[Chris Burkard]

You are over-propped! Being over-propped adds stress to the engines, shafts and transmissions and reduces efficiency and long term can cause premature engine and transmission failure.

I believe your engines should turn at WOT under a load, a minimum of 4000 and a maximum of 4400 (please verify this as I am going off of memory). Tiara usually propped the boats to achieve a WOT throttle with a moderate load (full fuel, water, 2 people and 400 lbs. of gear to reach somewhere in the middle (4200 RPM). This way if you are running lite (low on fuel) you won't be able to over rev the engines and if you run loaded down with full fuel and 6 or 7 passengers plus gear you won't overload the engine engines.

If the recommended WOT range is 4000 to 4400 RPM, I would shoot for 4200 with a moderate load just as Tiara originally did.

My guess is you need 18x18-4s or 18/17-4s, but a prop shop will be able to give you a better estimate. I'm not sure that you can remove 3 or 4 inches of pitch from a prop so that can use your existing props. I know they can be adjusted about 1 or so inches either way. The rule of thumb for a vessel of this size is 1 inch of pitch equals approximately 100 RPM. You may be looking at purchasing new props.

Good Luck and let us know how it works out.