This sounds like a great news. If at the very beginnig of the season your boat performed to the specs (engines and underwater gear tuned) and clean bottom, then it you know that you achieved optimum performance for your application. But, currently you're overloading your engines. I'm sure you're on top of the things, but I would just focus on "what has changed between spring and now?". As you know, this is very common situation to have change like that at the end of the season due to some slight marine growth on the bottom or the running gear. If you know that nothing changed with your props (no damages due to hitting something or grounding), then I would think that marine growth is the primary contributor. Even if you have diver cleanning the bottom I can't say that you can count on 100% results. The test time has to be timed correctly. For example, if your diver cleans the bottom on the 1st of every month, then at the end of each month you obviously have good level of growth, if you're not using the boat every weekend. There could be other variables, but one thing I know based on personal experience is that when all compenets remain unchanged the boat performs the same or very close to the same at the end of the season as it did at the beginning. I tuned my props on my 320DA and then didn't touch them for 2 seasons. I always do performance test at the beginning and end of the season (just like you, with full fuel, at least 1/2 water and gear). My boat always produced the same numbers. Having said that I think there's some variable in the picture that has changed and I hope it's just some marine growth, as it's the easiest to fix. BTW, last year I happen to test drive one diesel 400DB from your area at this time of the year and the boat hardly plane, but it barelly spun those props at WOT. The 2600RPMs rated engines barelly turned 1500+ RPMs and couldn't even trigger the turbos. The cause of the issue was a month old marine growth.