Steel Stringers?

[OldBoatGuy12]

Hello, my boat's(78 invader w/ evinrude 115) floor has rotted out. The transom is fine and the boat will only be run in fresh water lakes, no salt. I have gotten all the crap out and am at the point of where I am ready to put new stringers in and I came up with the idea of using a steel ladder-frame, much like those used in many race cars,in the place of individual wooden stringers. I would most likely use regular 1" or .75" .0625 wall square steel tubing. One reason for wanting to do this is that I have a pretty good engineering and CAD background and can design (and test for strength, weight, stiffness, etc...)a ladder frame system that will give a much better strength to weight ratio than the plywood that was originally in the boat. Another, is that I am by far a better metalworker than woodworker. And, heck, it might even end up being cheaper, although I don't know about that for sure.<br /><br />Obviously, rust becomes a problem instead of rot. However, I am pretty confident that a good paint or even powder coating will prevent this. As far as water getting into the tubes themselves, and eating them inside out: I can easily completely seal off any holes in the frames by welding caps on any open places. A good seal would be further checked by fitting an air valve temporarily to the structure, filling it with air, and putting it a tank check for leaks. Pretty fool proof. <br /><br />As far as attaching the structures to the hull, I would just glass them in like any ordinary wood stringer, except that only the bottom bar(which conforms to the hull) would be glassed. If I deem this unacceptable, I will most likely attach wide flanges to the bottom bar to give the resin more surface area to grab on to. Ideas here would be helpful. <br /><br />The idea is that since the frame shares distibution of ALL loads, it will result in a much stronger, much safer boat. Any and all comments or criticisms are welcome as I am sure I have not thought this out completely yet. If anyone needs to see a visual, just let me know and I'll send you a picture of my CAD renderings. Thanks!

 

[Always Broke]

Re: Steel Stringers?

Sounds like a good idea. Powder coating will rust though, I made a bird cage, sand blasted it and had it powder coated it took about ten years but I got rust and it was kept indoors. It happened mostly from the bird rubbing his beak back and forth across the cage but anywhere you put in a screw or chip it you’ve made an opening for rust. In a marine environment it may be a little quicker. Careful sealing up tubes and such, I’ve had one explode on me because of the heat and gases built up inside with nowhere to go. How about aluminum? The stringers in my Starcraft are made of it and it’s 32 years old and they still look good. They are kind of a Z shape if I remember correctly with the top screwed to the floor and the bottom screwed to the cross braces. By the way I can have a lot of fun with that CAD program, I’m not above using a pirated disk Let us know how you make out it sounds like an interresting idea.

 

[OldBoatGuy12]

Re: Steel Stringers?

Aluminum is out of the question. When all said and done, weight savings from using aluminum(if keeping the same design I have now) won't nearly outweigh the cost of strength. I could redesign specifically for aluminum, but that's alot more work, and alot more expensive to boot.

 

[OldBoatGuy12]

Re: Steel Stringers?

Oh, by the way...If you are looking for a fun little CAD program that is still pretty dang good, check out alibre design xpress. Its free. The demo for rhino is also very good, although you might find alibre easier to use.

 

[tommays]

Re: Steel Stringers?

I build a lot of steel Stuff thats gets really pounded in races <br />

<br /><br /><br />And you can get a far better all around stringer out of correctly used wood remneber the fact that it does have to flex without cracking is inportant<br /><br /><br />tommays

 

[Mark42]

Re: Steel Stringers?

How about using the cad to design stringers from plywood with as much open space (holes) in them as possable and still retain strength? They would be strong, light and allow the entire underfloor area to breath to help reduce any moisture that might collect there.

 

[OldBoatGuy12]

Re: Steel Stringers?

Doing something like that would take FEA analysis software, which I don't have access to. I could get a ballpark idea by just building many different structures, inputting all of the material specs, and testing them in a simple stress analysis though, same as what I was talking about in my first post. <br /><br />Tommays, remember that flex can be engineered INTO a structure also. This could also save material, and even more weight.

 

[Boomyal]

Re: Steel Stringers?

No expert here but I would think that corrosion would be the limiting factor here. Otherwise, properly engineered, I think it is a nifty idea. You'd sure have a one of a kind.<br /><br />How 'bout using stainless?

 

[OldBoatGuy12]

Re: Steel Stringers?

haha, I wish. A bit on the expensive side

 

[OldBoatGuy12]

Re: Steel Stringers?

Just going to bump this...Does anyone know of anything I have overlooked? I'm looking for comments specifically on mounting the frames to the boat. I know resin is supposed to stick to metal very well, but I haven't actually done it. <br /><br />I too think corrosion is the biggest issue, although I will NOT be putting foam in, so that helps alot with getting any moisture out.

 

[woody20]

Re: Steel Stringers?

why all this design of metal talk, stick to what we all know works, wood and expoy, it will last just as long and should out live you if you do it right.

 

[Always Broke]

Re: Steel Stringers?

I think I know where your coming from let’s see if we are on the same page. You want it strong but flexible, light, corrosion resistant, relatively easy to build or at least close to the design you already have in mind and here is the kicker cheep. Resin is cheep compared to epoxy but (there’s always that “but” and you can get a lot of different views on this) to me the excellent holding power and variations available with epoxy for specific uses out weigh the cost difference. I’ve had problems with resins as far as adhesion goes but when I’ve used the proper epoxy for it’s intended application it has yet to fail me. If you look back on the forum you will see the little flaming folder with people discussing the pros and cons of “poly VS epoxy” You might want to check that out. As far as metal goes I have no idea of expenses, quality or tensile strength of different grades I've seen welders use a rust resistant steel I believe it is called core ten, just a thought. Keep us informed I find it interesting someone is trying something different, you may be on to something. By the way, Thanks for the program advice, I got it downloaded.

 

[samsam]

Re: Steel Stringers?

Originally posted by coleasterling:<br /> Does anyone know of anything I have overlooked? <br />

Maybe different expansion and contraction rates and limits of steel and fiberglass will cause problems. Sam

 

[Bondo]

Re: Steel Stringers?

The Biggest Problem I see with your design is the Tubing itself..........<br /><br />You need Open Structure,... Not Tubing to get around any Rust Issues........<br /><br />Steel Boats were Very Popular back around the 30s to the 50s...........

 

[paulie0735]

Re: Steel Stringers?

Hummmmmm. <br />Steel boat – steel stringer! <br />Alloy boat – alloy stringer!<br />Glass and timber boat – glass and timber stringer!<br /><br />Personally I think your selling yourself way short man, you’re a smart guy with good skills, shaping timber stringers is not hard……..

 

[Solittle]

Re: Steel Stringers?

Well lets see - - - - they build aircraft carriers out of steel that lasts 40+ years pounding around the globe in salt. It oughta work for you.

 

[OldBoatGuy12]

Re: Steel Stringers?

Update: I just completed a preliminary stress test on my regular carbon steel design. This particular piece was one of the two middle legthwise stringers. Turns out that it is much stronger than what I anticipated. It it was sitting right around 32-33lbs when tested. Looks like a redesign is in order...Got to lose at least 10 lbs off it. I have videos in avi format of the testing if anyone wants to see.

 

[flashback]

Re: Steel Stringers?

sounds interesting! how well would the steel bond to the glass hull? my concern would be that the stringers would come loose.

 

[Bondo]

Re: Steel Stringers?

As far as water getting into the tubes themselves, and eating them inside out: I can easily completely seal off any holes in the frames by welding caps on any open places. A good seal would be further checked by fitting an air valve temporarily to the structure, filling it with air, and putting it a tank check for leaks. Pretty fool proof.

As I stated earlier,.......<br /><br />This is your Design Flaw...........<br /><br />"T" bar,.... "I" beams,...... You'll Have to use some sort of Structural Steel,......<br />Tubing is Not Foolproof,..... It Will Leak,+ Rot from Within..........

 

[OldBoatGuy12]

Re: Steel Stringers?

In resoponse to Bondo: I am very confident that it will not leak. Completely sealing a steel structure is alot easier and less prone to failure than fiberglass and wood, in my opinon. I have, however, come up with another design I will be testing soon which uses two lengths of angle iron back to back with strap in between them at strategic places. I am thinking this will be much heavier and have less torsional rigidity though. We'll see.