Anyone made a CNC centerboard?

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Anyone made a CNC centerboard?

Postby Slim » Thu Mar 09, 2023 1:59 pm

My old centerboard is having some serious issues and may not be worth repairing/improving.

I would love to make a new one using a CNC machine to shape a perfect foil. A local makerspace has the machine, but I lack the computer know-how to create the file/program for the machine.

Wondering if anyone has done this who would be willing to share the necessary programming?
Or if any of you computer savvy Daysailors out there would be up to the challenge. It would be awesome if we could create that file and have it be available to any of us to improve our boats!
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Re: Anyone made a CNC centerboard?

Postby tomodda » Fri Mar 10, 2023 2:37 pm


Sorry to read that you are having CB issues. Answering your question, no I have not made a CNC Centerboard, but I can outline the process:

-Make a 3D Virtual Model of your Centerboard, preferably using a NACA profile (which you may want to flatten a bit for when the CB is partially retracted). You can use CAD or your favorite modeling tool (Blender, 3DS Max, etc).
-Convert the model to .dxf format, if it's not native to your 3d Tool. Adobe has a free (I think) converter tool.
-Program Tool Paths onto your dxf file, this is highly machine-specific (which 3-axis CNC cutter table you are using), so there's a learning curve.
-Cut your blank, flip it over, cut the other side, cut a pivot hole, etc, etc.

Now, I'm assuming that the CNC table is big enough to cut the entire board, you'll still need to carefully index the positioning so that you can flip it over and cut the other side. What will your CB be made of? Wood Core then cover with epoxy and glass? Or HDPE (cutting board plastic), can you get a blank that is big enough? Either way, you have to consider weight, it can't be over 25 lbs but you also dont want it to float, so you may have to add lead (if wood). And then you may have to re-enforce the pivot point, again depending on your material.

Yes, I'm writing all this to "scare you off." I really, really can't see learning modeling and 3D CNC cutting being worth is JUST for this project. If you want a project to learn from and then do other CNC, then go for it, of course.

In the meantime, assuming that your CB is beyond the point of repair, what CAN you do? Best option is probably "just throw money at it." A new CB is $650ish at, add crating and shipping you're probably near $800. Painful, but so is building a new CB. However, if you ARE going to build, the way to do it (IMHO) is laminating vertical strips of a decent hardwood (oak?) into a black, then cutting that blank to shape with a router. You could also laminate together a blank out of marine ply, but I don't trust it not to eventually warp. Either way, you shape the foil portion by cutting half-profiles of your desired NACA shape into plywood, and then gluing them to the blank as a routing guide. Pictures will explain better than words, check out these links:

You'll still need to glass the blank afterwards, of course. The other way to do it, as I wrote above, is HDPE. I've never worked with it, but supposedly you can work it with normal woodworking tools. No idea if you can laminate it though!

Overall, between CB blank material, glass cloth, epoxy, router bits, clamps, blood, sweat, tears... you really may be better off paying the $800. Horses for courses....

Good luck and fair winds!

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Re: Anyone made a CNC centerboard?

Postby GreenLake » Sat Mar 11, 2023 12:51 pm

There are some CB that are aftermarket and properly foil shaped. I would consider purchasing one of those unless you enjoy building things. I would not consider purchasing a non-shaped stock CB.

I've built a rudder using a doubled sheet of plywood and using a power plane and belt sander for shaping, not going to the trouble with the router. Because shaping plywood results in visible banding you get some feedback and it's easier to shape it free hand with at least approximate consistency. I used a bit of fairing compound to even things out, and a simple cardboard template to verify foil shape. This is made easier by the fact that the foils on a DS do not taper. (So the same template works everywhere).

This works the opposite of the technique shown in Tom's post: you add the fairing compound and then, before it has set, push on the template. If you have too much compound, it will leave a groove. You can then sand down to that level.

Would I do a CB that way? I would probably not use plywood. It appears that it's not strong enough when flexed. For the rudder, it's possible to keep an eye on it and check for possible delamination. That's not that feasible for a CB, so I would use the laminated strips.

However, would I actually build one? Creating and shaping the blank is less than half the effort. A CB would require a suitable way to reinforce the pivot and as Tom noted, some though on how to counter buoyancy. Still, most of the time will be the finishing steps: adding a glass sheath, more fairing, and painting.

The benefits of CNC is that you get a more precise shape, and if you build several, the shape is consistent. But this affects only one stage of the process, and because CB's are not only used in the fully down position, your best shape is already a compromise compared to the theoretical NACA curves. At that point, it becomes guess work as to whether the necessary design deviation to account for partially raised CB introduces more uncertainty than manual implementation.

That said, CNC sound like a fun toy to play with, so if that's your motivation, why not?
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Re: Anyone made a CNC centerboard?

Postby Bandit » Wed Mar 15, 2023 4:48 pm

Where are you locate? I will soon have a centerboard available for sale. It is the original but I have faired and painted it. Very nice board in my opinion.
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