Shaft diameter vs HP

[Bob Bradley]

Is there a formula to determine minimum requirement for propeller shaft size? Is it simply xx horsepower, or does prop size and pitch come into play in the decision?

 

[SKYCHENEY]

Is there a formula to determine minimum requirement for propeller shaft size? Is it simply xx horsepower, or does prop size and pitch come into play in the decision?

Its all about torque and shaft length so gear reduction is important.

 

[Pascal]

It s torque, prop and also gear reduction. Prop shops can calculate the safety factor and I believe there are online calculators as well. Oh... and shaft material is critical too. When I repowered I had to use AQ22HS to be within limits.

 

[Cricket]

Everything Pascul said plus each shaft and or batch will be scored for actual strength. The right machine shop can cherry pick the strongest available if needed.

 

[Bob Bradley]

I have the AQ22 shafts. I take it that AQ22HS is a High Strength version of the AQ22? I'm hoping to replace my 435hp DD 6v92ta engines with Cummins QSM11 making either 645 or 670. I believe max rpm will remain at 2300. I'm guessing my 26 x 22.5 wheels (originally 26 pitch) will be repitched substantially to take advantage of the increased power. Obviously, this isn't something I have any experience with.

 

[Boatsb]

Hp, torque, rpm, prop size, vessel weight and hull type all go into the calculation. Cummins uses a engineering study to make sure all the parameters are considered.

Its part of the repower package and service.

 

[Pascal]

Your Cummins dealer and a good prop shop will Have to run the numbers. That’s a big jump in HP I would t be surprised if you have to upsize the shafts. There is only so much pitch you can adjust on props. When I repowered I was considering buying new nibral wheels (better than bronze for higher performance). Turns out my spare props were nibral and the prop shop Was able to repitch them. Just. They said if the RPM is off they were at the limit and couldn’t go any further in pitch.

 

[piripucha]

In boatdiesel.com they have a calculator that is quite ussefull. The best $25 investment. They also have a prop and HP calculators.

 

[Bob Bradley]

Thanks. I have nibral 4 blade props now. 26 square that have been detuned to 22.5 pitch to let me hit 2300rpm. I am on Boatdiesel, and have tried to use the calculator, but there seem to be too many variables that they need, many of with which I am unfamiliar.

 

[Cricket]

Always was told the pitch on a nibral prop shouldn't be modified more than 2 inches, anything more and they will try to return to the original pitch, and that much change could/will weaken the blades.
Probably need to count on new props. Hopefully there will be some you can give a try before going all in.

What's your current shaft size?
New gear ratio is?

For reference we're running 2 inch Aq22hs behind 800hp CATs with 1.5 -1 gear ratio.
Have had 1 cracked key way since 2001 with almost 6,000 hours.
As I posted before getting the strongest material available can really make a big difference, it's sort of shocking to see strength variables from 1 piece of shafting related to other pieces.
Think you're in NY, S&S Propeller in Queens did our shaft and propeller work. Props were perfectly sized the first time which I thought would be impossible.
The cracked key way was discovered while changing wheels in FL, never really trusted it was cracked but wasn't there to be involved, replacement came from Stuart Propeller.

Also, what's you cutlass size? You may be able to move up a shaft size with using thinner walled bearings.

 

[jap201]

Bob

Might want to give Tony Athens of seaboard marine (www.sbmar.com) a call. They're on the left coast, but specialize in cummins. He might've already done your type of repower and be able to give you the info you need. Also, not sure if its still active, but there was a subforum on the downeaster board for Tony (https://downeastboatforum.com/threads/tony-athens-seaboard-marine-technical-forum.19203/) a few years ago. might be some info there

Good luck
Joe

p.s. fwiw: still waiting to hear on that 48

 

[SKYCHENEY]

The new QSM11 Tier 3 are 660hp at 2500rpm. They can sell you Tier II engines but they have to be remans. Those are 670hp at 2300 but you only save about $2500 per engine. The warranty is the same so you have to decide what is more important to you.

Those props will never work. You'll need larger diameter props and most likely larger shafts and struts. Figure about $250k all in.