Bravo 3 mixing props/

[roccopep]

has anyone tried mixing prop pitches say a 20 and 22p?

 

[mwph]

I have not read anything about doing this but I can't imagine it being a good idea. Just for learning, can I ask what you would hope to archive by doing this?

 

[Henry Boyd]

Just thinking out loud here;
Pitch equals the distance the prop moves in one revolution.
Both prop shafts turn at the same rpm, just opposite directions,
Water in this case is an incompressible liquid.

As a result, it seems to me that using two different pitches would either tend to compress the water between the props (higher pitch leading), or expand the water (higher pitch trailing).

I seem to recall that jet engines use varying pitch arrangements to increase power by compressing the gas flow, but I don't think this will work in water.

I'm also not sure if this would also create a dynamic imbalance.

Henry


Sent from my iPad using Tapatalk HD

 

[src]

I feel that either option would lead to an increased chance of cavitation, either before the second prop (low pressure between props) or after (high pressure between props). It might not be severe enough to overspeed, but it could certainly cause damage to the props over time.

 

[roccopep]

The reason being with the new configurations of 3 and 4 blade combinations . Volvo uses a large 3 blade and smaller 4 blade .I don't know if this configuration would work with merc although if it does with Volvo way not.

 

[Henry Boyd]

The reason being with the new configurations of 3 and 4 blade combinations . Volvo uses a large 3 blade and smaller 4 blade .I don't know if this configuration would work with merc although if it does with Volvo way not.

Merc does have a four and three blade set up, but they would still have to be the same pitch, and the twin 3 blade props are also different diameters as well.

Henry

 

[Lazy Daze]

Rocco, what are your goals with changing props? You might be itching to spend money where you don't have to.

I have a smaller engine and I can pull up skiers (2 at at time on two skiis) and even pull a slalom skier with no problem. I've never tried with 10 people on board, but then this is the wrong style of boat to be taking a herd of people skiing.

 

[sdarc]

And should you wish to experiment I have a set for sale on here. (shameless plug). http://clubsearay.com/showthread.php/63434-Bravo-III-props-22P

 

[roccopep]

i'm trying to improve over all performance not looking for more top end just more efficiency. Would like to stay on plan at say 3500 rpm instaed of 4000

 

[techmitch]

i'm trying to improve over all performance not looking for more top end just more efficiency. Would like to stay on plan at say 3500 rpm instaed of 4000

My bet is that you have another problem if you cannot stay on plane at 3,500 RPMS.

 

[Lazy Daze]

I agree, something is amiss.

What gear ratio is your lower end? If you don't know, what's your engine serial number and I'll check.

With a light load, what is your MAX WOT and Speed (GPS, if you know it)?

Do you use your tabs?