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.

The boat obviously liked the blade area as it was downhill as you went to less. I agree that you should try 4 blades at the least. I don't understand why the RPM did not increase with the change from the 28 to the 25, are you sure the engine is making power? Good fuel flow? Plug your numbers in here:

http://go-fast.com/Prop_Slip_Calculator.htm

thanks a million you guys on this forum is so helpfull compared to the
**** british forums. but as props are so expensive and almost unheard of in this neck of the woods it makes it very difficult to try different props would you guys have any props that i could barrow or buy i would pay all the shipping costs
hull:27ft
beam:8ft
waterline:23ft
weight: 2.2ton
hp:440
fuel:diesel
max rpm:3300
shaft angle:fixed 7%
ratio:1-1

thanks again i really appreciate the help

i sent both props off to be cut down in dia and relieve some cupping when the props were scanned the 25 had a cup equal to 6inches and the 28 was 3 inches cup witch left both props 31 how much cup is normal

i got my 28 3 blade cou down to 16 dia took out some of the cupping and thinned the blade it came up 300rpm and went down 5mph in speed and then i tried my first rolla 17x28 62mph at 2700 rpm i think if i had a 17 or 16 24 rolla it would be good has anyone got one they would sell or lend or advice to give

cheers gerard