Grind away part of deck stringer to install backing plate?

[memelet]

I'm installing standup blocks for my running backs. The place where I need to mount them is partially right over a deck stringer.

Pics showing deck location and where the backing plate would be: https://picasaweb.google.com/memelet...PTOjvz3xbTPxwE

I suppose I have three options:

1) Grind away the stringer so the backing plate has a flat surface
2) Build up next to the stringer to make a flat area.
3) Don't use the backing I have and instead use individual washers (or two backing plates)

If I do 2, then I would have cut out holes in the coach-roof for the bolts and nuts to protrude. (My coach-roof is wood.)

If I do 3, then clearly I will not the full strength. 3 also has same the downside as 2 for at least one bolt.

So I'm curious about grinding out part of the stringer. Does the stringer need to be continuous to perform its function? Would a cutout cause a deflection point with excess stress? Would the backing plate make up for any loss of strength?

The boat is a 1985 Passport 42. (Brion T, you looked over my boat during the Oakland show.)

thanks!

-barry

[Stumble]

Barry,

Before I would touch a stringer with a grinder, I would have a navel architect certify that it wouldn't compromise the boat. There is just no general rule of thumb here.

But conceptually, you want to mount a highly loaded system into a newly weakened area of the hull? Sounds like a recipe for disaster to me. Without professional advice I would say only option 2 would be acceptable.

[Ian McColgin]

Build it up. Also, if you must locate a screw through the stringer, seal even more carefully than the others since you don't want water intrusion to travel there. This is really a trivial build up job anyway - a bit of wood settled in some thickend epoxy does it.

G'luck

[memelet]

Build it up it is. I do have drill one bolt thru the stringer, but I will pot all the holes for the bolts.

thanks!

[Auspicious]

Hope I'm not too late to the party. I am a naval architect (Webb '82) and did some structural work on GRP with stringers for US Navy minesweepers back in the late 80s.

As noted above, don't grind away a stringer. You will reduce strength AND create a stress point at the discontinuity.

Without running numbers and just shooting from the hip, I'd build up a round area 20% greater in diameter than the backing plate you intend to install. Make sure adhesion is very good between the new glass and the stringer. Fillet everything carefully to avoid corners and sharp edges.

[memelet]

Not too late all!

Even with the the buildup, I will still need to mount one of the bolts right thru the stringer. Should I avoid that all costs or will the build up account for that? If so, I will be over drilling and filling with epoxy. I can stop at the inner glass (pretty easy with forstner bit), or should I go all thru the stringer? I'm assuming the stringer is wood encased in epoxy.

Also, for the buildup: I'm assuming it is ok to use G10 for the base, epoxied in place. No?

BTW in case I was not clear, the stringer runs parallel to the toerail about 12 inches inboard. Its roughly 2" wide and 0.5" deep.

Thanks!

[Auspicious]

The importance of really good adhesion of the built up area to the existing stringer is specifically to carry loads around the penetrations. You should be fine with a reasonable sized hole through the stringer for a bolt.

Try to avoid sharp edges. Edges and corners are stress concentrators, thus the guidance to round over and fillet.