Monday, 5 May 2014

New rudder

I wanted the rudder parts to be 1½" thick, and thought I'd make it out of two layers of ¾". Then I thought there might be an advantage in making it out of four layers of ⅜". I chose exterior fir ply because the whole boat is that, so no point in adding fancy.

One reason to go with the ⅜" ply was the number of plies per sheet. At the lumber yard I looked at both ¾" and ⅜". The ¾" had six plies. The ⅜" had four. So 1½" of ¾" ply would have 12 plies, whereas the equivalent thickness in ⅜" (four layers) has 16, a third more.

Another factor that decided me on the ⅜" was that the pintles from the old rudder are for ¾" stock. If I wanted to use them and also have the rudder stock be 1½" thick, I'd have to install them on a core ¾" thick, and put a ⅜" cheek on each side. I needed either a half sheet of ¾" or a full sheet of ⅜". Simpler to go with the ⅜", even though it meant more cutting and gluing. And stronger, because of the additional plies. I had it cut in half at the yard so that I could manage it, another reason to go with the thinner wood.
The reason for such a thick rudder blade is to be able to make it into a foil shape. It's an empirical fact that making blades foil shaped improves sailing performance and boat handling. The official foil shape is NACA 0012, apparently. I tried to figure out how to make the correct shape, but it was too complicated for my little noggin, so I grabbed a jpeg of a NACA foil and adjusted it in my illustrator app to fit my rudder. If mine isn't a true NACA foil, it will still be way better than a straight-sided blade. The above drawing shows how I can get a jump on the shaping by making the outer layer of plywood shorter in the right places.
Another good thing about making the blade with four layers is that I can have a groove in the top for the uphaul and downhaul lines to travel in. Here's a view looking at the round top part of the blade, from in front or behind it:
The two core layers (⅜" + ⅜" = ¾") are shorter and the outer layers taller. The outer plies will be rounded and the inside corners will be filleted so it can all be glassed and sealed properly.
Yellow is the downhaul, green the uphaul.

Here's a picture of my bathroom floor. I chose it to glue on because I wanted to make sure the core layers were straight and flat, and I'm pretty sure my bathroom floor is good. The orange/red circle shows where the top of the blade is on the other side of the stock, and also shows the minimum contact area of the two pieces. It's a 12" diameter circle. I was working out how tall the whole rudder will be (53"). It will extend 12" below the skeg on the Firefly.
It takes almost as much time to illustrate and describe what I'm doing as it takes to actually do it! Much less mess and back ache, though. So far I have all of the rudder pieces cut out and the core two layers glued together. Next is to carve out recesses in the stock cheeks for pintles, install the pintles and then glue on the cheeks. The bolts attaching the pintles will not have heads or nuts: I'm going to cut them flush with the pintles, to make the recesses simpler and because the cheeks will serve the same purpose; those bolts won't be going anywhere.

No comments:

Post a Comment