G10 vs. Stainless Steel for Backing Plate

[JK_Boston_Catalina310]

Hello all,

I am beefing up my anchor cleat system to head out cruising. I will be adding the Schaefer removal cleat system to our boat for a stronger anchoring cleat with better angles than our current bow cleats.

My question is what to choose for the backing plate, G10 or stainless steel?

One thing to consider is that I have a free source for custom stainless steel backing plates. My cousin owns a machine shop with laser and waterjet cutters and can fit my backing plates in the scrap area for his bigger orders so he doesn't mind making me backing plates of any size or thickness.

Looking at the data it appears that stainless steel is slightly stronger than the G10 but the G10 can bond better to the hull plus you don't have the same corrosion issues.

So I am leaning towards the stainless but I am interested in feedback.

Thanks,

Jesse

 

[kloudie1]

Stainless is fine for that application, but it should be at least 1/8" thick (assuming a good core in the area) and it is really important to correctly seal the deck penetrations.. chamfer the hole top, use MaineSail's excellent butyl.. No leak = no corrosion.

 

[Ron20324]

Chamfer the edges of the SS that will be against the hull- to reduce contact of the square edges.

 

[edcarter517]

Material Choice

The backing plate has one purpose - to spread the load from the bolts into the deck material. Since stainless is at least 10 times stiffer than G-10, it will do a better job (if the thickness is the same). Bonding is not a factor.
I like to use an o-ring on bolts going through the deck. Countersink the deck using the same principal as an SAE hydraulic port o-ring prep. This must be a carefully controlled countersink and the bolt must have an un-threaded shank where it meets with the o-ring. Use sealer also - belt and suspenders. I have seen many of these o-ring deck joints go for 40 years with no leaks.

 

[centerline]

hands down, stainless if you have a choice.
if the G10 is thick enough it would be fine, but the stainless will give much more rigidity for the same thickness, which rigidity is the first requirement of a backing plate. the second requirement is the square inches it needs to cover to spread the load its going to be holding..

 

[Maine Sail]

If you have access to SS and a water jet this is a no-brainer, go SS. Otherwise G10 is the cats meow compared to working with SS.......

 

[gettinthere]

Are the surfaces where the backing plate mates up, perfectly flat?

If not, the ss might dig in and crack the glass at the edge of the backing plate. It would be much easier to shape a curve to the surface of the g10

 

[JK_Boston_Catalina310]

Thanks everyone.

 

[thinwater]

Are the surfaces where the backing plate mates up, perfectly flat?

If not, the ss might dig in and crack the glass at the edge of the backing plate. It would be much easier to shape a curve to the surface of the g10

What he said. Certainly SS is durable and strong.

That said, I have used FRP many times when the deck surface was not exactly flat (is it ever?) or was tender (Kevlar honeycomb). Remember, this is about distribution. Of course, this can be addressed with fillers if it's important, which it may not be if the plate is large.

Also, the stiffness calculation is not so simple. When you bond the the FRP to the deck, the strength and stiffness increase is 4x, not 2x, depending on the layup of each part. The bonding is more than just keeping it in place, it adds disproportionate strength and stiffness.

So there are a few variables. It all boils down to fit. I've seen an SS plate under a primary winch crush the skin due to poor fit. And with FRP you will need stiff washers, since the local strength is a little less.

 

[tommays]

water-jet and ss on deck for sure



How size could the original Not damage things

 

[Ukeluthier]

The size and fit of a backing plate are more important than absolute stiffness, as long as the stiffness is adequate... which should be the case for G-10 of any reasonable thickness.

The whole purpose of a backing plate is to spread the stresses caused by the load over a considerably larger area than the footprint of the loaded piece of hardware, thus preventing what we engineers call "stress risers." If a backing plate is extremely stiff and a bit too small or ill-fitting, it can actually CAUSE stress risers at its edges.

The IDEAL backing plate would have beveled or feathered edges to allow a bit of spread-out flexing over the areas at the edge of the plate when the fitting is overloaded rather than concentrating the stresses right at the edge of the plate.

 

[thinwater]

The IDEAL backing plate would have beveled or feathered edges to allow a bit of spread-out flexing over the areas at the edge of the plate when the fitting is overloaded rather than concentrating the stresses right at the edge of the plate.

... And I feathered the edges on the Kevlar boat.

Bottom line is that there should always be a back-of-the-envelope calculation that confirms that the fasteners will fail before a hole is jerked in the boat (sinking is bad).

 

[Ukeluthier]

... And I feathered the edges on the Kevlar boat.

Bottom line is that there should always be a back-of-the-envelope calculation that confirms that the fasteners will fail before a hole is jerked in the boat (sinking is bad).

Absolutely!

 

[Dan_Y]

Following what thinwater said above, ... If the contact surface is somewhat uneven between the g10 or SS backing plate could you backbutter the backing plate with some sort of filler and press in place and let cure before drilling to get a better distribution of load and reduce hot spots? Maybe put saran wrap on filler so plate can be removed after cure? Thickened epoxy might be too brittle?