props revisited

[spencer]

past posts on the subject indicate a 20x23 prop will be a good fit for the cummins 250
I kept the original strut angle, used high tide marine struts which are duplicates of the original set up for 1/1/2 shafts
the mechanic says he does not want to go bigger in diameter than 19 inches, is there a minimum allowance for clerance?
I told him most of the board members are swinging a 20 inch prop.

thanks

spencer

[IRGuy]

I believe I read here somewhere that you should allow at least 2" clearance between the tip of the prop blades and the hull. I am not 100% sure of this.. I expect Capt Patrick will post the correct answer.

[mike ohlstein]

My understanding is that the critical measurement is the distance along the shaft from the aft end of the strut to the forward edge of the prop hub. No more than the diameter of the shaft. That will prevent the wobble that would otherwise have the prop dig a hole in the hull. So with a 1.5" shaft, 1.5" max from strut to prop. After that, I would think that 1" clearance from blade to hull is enough. I have run 21" props with no problem. 22" would be cutting it a little too close. 20" is no problem at all.

There is also a maximum distance from the shaft log to the strut, but I don't think that you can reach it on a B31 with even 1.25" shafts.

[spencer]

Thanks for the education,

Have a nice set available for a good price , so im going to go for it , 20x23 4 blade nibral
Spencer

[CaptPatrick]

The "ideal" formula for clearance between the prop and the hull is 1:10, (clearance:prop diameter).

A 20" prop then should have a 2" clearance. However, like most other "optimal" equations, it's not something written in stone. There are several guys among us that are running 21" props and having no problems at all. In terms of clearance what's the significance of 0.10" one way or another...

In my estimation, for a diesel powered B31 with a stock shaft angle of 15º, a 20" prop is the ideal diameter. Remember also, that the "effective" diameter can be increased by adding more cup to the blade tips. I have always suggested a medium cup to a 20" prop, thereby making it react like a 21" prop.

Spencer, stand your ground...

[Carl]

More slip with a smaller prop, AKA less efficient. To a degree anyway.

I will not tell you how much clearance I have between prop and hull...but lets just say you should be more then fine with a 20" wheel.

[wmachovina]

I run 20x23 4 blades on my 250s. Turn up 2650. seem to remember taking a half inch pitch off stbd to fine tune my motors. Stock shaft angles, seems to work fine, Bill

[spencer]

Thanks again for the Help
Spencer

[Tony Meola]

I am running 20x21 with my 270's. We also added some cup. I hit 2650 on the top end.

UV used to run 21 inch props with his 250's.