Propwash October 2019 web.pub

My other design efforts are for a P Limited outboard lower unit. The design mounts the motor directly on the lower unit without an adapter. The plastic parts were printed in Taulman 910 nylon and are very strong. The problem was joining the plastic to the propeller shaft assembly. The fine threads did not hold up in plastic. I then tried a metal adapter in the plastic, but that vibrated loose as well. The last try was to run the propeller shaft directly in the nylon. It seemed to work, but I’ll need to see how it wears in actual use. The fin also turned out to be a problem. When printed as thin as an aluminum one, it was too flexible. A bolted on aluminum fin leaves an ugly attachment area. The best solution may be to make the propeller tube, prop shaft attachment and fin as a one piece, brazed together, metal part. There are several similar boats on our club for a comparison against the stock sponson equipped boats. I drew up a first attempt with aircraft style internal reinforcements. The original foam filled epoxy sponsons weighed 127 water soaked grams. The new sponsons weighed 73 grams. PROPWASH October 2019 17 Bulkhead and Stringer Construction However, their thin construction failed the first structural test; hitting a buoy. The next version used a standard 10% hexagonal infill and weighed 104 grams. Hex Infill The boat had a tendency to trip on the left sponson edge in tight turns so a nontrip was added. That design ran better, but still could trip. The worst part was cornering speed. Compared to a similar stock boat, the sponsons were very fast on the straight, but bogged down in the turns. A new design was de- veloped for the left sponson. It had a wider running surface as well as a non trip, similar to the original sponsons. It weighs 74 grams using a 5% gyroid infill. We’ll see how it does in the corners and structurally. Left Sponson Bottom Comparison Outboard Lower Units (Continued on page 18)