shaft dia for surfacing prop

[stupidbaker57]

I am building a surface drive for a small boat (9 foot runabout) that is powered with a 27hp motorcycle engine. The shaft needs to be 52 inches long. I would like to use a 3/4 stainless shaft. Would the length be too long for the dia., and would it have a tendency to "whip" in the center?
The boat would weight in around 550 pounds with driver. and shaft speed would be around 7000 rpms.
Any info will be welcomed.

[gonzo]

Unless you are making your own propeller, 1" shafts and propellers are more readily available and usually cheaper.

[tom kane]

Bearing support for shaft should be no closer than 20 times shaft diameter.
And no further apart than 40 times the shaft diameter. The twenty forty rule.
A 3/4 " shaft should be good. You may have to compromise.

[Frosty]

The probs with a surface drive shaft is that it is always pushing from the bottom, shearing the shaft off just in front of the prop will occure if insufficient thickness is not used.

Surface props also have large blade roots and unfortunately hubs for the same reason.

However as the hub is not in the water it is not a prob. Surface drives are good if a few silly rules are adhered to that would not be normally be obvious on conventional drives.

[powerabout]

Originally Posted by tom kane

Bearing support for shaft should be no closer than 20 times shaft diameter.
And no further apart than 40 times the shaft diameter. The twenty forty rule.
A 3/4 " shaft should be good. You may have to compromise.

Thats rpm based isnt it Tom?

[stupidbaker57]

Thanks, Tom. I have a 1" shaft that can be used on the runabout (cracker box) I'll spline it for and outboard prop. (I have a couple of 8 inchers)
As for the hydro with the rear mounted engine position, I will be able to use a 3/4 shaft and machine a taper for the 6 1/2" prop that came today from Hill Marine.
With a site like this, you get to save a lot of time in guesstamations.