Another cavitation question

[Sow&Pigs]

If I turn hard at speed my motor cavitates like crazy -- to the point where I have to back off the throttle until the boat settles down. Apart from the safety factor it's just plain annoying.

Boat is a 2520 MV; engine is already in the top holes so it can't go any lower -- it's a 225 Optimax with a 25" shaft and spinning a 15 1/4 X 15 aluminum Black Max (lots of rocks in our area...).

Apart form that one situation I have no issues: fuel economy is good, top speed on a glassy day is north of 35, and normal cruise is about 28 at 4500.

Any thoughts?

 

[HM5]

the aluminum prop is not helping your situation, IMHO. I have found that there is enough flex in the blade of an aluminum prop that is not present in SS props. Consider that your maneuver loads the prop considerably to the point that it just won't hold. I have experienced this condition with my old 20' Willcrack with a 200 blackmax on the back. Stainless steel prop cured the issue, and I just watch out for the rocks. Besides I can repair/straighten minor dings in SS, rather than trying to weld aluminum.

 

[DaleH]

Besides I can repair/straighten minor dings in SS, rather than trying to weld aluminum.

Yes, but with a BIG hit, S&P will just lose a $100 aluminum prop as compared to your $2,000 to $3,000 lower unit gearcase.

S&P, my rig is darn near identical, down to the prop sizing and mine too is mounted on the top hole. Just trim the OB in before taking power turns. Works for me!

 

[B-Faithful]

When I had a black max on my 225 opti cavitation was a HUGE issue. Going to a good Stainless prop like a Rev 4 or Enertia solved the problem. The black max is probably your problem.

 

[tbone65]

Besides I can repair/straighten minor dings in SS, rather than trying to weld aluminum.

Yes, but with a BIG hit, S&P will just lose a $100 aluminum prop as compared to your $2,000 to $3,000 lower unit gearcase.

S&P, my rig is darn near identical, down to the prop sizing and mine too is mounted on the top hole. Just trim the OB in before taking power turns. Works for me!

This may be not as big of an issue as some think...I know Conventional Wisdom says the aluminum prop will sacrifice itself in an impact where the stainless won't, but.....we just don't see that much here anymore. We send dozens of SS props out for repair every year, and sell dozens of replacement aluminum props...and don't do many gearcase replacements. In fact, I don't think we've replaced more than 3 propshafts in 3 years...and we sell 150-200 boats a year here.

Breakaway hubs (FloTorq, etc) have probably been the best improvement in propshaft protection. Whatever the risk, IMHO the benefits of Stainless outweigh the additional risk exposure. Because of the performance disparity, we do not rig V6 outboards with aluminum props, specifically to avoid the issues the original poster in having.

Just my .02.....

 

[Sow&Pigs]

I think I'll try what Dale is suggesting first, although I appreciate the conventional wisdom of SS v.s. aluminum.

I tend to run the boat with the engine trimmed up as far as possible, given the prevailing conditions.

I also tried a bunch of stainless props a while ago, and couldn't seem to find one that worked as well as the Black Max. Maybe I didn't try enough.

Most of the Parker owners on this site seem to be Yamaha drivers, so I suppose it's a long shot to assume that someone has the same set-up that I do... notch transom, 2520 running an Optimax. Anyone?

 

[HM5]

Back again. I ran your specs through the Mercury prop selector a couple of ways.
1st:
Boat Specifications
Boat Type: Offshore Fishing
Boat Weight With One Engine: 5500
People & Gear: 500
Fuel Capacity: 150
Performance Goal: Overall good performance
Altitude (ft. above sea level): 0
Engine Manufacturer: Mercury
Engine Type: Outboard
Year: 2002
Number of Drives: Single
Engine: 225 V6 (3032cc)
Drive: Large OB 200-300 HP 1.75 Gear Ratio
Gear Ratio: 1.75

Results:
model material blades dia.&pitch part number
BLACKMAX Aluminum 3 16.000 x 14.0 RH 48-16438A45

BLACKMAXxc Aluminum 3 16.000 x 14.0 RH 48-16438CP1

ENERTIA X7 Alloy 3 15.000 X 15.0 RH 48-898990A46

MIRAGE+ Stainless Steel 3 15.750 x 15.0 RH 48-19838A46

VENGENCE Stainless Steel 3 14.500 x 15.0 RH 48-16312A46

Changed Goals from Overall Performance to Handling and results are:

BLACKMAX Aluminum 3 16.000 x 14.0 RH 48-16438A45

BLACKMAXxc Aluminum 3 16.000 x 14.0 RH 48-16438CP1

It looks like you have what Mercury recommends for your application with the Blackmax prop. I saw that you took part in another Prop thread a while back,
http://www.classicparker.com/phpBB2/viewtopic.php?t=471&highlight=
You will probably have to do more testing or trim back in before you turn. All I can say is the 3 blade aluminum would blow out on turns on my Wellcraft and the 5 blade SS High 5 didn't. If you fish in the rocky shallows, maybe an electric trolling motor?

Just my .02

 

[MaxOut]

Besides I can repair/straighten minor dings in SS, rather than trying to weld aluminum.

Yes, but with a BIG hit, S&P will just lose a $100 aluminum prop as compared to your $2,000 to $3,000 lower unit gearcase.

S&P, my rig is darn near identical, down to the prop sizing and mine too is mounted on the top hole. Just trim the OB in before taking power turns. Works for me!

This may be not as big of an issue as some think...I know Conventional Wisdom says the aluminum prop will sacrifice itself in an impact where the stainless won't, but.....we just don't see that much here anymore. We send dozens of SS props out for repair every year, and sell dozens of replacement aluminum props...and don't do many gearcase replacements. In fact, I don't think we've replaced more than 3 propshafts in 3 years...and we sell 150-200 boats a year here.

Breakaway hubs (FloTorq, etc) have probably been the best improvement in propshaft protection. Whatever the risk, IMHO the benefits of Stainless outweigh the additional risk exposure. Because of the performance disparity, we do not rig V6 outboards with aluminum props, specifically to avoid the issues the original poster in having.

Just my .02.....

I would be willing to bet the 3 prop shafts replaced were not all related to SS or aluminum prop. The dealer I bought my boat from will not sell me an aluminum prop for my 2520SL-Yammie 250. Too much torque to weight ratio causes the prop to flex and becomes dynamically unstable. Get a stainless prop and keep the aluminum as a spare. Either way if you are in an area with the likelihood of hitting rocks is high you should have a breakaway hub and a spare prop. A stainless prop along with trimming your motor down will help your situation.

 

[tbone65]

I agree 100 percent...in fact, I'd have to dig through the records, but I'm sure at least two of them were on 115's or smaller, running aluminum props.

Also, this won't make me any friends with the Mercury guys, but: I think their prop application software is junk.

If you have one, a local prop shop with some experience with the type of boat you are running is probably your best resource for getting propped correctly.

 

[DaleH]

I dunnoh ... I personally know of more wrecked lower units and/or gearcases due to boats with premium SS props hitting rocks, with special hubs or not. Even many propshaft straightenings or replacements .. but maybe my experience differs as we have so much rocky coast up here in NE. Can't say I know of one motor with any of the above damage when they used an aluminum prop.

I even know of a 70hp Suzuki 4-stroke that hit a rock with the prop without damaging the skeg. While just 2 of the 3 blades were wiped out ... but the ENTIRE top-end of the motor was damaged. The rubber-insert prop was not enough to prevent damage to the top-end of the motor, just too many parts in motion and the Suzi's of that HP use an interfence cam to cam shaft arrangement.

It just comes down to ... use what works for you. But if Sow&Pigs is more comfortable with the aluminum prop, just make sure to tuck her down and in before carving a sharp turn.