Help with Cavitating Prop

[dpfeffer]

I just purchased a used 1995 Proline 220 Cuddy/Fish. It has the standard 5.7L V8 MercCruiser and upgraded Bravo I outdrive. I am having a big problem with the prop cavitating. The boat will not get on plane without using full down trim tabs. From idle (with trim tabs up), the boat starts to accelerate as you advance the throttle forward. As the boat begins to plow (prior to getting on plane), the prop begins to cavitate. RPMs go up, and acceleration decreases. The boat can never get on plane this way.

If you put the trim tabs down and repeat the same process, the boat planes fairly easily with only 2 people on board. I know the trim tabs should not be required to get on a plane however.

Two props, a 14x17p aluminum and 13.5x19p stainless came with the boat. I originally thought the hubs were shot, so I replaced the hubs in both props and tried both of them again- There was no change.

Given that both props cause cavitation, I do not believe it is propeller induced cavitation. I'm an Aerospace engineer- I know fluid dynamics, but can't for the life of me figure this one out. I don't believe the upgrade to the Bravo 1 outdrive should have made a difference. I know Proline recommends a 14x19p prop. Both of mine are in that ballpark, so they shouldn't make a big difference either.

Thanks so much for your help.

 

[hwsiii]

Re: Help with Cavitating Prop

If you will tell me what is the gear ratio for the bravo drive and what is the horsepower for the motor as well as the maximum WOT RPM and GPS speed with both props, I believe I can help. And also what brand and model of SS propeller. If you happen to know much about props I will ask you about how much rake, is it progressive pitch and is there any trailing edge cup in the blades on the SS prop.

H

 

[dpfeffer]

Re: Help with Cavitating Prop

The Bravo One is available with a 1.26:1, 1.35:1, and 1.50:1 gear ratio. I am not sure what I have, so lets assume 1:35:1 to stay in the middle.

Engine horsepower is 290.

WOT with the Merc Laser II SS 14x19p prop is ~4800 rpm @ 42 mph.
WOT with the aluminum 14x17p prop is above 4800 rpm (I didn't take it higher), with a slightly lower top end.

Unfortunately, I don't know the other details on the Laser II prop, but the prop number is 48-16544-19.

Thanks so much for the help!

 

[steelespike]

Re: Help with Cavitating Prop

I assume when you say cavitate you mean it loses its bite.
Actually ventilation.Cavitation is a different animal not likely to cause planing issues.
I wonder if the bravo is shorter requiring a lower mounting position.
You might check the position of the Antivent (cavitation)plate in relation to the bottom.

 

[MikDee]

Re: Help with Cavitating Prop

Good point steelespike!

His top speed doesn't sound right for the rpm with either prop, there appears to be a lot of slip. The way I figure it he has at least 25% slip with the lowest 1.5 ratio gearing even more if he has the other ratios?

 

[dpfeffer]

Re: Help with Cavitating Prop

Thanks guys. The top speed is Ok. Proline says top speed with the 5.0L (largest engine they listed) is 42-44 mph with the 14x19p prop, so that puts me in the ballpark.

I'll check for depth distances b/t the alpha 1/bravo1.

 

[wca_tim]

Re: Help with Cavitating Prop

Laser 2 is actually recommended for a shorter boat... and is a bow lifting prop. Seems like you owuld be much better off with a stern lifting 4-blade prop... presumably that has a little more diameter or at least blade area than the laser does...

 

[MikDee]

Re: Help with Cavitating Prop

Is your boat extremely back heavy? What's your bottom look like is it full of barnacles? Or, are you running into seaweed? Finally, maybe your engine coupler is slipping? Something don't add up

 

[wca_tim]

Re: Help with Cavitating Prop

I'm also betting that the gear ratio is 1.5....

 

[hwsiii]

Re: Help with Cavitating Prop

I show the Bravo 1 for 1995 with a 1.5 gear ratio, but the largest 5.7 I can find is only 280 HP. Using that as a basis I show your prop slip at 27%. In my opinion you need more blade surface area to reduce your prop slip.

H