Prop switch help

[racerman16]

87 formula pc25 with single 260hp. Last year I had a 3 blade alum 15.25x15 prop. Boat was a dog coming out of the hole but ran 36mph at 4900rpms. This year(I messed up and couldn't remember my prop numbers) i ordered a 15x16 4 blade alum prop. The boat comes out really fast and I love it BUT my WOT has jumped to 6200rpms. From 4900 last year. Even at 5600 rpms I'm cruising 36mph. At 4800- it's 27mph. Now I'm lost because I don't want to be anywhere near that rpm.

 

[MikDee]

Re: Prop switch help

A 141/2"X 17" 3 blade Black Max should be the best prop for that boat. Bigger diameter then needed can cause drag, smaller diam. can cause prop blowout, & falling off plane.

 

[racerman16]

Re: Prop switch help

How is that going to act coming up out of the hole? I do a lot of tubing and need that get up and go.

 

[MikDee]

Re: Prop switch help

How is that going to act coming up out of the hole? I do a lot of tubing and need that get up and go.

It should spin up faster because you've taken some of the extra diameter load off it, giving you a better hole shot then your old 3blade 16"prop.

 

[jestor68]

Perhaps you should get your tachometer checked for accuracy. Your numbers indicate that it's wrong; reading too high.

 

[racerman16]

I've checked it against two different timing lights that have a rpm mode and they all are reading the same.

 

[steelespike]

At 5600 rpm 36 mph I get 36% slip at 4800 and 27 I get 44% normal slip is around 13%
A 1" increase should knock about 200 off the rpm. Its normal for diameter to go down as pitch goes up.
It's highly unlikely for 1/4" to have that affect.If we figure 5600 at 13% we get 49 mph.
If we figure 36 mph at 13% slip we get 4100 rpm and your running at partial throttle so 4100 seems normal.
​ You either have a whacky tacky or a very poor prop. Perhaps you could borrow a prop and check the rpm.
Are your speeds by GPS?

 

[racerman16]

Ok. I've heard about "slip" before but what is it and how do you control/change it? My speeds are by the speedometer on the boat and by my gps. There is usually 1 or 2mph difference between them though.

 

[steelespike]

Slip is a way of describing the actual distance the prop moves through the water.Your 16" prop is said to move 16" in one revolution in a solid.
Water isn't a solid so slip describes the amount of slip.Very basically as rpm goes up slip goes down.If rpm goes up and speed doesn't progress as it should
slip will higher. If your tach is reading high slip will appear high. If the tach proves to be accurate then logic says something may be up with the brand new prop.
​ The increase in pitch and the addition of a blade should hold rpm down. Another reason to suspect your Tach.
Your relatively heavy boat may have somewhat higher slip than say a 17 ft runabout.But highly unlikely 44%.
​ Pontoon boats usually have slip around 20%

 

[racerman16]

Ok. I haven't looked yet but does the tach wire come straight off of the distributor like a car?. When I give it more throttle, she picks up speed with no issues.

 

[steelespike]

I assume the tach is off the distributor but I really don't know. Also the boat is very likely responding normally but the tach appears to be reading high.
Marine tachs are noted for poor reliability,both outboard and inboard especially with age.
There may be a selector 4,6 or 8 cylinder on the back of the tach.Note its position and operate it a few times to clean the contacts.