Boring the Prop Shaft Hole

Unicorn will be moved thru the water with a propeller powered by an inboard electric motor. While the drive system layout uses modern components, boring the prop shaft hole was done with very low-tech tools.

(Yes, I'm skipping over the fiberglassing effort for now because a)it's not quite done yet, and b)it looks awful. 😕 More on that in the next post. )

Here's a picture of the tools used to bore the prop shaft hole. The thing in the foreground is a modern version of an ancient hand drill, a Brace (sometimes called a Carpenter's Brace). This simple tool can produce the large twisting force needed to turn the 7/8" diameter auger bit I used.


The picture is from after the hole was bored, but you can see extending out of the Brace is a 24" drill extension. The 17" auger bit in this picture is already completely inside the keel and hull.

Here's the setup of the process. The goal is to bore the hole exactly down the center of the keel so the hole, obviously, does not poke out the side. I investigated a zillion different ways to do this, but in the end built up the layout shown below kind of as I went along.

In this picture, the auger bit has been started a little ways into the keel to hold it in place. The 2 clamps in the top right corner of the picture clamp 2 very straight boards against the sides of the keel. At the end of these boards are spacers that hold the drill bit extension so it can't move left/right. The orange C clamp holds the drill bit extension so it can't move down. The green strap is holding that C clamp from falling off. I guesstimated(!) the slight angle of the prop shaft hole. It's tilted up (down?) about 3 degrees.



This picture shows the drill bit extension and the auger bit. Boring the hole took about 1/2 hour with several breaks to give my tired arms a rest. I had to pull the auger bit out often to clear some chips. One time when I pulled it out the pilot threads on the end were completely jammed with sawdust. I had to dig that out with a utility knife before starting again.

At some point the drill seemed to stop cutting. No matter how hard I pushed on the Brace, I couldn't get it to cut. I knew I was near all the way thru so I got a flashlight (from my hiking gear which is on shelves just a couple feet away) and looked up under the hull, hoping, stressing, wondering if I had made it thru and more importantly if I'd made it thru -where did it come out?

Shockingly, it was dead on! Right in the middle of the red oak keelson (the inner keel)!!! The picture is at a weird angle, but the silvery helix in the middle is the tip of the auger bit. I can't even measure any deviation from the exact centerline of the keelson. Huzza!!!



The unusual drive system is designed to be completely water proof. Typical inboard drive systems use a stuffing box that allows a small amount of seepage of water into the boat. Outboards use a different seal method which somehow keeps all water out. That's what I'm going to try to do. The other deviation from the norm is I'm going to have ball bearing thrust bearings to minimize drag on the shaft. This picture shows most of the components, minus some 3D printed plastic parts to hold it all together.


The 5 blade propeller is about half the diameter of what would typically be used on a 14' low speed boat. The advantages are it can be spun faster so no gearbox is needed on the motor and the boat has less draft (it can travel in shallower water). The downside is the propeller itself is less efficient at the low speed the boat will be traveling at. There is, however, a method of increasing the thrust of a prop in this situation, something called a "Kort Nozzle".

A Kort nozzle is a ring around the prop, often used on tug boats to increase thrust without having to use a larger prop. The propeller I selected is designed to operate in a tube inside a bow thruster, so it should work well in a Kort nozzle. The Kort nozzle will hopefully deal with the inherent inefficiency of the propeller's small size. It will be mounted such that I can try different types and sizes to get the most efficient design.

Back to the dreaded fiberglassing.







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