagnosing this? Now I think of it, I believe we ran the generator the rest of the trip home.
PC BYC, Holland, MI