B3 prop alignment

boatman37

So read somewhere before about aligning the B3 props to eliminate vibration. Then remembered reading something about only the 3x3 props need properly aligned? I have the 4x3. Even if I try to align them they change after a few revolutions anyway?

Also, I added grease to the rear cone washer (first washer that goes on). None on anything else other than the shaft and splines. Pretty easy to pull them back off if I need to change something.

LaRea

Right.  If you have a 4-blade plus a 3-blade, you don't have to align them.  If both props have 3 blades, you should align them.  

boatman37

Awesome. Thanks! Yep, one 4 blade and one 3 blade

raybo3

LaRea said:

Right.  If you have a 4-blade plus a 3-blade, you don't have to align them.  If both props have 3 blades, you should align them.  

Not true. I contacted merc a few year ago and they flat out told me that there is no such thing as aligning your props. I will see if I can find the e-mail and post it. 

aero3113

Check out this thread. I know 4x4 prop set up it doesn’t matter.

https://rinkerboats.vanillacommunities.com/discussion/861/prop-timing-torques-specs

raybo3

Found it:

MERCRUISER CUSTOMER ASSISTANCE <mercruiser.customer.assistance@mercmarine.com>

Fri 4/21/2017 10:23 AM

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• You

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Ray,
There is no special timing of the props. Service manuals for engines and drives are available from local dealers or by calling our Publications Department at 920-929-5040, option #4. Have your serial number ready when ordering.
Thanks,
James
MerCruiser Consumer Support

2017-00054090

-----Original Message-----
From: raybo3@live.com [mailto:raybo3@live.com]
Sent: Friday, April 21, 2017 8:39 AM
To: MERCRUISER CUSTOMER ASSISTANCE <mercruiser.customer.assistance@mercmarine.com>
Subject: 

Contact Request:
Name : Ray 
Address :
City/State : 
Zip : 

Home Phone : 
Work Phone : 

Email Address : raybo3@live.com

Serial Number : 
Model Number : 
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Contact By : Email

Comments:
I have a twin Bravo 3 drives. My question is do the props have to be "timed/aligned"? If yes is there an online manual for this. Thank you for your help.

Mercury Marine
mercruiser_customer_assistance@mercmarine.com
https://urldefense.proofpoint.com/v2/url?u=http-3A__www.mercurymarine.com&d=DwICAg&c=tjPg1sbLxZ1bjgTYa5hhdw&r=FEQWMy8t0qVEVOymqTeqBMVeLlfR66Wd26pDhpbyAhJy52Gjwpo5ZejkaibZ_Oj7&m=QgsNEbY6VvzuK92b0MB4xtuZmHSs-23Fe4yO09LJ4w0&s=Klr2U2CSt7hoq_R2C1JyFY8t_6PVhoHuQ2JHfIG6eUQ&e=

boatman37

Cool. I spun them and they are never in the same place each revolution so didn't figure it mattered

LaRea

Well, this is what @Alswagg said a few years ago on this topic:

Alswagg said:

Prop alignment is critical for proper thrust wash.  This is basic hydrodynamics taught in Merc drive class  

boatman37

So where do we find how to properly align? My manual doesn't mention anything about it. I have the whole satchel full of all the booklets that came with it. I have one 3 blade prop and one 3 blade

reneechris14

Make sure the 2 blades don't cross under the cavitation plate at the time.

LaRea

This is the manual page that has been posted here before.  @raybo3 that Merc tech must have assumed you have 4/3 props.

Trashman said:

  

raybo3

@LaRea maybe but I did give him my serial numbers. 

Cableguy Greg

LaRea said:

This is the manual page that has been posted here before.  @raybo3 that Merc tech must have assumed you have 4/3 props.

Trashman said:

  

This is how I align my props every spring. 

LaRea

I'd love to know the real explanation for this.  Those I've read seem to be idle chatter that is uncluttered by facts.  It's for vibration, it's for cavitation, you need it on a Merc but not on a VP, you need it on aluminum props but not stainless, yada yada yada.  

raybo3

OK take a look at this. See if you see the same thing I see.......
https://www.youtube.com/watch?v=Zo1l1RncrvI

boatman37

Seems like with a 4/3 no matter what you do they will cross together at every point around the circumference at some point with enough revolutions?

Ian

Cableguy Greg said:

LaRea said:

This is the manual page that has been posted here before.  @raybo3 that Merc tech must have assumed you have 4/3 props.

Trashman said:

  

This is how I align my props every spring. 

Why - do you take them off? They shouldn’t go out of alignment....

LaRea

I take mine off to scrape and paint the drives and clean the props.  It's a ritual of spring.  But I have 4/3 props so no alignment.  

reneechris14

I take my drives and props of every year. Shine them up and check things out

LaRea

@raybo3, that video is not what Merc recommends.  You don't align the props relative to each other or to the logo positions.  You align them relative to the cavitation plate. 

Install the forward prop.
Rotate it until one blade points up.  
Install the rear prop with one blade pointing down.  It doesn't matter where the logos are.

raybo3

@LaRea that video is the complete opposite of what the timing "should" be. In the video both blades are under the cavitation plate. 

Cableguy Greg

icoultha said:

Cableguy Greg said:

LaRea said:

This is the manual page that has been posted here before.  @raybo3 that Merc tech must have assumed you have 4/3 props.

Trashman said:

  

This is how I align my props every spring. 

Why - do you take them off? They shouldn’t go out of alignment....

My boat is in a storage facility. They are rather expensive to grow legs. Call it cheap insurance. I have always removed my props when the boat is being stored, whether it is in a boat yard or a storage lot. With them off, I am able to clean up and regrease the splines.

LaRea

@raybo3 the video from On Board with Eddie is kinda funny.  When he starts, the props are aligned backwards (rear prop is upside-down).  After removing them, he carefully reinstalls them the same position (and it's still wrong).  

Wrong way:

raybo3

@LaRea when I posted the link that's what I meant when I said see if you see what I see....lol

boatman37

I remove my props every year to clean and grease the shafts and clean the props

rasbury

I don't get it, they counter rotate..they are never in the same position to each other, how could you possibly align them? Not saying anyone is wrong, I'm just not grasping it!

Willhound

They counter rotate, but at a fixed ratio. So the points that they pass each other don't vary.

raybo3

Why wouldn't they "key" the props if align is so important? Very easy to do with the spline................ Just say'n

Cableguy Greg

rasbury said:

I don't get it, they counter rotate..they are never in the same position to each other, how could you possibly align them? Not saying anyone is wrong, I'm just not grasping it!

You don't want the blades creating the wash/thrust under the cavitation plate at the same time. That would happen if you put both props on with both props blades lining up at the 12 o'clock position. 

LaRea

Yes, that's it exactly.  The top position (12 o'clock) is different from the rest of the circle for two reasons.  (1) The water flow there is turbulent from flowing around drive body.  (2) That's the place where you're most likely to get cavitation badness (or ventilation) because the hydrostatic pressure is lowest there.  

Apparently, if you have the front and rear blades passing through that space at the same time, it creates vibration, or cavitation, or both.  

LaRea

I was stewing about this last night because I couldn't figure out an easy way to explain it.  So I geeked out and created these animations.  

Each one shows counter-rotating props viewed from behind.  Any time there are two blades passing each other at the 12 o'clock position, the animation pauses and the blades turn red.  

First we have two 3-blade props (3x3) that are aligned incorrectly.  You can see that every time a blade of the rear prop passes the top, a blade of the front prop is also there.  It happens three times during every revolution.  



Now a 3x3 correctly aligned as specified by Merc.  You never get both blades passing the top at the same time.  



Next is a 4x3 setup (4 blades on the front prop, 3 on the rear).  They never line up at the top, although it does get close a couple times.  



Actually, it is possible to mis-align a 4x3.  You can have two blades line up at the top.  However, it only happens once per revolution, and only if you line them up perfectly wrong like this.



(updated the graphics, added cavitation plate and skeg)