Backing Plate Testing

Breaking stuff in the name of science is always fun! Parts of this appeared in Practical Sailor in 2015, some not.

The test rig

The bolt is tightened and torque recorded until something gives.

  

I tested laminate samples to make sure the failures were appropriate (good bonding). As you can see, the core is failing in shear.1 layer 17-ounce biax on the bottom, one layer 17-ounce biax and one layer 6-ounce cloth on the top, 1/2-inch end grain balsa.

I replaced core with plugs--standard practice.

The Poor

Starboard showed micro cracks after 3 week. It also creeps a lot--I had to keep re-torquing for 48 hours.

 

Teak is a big failure. Note how the distorted washer became a wedge.

 

 The Best

Just 2 layers of 17 ounce biax were very impressive, holding 1.5 tons. 4 layers would be correct for 1/4-inch bolts.

1/8-inch FRP bonded  did better than 1/8 SS, particularly on rough surfaces. 1/4-inch FRP would be correct for 1/4-inch bolts.

 The Bottom Line

• 1/8" aluminum for light duty in dry locations. Thickness = 1/3 bolt diameter minimum.
• 1/4" FRP most places, bonded if there is high load. Thickness = 2/3 bolt diameter minimum.
• 4 layers of 17 ounce biaxial cloth for every 1/4-inch of bolt diameter, where there is high load and irregular surfaces.
• Diameter of plate = 5 bolt diameters minimum.

And never, ever use fender washers. They bend into cones and are LESS effective than standard bolting washers in actually distributing the load.  Buy extra thick (Bolt Depot).

 A little more if the skins are very thin. Less with solid glass.