prop problem
 

hello everyone i have a problem with my prop.
i have a 27ft fletcher with 440hp yanmar and fixed surface drive. the first prop i tried was a 17x28 3 blade with 73% blade area it was doing 60mph at 2700rpm 700rpm less then maximum so i tried a 17x25 with 63% blade area it was doing 54 at 2700 rpm still way down in rpm so i tried a 17x23 with 53% blade area it was doing 56mph at 3100rpm. so what do i do now i need as much top end speed as possible. the boat is only 2.2 ton do i go for more blade area less cupping or what bearing in mind all these props is heavily cupped . i hope some of you experts can help me any help will be much appreciated

try smaller diamiter

thanks mate i suppose its worth a try

gerard get up bill at btm marine he used to run a desel fountain .. he should know what you need . robbie

cheers mate ill try that

Agreed, 17 inch diameter seems a bit large for your hull. Try a smaller diameter, possibly. You may also consider releiving some the cup if it is heavily cupped.

should i reduce the dia of the 28pitch that has the larger blade area. and guys i really appreciate the help im getting here i was on the boatmad forum and got nothing but smart arse replys
thanks again

What is the reduction in the transmission?

it is a zf 2speed the first gear is 1.3-1 and the second is 1-1 direct drive

If you want top speed reduce the diameter (as suggested) and run with less blade area and more pitch. That is generally the formula for speed. With surface piercing setups, drives tend to like more blade area in the form of more blades, so instead of 3 big blades you might want to try 4 or 5 smaller ones.