Bravo 3 Prop Alignment?

[OllieC]

Good Afternoon.

This last weekend we had a break in the weather so my buddy and I took the Sara Belle on a three hour tour down the Mississippi.

Coming back upstream at about 3/4 throttle, it felt as if I spun a prop, so I backed off immediately. I had the drive trimmed while under way.

I lifted the drive to see if anything was wrapped or if my props spun off...nope. So I fired her back up, slowly increased the throttle and it never happened again.

Googling this event, there was a mention of the coupler, however it was said, "once they go, they go"....and that hasn't happened.
The other was cavitation. Which it could be? Going upstream in hi current, trimmed up at 3/4 throttle.

There was mention of tuning the props with the first prop "stern or forward" single blade pointing at 12o'clock and the second "aft or rear" two blades at 10o'clock and 2o'clock. This way they don't line up at the cavitation plate. Is that true?

Thanks,
Ollie

 

[Don S]

Re: Bravo 3 Prop Alignment?

On a cruiser, that prop timing means nothing. Might be some truth to it for hi speed racing, but not in a cruiser.
Do you use the trim at all, or just set your trim by the gauge and forget it? If you are just going by the gauge, you probably found the problem. Trim angle depends on speed, load, rpm, etc. and is always a moving target. Trim for best rpm and speed, and forget the gauge.
You may only need to trim down a slight amount and everything would be fine.

 

[OllieC]

Re: Bravo 3 Prop Alignment?

Yeah.... I use the gauge :facepalm: I trim it as the bow rises to full trim at speed - at the first stop or notch on the throttle trim button as well - which also corresponds with the gauge....

(edit: sorry about the nomenclature - still (always) learning)

 

[HT32BSX115]

Re: Bravo 3 Prop Alignment?

Yeah.... I use the gauge :facepalm: I trim it as the bow rises to full trim at speed - at the first stop or notch on the throttle trim button as well - which also corresponds with the gauge....

The gage will not help you for trimming except for indicating when you're full IN/Down or UP etc.

You should always trim for best performance at a given RPM. that is done pretty much by "feel" and bow angle/speed etc.

Too low and you'll be "plowing"............ too high and you'll notice roughness, "porpoise" and you might have cavitation....



The trim gage is simply not accurate enough to "set & forget"

Bravo III props do not have a rubber center "hub", it's all metal-to-metal ......... "spin" a Bravo III prop and you'll have FAR more problems than just the "prop"

Regards,


Rick

Oh, and by the way, Mercruiser does recommend "aligning" or indexing the props but only on the higher speed model Bravo III drives. My top speed is 57 MPH, I have tried it both ways and couldn't tell any difference......

 

[Mischief Managed]

Re: Bravo 3 Prop Alignment?

I caught a small branch on the drive leg once that caused the props to cavitate simply by disrupting smooth flow to the props. I bet you caught some flotsam that did the same thing, but if fell off as you slowed/stopped.

 

[OllieC]

Re: Bravo 3 Prop Alignment?

There were plenty of dead heads. As I mentioned before, it went away.
Thanks for the trim advice. I'll give it a try this weekend.