wrong prop? cavitation?

[Shea_G]

I swapped out the 4.3 from my 97 regal 2100 (fastrac hull) for a mid 90s auto vortec 350. It used to have a 17" 3 blade prop in it with its second gen alpha leg. The gear ratio is 1.84 so i reproped to a 21" 4 blade SS. The prop has four holes around the diameter. Itll run up ok at slow sleeps but as soon as i gun it the rpms shoot up and the speed down. I dont think its spinning on the hub, i took it all apart and it looks fine and when i grab the prop on land and spin it in drive it just turns the engine over. Think i need a much high pitch prop? Or any other ideas?

 

[steelespike]

Some pics would probably help.Is this a new prop?
From your description it looks like someone tried to vent the prop and did it wrong.
Can't tell much about it if you can't do a wot gps speed and rpm.
Probsably need to get the holes filled.
Your going from 190 hp to about 300. and your gear ratio is close to that of the 135 hp 3.0. 1.98.
The 3.0 will turn up to 4800 with a 21" prop. My guess would be for a 23" 4 blade. And may still be under propped
But you might be able to live with it till you could figure the right one.

 

[Shea_G]

Not new but mint. The holes are factory, seen em before but dont know there name. They vent exhaust in front on blades. Speed wont tell anything cause its cavitating or spinning. I dont know, literally 10-15 mph would be my guess. Maybe less. Obviously no where near a plane. I got it planning by working my way up slower but it was still slow as hell, maybe 30 if im lucky. I should be doing 50+ no problem. You're thinking its way under propped?

 

[ondarvr]

Not hard to tell if it's spinning, so ruling that out should be easy. It wouldn't be cavitating, it would be ventilating if the RPMs build and you don't speed up. So you have two things to look at right now.

 

[Shea_G]

Well i made a mark on the shaft and prop and they look like they moved maybe a couple degrees. A tiny bit and the plastic piece would should if it was spinning i would assume so its ventilation then? What remedies are there?

 

[fhhuber]

If you marked the hub to check for slippage and it moved its wrong.

You might see a few deg... but it might have slipped hundreds (thousands) of revolutions. If that's what is going on it won't be long before you'll be able to
turn the prop on the hub easily since it will be wearing the rubber insert out.

 

[ondarvr]

Like he said, there should be zero movement of the prop.

 

[Shea_G]

Yea i get that but if it were too slip would there not be wear? It was tortured down to 50ft/lbs and there isn't a mark on the plastic peice.

 

[ondarvr]

The torque applied to the nut is irrelevant, it's the internal hub that slips, you said it moved, if it moved it's bad, if it didn't move then it's ventilating.

 

[Shea_G]

Im worried that the slight amount it appeared to move could be human error in setting the marks (not done by me) or play in the hub thats why im wondering if theres another way to test or inspect the hub on land. What causes ventilation and what remedies are there? The boat worked fine with old prop and engine.

 

[fhhuber]

Your engine is applying a LOT more than 50 ft-lb to turn the prop itself on the rubber insert.

The rubber insert is not squeezed tighter by tightening the prop nut. Its pressed in with a hydraulic press and the center is also pressed in. That center is essentially a tube with splines on the inside.

Eventually the rubber degrades and you have to have the old rubber pressed out and a new one pressed in (if the prop is worth restoring) that rubber can break down just from age even if its never been in the water.

 

[ondarvr]

There should be zero play in the hub. Ventilation can be from the wrong prop, too much HP for the prop, the vent holes, wrong trim setting, outdrive mounted too high (not likely in this case), plus other things.

 

[steelespike]

If the holes are factory you should be able to get plugs either to seal the holes or to restrict the holes.

 

[Shea_G]

Wait a second, the hub and rubber hub are hydraulically pressed in?

 

[ondarvr]

Yes, it's a press fit designed to slip if you hit something, the rubber eventually degrades to the point of failure even if you don't hit anything.

 

[steelespike]

Some props have a separate hub that can be moved from prop to prop.

 

[Bondo]

90s auto vortec 350. It used to have a 17" 3 blade prop in it with its second gen alpha leg. The gear ratio is 1.84 so i reproped to a 21" 4 blade SS.

Ayuh,..... With that gear ratio, yer never gonna get it to run Right,.....

Ya really need the 1.5:1 gear ratio with the V8 motor,.....

 

[Shea_G]

Yea i know. Im gonna switch it next season but i can definitely get it usable with the right prop. Anyways it looks like its not a prop issue after all but possibly timing or carb so off i go to the appropriate forum

 

[ondarvr]

Yea i know. Im gonna switch it next season but i can definitely get it usable with the right prop. Anyways it looks like its not a prop issue after all but possibly timing or carb so off i go to the appropriate forum

From your description of the problem, if accurate, it would make timing or carb issues not a possible solution. You said that at 15MPH or so the RPM's would increase but your speed wouldn't, those symptoms have nothing to do with timing and/or the carb.

 

[Shea_G]

I dont have a tachometer so i think i was mistaken. Its backfiring like crazy out the carb after half throttle.