Now that I would pretty much say that the new one piece sail is pretty much done in the way of detailing and the appendages as well, I thought it was about time to tackle the new Jack screws (I’m doing a high pitch Permit one as well. I actually built screws for the Jack way back when, but looking at them now and comparing them to the pictures of the actual screws made me unhappy, they just had to be redone. This time around because of more thorough research I am also doing different things with the blade pitches on the two props. Before I made both props with the exact same pitch as each other, and to my folly, I discovered that this wasn’t the theoretical best way to go. The front larger prop’s pitch has been increased by a quarter diameter and the pitch of the aft prop has been made to double that of the front one.
Some technical data for you all:
Permit screw diameter: 15’
Jack fore screw diameter: 12’
Jack aft screw diameter:10’
The actual design of the twin screw configuration for the Jack wasn’t planned on from the beginning. Portsmouth engineers asked for, and were granted to do the modifications. Perhaps this was why the boat took so long between the laying of the keel and the actual commissioning, or maybe it was due to extra testing needed because of all the extra stuff on board. None the less, the real boats didn’t quite produce the results that were desired by only giving a fraction of a knot in speed increase and around 15% efficiency gains
To get started, here are the charts I made to produce the helical twist required to make constant pitch propellers:
The angular sections were glued to circular pie charts printed out on the computer
All three prop pitch and position jigs set up on a prop making board
The gaps between the cardboard were then filled with Bondo
Now this time in my prop making endeavors I am doing something quite different than last time. I am not wasting time or money making moulds from masters of each prop blade. I am making a master blade from aluminum and then forcing the twist shape by simply pressing the blade on my pitch jigs. I then take Styrene sheet cut roughly to size of the master and then clamp it in place. I heat the styrene with my hot air gun and press it to the shape of the master blade. The edges of the styrene are simply then filed to the edge of the aluminum blade producing and exact replica…something that I couldn’t achieve perfectly with a mould, believe it or not!
With the hub and shafts in place on the jig each blade is carefully glued onto the hub. Not much glue (CA) is used as I don’t want to make a mess or have it drip down and glue the hub to the jig. Baking soda was also used very sparingly to speed up the process.
Here is everything assembled:
Next, I’ll add blade root fairings and do some blade profiling….Happy new year!