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Props blowing out

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Old 07-24-2010, 09:09 PM
  #1
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Default Props blowing out

Ok heres whats going on. I have a 1988 32' Hydra Tech Cat. Repowered from 468's w/ about 550hp, Bravo 1's, 1.5 w/ 30P 4 blades. Topped out at 96mph at 5800rpm. Had a slight prop cav while getting up on plane just backed off throttle a bit, props bit and away we went.

Over the winter we upped the engines to 540's w/ 700hp, moved the engines forward 2', Stellings 12" ext. boxes set at neutral(7/8" up), Imco SC drives, 1.5 w/ -2 shorties. So we're up almost 3" with alot more power.

So far we've tried a 26P Hydro 5 blades. Way too small. Reached 6K at half throttle, props blowing out before on plane, bad porpousing after on plane at 70mph.

Next we tried a set of 34P 4 blades. Still too small. reached 6K at 3/4 throttle. Blow out seemed better and porpousing seemed better. Got to 102mph but boat scary loose in the stern.

Third try with a set of 33 Throttle up 5 blades. Blow out real bad, once it bites it takes off. Great punch at 75 but around 85 starts to porpous again, bad! Think it would go past 102 point but trying to power thru it starts to get real loose and jumping out of the water. Trim doesn't seem to help with any prop.

Knowing that I will most likely have to re-gear the lowers to 1.36? would help with prop pitch selection. I was considering spacing the lowers down 1.5" to see if it helps the looseness at speed and possibly help the blowout? Am I on the right track here? Any suggestions would be appreciated?

Sorry for the long story.
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Old 07-25-2010, 05:15 AM
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man, that was a really long story...
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Old 07-25-2010, 05:58 AM
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Thanks Gov. very helpful. LOL
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Old 07-25-2010, 07:17 PM
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Do you have cavitation plates on the shorties? This will help with blowout on take off. However if your breaking free on the upper end and having handling issues, as well as blowing out 5 blades Id question if your X dimension is too high.
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Old 07-27-2010, 11:38 PM
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With a bravo setup your probably too high. Mine started out with prop shaft 3" above bottom and boat wandered all over the place. I found my best slip #'s with the prop shaft even with the bottom. I have 12" imco neutral boxes 725 per side. May wanna try spacing down. I spaced down 1" at a time till i found what made the boat happy. I never had any blow out but i also started with 36p bravo one. I also have 32p maximus and they work about the same but the 4 blade is about 2-3 mph faster on the top.

Last edited by bobsbillets; 07-27-2010 at 11:41 PM.
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