4" Prop Hub on Bravo 1 Drive, Outboard verses sterndrive prop hub size

[JC766]

I currently have a Bravo 1 drive with a Bravo 1 22p prop. I came across a decent deal on a Hydromotive Quad IV OT 23P that I wanted to try but it is the outboard version. Supposedly the specs are the same but the hub size is 4" versus 4.5". I have read that if your exhaust goes through your drive it will create poor cavitation as it escapes around the hub. But what about a sport cruiser with thru hull exhaust?

Would the smaller hub create a bad effect as the water runs past the drive to the smaller hub size? My thoughts are that the smaller hub would decrease drag but the 1/4 gap all the way around from the drive to the prop may be an issue.

I know the diameters are slightly different too between Hydro's Quad IV 15.25 versus the Quad IV OT 14.75. However, I'm hoping the smaller diameter would keep me in a higher RPM for the increase in pitch.

2001 Kachina Bandit, 23' (stepped hull, pad bottom)
454 MAG (385 hp)
About 3,800-4,000 lbs med load
Topping out at 58-60 at 5,000 rpm with Bravo 1 22p

 

[steelespike]

From my limited reading here in the prop forum; the "outboard" props sometimes "work" but others fail and all are likely to produce higher than normal slip numbers.

 

[alldodge]

See a couple of issues, as you say the exhaust will come into the area of the blades and ventilation will happen. The next is the water flow going to a smaller diameter will create turbulence and cause more problems

 

[Bondo]

Ayuh,..... It'll ventilate, 'n blow out,.... Been there, tried it, it don't work,....

 

[JC766]

But my exhaust is through the hull so ventilation should not be a problem. It's the "turbulance" as Alldodge referred to that I am wondering about. As the water runs past the drive to the narrower barrel diameter will it create a big effect.

Thanks guys.

 

[alldodge]

If your exhaust boot has been removed your going to suck air thru the prop just like a carb does due to the venturi effect. A stepped hull does the same thing as a reduced dimension would. As water passes to the smaller diameter, the turbulence and reduction in pressure will cause air bubbles to form.

Go try the prop and prove it to your self, we won't say we told you so