loss of mph and rpm boat

Question
Hello and thank-you for your time.  I have a 1991 Sea Ray 225BR.  5.7Ltr
engine, alpha 1, 374 hours, use on fresh water, boat has been in water since
April.  Last year had the lower unit rebuilt.  
This year boat ran fine until now.  In spring boat would reach 48MPH and
4500RPM at WOT.  Last week it was 38MPH.  This week 32MPH and only
4000RMP at WOT   I do not think that the bottom could get that much crud to
lower the MPH that much.  Any thoughts.

Answer
Hi Mark...Thanks for selecting the undersigned to respond to your question.

I presume you didn't change the pitch on the prop, which can account for decreased rpms & mph, and the bottom of the hull is not severely covered in algae or other organisms of any kind. Do you wax the bottom of the hull or use an anti-foul paint each spring?

I'm not a mechanic, but a 16mph speed reduction is significant, particularly in such a short period of time. I'd return to the repairer and request they diagnose the problem. I'm inclined to think there's a problem in the lower unit rather than with the engine. Did you check the color and viscosity of the oil in the lower unit? Did feel gritty? Is there evidence of the lower unit leaking oil?
Just make sure the source you select to troubleshoot the symptoms is a knowledgeable, reliable, and reputable one.

Safe Boating
Ron