DIY Small Aluminum Boat Prop
Can be done in one evening

This is intended to be used on the Weed Eater conversion, but could be used on other motors.

The first requirement is a piece of aircraft quality aluminum 1/8" x 2" x 4".  Your local shops may have some scrap pieces. I got mine on eBay (cost too much though). It is 6061 T5 alloy.
 

Locate the center with a square and scribe lines from all four sides.  Center punch lightly (use a nail if you have to) with one blow from a small hammer. I predrilled with a 1/16" bit and then drilled for 5/16" with an almost new bit.

(below) A bolt holds the Weed Eater nut from the rear so that the outline can be scribed on the plate.

Next we will print out the template. It was made from an actual photo of an early model Sears weedless trolling motor prop.  Note the odd line in the center where I copy and pasted to make sure each side was identical. It took me four tries to get the printout scaled to 2" x 4". The one below should be close for you.

Cut out the pattern. I used Elmer's Ultimate glue to attach the pattern.

Hold it up to the light to make sure the pattern is centered over the hole. Allow to dry.


Use a combination of hacksaw and disk sander to shape it, staying outside the hub lines.


I decided to make it easier to bend I would remove a little more material near the hub. The purple lines are 5/8" apart. The area in blue was removed and smoothed to match the hub.


MAKE SURE WHICH SIDE YOU ARE WORKING ON FOR THE MOTOR ROTATION.

A disc sander was used to hollow out the aft side of the blades about .020-.025" deep. They were smoothed further with a Dremel tool and flap sander.

Make a hook out of bailing wire, or a piece of coat hanger wire. Suspend the prop from the hook and use a propane torch to heat it until it just barely glows reddish, but NO hotter. Better to make it cooler than warmer. This will anneal the aluminum so we can bend it with out fear of cracking.

Bending the blades to the proper angle

Place the prop in a vise and using a crescent wrench (has adjustable smooth jaws), carefully bend both blades 5° in what would seem to be the wrong direction. I.E., it the photo above away from you. The exact location is not terribly critical.


Now reposition the prop in the vise, and using the wrench, bend the blades 10°  toward you. The previous 5° is canceled out and now faces forward by 5° due to the new 10° bend. It should now resemble a store bought prop. The reason this is done is because the velocity in feet traveled is greater as we go farther from the center, so to try to make it balanced hydrodynamically, the blades have less pitch farther from the hub.

Almost smooth enough.
The other side, the one that faces the boat, is rounded to give each blade an airfoil shape. Disc sander does this nicely. Finish with filing and hand sanding using progressively finer papers. The inner cupped shaped surface can be done quicker if you have a Dremel tool with a flap sanding disc.
The edges should be almost sharp.

Balancing
Locate a 5/16" metal rod and support the prop between two sharp edges to see which blade is heaviest. Alternately, you may be able to support it at the hub hole by an edge.  I supported it on a piece of stainless tubing supported by the rims of two cans. Occasionally tap on the cans with the file. Remove a bit of metal from either the front or rear, but not the edges to balance it. When balanced, it will stay in whatever position you turn it.

Hub spinner
I found a shoulder bolt with 5/16-24 thread and cut the head off so it can be used as a mandrill in the drill motor or drill press.

A block of white oak 1 inch thick was used. Don't try to make the spinner too short to allow for a hole that will accommodate the threaded shaft. The larger hole was made with a 1/2" spade bit and then drilled for clearance for the 5/16" shaft end.  A nut was pressed into the hole to make it hex shaped.



The shape was roughed out by holding the spinning block against the table saw blade. THIS IS DANGEROUS. Do it at your own risk.

Finish shaping it by holding course sandpaper against the spinner as it rotates in the drill, followed by fine paper.

 
A nut was glued in the hole using J-B Weld. When dry, I will use wood filler on it, sand and paint with black auto undercoating spray. That's it!

Copyright © 2007 Bill Weller