Stringer Strengthening Question

[WalleyeSniper]

So, on the 270 Sportcraft we're restoring we're getting ready to remove the stringers and noticed that one of the main stringers has a failure crack in it and when looking at it, and you can easily understand why it failed when realizing how it was designed...

The stringer is 17" tall, and then abruptly steps down to 4" tall throughout a good 5-6ft. section of the stringer. Which, to me, is just asking for trouble at the points where it steps down and up so radically. I'm assuming the factory did this because the fish-box that's in the sole/deck directly above it. The floor of the fish box sits within 1/2" of the 4" section of stringer... So, I can't really just make it 17" throughout, and am trying to figure out a way to make it substantial enough to be more reliable and not end up as a crack-prone area like current.

Thus, how can I strengthen the living hell out of this design, or add to the design while still keeping the fish box, in a way that's efficient and will prevent a crack-prone area like what has already happened? (Pictures and diagram of it below)

Stringer drawing.

The actual Stringer section in question.

Closeup to where the crack is rather apparent, and exactly where you would assume it'd be on that design.

 

[Will Gilmore]

This stringer was cut out to accommodate a fishbox? Is the fishbox intended to be removable? If not, is the box as part of the reinforcing structure. Otherwise, Marry a couple of metal plates to either side.

That looks like a crack from an impact against the bottom of the boat. It may have been preventable, but maybe not. I don't think it is inherently weak there, just the weakest point for failure from trauma.

 

[TANSL]

Whatever the reason for the cut in the reinforcement, a sudden change in section always gives rise to a stress concentration point. The breakage at that point, whether due to impact, material fatigue, or whatever, was predictable, as the OP points out. It would be necessary not only to repair that crack but also to correct that abrupt change in section.

 

[WalleyeSniper]

This stringer was cut out to accommodate a fishbox? Is the fishbox intended to be removable? If not, is the box as part of the reinforcing structure. Otherwise, Marry a couple of metal plates to either side.

That looks like a crack from an impact against the bottom of the boat. It may have been preventable, but maybe not. I don't think it is inherently weak there, just the weakest point for failure from trauma.

-Will

Thanks Will, I appreciate the insight and help.

The fish box isn't removable and is molded into the deck. But, it doesn't touch anything underneath to provide any structural support other than with the deck itself.

I assume it was probably from trauma of some kind, but no way of knowing for sure as I never ran the boat or etc. The previous owners were definitely the "careless boater" type, for sure, so wouldn't be surprising.

It just seemed like a 17" tall stringer throughout, with only a 4" vertical section a few feet inbetween, would be a point where stress cracks could easily form and lead to the stringer failing prematurely.

Then again, I'm not too well versed on stringer dynamics and tend to overbuild the crap out of most things.

 

[WalleyeSniper]

Whatever the reason for the cut in the reinforcement, a sudden change in section always gives rise to a stress concentration point. The breakage at that point, whether due to impact, material fatigue, or whatever, was predictable, as the OP points out. It would be necessary not only to repair that crack but also to correct that abrupt change in section.

What are some good options, when putting in the new stringers, to eliminate that from being a focal stress point (while still including the fish box in the sole above it)?

I have the boat completely gutted redoing it all from the hull up anyways. So, may as well do it right and know my family and passengers are safe when finally putting her into service.

 

[TANSL]

The typical solution is to gradually reduce the height of the reinforcement for a length equal to 30 times its minimum height.

 

[WalleyeSniper]

The typical solution is to gradually reduce the height of the reinforcement for a length equal to 30 times its minimum height.

That's where I'm running into a dilemma. With the fish box built into the sole directly above the 4" tall stringer section, I'm not sure how I can gradually taper from the 4" up to 17" section without eliminating the factory fish box.

Is there any clever ways of tying it into the neighbor stringer that's 12" away (which IS a full 17" throughout the entire stringer)? Or possibly sister some more support on each side of the 4" tall section leading well into the 17" section, kinda like with butt blocks or splices?

A little better view of the shoddy grid they had installed.

Near the rear 3rd of the stringers you see the step down. Any good ideas on strengthening it up? Or, am I overthinking it and it's more of just a sole support above it as to why the 17" height is there?

 

[Blueknarr]

You don't have any 17 inch tall stringers.
You have 4 inch tall stringers with some of them topped with 13 inch tall sole supports.
4 inch tall sisters would suffice.

 

[WalleyeSniper]

You don't have any 17 inch tall stringers.
You have 4 inch tall stringers with some of them topped with 13 inch tall sole supports.
4 inch tall sisters would suffice.

Ohhhhh ok. So those are really just for the sole anyways, and are more or less glassed ON TOP of the stringer itself then?

So when I go for a new install, should I run a 4" stringer, close it off, and then run the sole support on top of that? Or do I create the stringer timber, bond the support on top, then glass it all as a single unit as if it were cut out of a single piece of timber?

Probably going to go the coosa route, and just trying to now understand that layup schedule because I've never worked with one of these with some supports on top.

PS: I am extremely grateful for the help and advice you guys have given me. I yearn to learn even 1/3rd of what you guys have probably already forgotten, haha. So, thank you.

 

[Blueknarr]

The factory used a single piece of wood.
You can semi-replicate the original construction. BUT definitely add extra support at that high stress area. A 1x4 sister several feet long would do the trick.

 

[Will Gilmore]

Make the fishbox part of the stringer support. Bed it in with 5200 and screw it to the 4" stringer. So it all becomes one unit. Honestly though, 4" stringers should be plenty strong. There are just some collisions that can't be braced against.

 

[WalleyeSniper]

Thanks guys, those sound like great options to giving at least a little more rigidness to the system. I can't believe how the factory sent these boats out like this. Missing chunks of stringers, shoebox size through holes in the stringers and none sealed whatsoever, and exposed ply and bare timber everywhere under the deck in the bilge area.

I look to make it much better than original and hopefully long-lasting. So, looks like I'll be bedding the fishbox AND sistering a little more width and length on there, just for peace of mind if nothing else.