bedding stringers

[fibersport]

I've been doing a lot of searching and reading here and I think I know what I want to do but thought I would run it past the experts. I have a Coronado Cardel, which is the V-drive model (not the I/O) and it's in dire need of all new stringers. Most of what's left is black and crumbly except the engine stringers which are almost good enough to leave - but they are not. For those that don't know, the engine stringers are 2x6's capped with a 2x3 and tabbed to the hull intermittently for about 6 feet. The rest of the boat has 1/2" plywood for stringers, this was only tabbed on one side with a single layer of woven roving.

Briefly, my plan is to replicate the engine stringers but for the rest I'm planning on 3/4" plywood tabbed on both sides. I may add a secondary stringer but by using 3/4" rather than 1/2" I've already strengthened it. While I haven't researched the materials, it will be either polyester or vinylester. All wood will be pre-coated with resin. Since the boat was designed with foam, it will be going back in to add the stiffness, plus I like the idea of tying everything together.

Now finally here is my question. I've read in a design book that stringer should not be in direct contact with the hull and should actually float so hard spots are not formed. I've read in some posts here basically the same thing. I was wondering though if it wouldn't be a good idea to use a bead of 5200 between the hull and the bottom of the stringer to provide a flexible layer. I would apply it to the stringer first, lay it in place and clean up any that squeezed out, then use the peanut butter to form a fillet before tabbing everything in place. I know the factory didn't do this but since I don't have to have my repairs approved by bean counters I can afford a few tubes of caulk if it will help. Thoughts or comments?

 

[ondarvr]

It really doesn't make a difference what you use, 5200, putty, foam, nothing, just so the wood doesn't come in contact with the hull. But...almost no small boat builders space the stringers off the hull in the original costruction, they just set them in place and glass over them, so whatever you do will be an improvement.

 

[Woodonglass]

Yup, I agree with ondarvr.. You might find this beneficial...Fabricating Decks, Stringers, and Transoms

 

[Ned L]

As a point of clarity, the engine bolts to "engine beds",.. not "strangers".
I have a question too, relating to this ...."stringer should not be in direct contact with the hull and should actually float so hard spots are not formed."...., but I'll probably (properly) start another thread.

Good luck,.. and enjoy your project!

 

[mickyryan]

its not that the stringers shouldn't be touching as much as you could never make a perfect cut to evenly touch therefore using a bedding material makes sure that the weight is evenly dispersed and not on hard spots use pb its good and hard and makes for solid bedding for stringers.

 

[fibersport]

Hmm, some good comments but a little contradictory? I will agree that no matter what is done, it will be much better than original. In my thought process, something "soft" between the hull and the stringer material would prevent the two from touching and would also force all loading to be dispersed through the fillets and then into the stringer itself. This is starting to remind me of a uni-body car where everything is structural.

 

[JASinIL2006]

I don't think the bedding needs to be soft at all. You just don't want small point of contact between stringers and the hull, or you will have cracks form there. You want to disperse the points of contact as much as possible. I've read of folks who tried to have their stringers 'float' over the hull ( resting them on small pieces of foam, for example) and then using the tabbing only to connect the stringers to the hull. That always sounds like a lit more work than just bedding the stingers in something and then covering with tabbing.

 

[jbcurt00]

Yep ^^^

Another advantage of using polyester resin based filler (peanut butter, or PB) for fillets is that it kicks off and cures fairly quick and is nearly immediately ready to bond w the poly resin and fiberglass tabbing. Many alternative bedding materials arent very compatible w poly resin and take a long time to cure. Poly resin doesnt bind well to Liquid Nails or similar, and takes a long time to off gas and cure. Not sure how well poly resin bonds to 5200, but 5200 is running and does take some time to cure.

Good luck w your project.

 

[Ned L]

Considering things have drifted a bit anyway, I'll make my comment here on bedding stringers. .......
I don't at all understand the idea of "not allowing them to come in contact with the hull", and "letting them float", and " setting them on foam", or anything similar.
The ideal situation is for full, solid, non-flexible contact between the stringer and the bottom of the boat. The intent is for both to be bonded as a single unit. As 'most' people aren't capable of shaping the bottom of the stringer to a perfect profile to fit the bottom of the boat (and even for those who can and do), you want a solid non-flexing bond between the two, so "PB" as jbcurt100 calls it (thickened resin or epoxy) is the easiest way to ensure this. By using a solid, hard setting material it really doesn't matter if you have a really nice fit, or contact at only a few points. Once the 'bedding material' (PB) is cured it makes no difference how close the wood is to the hull.
The idea of putting something like "foam" under the stringers is really the opposite of what what you want to achieve. Besides, if you did set the stringers on foam and "IF" the foam did allow the stringers to "float", the glass tabbing between the stringers and the hull would crack and shatter very quickly with any movement.

You want the stringers and floors to form a solid matrix that are immovably bonded to the hull. (I did use the correct term "floor" here. ----- In a boat, a "floor" is not what you walk on. A floor is a structural member that runs athwartships and ties the two sides of the hull together across the top of the keel. Stringers run for and aft, floors run side to side. Bulkheads can run in either direction and are the "walls" that divide the boat into compartments. Bulkheads can also be structural, but don't have to be. Floors are strictly structural members that are below the deck or sole (the two things you walk on).

 

[Woodonglass]

Here's some great info on the topic by some of Iboats leading experts...http://forums.iboats.com/forum/boat...ir/471668-hard-spots-on-hull-bedding-question

 

[Ned L]

Here's some great info on the topic by some of Iboats leading experts...http://forums.iboats.com/forum/boat...ir/471668-hard-spots-on-hull-bedding-question

Hmmm, ......... I agree with a good bit what is there, can't say I agree with all of it. I think I'll just let people believe what they want, and of course there are always lots of solutions to a single problem.

 

[Woodonglass]

I've read about and researched this topic for 6 years. What I've learned is that the Glass and NOT the wood is what gives the stringer it's structural integrity. That's why most of the new stringers are most often a Pre Molded Fiberglass grid with no core at all. Some of the MFG boats in the 50's and 60's did basically the same. If the laminated glass is at least 1/4" thick and the base is 3-4 inches wide then for boats under 20 ft they will do the job. My understanding is that even though Peanut Butter cures to a very hard component it will still flex more than wood. Wood in direct contact with the hull will cause cracking and checking if the hull is being pounded by waves, wakes,and heavy chop. The most important thing when it comes to fabricating the stringers for a fiberglass boat is the attachment to the hull and the width of the base. the larger the better. That's why most recommend extending the glass from the base of the fillet 3-6 inches on both sides. The wider the base the better distribution of the load. That's all this Old Dumb Okie knows about it. I learned a lot from those "Experts" (Mostly they are Professionals working in their own shop or in the Fiberglass Boat or Resin industry) Mostly all of it made total logical sense so that's why I've been carrying the torch for NOT allowing the wood to come in contact with the hull. Like ondarvr says, A LOT of the Manufacturers just throw em down and glass em in and they seem to sell a LOT of boats. I'm thinking that most of the guys here on the forum try to do a better job than some of the work the MFG's did!!!

 

[fibersport]

All good stuff here. I really agree with the comment in one of the posts that bonding the 3/4" plywood really won't make a difference compared to 6" tabbing. I'm sure glad I'm not doing this now and have all winter to think and plan. Woodonglass, I think you summed it up perfectly, thanks.

 

[MILKWEED]

No matter how you set the stringers its a good idea to sand the edges after they are cut, just to eliminate the possibility of sharp edges if they do contact the hull at any point. Just my .02 worth.