406 won’t get to speed

[One Moor Time]

Hi everyone. Hopefully someone has some ideas here:
Bought a 2001 Carver 406 with Cummins 330’s last April. She was exceptionally well maintained, and the 2 surveyors who inspected her were highly qualified with one specializing in Cummins. She was documented at achieving 19 knots at 2600 WOT and 17 at 2400 cruise with 3 full tanks and 4 adults aboard. Due to other stuff going on, we were not able to take her out for about 2 months at which time she could only make 12 knots at a little over 2000 with a WOT AT 2400 making about 13 knots. She is berthed in Baltimore. I had a diver clean the bottom with no change. No smoke on startup or underway, and all gauges are normal. I cannot detect any rough running. Both engines fire right up with no smoke.
She is scheduled to be hauled 11/19, so I will see if we are dragging anything, but I would have thought the diver would have noted something that dramatic.
If someone has a similarly configured 405/406, what is your cruise speed? Anyone had a similar problem or ideas? The aftercoolers were serviced last winter.
Thanks!

[g36]

Trim tabs operating properly? There's a breaker for them in the electrical panel by the dinette.

[One Moor Time]

Trim tabs seem to be working. I have inched them up and down while underway, but the impact is minimal. Going out tomorrow-I will confirm.
Thanks much for the idea-keep them coming.

[g36]

I can definitely tell the changes they make when using them on my 405. I belive you should notice more than minimal change. May try making larger move rather than "inching them up or down"

[One Moor Time]

Good info. I will check their position and operation when I haul her on the 19th.

[tomschauer]

Sure sounds like a dirty bottom. It happens quickly in MD waters.