DN NA Class

So, I would like to change my current external plank attachement to the inside the cockpit style.  Frankly, I don't even know where to start.  Can anyone give any direction on this?  How to do it? How difficult is it?  Additional parts needed?

Flip hull upside down and remove external plates.  Use a router with a flat bit set at shallow depth (depth of bottom skin) to remove bottom skin around the area where hull and plank overlap.  Install blocks on insides of hulls with several stringers in between.  Replace bottom skin.  Drill holes for internal attachment hardware.  Refinish this section. 

Really quite simple.  Feel free to call me with questions.  608-692-4007

Best,

Daniel

You may only need to remove the bottom where the new blocking will go.
There should already be crossways stringers. If that's the case then just trim them to fit the new blocking.

As Daniel says, it really is quite an easy procedure.

The on-line DN plans should give you an idea of the size/location of the blocking:
   https://ice.idniyra.org/sites/default/files/DN_english_sheet2.jpg
You should layout the new hull hardware on the bottom and make blocks that extend at least 1/2" outside all the screw holes.

Thanks guys.
Are the any tricks to keeping the alignment square?  How critical is the alignment, what kind a of tollerance is acceptable?

How critical? I don't know, but it's not hard to make perfect, so why not do so?!

Here's what I'd do.  Mark the centerline of your plank on top and bottom.  From the top side, now measure 8" in both directions from this centerline.  Draw two more parallel lines at these points and mark their centerlines.  Since your posts will be 16" on center, these intersections will be where your posts are on the top of your plank.  Now, drill a small hole all the way through your plank at these points. 

Next, flip your hull upside down.  Mark its centerline.  On the hull centerline, mark the locations where our plank studs are going to come through the hull.  Use a square to extend these locations to the edges of your hull.  Drill the same small size hole through ONE SIDE of your hull.  Now, flip your plank upside down and use a nail close to the same diameter of the holes you drilled to hold plank in place.  Line up centerline on plank and hull.  Now, use your steering chock to triangulate the the plank and the hull to make sure your centerlines are perfect.  (I tape my tape measure to the runner bolt and measure to a common spot to each end of the plank).  Move the "unfixed" side of the plank fore or aft until your hypotenuse measurements are identical.  Using the other hole in your plank, now drill through your hull to mark the other side.  You now have reference points for a perfectly aligned hull and plank.  Using these reference points, mark the locations of the two or three additional holes you'll have on each side.  Use a forschner bit to drill 1" holes all the way through your hull. 

Mount studs on plank directly over reference holes.

Mount internal plate on one side of hull.  Place second plate over holes on other side.  Place plank on hull.  Triangulate one last time before mounting second plate.  If your measurements were all really good, studs will be dead center in all of your holes as plank moves fore and aft.

Using this method, I've been able to get to within 1/16", but "use at your own risk."  Good luck.       

While you're taking the time to convert to inside attachment, it is  worthwhile making sure that when you have completed the job and fitted the plank to the hull that you have complied with the official DN specification F.1.   With the hull and plank  mounted up-side-down, stretch a string between the centers of  runner pivot bolts in the side chocks.   Then measure from the center of the runner pivot  bolt in the steering chock, along the centerline of the hull, to the string between the side chocks.   Maximum allowed dimension is 103" , minimum is 97".  If the steering chock is mounted on an angle, be sure the measurement is taken with the chock spring extended to make sure you will not go beyond the 103" maximum no matter what the load on the chock.  Most of the top sailors are 1/4" to 1/2" under the maximum allowed, just to make sure they comply with the specifications.

There really is no particular speed advantage to the inside attachment.   Meade Gougeon has one of the best DN racing records and he used outside mounts.   

Biggest advantage to inside connection: the cockpit drains when it rains or the ice is wet.

Biggest advantages to outside connection:  theoretically stronger.  Easier to set up.

Most important:  if you have two boats, make plank interchangeable. If you go with 16 inches as recommended by Dan, you can borrow a plank!

16"  centers also allows you to swap or borrow planks from fellow sailors.  It can save you from loosing a day (or weekend) of sailing, or it can help you learn a lot about the dynamics of your boat when you borrow a plank that is stiffer or softer than what you are used to using.