I've been racing RC boats for 40 years and one of my top boats, which runs in Multi Engine Hydro with two 91's for power weighs 15 lbs, and steering servos are a problem. On a friend's suggestion I purchased DS470BLHV for rudder, the DS90DLHV for throttle, and DS100DLHV for mixture.
Just powering up on the bench I could tell the difference right off the start. They were very smooth and quiet, so off to the lake we go. Fired up the pair of 91s on 55% nitro. Boat in the water and with ProModeler DS470 servos for steering, DS90s on throttle, and tiny DS100 micro servos on mixture the boat was driving straight and great. Never had a boat drive this smooth before. Exit off the turns was true and straight. No course correction was needed. Clocked with radar at 88 mph. Took home the hardware in 1st. These motors turn about 23,000RPMs. One run was about 15 laps. On our course 2 laps is 1/3 mile so 6 laps is one mile. This was a full fuel load of about 48 oz. total of fuel.
I like the way the boat ran so well the next week I ordered more servo's to replace the ones in another boat. Guess I'll be replacing servos in all of my boats as I get to them.