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Thistle Centerboard Drum Rigging

When I was re-rigging some systems on Thistle 3669, Weatherly, I asked for advice from the Thistle Class list serve. The results were mixed on whether or not to discard the drum. I was surprised at the number of Thistlers that believed the drum was superior to the block and tackle system described on Dan Winchester's website. Dan himself even mentioned that at one time drums were in short supply and, given the decision to do it again, he would choose a drum.

The drum has a few shortcomings. While raising the board the leverage ratio changes as the line or webbing spools onto the axle. The drum has the highest leverage ratio when the board is down and the mechanical advantage is needed least. As the board approaches it's fully retracted position and the crew needs all the help they can get, the leverage ratio is at it's lowest. Another common complaint is that the line will jump off the drum. The drum takes up valuable space between the stanchions. Last is the failure of the bushings between the axle and stanchions.

I can't take credit for my first change. A post on the list serve mentioned moving the axle forward. I accomplished this by cutting some brackets out of 3/4 inch thick HDPE plastic. This seems to have solved two problems. More space is opened up between the stanchions and the brackets are made of a material that acts as the bushing. The brackets are more or less a capital 'D' shape with the straight vertical portion of the 'D' screwed to the front of each stanchion and the rounded side facing forward. Each 'D' has a 3/4 inch hole drilled in the center.

My next change was to the leverage ratio. From experience with my previous Thistle 2285, Deluxe, I knew that a 4:1 block and tackle was just adequate for raising the board. My 7 1/2 inch drum and 3/4 inch axle had the potential to be as high as 10:1 but the 3 or 4 starter wraps of webbing probably had it starting out at 8:1 or so and decreasing numerically as more webbing wrapped around the axle. My goal was 5:1.

Increasing the diameter of the axle would be my means of changing the ratio. I did this by adding two short sections of PVC water pipe. The first piece had an inside diameter of just over 3/4 inches. I added a couple wraps of electrical tape to the axle so that it was able to slide on but was a snug fit. The second piece had an inside diameter roughly equal to the outside diameter of the first. Both sections were drilled with a hole near one end and then cut lengthwise. The drilled hole lined up with a new hole drilled in one end of my axle. The lengthwise cuts allow the PVC to clamp down on the axle as the line is wrapped and tensioned. These short lengths were then slid over the axle with the holes lined up to allow the centerboard line to enter the axle where it received a stopper knot. The end result was an axle that measured about 1 3/8 inches in diameter. This calculates to roughly a 5.5:1 ratio. Close to the goal and my crew reports that it works without excessive effort.

Keeping a constant ratio was also a goal. To do that I needed to keep the drum from shrinking in diameter when it loses line and keep the axle from growing in diameter when it gains line. I ignored the drum. The diameter change is minimal and I didn't see an easy solution. On the axle it is all about the choice and placement of the line. I discarded my webbing and looked for a small, stiff, strong line. Samson's Ultra Tech with it's Technora core was chosen in size 3/16 inch. The plan was to have the line start spooling on one side of the axle and work itself to the other side in a single layer as the board was raised. By never having it ride over itself the leverage ratio would not change. Getting the length correct was important. Too long and it would have spooled from one end of the axle to the other and be forced to ride back over itself. The stiffness also helped it to not override. This probably wouldn't have worked at the original axle size as a smaller diameter axle would have held less line before needing to start a second layer of wraps.

The change in ratio may have had a secondary benefit. I don't believe we have had a round of line jump off the drum while racing. Maybe the slower drum speed when dropping the board helps the line pay onto the drum correctly.