Towing vs Davit.....

Your 440 should be cruising quite a bit faster than 12 knots.

Cruise speed on several that I am familiar it's is anywhere from 17-25 depending on engine options as long as it isn't gas.

Run the boat without the dinghy and tell us what it does at cruise. If it gets up and runs on plane, your best bet would be to put larger trim tabs, or if that's not possible. Either put spacer blocks between the rams and the tab to get a few inches more travel. If the boat doesn't get on plane and run considerably faster, I would slow down to 9-10 knots and run it that way.

I wouldn't try to tow an inflatable, especially that size. They track horribly and usually do not tow very well.

Hopper5

You said that the prior owner added a lot of weight...tile, granite, etc. How much weight?? I certainly am no expert, but I would think that twin 3116's would give you more than 12 knots in that boat. I agree with the others above - leave the tender at the dock for a first test run. Also dump some water and run with a half tank of fuel. It just may be that someone overloaded the boat with improvements. Where is the waterline?

This is simply untrue for trim tabs. On almost all yachts that I run, they run faster at cruise speed with some trim tab. Even 35 knot cruise sportfishes usually achieve best cruise speed with some trim tab. The trim tabs give the stern a lot of lift, which allows the boats usually to run faster. Slower boats such as 20 knot cruise, tend to like even more trim tab to achieve their best cruising speed.

To the OP, I fully believe in your situation the larger trim tabs will most likely give you a lot of advantage.

I have an old Carver factory spec book that shows a 1994 Carver 440 with 350 HP 3116 Cats cruising at 19/20 Knots and getting 22/23 Knots at full throttle.

I have run a few of this model over some time and this is my experience.

-1993 Carver 440, Full fuel and a summers load of stuff with 300 Cummins cruises at 17 knots with a WOT run of 21 knots

-1993 Carver 440 with 3116 Cats 350 HP, loaded up with all kinds of stuff including hard bottom dinghy and 15 hp. mercury. I was running 18 knots for a cruise at 2500 RPM and getting 2840 on the digital tachs for a WOT run at 22.5 knots

-1995 Carver 440 with 420 HP 3126TA Cats, same as above with no dinghy. 22 knot cruise at 2500 rpm and a WOT of 25 knots at 2800 dead even. This boat was propped perfect for their load.

-1995 Carver 440 with 450 HP Cummins, RIB on the aft deck hard top with center console, large electric davit, davit system on transom (Roskelly) and a boat load of clothes, food, knick knacks and tools. Ready for this one... 24 Knot cruise at 2400 and a 28 Knot WOT at 2650.

I think that your 440 needs some prop work and or there is a lot of growth on the bottom. If your engines are running up to the proper RPM there has to be something else going on. The boat should simply be faster, the older gas 42s were as "fast" as your Diesel 440 is!

Sounds like something is wrong with your props. Assuming rated for your engines is 2800 and you make that on digital tachs. If not digital you can ignore the readings and check with digital. Also need no-load or high idle WOT rpm readings. Also verify transmission ratio or shaft rpm.

Select a good prop shop to deal with. They will have a sea trial sheet for you to fill out. Speed/RPM table etc. Pull the props and send them to the shop. You are either under propped or have serious cavitation issues.

Suggest you report findings to the knowledgeable folks on this site at that point and before making any large purchases.

PS. Your boat may have had the props damaged at some point and cheaper (wrong) ones put on. Mine did.

Although I have a smaller aft cabin, I thought it beneficial to add that I run with my tabs down all the time [except for a week or more at the dock, to keep the piston shafts clean]. If after I'm on plane cruising at 18 knts and up the tabs, I fall off plane and push water normally. Tabs down, even if causing some drag, is overshadowed by superior performance by lifting the aft.