Wednesday, April 27, 2011

Back of an envelope calculations

Brede RuddersI have a plan of the Brede lifeboat. I have photos. I needed rudders.

The plan is 1:20th scale and the boat I need rudders for is 1:12th. My plan was to make the blades up from some brass sheet I had for model railway work. The shape is odd with not a right angle to be found, but by sketching it out on literally the back of an envelope, I was able to calculate the new dimensions by multiplying those on the plan by 1.6.

The results looked like they would be OK. I tweaked a few numbers for a slightly larger rudder to improve turning on smaller pools and started hacking away.

First lesson learned - 1mm thick brass might seem like a good idea but it's bloody difficult to cut compare to the thinner stuff. My Olfa Plasticard cutter scored it but I still needed the attention of a piercing saw (2 blades broken) and tin snips to get the basic shape. Then there was some filing to smooth up the edges and round the corner a-la the prototype.

The pivots are 3mm brass rod. I'd hoped to cut a slot up the middle with a junior hacksaw and slide the metal in. It quickly transpired I don't the skill for this and so I just filed half the rod away and soldered it to the blade. That seems to be nice and strong. This is topped with a little collar made from tube. This rubs on the tube set into the hull and should help keep water out.

These turned out quite nicely. Proper scratchbuilt jobbies and not too bad for a couple of hours work.

Lesson 2 - all that metal holds heat. So I type with burn fingers because I tried to pick the newly soldered parts too early. Twice. I think that probably says something about me.

1 comment:

CF said...

Wet tissue paper at one end - keeps that bit cool enough to play with.
I soldered up a 12mm dia wagon turntable for Wood End and needed fingertip reconstruction afterwards -we learn.