Help With Rotted Stringer

[jason58492]

I hope someone can give me a good answer to my question. So here it goes.

I have a 1979 18" Caravelle that I bought to restore. The floor was rotteed so I pulled up the floor and noticed that both stringers on the drivers side were rotted. Only 1 stringer was useable. I cut the other 3 out and instead of replacing them I used 2x4's in a grid pattern that are attached to the bottom of the once existing fiberglass floor with screws. The floor was glassed in so when I cut it out and left a 1 inch flange aroung the entire inside of the boat and then screwed the 2x4;s in to the flange. It looks good and the 2x4's do run the width and length of the boat with the same priciples as the stringers do except they dont sit on the bottom of the baot.. How important is it that the stringers be fiberglassed to the bottom of the boat? My opinion is that it is sufficiant strength. Any opinions?

[SamSam]

The stringers tied into the hull and floor creates a torsion box, a sandwich of sorts with the two 'skins' of the hull and floor separated and connected by the 'core' of stringers. The boat is only strong enough for safe use when all the parts are working together which means securely connected together. Your hull without the stringers will change shape sitting on a trailer or even sitting in the water. Your floor will probably disconnect itself from the 1" flange real quick. You are not 'restoring' the boat, you are 'rigging' it. That in itself is OK, but it still should be done so it works. I don't think what you have described will work for very long, if at all. Sam

[tja]

Please don't take this wrong but am I the only one laughing at this post? Sam has it right. Everything must be glassed in place as a unit. Don't half ass it. If you do make sure you where a good life jacket. Good luck. Sincerely, Tom.

[bilgeboy]

Ugh. I sweat this kind of thing. When you watch your boat in the sling, ready to launch, you can sometimes just wait for it to snap in half in front of all the audience, and you know your insurance company is not going to help.

If the gap is real small, like less than 1/16", I'll bet you could bond those stringers down with a thickened epoxy, injected via a syringe and maybe a short rubber tube. A "Tumi" syringe will hold 60ml, and can quite easily squirt a heavy epoxy / wood flour mix anywhere you can fit the tubing.

Not just a cheap fix, but I hope this might help you. Kind of an like an injected fillet. Obviously, your repair is better than what you had, or you wouldn't have bothered advertising your handiwork here. I'm just trying to help think of a solution to make it better yet.

Any other ideas?

Mike

[SamSam]

There is a lot of threads here about rot problems and how others fixed them.
http://boatdesign.net/forums/search.php?searchid=709030
This is another good site. Sam
http://forums.iboats.com/

[jason58492]

I appreciate all your help, but does it help that I will be glassing the plywood floor in and I will extend the glass up the sides of the hull? I understand what you are saying about the stringers holding everything together, but isnt that what I am doing just instead of stringers being glasses to the bottom of the hull I am glassing them to the sides of the hull. In theroy I am accomplishing the same thing, just in a different way. Am I correct in my thinking?

[jason58492]

To me with the 2x4's going length and width of the entire boat and being fastened to the hull itself and with the floor being fastened to to the 2x4's and also having a layer of glass securing the floor to the hull that would be enough to hold everthing together. I cant see how any flexing of the hull could be possible. If anything to me it would be stronger as I have added more braces.

[tja]

Hello, You asked for thr advise of people on this site of which many have years of exsperience. I've been building and repairung boats for over 25 years as have others here. Take our advise and do it right. Sincerely, Tom.

[SamSam]

The part of the hull beneath the floor seems like it will have no support and that is the part that might be too weak. To keep boats light, the hull and the floor aren't built strong enough to be self supporting by themselves. The bottom of the hull without stringers or some kind of support will flex as it smacks into waves and eventually start cracking and/or flat areas will become concave and the whole aerodynamics of the hull won't be the same so handling and efficiency will suffer.
How far along are you? Has the floor been permanantly attached to the 2x4's yet? Sam

[jason58492]

In regards to your comment about not having any support on the bottom of the hull I did think of that and that was my main area of focus when using the 2x4's. I have 2 (2x4"s)that run the length of the boat, these will be glassed in and the I have 2x4's that extend straight up and and are attached to the wood grid that I have for the floor to be attached to. To me I think it probally would have been easier just to put new stringers but I feel that is one of the reasons the floor failed because of the lack of support the floor had.

[Willallison]

The process of designing what are known as scantlings for a boat (the amount and placement of the materials that you build it with) is a fairly complex business; and there are a number of ways to achieve it.
But essentially, when you design the bottom of a boat you consider (amongst other things) the longest unsupported panel length. In other words, the distance between your stringers (running for and aft) and/or your bulkheads, frames and floors (runnig transversely, or across the boat). The result of all these calculations will tell you how thick the panel - the bottom in this instance - needs to be in order to stand up to the pressures that are going to be exerted on it. The stringers are there to support the bottom - not the cockpit sole above it. It's fine if you want to extend them up to support the deck as well as the bottom, but they MUST be glassed to the bottom.
Otherwise, sadly, all your work will be in vain - your boat will at best flex to the point of getting stress cracks and at worst sink.

[SamSam]

Originally Posted by jason58492

I feel that is one of the reasons the floor failed because of the lack of support the floor had.

Well, that may be, or maybe it just rotted. The boat is 27 years old. Maybe what you tore out wasn't original anyway. Post some pictures. Have you checked the transom for rot? Sam

[bilgeboy]

It wouldn't sound like that much work to back the screws out, remove the stringers, and bed them in a thickened epoxy, and pop the screws back in. Would that solve the problem that so many are seeing with the repair as it now stands? Any glass work on top of that would be icing on the cake.

Pics would be really helpful both to get precise advice and to let those of us trying to learn more about this kind of thing visualize the situation.

Mike

[Willallison]

Generally speaking, the stringers need to be effectively fastened to the bottom - ie - they become a part of the bottom. In many cases, the timber (or more often these days, foam) that makes up the bulk of the stringer is just there as a core: something to help keep the shape of the FRP skins, if you like. So the strength is in fact in the combination of core and FRP. Timber stringers, simply glued to the bottom will likely part company over time.
There are plently of books out there that describe the correct way of constructing framework. Dave Gerr's Elements of Boat Strength will certainly suffice for the job being undertaken here. And it doesn't require a detailed knowledge of all the engineering invovled in calculating the correct scantlings

Do it properly. Do it once....

[bilgeboy]

Will,

Of course wood stringers will part company over time. That has already happened to him. He already has this problem, and replacing with wood guarantees that this will happen again...eventually. It happened to me, too. I bet it happens to every wooden stringer out there!

But lets face it...some stringers are "active" and some are "inactive". The wooden stringers in general are active, and the foam cores are inactive.

I don't think we've really addressed the situation. He's putting wood down to strengthen the hull, and it seems like most are arguing that without these stringers fastened to the hull, he is looking at potential hull damage. Can't he fasten with epoxy, and use the innate strength of wood to create active stringers?

If he encapsulates with enough glass and resin, he may create an indestructable stringer, but that is up to him.

If it were me...I'd wrap 'em with so much glass that fungi would only lighten the hull over time, and I'd make more knots with age.

Thats a good place to be!

Mike