Tuesday 26 November 2013

Centerboard lifting mechanism

I decided to go a different route for the centerboard lifting mechanism. Basically it took us so long to finally have a small sailing boat with a potential fore "sunbathing platform" for my wife and I didn't want to spoil the dream by having taught ropes going across it.
In the original arrangement a 2 or 3 part block system is used to draw lines from the centerboard lifting point to the mast base.

In my version I will rather have pulley wheels fitted to the centerboard lifting point, to inside the forward log of the centerboard case assembly and on the deck at the base of the case itself.

Here's a sketch of the principle idea. The system of ropes is basically doubled up yielding a 4:1 purchase on the lifting effort. The lift line can be fed directly to the cockpit are or to a jammer on the side of the centerboard case.... no ropes fed to the mast step!

Here is how the implementation of the design look in a prototype set up. You can see the pulley wheels in slots routed in the case fore log. Also it should be quite clear how the system will eventually work.
For the moment it seems to be behaving extremely well and I am very pleased!!

Monday 11 November 2013

Centerboard shaping

As I mentioned in my previous post I was a little worried about how to shape correctly the centerboard stock.
Scouting a bit the boatbuilding forums I came across a very clever idea proposed by Ross Langford and appearing on duckworks magazine pages:
another approach to shaping foils

Great idea I thought right away, and I decided to follow its principle. Ross provides the complete solution including an excel sheet to design the router gig he proposes for the purpose.
For the fun of it I decided to apply the method but to redo the calculation, using 2D cartesian equations rather than trigonometry. I therefore wrote a MATLAB script that computes the foils NACA profile, its offset for the router cutter and finally outputs an AutoCAD script for printing purposes.
I am happy to provide the scripts to anyone interested ... they worked out pretty well!

Here's the model for my pathfinder's centerboard NACA profiler:

which is then used to build the router support:
And here we go, with the gig cut and assembled the router comes into (loud) play. The idea as that the router slides over the guide it cuts away the unwanted material from the centerboard stock, to obtained the required NACA profile.

The process is into two phases and mall portions of the centerboard are left unshaped to allow to turn the board around and shape its other face.   

Its great to see the NACA profile appear through the sawdust and the noise of the router.

I have no idea how I could have achieved a precise and constant profile throughout the board otherwise!