TOPIC: "Book" pt 2 questions

Been reading the restoration "book" on the main, and I am rather confused in the reads in part two.

just made some c/p notes...

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Floors

Not yet written.
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Stringer Repair

Not yet written
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Note # 1
Plywood will also work just fine as a core material. It is much more stable than solid wood. It's also available everywhere. It's cheap. And, though it's harder to shape than urethane, it's not beyond the ability of most DIYs. The problem with plywood is that it's susceptible to rot.

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The only medium for stringer construction I absolutely do not recommend is solid wood. Because plywood is made from alternating grain directions of lumber, it is orders of magnitude more stable than solid wood. Solid wood will expand and contract with humidity and temperature variations and can ultimately fracture the fiberglass lamination to the hull. Do Not Use Solid Wood As A Core Material!
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Cardboard is actually a fantastic core material. It is light weight, readily available everywhere and so cheap it can often be found for free. The downside it that it is already rigid enough that you may have a hard time getting it to conform to the shape of the hull.

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Now on this read, it basically states the core of the build really as no influence on the strength of a stringer, being about half way through the cobia's over haul this read has left me confused.

should we have just top off the stringers that where there and vacuumed out the shell which was nothing other than crumbled sawdust left by the plywood that was used and recapped the stringers.

in order to redesign what the stringers where we used cardboard blanks from the box factory to get the curve of the sole to the level of the interior cabin "deck", should we have just used those instead to make the stringers. and just recapped that and would it have been better for the cobia.

according to the read by my understanding, if you made what they call a super truss out of Marian plywood and ran rove and mat 1/4 thick down both side it would work better than the doubled up plywood that they used to begin with.

will the "book ever be finished, got some pointers on the stringer "replacement" that hasn't been written yet as that what we are doing.

should of kept the funliner, you wouldn't have to worry about how to build stringers.

Hi Houston,

I'm sorry I haven't finished those elements of the book yet. I'd hate to think you're at a sticking point because I got distracted.

I'm in the process of reformatting and, to a degree, rewriting what's already there.

I should clarify a number of points.

The "core materials" section was written with transoms in mind. Not necessarily stringers. I haven't made that clear yet, so you have my sincere apologies if that's been a point of confusion.

Part of the reason the Stringers and Floors sections haven't been written is because there are many variables involved with both of them.

With regard to stringers: Yes, the shape of the fiberglass stringer does the heavy work of stiffening the hull, while the core of a stringer is essentially a plug on which to lay up the fiberglass.

*except when it's not!*

Yes, yes. I know that's a horribly confusing to sentence to follow the preceding paragraph, but here's why it's necessary:

Not every boat builder built boats the same way. I have seen stringers that had no core material whatsoever, and I've seen stringers that were basically pure solid wood that were only tabbed in place with a few strips of glass mat.

Your boat is somewhere in the middle. If you choose to reuse plywood as a core material, make sure the plywood is sealed with resin prior to laying up glass on top. Make sure to really soak it into the end grain of the plywood because that is where it's going to be "thirstiest".

sidenote: penetrating epoxy, which is simply regular epoxy that has been thinned down to a watery consistency is designed to leech into wood. It's made to seal it. It's great, but there is a downside to it.

If you use penetrating epoxy to seal the wood, you'll have to use epoxy to lay up the fiberglass. As mentioned in other parts of the book, epoxy will stick, well, like glue I guess, to cured polyester resin. HOWEVER! polyester resin doesn't adhere very well to cured epoxy.

Since penetrating epoxy is actual epoxy, laminating fiberglass on top of it with polyester resin will have a questionable bond at best.

If you choose to use an non structural element as a core material, such as urethane foam or a cardboard form, make sure you laminate the fiberglass thick enough that it can do it's job. (As a rule of thumb, 1/4" of thickness per 10' of stringer length.)

Another sidenote: Make sure you don't just use mat during your layup. Mat is weak by itself, no matter how thick the lamination. You'll need to alternate between mat and cloth, or mat and roving when laying up the new glass. It is the cloth (or roving) that gives fiberglass its strength. The mat is a binder between layers.

and lastly, always start and finish your lay up schedule with a layer of mat. Mat, while not structurally sound by itself, is more waterproof than cloth and roving, and it creates better surface contact when mating a stringer to the hull.

Also, make sure the mating surface (the hull in this case) is free of any contaminants (grease, dust, dirt...)and the surface has been abraded to bare glass, or else you risk having a poor bond to the hull.

Lastly, whatever you use as a core material needs to be bonded in place to the hull. If you are using a core that is susceptible to thermal expansion like plywood, bond it place with an adhesive that has some "give" to it, like 3M 5200, which will remain pliable after it cures.

I hope that answers your questions and yes, I promise, the book will be finished in its entirety. Until then, feel free to email me directly if you have any questions and I'll do my best to answer them.

Good luck and let me know how it turns out.

Jamil

P.S. Part 2 of the book has been moved back to the research/restoration tab in case you can't find it anymore.

na, your topix book didn't bring the project to a halt, I did and everything around what happen brought it to a halt.

where it got left off at was two out of the four damaged/destroyed stringer have been replaced, everything was all guns to the wall till while removing the remains of one of the other stringers off the stern the stern got "impelled" with a saw blade and the wood in the stern kept the saw blade. which for me it had the next step "replacing the stern" written all over it.

so, then there was this slight delay...

keep reminding myself that now is as good of a time to get the cobia finished, but my short fall is I can't find any of the project pictures.

one of the problems with the hollow stringer concept would be simply where would one anchor the deck?, and this is where one of the purpose for the wood in the stringers lay and the #1 cause for stringer failure is the screws from the deck breaking the seal of the fiberglass that surrounds the wood. the wood in the fiberglass is not the problem the type of wood and how it was prep is, and of course "air" in general.

what we have done so far, was uncap the two stringers and replaced the sawdust that was plywood with a marine grade solid board on advice of a builder. the boards where sanded down and then "treated" (how I don't remember) and then completely coated in risen and inserted in to the slot where the old stringer was and re-fiberglassed. seeing the decking issue we also installed a sider to the top of the new stringer that is bolt to it and sealed separately just for mounting the deck to the stringer.

the cobia is not a "little" boat btw.

edited: and it not a BIG boat either, it is what they use to refer to as a pocket cruiser would be the best description.

The cobia project first step this year will be the removing and replacing the stern, what I am looking to learn about is Coosa Board, seams logical if I am going to get back in there and I have to move the motor and remove all the components for the motor out of the stern "pocket", by the way Coosa Board sounds it would be only one stern "overhaul" for a long time to come.

is there any special blade to cut it, does it cut much like plywood..

probably again, we will have to use the blank sheet cardboard from the box factory to make a template to rebuild the stern.

Hi Houston,

In regards to Coosa Board; it's a great product and, if installed correctly, should outlive the boat.

Here are the drawbacks:

It's really expensive. For an average transom, you'll need to build up thickness by laminating two (or more) plies together. The thickest you can get it is 3/4" (maybe 1", but I don't think so). That means you have to buy a full sheet in order to have enough material.

I was looking at the display this morning. A full sheet of 5/8" #26, which is what I was there to price, was over $300.

From a long term perspective, it's a permanent fix; it's worth it. From a practical standpoint, it's hard to spend $300 when you can spend a third of that and make a perfectly sound transom with marine plywood.

It doesn't cut easy. It will kill your blades. The company says you can use wood working tools with a new blade. They say that because most people have wood working tools laying around somewhere. What they should actually say is that metal cutting blades will last a lot longer slicing through coosa board than wood cutting blades - from experience.

Regardless, you'll be spending some money on new blades by the end. Again, the pay off is a permanent fix.

It's made of continuous strand fiberglass that is lighter and stronger than plywood, perfectly compatible with any resin, and nearly impossible to degrade.

It varies in density from the extremely light weight Nautical #15 to the extremely strong and heavier, bluewater #26. For a transom replacement, there are variables. The most important of which is the strength of two fiberglass skins which make up the transom "sandwich". In the end, stronger is always better.

One last suggestion.

If it's available to you, think about using door skin (Thin veneered plywood) instead of a cardboard template. It's harder to cut (I use a jigsaw and then either a razor knife or some aggressive sand paper to shape it), but you get a more rigid and more precise template.

If you can hold off for a week or so, the transom replacement pictorial I'm working on is going faster than expected, it should be up next week. It covers everything in this thread except I'll be using marine ply. Other than that, the replacement is identical.

Dude we gotta work on a profile picture....I think I may have something right up your alley.

right now the project can't be touched, it buried in snow, BUT I think your understand why I am looking at coosa board the only excess to the "lay-up" involves unbolting the motor and moving it forward (on skids) after making all kinds of measurements of it's exact position and removing the "through-out" and all the extra stuff that is mounted on the stern for the operation of the mechanical's.

least to say I don't want to have to do this ever again, but if you got a better course of action I am all "ears" and can follow a nice procedure list...

did the Pryor owners of you current affair happen to explain why the keyhole in my handcuffs where welded over...hehehehe