Help! - Drive Height/ Props? - 32' Skater I/O
05-10-2004, 12:54 PM
#1
Guest
Posts: n/a
Help! - Drive Height/ Props? - 32' Skater I/O
Can somebody out there can shed some light on this for me?
Trying to dial in a 32' Skater with twin bravos. Have the following data from last week's sea trial. Motors and complete driveline is new.
Getting excessive prop slip.
3000RPM = 31 mph, 70+% slip
4000RPM = 50mph, 42% slip
5000RPM = 65mph, 35% slip
5500RPM = 70mph, 38% slip
Drive height is suspect here.
My questions are: What should the drive height be on bravo drives in reference to the bottom of the boat?
What size drive spacers or different props would one recommend to try?
Here is some other useful info:
Drives = HP Bravo 1 with water pickup in the bottom nose of the drives (1.5:1 ratio)
Drive height= 3/4" above bottom of boat(measured to centerline of prop shaft)
Engines = 502 CI /665hp each, N/A. (6500max rpm)
Drive-shafts = 18-1/4" long.
Drives width separation = 51-1/2"
Notched transom hull. (36" distance from hull to nose of drive)
Props = Bravo 1 four blade - 28" pitch x 15" diameter (turning inward)
Appreciate any help one could offer,
GNJ2
Trying to dial in a 32' Skater with twin bravos. Have the following data from last week's sea trial. Motors and complete driveline is new.
Getting excessive prop slip.
3000RPM = 31 mph, 70+% slip
4000RPM = 50mph, 42% slip
5000RPM = 65mph, 35% slip
5500RPM = 70mph, 38% slip
Drive height is suspect here.
My questions are: What should the drive height be on bravo drives in reference to the bottom of the boat?
What size drive spacers or different props would one recommend to try?
Here is some other useful info:
Drives = HP Bravo 1 with water pickup in the bottom nose of the drives (1.5:1 ratio)
Drive height= 3/4" above bottom of boat(measured to centerline of prop shaft)
Engines = 502 CI /665hp each, N/A. (6500max rpm)
Drive-shafts = 18-1/4" long.
Drives width separation = 51-1/2"
Notched transom hull. (36" distance from hull to nose of drive)
Props = Bravo 1 four blade - 28" pitch x 15" diameter (turning inward)
Appreciate any help one could offer,
GNJ2
05-10-2004, 06:38 PM
#2
Registered
Do you have dyno numbers on the motors?
The rpm is 1000 below the max so you have to make sure you are off the torque curve.
You may want to try dropping the drive 1" with a lower pitch prop. If you drop the drive down, with the same props, the slip will come down and the WOT rpm will be even lower than it is now.
What year is the boat?
What's the history of the props? Are they stock or have they had work done to them?
Matt
The rpm is 1000 below the max so you have to make sure you are off the torque curve.
You may want to try dropping the drive 1" with a lower pitch prop. If you drop the drive down, with the same props, the slip will come down and the WOT rpm will be even lower than it is now.
What year is the boat?
What's the history of the props? Are they stock or have they had work done to them?
Matt
05-10-2004, 08:10 PM
#3
GNJ2
Try this formula http://go-fast.com/Prop_Slip_Calculator.htm
Your boat should run over 110, but you won't get there with 28's and a 1.50 ratio. My Skater 36 with 500's ran 112 with 1.36 gears and 34 B1's at 5600. This with the prop 1.75" above the bottom, Stellings boxes, and notches.
You may be a little high but my slip #'s were still in the high teens (best) at top end. Slip #'s will be higher at lower speeds, 20%'s are not unreasonable on these boats. Super cat lights are running (I believe) 1" above like you are without boxes, and not having major problems.
I would try some 34/36 Bravo ones before anything else.
Is you prop completely shielded from tunnel water? If so more slip.
Try this formula http://go-fast.com/Prop_Slip_Calculator.htm
Your boat should run over 110, but you won't get there with 28's and a 1.50 ratio. My Skater 36 with 500's ran 112 with 1.36 gears and 34 B1's at 5600. This with the prop 1.75" above the bottom, Stellings boxes, and notches.
You may be a little high but my slip #'s were still in the high teens (best) at top end. Slip #'s will be higher at lower speeds, 20%'s are not unreasonable on these boats. Super cat lights are running (I believe) 1" above like you are without boxes, and not having major problems.
I would try some 34/36 Bravo ones before anything else.
Is you prop completely shielded from tunnel water? If so more slip.
05-11-2004, 08:00 AM
#4
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Posts: n/a
more info
Thanks for the replies.
Matt:
5500 RPM was not WOT.
I did not push it any farther.
In answer to your question - Stock new props.
John B:
The 28's weres all I had at the time to test with. The props are not sheilded from the tunnel and 3-1/2" of the blades are into the tunnel.
Question: How much prop is into the tunnel on your 36?
Gary
Matt:
5500 RPM was not WOT.
I did not push it any farther.
In answer to your question - Stock new props.
John B:
The 28's weres all I had at the time to test with. The props are not sheilded from the tunnel and 3-1/2" of the blades are into the tunnel.
Question: How much prop is into the tunnel on your 36?
Gary
05-11-2004, 08:26 PM
#5
GNJ2,
I would guess about the same as yours, around 3" in the tunnel. I just sold it, so i'm going from memory.
You can always add spacers to lower the props after you get close. I went to labbed 5 blade Hydromotives to improve the slip and drivability. The speeds were about the same compared to llabbed Bravo Ones.
I would guess about the same as yours, around 3" in the tunnel. I just sold it, so i'm going from memory.
You can always add spacers to lower the props after you get close. I went to labbed 5 blade Hydromotives to improve the slip and drivability. The speeds were about the same compared to llabbed Bravo Ones.
