Prop flex...how much and when

[smartwork]

While reading various threads/debates about aluminum vs ss props, I see different perspectives on when there is enough stress being placed on an aluminum prop to make it flex (which is one of the negatives I see posted).

If we take something like an Alpha II drive, what are the engine sizes and drive ratios that make up the point where "flex" starts becoming a recognizable differentiation in performance. I realize the other aspects between the two materials such as saving the outdrive during collision, etc. that people have an appreciation for, but I'm really trying to conclude where the flex point is. Is it with a 3.0, 4.3, 5.0 - with what gearing?

 

[steelespike]

Re: Prop flex...how much and when

I don't think you can figure much about flex.I think the percentage of flex
is probably about the same through all hp ranges as the props are designed
to work with the power requirements.Just be secure in the knowledge
that ss flexes less.There are aluminum props that deliver close to ss performance.
I think I would worry more about selecting a prop that give the best performance for my boat and motor combination.ie bow lift,stern lift,holeshot venting resistance etc.

 

[4JawChuck]

Re: Prop flex...how much and when

Its not so much a flex issue as much as it is an issue with creating the shape and making it strong enough in AL so it doesn't flex and break in normal use. A steel prop is easier to create the shapes, make them thin enough and take the stresses without breaking.

This is why you don't see high rake props with lots of cup in AL, they would crack and break.

 

[hwsiii]

Re: Prop flex...how much and when

Smart, Spike and chuck are both right in their postulations on aluminum props. The biggest differences in aluminum and SS props has to do with blade geometry as under 45 MPH aluminum props do not show enough deformation to account for any change in pitch because of blade flex. IF you are planning on running your boat over 65 MPH then you might want to start considering blade flex.
But if you are not going to be doing that then you need to decide what particular blade geometry is best for your hull form, HP and intended uses, that is MUcH more Important.


H