Backing Plate

I had a loaner last season. I had backed it with a 3/16" piece of Lexan. It was cut to fit the full width inside the cover and was about 4" high. It was not bonded in place. I didn't know if that was adequate or inadequate. Hence the question in Post #1.

From this exercise, I now realize that the Lexan did spread the load to the structure. But, it did not add any strength to the structure. If it (or something of similar dimensions) had been epoxied to the keel trunk cover, then the cover itself would have been stronger for the same weight.

Interesting stuff.

Have a good weekend bntii!

 

All that and you used phillips heads?

I would have run more thread through the nuts (i.e. longer bolts).

No washers on the front are necessary but certainly would have done no harm.

When/if this set-up fails, it will be at the top of the backing plate as there is no feathering, this creates a stress point (line).

Overall, nice job, I don't think you'll be disappointed.

Must be a very short stove or are you planning on recessing the floor?

-Tom

 

You too Piper- the boat is lookin good.

 

I guess I should have looked at what stove was being mounted. That was the stove this thread was devoted to?! - Double-sided tape/chewing gum/a radar detector suction cup. Good grief, what a waste of time.

 

"Good grief, what a waste of time."

...yep, that just about sums up the whole thead.....ya got ta larf....

 

Tom, what is the comment about phillips heads?

How many threads should have been through the nuts? Are you OK with nylocks?

 

It's fine SP. I love nylocks. Phillips heads suck, but that's just my personal opinion. I use robertson, a Canadian invention not commonly available down your way. I usually leave three threads showing through the nuts.

It's all good big guy, you did a nice job, have a good sleep.

-Tom

 

Not The First

I was reading the ABS 1994 Guide For Building And Classing Offshore Racing Yachts. There is probably a newer version in the public domain. I didn't look. I kind of stumbled upon the 1994 version. That is the era when NAs like Bruce Farr were on the International Technical Committee for the Offshore Racing Council.

In Section 7 - Plating: " . . . the thickness of the bottom shell . . . is not to be less than the diameter of the keel bolts."

There it is. Thickness = bolt diameter. The rule is obviously good in the 1" bolt range.

The document also included the attached Table 4.3 - Basic Laminate Properties. It listed Shear Strength as being between 9,000 and 11,000 psi. That is about 1/3 what I quoted back in Post #10. The lower shear strength actually validates the 'fender washer' part of the rule. I had arbitrarily inflated the washer diameter to 'fender'. Using the lower shear strength actually computes the diameter of a fender washer.

I thought the rule had been developed here on BoatDesign.net. Someday I'll recognize that the planet is populated with folks smarter than me.

 

Finishing Washers

I'm still working on some repairs and modifications to the keel box cover. Here is a straight forward question.

When/why do I use FINISHING washers? For cosmetics? To jump the holding pressure to the outside diameter of the hardware? Are there advantages/disadvantages vs flat washers?

Thanks for the help.

 

Spreading out the load, avoiding pull through, etc.

Why?

-Tom

 

There are a dozen and a half #6 screws holding the keel box cover to the liner of the cabin sole. The holes in the cover are mostly busted and split. I'm in the process of removing the gelcoat and applying some glass along the bottom of the cover to re-enforce it.

When I screw the cover to the liner, should I use panhead screws with flat washers? Or, should I use flathead screws with finishing washers? I'm not sure of the differences.

 

Without a picture, I would say pan-heads with flat washers. Providing of course, you do your fiberglas repairs. Why did it fail in the first place I wonder...

-Tom

 

Superpiper,

You are mixing things up. The rule of thumb was not developed here in the forum but rather by builders and designer who had years of experience building a boat. The thumb rule would most approximate the Class Rules.

Seaking of Class Rules, the bottom plating thickness is different from the keel thickness. The keel plating is the thickest part of the boat and sometimes designed to "bolt on" a keel, not a mere keel cover. The plating thickness is designed first then a bolt diameter is chosen, not the other way around. Without picking up my ABS guide book, I know the rules specifies minimum bolt spacing for different bolt diameter. So first thing first, design the thickness then choose a bolt diameter not to exceed (not equal to) the plating thickness, then determine bolt spacing.

When we talk about fender washers, we do not consider only shear strength but compressive strength as well. FRP is poor on compression and has a tendency to "creep" or relax under prolonged load. Some bolts are known to loosen up over a period of time due to creep.

As for the panhead and flathead, it is easy to see that the panhead is flat after the head while the flathead is countersunk and would dig in the laminate without the finishing washer.

 

rx,

SuperP is talking about #6 screws...

-Tom

 

yes...well stated.


also for fasteners...countersunk vs pan head is a critical detail when mounting technical...highly loaded gear.....always consult the scantling, material rules when chosing between the two