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Drive spacer or not?

Old 09-17-2022, 03:00 PM
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Default Drive spacer or not?

Hi, have done some testing with my new build, 26ft 24 degree v-hull, no notch or step, bravo 1.5 ratio, 4.7" propshaft height. Single big block 509cid, dynoed 520hp @5300-5400rpm.

Hydromotive p5x 24"
2700rpm=30mph=27% slip
3000rpm=33mph=27% slip
3500rpm=42mph=21% slip
4500rpm=60mph=12% slip

Boat planes nicely but trim has little to no effect under 4000rpm speeds, still it does not feel bow heavy, and rides like its in rails. Also have a bravo 24" blade, but those slip numbers make me think it is not even worth to try. Have not tried WOT speed yet, but i have feeling that the slip would lower than the 12% @4500rpm. Considering to try 1/2" drive spacer. What is the typical slip @ cruising speeds in these hulls, propshaft 4.7" below?
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Old 09-18-2022, 10:49 PM
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12% slip is not bad but it looks as you need to get your rpms up.
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Old 09-18-2022, 11:10 PM
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Originally Posted by badluck View Post
12% slip is not bad but it looks as you need to get your rpms up.
There is throttle left @4500rpm. Engine is fresh build so i want to take it easy.
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Old 09-19-2022, 12:19 AM
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Your slip numbers are similar to what mine were on my 28 Pantera, except the best the Pantera got was to about 17%.
I ran a 26 Bravo prop and had about 560hp and prop shaft was 4.25" below bottom with a notched transom.
I would be less concerned with slip% and focus on what actually performs best. I would also try a P4X or Bravo 1 prop.
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Old 09-19-2022, 12:20 AM
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You shouldn't need a spacer at 4.75 below and 1/2" more prob won't do much, but every boat is different
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Old 09-19-2022, 01:25 AM
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Originally Posted by Griff View Post
Your slip numbers are similar to what mine were on my 28 Pantera, except the best the Pantera got was to about 17%.
I ran a 26 Bravo prop and had about 560hp and prop shaft was 4.25" below bottom with a notched transom.
I would be less concerned with slip% and focus on what actually performs best. I would also try a P4X or Bravo 1 prop.
I have a bravo 24" and powertech ofx4 24" props in hand to test. Every time i go to test the sea is like glass, zero wind/chop. The bravo 24" had 18% slip @5000rpm, trim fully down.
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Old 09-19-2022, 06:04 AM
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Why are you testing at full trim down ? On the other hand, Why are you driving at full trim down ? Btw: Bow steer can be dangerous.
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Old 09-19-2022, 07:02 AM
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Originally Posted by SB View Post
Why are you testing at full trim down ? On the other hand, Why are you driving at full trim down ? Btw: Bow steer can be dangerous.
Trim up did not work when on plane, found out that the hoses at the trim pump were connected wrong. That was at first test run with bravo prop, now the trim is working and i have to give another change for the bravo prop.
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Old 09-19-2022, 07:08 AM
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Voice of reason says Do proper testing for proper results.
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Old 09-19-2022, 08:07 AM
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Originally Posted by SB View Post
Voice of reason says Do proper testing for proper results.
Yes sir! Those numbers with p5x are highest speed for the given rpm ( trimmed). I try to test the bravo 24" again during this week. I think the main reason i did not realize that the trim was not working, is beacause it feels like the boat does not need any positive trim, it has bow sligthly up attitude drive fully down. New boat that rides complitely different than the old one, you know.

Last edited by JaniH; 09-19-2022 at 08:18 AM.
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