Dual prop installation

[Ryland3210]

When installing the second prop, is it important to pick a particular angle relative to the first one?
A local mechanic told me there is a particular orientation to maximize performance.
Thanks,
John

 

[alldodge]

When installing the second prop, is it important to pick a particular angle relative to the first one?
A local mechanic told me there is a particular orientation to maximize performance.
Thanks,
John

Yes, I always just thru them on until someone mentioned it to me some time back, and sure enough there is. If it provides more performance I have found none but here is how.

Install fist prop and tighten down the nut. Now position one blade facing up inline with the edge of the drive. Now install the rear prop with one blade facing down with the edge on the lower section of the drive, install nut.

 

[alldodge]

Here is a pic, not of a B3 but you should get the drift

 

[Leardriver]

The Bravo 3 service manual doesn't know about that sort of trick. Bravo service manual 11, page 3D-38 describes the process.

I've heard it also, but since both props continue to spin counter to each other, and are never positioned in any direction unless you move them there, I always chocked it up to urban legend or an ongoing practical joke.

If someone has a Mercruis​er recommendation, I would like to read it and try to understand the logic.

 

[fhhuber]

The only thing you'll be adjusting is at what angle relative to the skeg the blades are when the leading edges cross.

That actually might matter for 0.01% extra thrust.... but it would take a fair amount of experimenting to prove it.

There are a lot of people who believe in astrology too.

 

[Thalasso]

It doesn't matter how the props are oriented. At some point both blades will be in the same position no matter what.

 

[Scott Danforth]

clocking the prop blades helps with cavitaion burn on vp dp drives. One up, one down. Has to do with reducing potential for cavitation.

 

[jimmbo]

Can I have some of whatever it is some of you are smoking?

 

[Scott Danforth]

dont believe me, do your own search. prop burn on volvo pentas is well documented on iBoats and other sites. the leading edge of the drive causes a low pressure area, which is further reduced due to the leading edges of the prop. this causes cavitation on the prop blades at the root. to minimize it, the props need to be clocked.

http://forums.iboats.com/search?q=prop+clocking&searchJSON={%22keywords%22% 3A%22prop+clocking%22}
https://www.google.com/search?q=volv...utf-8&oe=utf-8

 

[Ryland3210]

Picking up on thhuber's comment, the angle relative to the skeg at which the two prop leading edges synchronize has to make some difference, it seems to me, because the leading prop's propulsion is not symmetrical relative to the skeg. Nor is the trailing prop's force on the water.
Scott, is the method shown by AllDodge the same as the one you are familiar with?

 

[bajaunderground]

I did this... It made an improvement of .00008% so, there's that? The numbers don't lie!

 

[Maclin]

Original Poster has not given the application. VP drives with the duoprop are prone to cavitation, the real cavitation (not blow-out), where supersonic bubbles burn the edges of the props. I know this as fact for the "white" duoprop drives with aluminum props. So if they have one of those (280DP,290DP/DPA/B...) with alum props then this clocking can prevent the blade edge burning. I only have the experience with the older VP drives and aluminum props. Though I am pretty sure it would happen with ss props on those older drives.

Diesel duoprops have a 4 blade aft prop so the clocking would not apply. Now even non-diesel duo's have the aft 4blade so there's that.

The actual phenomenon occurs if there is a gap at the top of the cav plate as the blades pass. No measurable thrust advantage, or disadvantage either , so the clocking is a free thing and may help on a particular drive with the cavitation.

 

[Maclin]

The method is similar to what AllDodge showed.

 

[Maclin]

It is not astrology, some of you guys are being sorta dismissive about a real issue.

 

[bruceb58]

I have timed mine the way All Dodge says but I have also read that you want the trailing edge of the front to be aligned with the leading edge of the rear at 10:30 which makes them overlap at 12:00 so who knows. Personally, I don't think it really matters.

 

[Thalasso]

Volvo has no position on this. Even Mercruiser doesn't say you have to time them.
Timing has nothing to do with anything.
Venting can lead to cavitation, which over a period of time causes blade erosion, or cavitation burn

If your getting prop burn your over trimming causing the water to boil on the prop edges
8 years of installing my F5's just by slapping them on every year in whatever position they go on and none of these issues

 

[Thalasso]

I did this... It made an improvement of .00008% so, there's that? The numbers don't lie!

I could only get .00006. What am i doing wrong :grumpy::happy:

 

[bruceb58]

Volvo has no position on this. Even Mercruiser doesn't say you have to time them.
Timing has nothing to do with anything.
Venting can lead to cavitation, which over a period of time causes blade erosion, or cavitation burn

If your getting prop burn your over trimming causing the water to boil on the prop edges
8 years of installing my F5's just by slapping them on every year in whatever position they go on and none of these issues

I would tend to agree with you. People complaining about this issue are likely causing it themselves.

 

[Maclin]

I did a lot of research on this, and experienced it myself on this vey specific drive. All of the prop edges were very finely bubbled and burnt, was not sure what it was. Looked up all over the web, found a bunch of Albermarle users that had it, and clocking fixed it.for them. A couple of them had some pretty convincing evidence of back to back runs. I reclocked mine after a refurb and it went away. In that research by the Albermarle users there was a Volvo tech that had contributed. I did not think much about "keeping" it all then since it was such a simple thing. I do not know if Merc has this or not, don't care. Mine did, reclocking was the solution. Thing is, if the props are off then clocking them on the reinstall is no effort at all, much less effort than panning the guy who tries it.

 

[bruceb58]

All of the prop edges were very finely bubbled and burnt, was not sure what it was.

If all the prop edges were burnt, then the theory of the clocking falls apart. If it's the one blade creating the vacuum for the following blade, only one of the props will have its edges burnt. If the blades on both props are burnt, it's not the clocking.

I think I know which post you were referring to regarding the Volvo tech. The Volvo tech himself didn't post..was someone who supposedly heard a Volvo tech say it.

If it really was an issue, Volvo would issue a service bulletin. It's not going to cost them any money to do it so if there is no reason for them not to issue it. I know many Volvo and Bravo dual prop owners. I don't know a single one that has an issue with burnt prop edges.