We own your same set up. A 2006 Tom Cat 25.5with bottom paint and Honda 150s having about 400 hours total time.
I have charted the speeds at different fuel burns and found them to be consistently lower by 3 - 4 knots than shown in the sales literature, so suspect the factory was a bit generous in their speed estimates.
But, having said that, I have also found the boat's speed is very senstive to the following:
1. Engine control software. Honda engines are fuel injected electronic wonders, but if the software gets even slightly out of wack, it really effects power output. Take the engines to a Honda dealer and have them check it out with their computer. The needed computer software seems to be proprietary to Honda, and so only their authorized dealers have it. Once on the computer, it is easy to re-set the engine controllers electronically. Cost is $200 or so. Would certainly do this before changing engines.
2. Propeller size and condition. Not all the boats recieved the most efficient combination of engine and propeller size and pitch when new at the dealer. Take a careful look at what you have, supply that information to this web site and get some feedback from what others are running. (We have a set of high altitude props, an aluminum set that came with the engines, and a stainless steel...don't have the numbers here). Of those, the SS set work better than the others in salt water at sea level.
3. Height of engine at transom. Look carefully at the bolt patten on the transom mount and see how far "out" of the water your engines are. In our case, they were mounted a couple inches too high by the dealer when new. Among other things, this occassionally caused the engine cooling water intake to be inadequate, causing an over heat alarm. It took some time to figure this out and then lower them 1 or 2 bolt holes, which seemed to improve speed by a couple of knots and also resolve the cooling problem.
4. Weight of the boat. Cats of all types are very weight sensitive vessels. You will easily see a 5 or 6 knot speed penalty just by having the fuel tanks and water tank topped. At about 12 gph (which we figure to be average with speeds in the mid 20s mph), you have nearly 8 hours of fuel on board, yet most runs are rarely more than two. If you are looking for speed, manage your fuel load with this in mind. Same with fresh water.
5. Water in transom engine mount. The transom mounts have a hatch in the top on each side that is not adequately water proof. In certain conditions you can easily accumulate 40 or more gallons on one side or the other. That is in excess of 300 lbs, all as far aft as you can get, and it will really slow your boat down. There is a drain plug for each side. Pull it and see what you get.
6. Boats longtitudinal center of gravity. Weight aft seems to have more of an adverse effect on these boats than one might expect. I have moved my house batteries forward, and keep as much on board 'junk' in the bow lockers as possible. Also have anchor chain, which shifts the longtitudinal CG forward a bit. All this seems to help.
7. Altitude and fresh vs salt water. Our boat turns into a pig at high altitude and in fresh water. Will not even plane with the props that work just great in salt water at sea level. When in fresh water we change the props to a smaller size and pitch, and give the engine computers a couple days to 'adjust' to their new environment.
We have owned a number of boats over the years, and lived on board and cruised a Nordhavn for a while. Although the Tom Cat has some design issues, in the main we really like the boat, and have found most other owners feel the same way. You will get lots of useful feedback from those folks if you inform the group of your prop combination and engine mount configuration.
Kevin Ware
Tom Cat 25.5
