Plank Stiffening

[tasar]

so you get some poly fab, roll it over a wetted glass, let cure, peel and presto......seems hard to believe the fabric would remove so easy.....I like "nude" for color choice

[Bob Rast DN1313]

Cheap peel ply Make your own, use 6mil viscuine Cut to fit project, Fold up several times into small square, take a Ice pick or similar sharp pointed object, become woody the woodpecker and punch a lot of holes in plastic, Probably not on your plank, the more holes the better, works with vacuum bagging. for breather material get quilt batting at the fabric store.

Jeffs Idea of the shop vac would probably work but you might burn up your motor it would have to run until epoxy sets,

Jeff I have a industrial stength Gast vacuum pump if you need one.

Rasty 1313
414-791-3056

[Bob Rast DN1313]

You will need several layers of uni  to get down to 1/3/4 or 1 5/8 deflection. 1 layer of uni or triaxle will only take out about 1/8 of deflection.
You might want to add a thin layer of venner or okume plywood  or some strips of Ash or similar to get there quicker


Good luck
Bob Rast Dn1313

[Bob Gray]

  I stiffened several planks in the past by doing the ash strip trick. If you want to try this, make up two strips about 3/16 inch thick by 1 1/4-
1 1/2 inch wide. Epoxy them on the bottom at each side of the plank, not in the middle. If it ends up too stiff, just sand them down a little. If it's not stiff enough, put another strip down the center. The strips should run most of the length of the plank. A little wood does a lot.
                                                                                                 Bob

[Ken Smith]

Additional wood works as well.  Unlike Bob Rast, I stiffened almost 3/8 inch with the additional I described (3 four inch roving tows, two side by side, one in the middle).  Remember, I only added it on the bottom, which has the magical effect of moving the neutral axis of the theoretical beam that is your plank.  I think you'd get less if you used layers both top and bottom.

Ken

[tasar]

all above considered, I like Bob Gray's idea. epoxy wood strips (at ends) running full length....add strips as needed.....when you get the right deflection epoxy full length or perhaps lay over the top with some bi or triax glass.....if you add the glass over top, does it really matter whether you use ash or balsa, the strength now becomes the layup, not so much the core ?

[Ken Smith]

sorry, tsar, I have to disagree somewhat.

The wood near the top and bottom surfaces matters big time.  Unless the glass layer is strong enough to carry all the loads, which requires a multi-layer schedule, then the glass will deflect with load and make the wood also pick up load.

An all-glass  plank (glass schedule plus bulsa core) is a different animal than a wood plank plus some glass stiffening.

Making the plank thicker always will stiffen it. stiffness is strongly influenced by the cross section moment of inertia "I."  For a rectangular section, that I is 1/12 thickness cubed times width.  Adding a wood veneer that is 50/1000 thick to a minimum thickness, 7.5 inch wide plank plank takes the I from 0.889 to 1.014.  Making part of the plank of a stronger material (wood-glass composite) makes the calculation of I more complicated, beyond the scope of this post, but adding a layer of glass drives I up much more than the wood because the stressed fibers have higher properties.

Maximum stress at an extreme fiber in bending is the bending moment times the distance from the neutral axis to the fiber of interest divided by I.  If the all-wood plank is thicker as described, for a given bending moment on the plank, the stress on the plank-plus-veneer is 8 per cent lower. 

The deflection at a given load is reduced by 12 percent. 

[Sunfish1909]

I had good luck using unidirectional glass roving from Wicks Aircraft.

http://www.wicksaircraft.com/catalog/product_cat.php/subid=8477/index.html

This 3 inch wide product, two lengths, were applied to the bottom surface of the plank. 

I know that the glass is more effective in tension then compression, and the bottom surface of the plank is in tension.  The analysis of a wood-glass composite shows that glass on the bottom lowers the centroid of the plank, making the compression surface (top) in a lower stress state for the same weight on the plank.  That plank was discussed in an earlier string, use the search in the upper right for a look, if it is still there.

Wet the bottom of the plank, and lay on the roving starting at one edge.  The two layers may overlap slightly along the centerline of the plank.  I used peel-ply over the roving which helps get the excess off.  Once set, just peel off the peel ply (if used)

Ken.....is It meant that the added glass or carbon fiber should be added toward the outer edges of a plank and not so important along the center axis of the plank? Is the center of the plank failrly neutral along the center and the stiffening is benefited towards the forward and aft edges under the plank?

[Bob Gray]

 I don't want to get into the glassing conversation but I want to make sure you know where I added the wood. It's on the bottom of the plank along the leading and trailing edges running lengthwise and stopping just shy of the chocks. I'm presently building symmetrical cross section planks and the only way to get the proper stiffness is with glass. Let me tell you it's a lot more work, more expensive and difficult t get the exact stiffness you want. With the strips, put them on a little thick and plane or sand them till you get what you want. This wasn't my idea, Jan Gougeon told me how to do this.      Bob

[Ken Smith]

Let me answer both questions....

The plank bends so top and the bottom surfaces se the highest loads.  Adding material to these surfaces makes the plank stiffer.  The material can be wood or glass (carbon is not allowed in planks per our class rules).  Adding strips on part of the bottom also makes the plank stiffer.  Certainly adding strips on the bottom between chocks on the front and rear edge will make the plank stiffer.   


But I also want the least aerodynamic drag as is reasonable.  This requires smooth surfaces top and bottom (as opposed to a surface with a wood strip bump) as well as a radius or faired leading and trailing edge.  My last step will be to wrap the plank in fiberglass for protection.  THe achives both in teh main section and in collective wisdom say how much deflection the outer glass will add.  I will not recommend from memory...

When I fabricate a plank, I make it rectangular cross section.  Then I measure the deflection.  If it is too stiff, I will first put a 1 inch radius on the top corners fore and aft the width m(long dimension) of the plank.  I also use the router with the one inch radius on the bottom from where the chock will go side to side and front and back.  This changes deflection little and knocks almost all the aero drag off.  If the deflection is still too stiff, I use a power planer and start fairing the lower surfaces to change the plank profile, moving toward a more elliptical cross section.  Taking this material off changes I, and makes the plank less stiff.  When deflection is at your desired ideal, stop planing material off.

On the other hand, if the initial stiffness measurement indicates the plank is too soft, then my first step is to cover the bottom of the plank with glass fibers running the long dimension of the plank, covering the whole bottom of the plank.  First two strips go on butted together in the plank centerline, covering the whole bottom surface thoroughly.  If a third or fourth set of glass fibers, that would be added at the centerline of the plank.

Then I radius the front and back, as above, until the deflection is as desired.

Is that clear?  If you don't thicken the plank with either wood or glass covering the whole bottom, keep it symmetric fore and aft, or the plank will warp as it deflects.

I tend to decide how much is needed by calculation first.  Calculate I and then the new revised I after my stiffeneing plan., calculate I before is the base line.  I after modification is teh revised.  Stiffness is proportional to 1/I,  adding material increases I.  So I calculate how much glass  (or wood) is needed.

Ken

If teh

[JOHN BUSHEY]

If you are adding wood strips to the top or bottom, there is no need to carry them all the way to the chocks.  It may be easier to leave the chock mounting area alone if it is already set to your chocks and mounting hardware. 
The stiffening effectiveness is greatest near the center of the plank and decreases as your get to the ends.

I had a too soft plank that I stiffened with 1/8" oak mullions on the top of the plank and tapered their width and thickness to fair them into the existing plank top stiffness for a total length along the plank of about 5 feet, leaving the remaining length of the plank untouched.

[Sunfish1909]

Thanks guys. I am stiffening an 11 ft plank on a Sprinter, a recreational boat, not a DN. I do have some uni carbon fiber tape strips. That is why my earlier post mentioned carbon. I have a six inch wide carbon strip about eight feet long. Should I lay the whole strip centered under the plank? Should I cut it length wise so I have two, three inch wide strips and set them along the aft and leading edge?  My plank is only a two-ply plank and so I am looking at a stiffening method. Your "collective wisdom" is very much appreciated .

[Sunfish1909]

I am reposting this question before I do this work...this week.

I am planning on adding the carbon fiber strip right down the center on the bottom side of my plank. If there is a better postion for this reinforcement, like two strips at the fore and aft edges, please respond. Thank you ahead of time...........

from a previous post

.........I am stiffening an 11 ft plank on a Sprinter, a recreational boat, not a DN. I do have some uni carbon fiber tape strips. That is why my earlier post mentioned carbon. I have a six inch wide carbon strip about eight feet long. Should I lay the whole strip centered under the plank? Should I cut it length wise so I have two, three inch wide strips and set them along the aft and leading edge?  My plank is only a two-ply plank and so I am looking at a stiffening method. Your "collective wisdom" is very much appreciated .

[Geoff Sobering]

I am reposting this question before I do this work...this week.

I am planning on adding the carbon fiber strip right down the center on the bottom side of my plank. ...

I don't think the location matters on a rectangular plank, but if it has any profile to it, I would certainly keep it down the center.  Hopefully someone else will chime in on this specific.

Here's something from an item I recently posted on a similar thread in the J-14 Yahoo-group:
If you plan to do any sanding on top of the carbon strip, I would recommend either laying a sacrificial layer of glass (6 oz fabric works well) or a fairly thick layer of epoxy thickened with some kind of fairing filler (I like WEST Microlite).

The fibers in the carbon are very small diameter so any sanding will cut many of them and degrade the strength/stiffness of the reinforcement. In my experiencea little bit of damage will weaken the carbon very dramatically.

If you choose to go the route of adding filler, the best procedure I know of is to roller/squeegee the carbon and then wait for the epoxy to cure into the gel stage (usually a few hours, depending on the hardener and temperature). At that
point the first layer of epoxy can still bond chemically to additional epoxy placed on top of it, but the first layer is strong enough that you can't inadvertently shift the carbon around when you're laying on the filler.

Cheers,

Geoff S.
DN 5156

[Sunfish1909]

I spoke to a gentleman at West and he suggests running the fiber on “top” of the plank not the bottom. He said the wood would fail first in compression (topside) and be stronger in tension , the bottom side, ( like breaking a stick over your knee). Also that if the strip is shorter than my plank length to taper the width of the tape(over 24” or so) to “nothing”. This would“give back the stresses to the wood alone gently versus all at once with the fiber in it’s full width. Pretty cool analysis. Geoff's use of the sacrificial glass or filler over the carbon and epoxy in the gel state is very good practice. If that can not be done, West suggests using water and a "ScotchBrite" pad , not a fiber cutting sandpaper,over the cured epoxy to develope a tooth for paint or additional epoxy work to take hold Thank you all for your input here. Cheers.