Core / Stringer question

I'm currently rebuilding a boat that probably should have been taken to the dump (my family accuses me of enjoying working on boats more than using them). It's an 18 ft. bass boat, and will ultimately end up being my son's when he graduates from college in a couple more years.

The hull originally had 7 stringers , and 3/8" PLYWOOD core. The core was non-continuous, it was only laid in between the stringers with about a 1 to 2 inch lengthwise gap between the core and stringers. The factory did the best they could to get fiberglass cloth to follow that contour, but there were a lot of areas where there was a gap between glass cloth and the hull below it. Here's a rough sketch of how it was originally, as well as a photo.







Instead of using plywood as core material I've used 3/8" balsa, and I've laid it in continuously instead of just fitting it in the areas between stringers as before (That has to be significantly stronger than it was originally). Now that it's time to start fabricating stringers I'm pulling my hair out trying to decide if I really need to use 7. I'd like to go with a center stringer along the keel and one more stringer on each side (in the same location as the 2nd one from the left in my sketch, and it's twin on the starboard side). I'm not concerned about the stiffness of the deck, as I'll be glassing the top and bottom before installing it, the original just had csm on the top and nothing on the bottom.

Opinions?

 

The gaps were intentional and prevented stress risers and "oil canning" from occurring in the hull shell. The plywood wasn't so much a core as it was a former to lay the CSM over. The balsa will serve the same role, though likely will not last as long as the plywood did.

Without a year, make and model, it's tough to tell, but most bass boats have a fair bit of deadrise and are intended to carry huge bottom loads, from the massive weight and HP hanging of their butts. This is why you have so many stringers and why they are so deep (very important). You'd be well advised to replace the stringers as they were, with at least as much material as you've cut out.

Also insure good tabbing with sufficient overlap (you can't have too much), as this is an area where the manufactures often skimped. Coat the plywood sole on both side with resin well to help seal it and make sure you have limber holes to permit moisture to drain aft out a drain.

 

It's a 1993 Vision bass boat, they were only built for a few years before the manufacturer went out of business.

I've always heard that end-grain balsa is a far better core material than plywood for several reasons, one of which is that water migrates laterally through balsa (with the grain being vertical) much less readily than plywood (with the grain horizontal). Also, when properly installed balsa can last a very long time as evidenced by my 1979 Checkmate which I rebuilt - every last bit of wood in the hull was mush but the balsa core was still perfect. I've heard all the horror stories about certain high performance boats with rotted balsa core, but in talking with other Checkmate owners who have rebuilt their boats I've only heard of one bad core (and that was likely because of multiple screw hole penetrations into the core - so balsa can certainly stand the test of time if installed correctly). In any case I'm using epoxy and have done a good job installing the balsa to ensure that water can't get to it, so I have every expectation that it won't be an issue in the future.

 

Listen to PAR and you won't go wrong. He (and others here) taught me a lot as I was doing similar work. You can also make stringer formers out of closed cell foam such as is made by Dupont. I'll post a link to another thread where I posted some photos. The stringers and floors that I made up with those foam formers have been in there for 4 years and are as solid as the day they cured.

Just click below...BTW, after I did this job I learned that I should have used fiberglass cloth rather than biaxle mat in this application. The mat is OK but the stringers are somewhat heavier than they need to be. Pound for pound they are not as strong as stringers built with laid up cloth but I overbuilt them anyway so they are OK. Bottom line use woven roving or cloth.


http://www.boatdesign.net/forums/fi...building/yet-another-stringer-post-28361.html

MIA

 

Definitely.

I looked at the stringers you built and will certainly consider that as an option. I was planning on doing wood stringers (with glass cloth over them of course), which I feel should last a long long time given that I'll put more care into building and installing them than the manufacturer and I'm using epoxy for everything instead of polyester. But I must say that yours definitely look nice.

 

Balsa has a huge (bad) reputation issue and nothing approaching plywood, though you can hear good and bad about both. Most agree in decks and above the LWL, it's a great core, but below the LWL, well this is where opinions will dramatically go negative on balsa.

Balsa is just a core material, but plywood is a structural material, so the engineering approach are different. It doesn't appear the plywood stringers were structural in your boat, so you can use anything you want for the core, including your aunt Millie's mash potatoes, so long as you have sufficient tabbing over lap, contact with the hull shell and bulk. These types of stringers get their strength from the fabric, not the core. The core can rot away to nothing after the fabrics cure, with little change in strength and stiffness.

Epoxy is a good choice and if using epoxy, save yourself lots of resin by eliminating mat from the laminates. You don't need it, it's has no real strength and is just a bulking agent used in polyester laminates. Mat can be used to help prevent print through on finished surfaces, but you only need a very light top sheath for this (less then 2 ounces).

 

OK, then why are there so many warnings about balsa rotting?

That confuses me. Is it just some balsa core that is badly built? That could be the case since balsa is glued together. Maybe some use greatly inferior glues?

wayne

 

If that is the case, I read that balsa core often gives a much greater strength to weight than foam ....

Is balsa core a better product if made correctly?