Changing plank stiffness

[ktb11]

I know to stiffen up a plank you glass wrap it, but what is the preferred method to decrease the plank stiffness and get some more bend in it? What tools are used to "shave" or remove material?

[JOHN BUSHEY]

It is the very top and bottom of the plank that has the biggest influence on stiffness.  If you thin the plank overall by removing material top and bottom with a belt sander, a small change makes a huge difference because the plank stiffness is proportional to thickness cubed.  Sanding evenly is kind of a pain, too.

  A more subtle and satisfying method is to sand chamfers into the leading and trailing edges of the plank, both top and bottom.  This effectively reduces the width of the plank at the critical top and bottom.  Stiffness would change more gradually with material removal.  There is an approximately linear relationship between width of top and bottom skins and stiffness, so if your chamfers reduce the effective width by 10% top and bottom, you will get roughly that much stiffness change.

It is easier to remove material on the edges with a router or belt sander.  There is no need to chamfer it over the entire length, though that will reduce the amount of softening you get.  You can leave the area between the hull mounts flat as well as the chock mounting surface.   Removal of material inboard on the plank has more effect than outboard by the chocks.

Bonus:   plank is more aerodynamic and looks cooler! 

[ktb11]

John, thank you very much for the input!  I believe I'll follow the more subtle approach.  Is the ideal plank deflection 1 3/4" with an additional 20-30 pounds? 

[JOHN BUSHEY]

Many use that deflection.  Others can comment more authoritatively that I can.

Years ago I put together a spreadsheet for calculating plank stiffness.  Paul Goodwin tweaked it and put it up on IDNIYRA.ORG.  I have always hoped to polish it up to put in real chamfer calculations.  Hasn't happened.  But you can approximate the effect with adjustments to skin widths.

https://ice.idniyra.org/Articles/Planks/Plank-Design-Calculations

[Geoff Sobering]

There are two ways I've used to thin a plank. Both involve making a series of cuts across the plank at regular intervals along the entire length to set the thickness of material you want to remove, and then using another tool to remove the material in between the cuts.

The "classic" technique uses a circular saw to make a series of narrow kerfs every couple of inches and a power plane to remove the bulk of the material.

I prefer using a router to make the depth-cuts. The biggest benefit is that most routers have a very nice depth adjustment so it is pretty easy to set an accurate depth as small as 1/32" (and even 1/64", but most planks don't have a smooth enough surface finish to make that practical). I've found a really sharp bit is critical to the operation.

Since I'm not very good with a power plane, I also use a wide bit (~3/4" dia.) and remove most of the material with the router, leaving ~1/8" wide strips between the cuts to give the router base something to ride on. With a small amount of material remaining, a belt-sander easily removes it.

Here's a photo of the router process:
   

[ktb11]

Geoff, thank you very much explaining your method. Certainly another good way of doing it.  I'm expecting to need to get another 1/2" of deflection on my plank.  Have you found that a certain depth on the router bit will give a certain change in deflection?  (obviously there are plenty of factors but maybe you have found a ball park number) 

[JOHN BUSHEY]

Be careful if reducing overall thickness!    A little material removed will make a big difference.   Play with that numbers in that spreadsheet to see what I mean.  Even if it is not absolutely accurate, the affects of changes will be obvious in the calculated deflections.

[ktb11]

I'm going to follow the chamfer method and work on the leading and trailing edges little by little until I hit my desired deflection.  It's a lot easier to remove material than put it back on! 

[ktb11]

The question is, will I be able to get the 1/2" change in deflection that I need just by working the edges?

[Geoff Sobering]

The question is, will I be able to get the 1/2" change in deflection that I need just by working the edges?

I doubt it, but it's an easy/quick process so I would start there.

The plank stiffness spreadsheet is a good place to start with getting a feel for how much thickness change affects the deflection.
I'll hazard a guess that you'll need to remove somewhere between 1/32" and 1/16" from the plank to soften it by 1/2" at your-weight+30 lbs.

Bill Mattison says removing material from the top skin changes the stiffness more that the bottom, so I always start on the top of the plank. If you remove more than ~1/16" from the top, I would move to the bottom to keep the top and bottom skin thicknesses approximately equal.

[Bob Rast DN1313]

if using the router to mill grooves try a v bit or cove bit or small straight bit VS wide straight bit  sand  the remaining tops off.

Check the plank width and see if you can take a little off front and back edges with skill saw or table saw  and still make min width

Happy sanding
DN1313

[ktb11]

I think I'm pretty set on chamfering the edges with a router then checking to see where I am in comparison to my target deflection.  If I need more I plan on making a pass on the underside of the plank with a belt sander, then continuing the sanding process until I hit my target dead on.  If I need a considerable amount more after my first pass with the sander, I'll alternate between doing the top of the plank and the bottom to keep everything pretty even.  I'll check frequently to make sure I end up spot on.  Seem like a pretty good method? 

[Geoff Sobering]

The only issue with just using a belt-sander is removing a uniform thickness across the entire side of the plank.

I've seen some people who are very (very) good with the planner run it from one end to the other in a single pass to remove a constant amount of wood.

[Bob Rast DN1313]

Planer might work, Lou and I sent a Nite Mast Through
 the planer Once worked pretty good

[Geoff Sobering]

Planer might work, Lou and I sent a Nite Mast Through
 the planer Once worked pretty good

I meant a hand planer! 

(although I have trimmed planks to width by running them through a table-saw...)