tssteph

Ok, I had some time to do some testing after getting my fuel pressure issues resolved.

Problems so far:
Prop blow out pretty much everywhere in low to mid range, especially bad as the boat breaks up on plain. SB prop is much worse. boat chine walks a lot with any amout of trim over 70. 280 K plane dropped down a little seattles it down.

Props are turning out.

Comes on plain just fine, no problems there.

3200 rpm 50 mph gps
4900 rpm 77 mph gps tabs down for chine walk problem.

Requires trim to be around 5 to get good speed, I have not trimmed up to the point of rpm gain.

6400 lbs + 80 gallons of fuel and two adults

1998 29 Scarab twin 350 mags mpi motors, 5 psi procharger kits, stainless marine exhaust, nose cones, bravo drives.

Props are 27 pitch mirage plus completely stock.

What I would like and in this order:
3 mph on the top end, no blow out in the mid range, no blow out in the low end.

Should I switch to a 4 blade prop? Can the 27's I have be re-worked to get me what I want? Or would I be better to get 4 bladed and have them worked? If so what pitch?


Thanks in advance.

cobra marty

My guess is to try the 4 blade bravo 26" or maybe a 24" depending on if you want to torn about 400rpm more if you have the TQ which you probably do with the prochargers. Get the rpms and blow-out where you want and the 3mph is only a 1/4" smaller pully change away.

throttleup

At what RPM is your peak HP produced?

Julie

tssteph

Julie,
I am not really sure. Rev limiters are set at 5400 by Mark at Percision Marine.
I think I would like them to turn around 5100.

Brian

bbladesprops

Pulling an additional 200-300 rpm on the top and maintaining the efficiency you currently have with the 27's wouldn't be a problem. This should reward you with the speed gains your looking for.

The blow out issue is a different animal. Some of the blow out coming on plane is the nature of the beast for your boat. The 4BL would definitely help this problem, but most likely won't make it completely disappear. Neither will Labbing the 27's. The 4BL will improve your performance most everywhere but because of your WOT slip numbers being good, the top end is somewhat elusive. 26 Bravo 1's Lab Finished by us would take you to 5200 rpm and if we drop 1% in slip you will gain .4 mph. But,,,,,more blades mean more drag too. The Bravo 1's will help the chine walking also.

Try to spin the props in and see how it affects the blow out issues and the chine problem. Then we take that out of the picture. Do you have any 4BL props you can try? Do one test at a time so we know what works and what doesn't.

tssteph

I will try them turning in. I did this once prior to the chargers and it porpoised so bad I hated it. I can deal with the blow out coming on plain, this I can drive around. I would like to have a little more bite through the mid range and a couple MPH on top. Can this be done with the 27's?

Brian

throttleup

If you want to prop for top end then you need to run at the peak HP.

Julie

bbladesprops

In order to gain top end speed we need to use the tallest pitch we can, turning the max rpm the motors are designed to run at. With 5400 rev limiters and knowing those motors, 5200 is probably the place to spin them. Lab Finishing the current props will maintain their pitch and should produce the speed your looking for. Since your mid range #'s are good, you shouldn't lose anything there.

The personality of the Mirage Plus's in the mid range can be adjusted to give you more bite but may affect the ultimate top end numbers too.

tssteph

Ran the boat on saturday timming for complete speed.
5000 RPM 78.5 MPH with trim around 7
3 blade mirage plus 37 pitch stock.

What will happen to top speed if I switch to a 28 pitch Bravo I prop?
What about mid range?

Brian

cobra marty

I think that you would want to go down 1" pitch when changing from 3 blade to 4 blade bravo prop. A set of 26" would be better and then have them worked later.