I got a 25 foot donzi ragazza it had a 5.7 260 hp v8 in it. with a 14 !/2 19 pitch NOW i replace the motor with a 300hp v8 adding 40 hp to the boat. what prop should i use. the new engine factory WOT is 4900 rpm. im getting 4000 rpm with the 19 inch pitch help me plz smart guys. i know it is a big boat at 25 feet but im getting crap speed for having 300 hp on gps i cant even get 40 mph HELP

What kind of RPM and speed were you seeing with the old motor? What does your boat weigh? IMO a 5.7 litre seems small for a 25 foot boat. You may have a problem with just being underpowered (are you sure your getting 300 hp?) or with too much weight for that engine. Even with a 17 pitch prop your only going to get to 4400 RPM's and will most likely still be around 40 mph.

well the boat came with a 260 hp in it from factory. dont see why adding 40 more hp in it would be underpowered. with old engine i got 3000 rpms sometime a bit more. dont know the weight of the boat. people with other donzi with the 260 tell me they get 43 to 48 mph. im using 300hp on the same boat why on my gps i see like 30ish on clock. i think i should be getting more rpms and more speed with another pitch. just dont seem right.

You want to spin it as close to the max recommended WOT rpm to make the most power. What gear ratio is the drive????

All 19" props will not turn the same rpm on the boat. You need to make sure you have the correct make of prop for the boat.

If you were only getting 3000 rpm with the last engine and are now getting 4000 you are going in the right direction!! Griff is 100% correct--you NEED to run that motor in the correct operating range (4400-4800 RPM). I would take your boat and trailer to a truck weight station and see what it weighs and then compare that to what it was when new. You either have a very heavy boat (they do gain weight as they get older) or are not making enough power to move that boat at the speed you desire. Also, the 300 HP your motor is rated at is at the crank--at the prop its probably closer to 250-260.

I have a feeling you are going to have to drop to a 17 pitch prop. Ill bet a mercury High 5 (5 blades) 17 pitch prop would make you very happy with rpm and getting on plane but you will probably still be around 40-43 mph.