Anti Cavitation Plate Height

[gfb220]

I have 1448 Alumacraft with a 81 Johnson 35, AC plate 1/2" above bottom of transom, 10X13 -3 blade prop on it, 5.5 inch setback on jackplate and trim tabs. With motor trimmed out to next to last outboard hole my WOT is 4810 rpm at 26 mph. If I trim to last hole out from transom speed stay the same but rpm comes into 5670 range ( I believe 5200-5800 is rpm range) but seems like a lot of cavitation till on plane and porpoises . Motor came with 4 blade 10X14 which ran like 22 mph at 4270 rpm. Any suggestion which position would be better for engine life would like to operate with optimal rpm range . Thanks

 

[JimS123]

Even though you are getting 5670 (a proper number) if it cavitates you are trimmed out too far.

Trimmed out one hole is "usually" where a manual trim motor likes to run. But at 4810, looks like you have the wrong prop.

However......, a quick search of the Michigan Wheel site, and what they would recommend, would indicate that maybe your tach is not accurate.

 

[jbcurt00]

Just for accuracy you're describing ventilation not cavitation.

 

[gfb220]

Just for accuracy you're describing ventilation not cavitation.

thanks wasnt sure

 

[gfb220]

Just for accuracy you're describing ventilation not cavitation.

Even though you are getting 5670 (a proper number) if it cavitates you are trimmed out too far.

Trimmed out one hole is "usually" where a manual trim motor likes to run. But at 4810, looks like you have the wrong prop.

However......, a quick search of the Michigan Wheel site, and what they would recommend, would indicate that maybe your tach is not accurate.

Im gonna try trimming in one hole check the difference thanks

 

[roscoe]

I think with a 5" setback from the jack plate, that you will need to raise the motor.
Raise it an inch and then try it.
Then try moving trim pin in one hole at a time.

 

[Texasmark]

thanks wasnt sure

Don't feel slighted. I have been boating, with my own boat since about 1957 and it has been Cavitation Plate all my life till I got on here. The big guns straightened me out. Once you objectively look at the two words, its easy to see that they were right. Wink!

 

[JimS123]

Just for accuracy you're describing ventilation not cavitation.

I use AC plate and AV plate interchangeably, depending on what the OP says. I DO know the difference, but when I reply to someone that doesn't, I don't want to cornfuse them.

Just like mag and clip, or 45 colt and long colt.

 

[gfb220]

Will do thanks

 

[JimS123]

I think with a 5" setback from the jack plate, that you will need to raise the motor.
Raise it an inch and then try it.
Then try moving trim pin in one hole at a time.

The AV plate is already above the bottom, maybe even fine for a 5" setback. If the engine is raised with it still trimmed out it will most likely make things worse.

Sure, try raising it, but start with it trimmed in and then move it out, not visa versa.

As I said before, the pitch and rpm don't compute.

 

[gfb220]

The AV plate is already above the bottom, maybe even fine for a 5" setback. If the engine is raised with it still trimmed out it will most likely make things worse.

Sure, try raising it, but start with it trimmed in and then move it out, not visa versa.

As I said before, the pitch and rpm don't compute.

do you think a 12 pitch would be more in line

 

[JimS123]

do you think a 12 pitch would be more in line

Based on your numbers it should be more like a 10 pitch. But the prop selector guide says a 15 is what your boat needs. Obviously, the tach is wrong or the motor isn't running well.

Aside from the rpm dilemma, are there performance issues, like hole shot.

 

[gfb220]

Based on your numbers it should be more like a 10 pitch. But the prop selector guide says a 15 is what your boat needs. Obviously, the tach is wrong or the motor isn't running well.

Aside from the rpm dilemma, are there performance issues, like hole shot.

turning points calculator says a 10.5 X11, hole shot is ok gets up quickly have to see if I can get another tach to try meanwhile I will trim it in first.. Thanks Gary