Backing Plate

It is a rule thickness (local reinforcement) based on the fact that the panel was designed to support a load within the allowable flex. Gerr's rule approximates this, hence differrent constant for diferrent loads.The fender washer was to spread the load and decrease shear (larger area), the increased thickness to compensate for compression load when the bolt is tightened.

Two conditions must be satisfied, strength and flexing. Most of the response was concerned about flexing, hence the cored approach to increase stiffness and reduce flexing. The response was based on years of experience in the back of their mind. Vertical acceleration when the boat is underway.

If the panel was stated to be 3/4" thick, I am sure most members would say just "hang it in there it will hold, that is a pretty robust panel you have."

It appears that the panel size is 6" x 36" (over 1/8" thick as stated) with the edge folded to make up the 48" sides. That makes it relatively stiff.

 

12 layers of 6 oz glass and epoxy inside the keel box cover will bring the thickness up to 1/4". The centre plateaus will be just a little larger in diameter than the 1-1/4" OD fender washers. Gerr specifies a 10:1 runout on glasswork, so 1/8" of additional thickness will have a runout of 1-1/4". The 2 patches overlap, intersect the bottom of the cover, and just reach to the corners.

I'm cutting glass.

 

It sounds like you are doing a nice job of it Piper.
Remember that most replied with two factors in mind- the part you are hanging and what you are mounting it to.

The stoves manufacturer is specifying #12 screws into ply at min. 1/2" depth.
That gives a fastener spec of ~600 lbs for this 1.5 pound stove!
You have increased it to 9000 lbs to meet the breaking of those 1/4-20's.

Matching those #12's is not much trouble and is in line with the responses largely saying to just hang it to the existing with 'just about anything in the scrap drawer'.

 

SuperPiper,

Have you been this "careful" with everything else on the boat?

There is a practical and an "engineered" response to the question, the practical comes from trial and error, mostly not done by ourselves but by others in the past....same as boatbuilding in general had done for centuries.

Both Gonzo and myself saw the need for some sort of practical backing plate that would not warp under load the side of the case box, as 1/8th glass could do easily if not supported, or at least attached to the corners which are of course stronger in sheer than the centre piece.

It is interesting to me to see your responses to our replies, the initial question seemed quite simple, you obviously did not want a simple reply, but were looking to be technical...as practical workers we tend to do many things from experience (both our own or others), looking up every little attachment to work out the "required" fastening would make the cost of any boat or repair at least tripple......you have to remember that ALL things are done with costs factored in...even the homemade boat the is perfectly put together has cost factors involved.

 

Some guys beleive "why make it simple when you can make it difficult?"

Me, I would grab a piece of fiberglass, cut it in rectangular piece covering the two holes, epoxy it and presto, I have the backing plate. Or I can use piece of steel plate to eliminate fender washers. "anything in the scrap drawer" that fits the "rule of thumb".

Also, the glass panel where it will be mounted is 4 mm. thick. 1.5 x 4= 6 mm. total.

 

Yes, I did not even think of scrap glass for a backer - There's always a piece of 1/4" stock leaning against the wall. It's been a while since I have been in the industry but that would be my first choice.

 

Thanks for the lecture, guys. You're right, I did get carried away and I turned this into a crusade. But we now have a rule for through-bolting. Maybe it needs to be adjusted or varied for different applications, but I have a starting point. Now if I'm mounting a boarding ladder, or an outboard motor bracket, or a stanchion base, or a clip point for my safety harness, I don't have to buzz Gonzo each time.


The Rule:
The solid glass thickness should be 1.0x the hardware diameter. If you would rather use a plywood core, the core should be 2.0x the solid glass thickness calculated by the rule. Don't forget to apply 30% of the calculated solid glass thickness over the plywood to make it a true sandwich construction. Use a fender washer as a backing plate. Taper glass at a run-out ratio of 10:1.

 

....no comment........

 

Difficult to understand exactly what you are doing. Important to remember that the keel bolt fender washers must be perpendicular to the keel bolt and have 100 percent surface contact with the substrate. Very difficult to achieve with your technique. . The easiest way to do this is a mold release or Mylar tape covered ,metal backing plates, bedded into thickened epoxy or chockfast , then fender washered and gently torqued. The load will be evenly distributed on all bolts then transferred evenly into the structure. .

Ive never seen your system employed. Simply drilling tthe keel bolt holes with presicion will be a challenge.

 

Are we still mounting a stove here?

Nice load distribution. Attractive breast-like resemblance. I like it!

And to think, I thought fender washers would have done the trick...

-Tom

P.S. I wonder how many more pages this thread will go on...

 

How will the rapidly changing climate affect BACKING PLATE RULES and in which file should I keep this (So I never forget how to do it)?

 

I have noticed also the roundlike markings in the center of the mound. The lttle detail seems to reinforce your way of seeing things.

 

Job Done

The local thickness is 1/4"+. I'm very pleased.

After all the focus on the backing plate, I was wondering if there should have been washers on the outside of the brackets.

BTW, there is a fundamental error with THE RULE: glass strength is proportional to its thickness. Bolt strength is proportional to its diameter SQUARED. This thread may have to continue for a few more pages.

 

Oh- you are definitely going to need washers on the outside too- big ones.
That is probably cast & we have all seen it break at the worse possible times.

Is the bracket mounted high enough???
Can we see a photo of it all mounted up ready for a cuppa?

Also- pick up a spare bracket and mount it in the cockpit somewhere perhaps.

 

Im warn'n Ya' bntii.... no shortcuts ......or youre gonna be before the judge with a team of lawyers defending against a class action.

Minimium spec by my calc's are .... make it strong enough to support a chopped steel drum full of jumbo , angry ,snapping , crabs ,with a few concrete blocks on top the lid to keep the buggers from escaping , as you gyrate downwind under reefed main and full spinnaker . .