Cutting out an airfoil shape

oscarBravo

The boat build has taken a back seat over the Christmas (largely because I'm out of the country), but I've been doing some thinking about the upcoming steps.

One of those steps is shaping the daggerboard and rudder. These are made up of 19mm planks glued edge to edge, and below the waterline they need to be cut to an airfoil (hydrofoil, strictly speaking) shape.

Any ideas on a good way to approach this? I'm planning to use a standard NACA foil shape, and I've plotted the thickness it needs to be at various points along the profile. What I'm looking for is ideas on how to reliably cut it into this shape.

I had thought about marking lines top-to-bottom at various cut depths and using the router to cut along those lines at the appropriate depths, then finishing the stair-steps back to the curve by planing and sanding.

Any other ideas? Bearing in mind we're only talking 19mm overall thickness.

oscarBravo

In case the above isn't very clear, here's a sketch of the rudder cross-section, with the three 19x75mm planks glued edge-to-edge, and the shape that I need to cut out from it:



The daggerboard is similar, but with six planks instead of three, and a correspondingly narrower foil profile.

FarmerGreen

How about something like this...

https://www.youtube.com/watch?v=z5X61XzJ9XY

oscarBravo

Interesting, thanks. The technique would need some adaptation as the pieces are much bigger (the daggerboard is 450mm x 19mm) and the foil shape isn't required along the full length. Still, food for thought.

Fingalian

Sounds like the way to do it if you using solid planks.Route to the depth and power plane and sand. Do you 'glass it up once it has the required shape?

Or there is this way but it is probably more work :-

http://www.duckworksmagazine.com/06/howto/foils/index.htm

oscarBravo

The plans don't call for glassing the finished foils, but it's always a possibility.

I've seen the technique of laminating it up from plywood. That could be a fallback if I frig it up completely. :pac:

fergal.b

Here is another way from duckworks using two wooden jig's for a router.
http://www.duckworksmagazine.com/09/howto/foils/index.htm

oscarBravo

Hm, also interesting. I was going to say that it would only work for a parallel-sided foil (mine taper somewhat toward the bottom) but I see that his tapers too. I wonder how he managed that?

FarmerGreen

Too technical.
It really doesnt have to be that precise.
A rounded leading edge makes all the the difference between a cammy nightmare or a nice flight.

fergal.b

Here is a link to his build you might like it. I know him from the Glen-l site if you want to get in contact with him, he lives in Dublin.
http://davesboat.blogspot.com/

oscarBravo

FarmerGreen wrote: »

Too technical.
It really doesnt have to be that precise.
A rounded leading edge makes all the the difference between a cammy nightmare or a nice flight.

True, it doesn't have to be perfect. But a NACA foil is as good as it will get, and if I don't at least aim for perfect, I'm going to fall well short. I mean, I have no plans to race this boat, but I don't want to build a complete dog either.

1chippy

Make a template of the external of the profile.
joint your flats together. marking the profile on the end. then use a high throated bandsaw to clean the rough off the piece leaving aprox 1.5- 2mm. then clean back with a good grade 40 grit paper. you would be surprised in the difference between cheap and dear sandpaper. check the profile regularly using your external template.

oscarBravo

...if I had a bandsaw.

1chippy

oscarBravo wrote: »

...if I had a bandsaw. [/QUOTE

A LOT OF SANDING I GUESS
If you have a good belt sander and a bit of time it woud work