Bedding stringer question

Manny I would stick to what you found on here,if there is a problem it would have come up with the number of boat that have been restrung...I think you have been talking/reading old school ways.
I had dabs of foam under my old stringers too I think they used it to hold them in place while they wrapped in glass back in the day.
Others will chip in with there thoughts,welcome to CCF.

8122pbrainard wrote: That layer of foam was simply a gap filler due to CC's poor workmanship!!

Come on now, Pete- thats not fair! Yes, the foam was used as a gap filler. The whole point is to reduce stress concentrations created by high/low points between the stringer and hull. Getting a stringer shaped perfectly to the hull is a nearly impossible task, as Im sure you know! Thats the same reason most of us are bedding with mat or thickened resin. We have the benefit of taking our sweet time when rebuilding... the factory had a pace to maintain and its not like they were cutting the stringers with a CNC machine.

While the foam seems to be an acceptable approach, there is no question that bedding the stringers with mat or thickened resin is stronger and more rigid. This may in fact be the way to go in areas that deserve more support (like primary stringers). Pieces that dont need such rigid attachment may be candidates for lighter bedding schedules, or none at all.

TRBenj wrote: 8122pbrainard wrote: That layer of foam was simply a gap filler due to CC's poor workmanship!! Getting a stringer shaped perfectly to the hull is a nearly impossible task, as Im sure you know!

It's not as "nearly impossible" than you may think. If you really want to get close, you place carbon paper betweenthe hull and pre shaped stringer. Move the stringer around some and the carbon paper black spots will show the high spots.
BTW, carbon paper isn't easy to find anymore. The last time I got some, I had to buy a big box of it!!!

He is not reading old information or bad information. Firmly bedding a stringer to a hull is in general bad form in boat building or composite construction in general.   Relative to a flexible hull a stringer can be considered essentially rigid and some where between flexible and rigid a bunch of energy must be dissipated or it will cause problems. Some of the methods and materials described here are far from best practice.. however there are a few things that keep this from being a critical error. One.. the hull on these boats in general is very think relative to the size of the span between the supports, this creates less flex and more ability to withstand the stress concentration. Two, the foam being poured into the hull further limits the motion of the hull, making the fact that the stringer is rigid less of an issue. Three the lengthwise orientation of the stringers is perpendicular to the direction of the cyclical loads and the full bunks generally employed on trailers and lifts with these boats are helpful. I do believe that should a few of the rebuilds I have seen on here end up on a rock at somepoint the point loads would not be tolerated as well as a more flexible mounting system.. but with a foam back up even that might not be critical.

I have used foam stringers in my rebuilds so I have been less concerned about their absolute rigidity, but the vertical fiberglass walls are also a significant rigid load. I deal with this as best as I can by tapering the joint with overlapping layers of glass over 6 inch area.   I have also gone to great lengths to not bed or very rigidly mount the various supports I have added perpendicular to the stringers… and certainly to keep them close enough together to limit the hull flex..

Anyway if you were building canoes, sailboats, or any craft where you were concerned about getting as much strength as possible with the weight of the materials you were using you would indeed not try to create any rigid (relatively) structures .. but with an old correctcrafts these rules are much less rigid because the boats are much smaller and more rigid.    It is fallacy to declare them out of date though anyone coming through auburn I can show how firmly bedding stringers in a classic sutphen will cause some serious trouble.    

JoeinNY wrote: BTW, Sutphen didnt screw up the stringer system.. the "professionals" who were well paid to restore the boat did.. now it goes into the backlog of things for this hobbist to repair.

Joe if you have time post some pictures of the problem boat it would be good to see the issues.

My thinking on this is back in the day the gap was good under the stringers or filled with foam, as the stringers soaked up water through the polly they would swell stressing the glass around it till they rot and letting off the stress + us in cold part will have freezing too adding more stress.
When I took my old stringers out I found places where the glass had parted from the hull along the stringers.
As you point out these boats have strong hulls and thick glass so flex is not a major problem unlike other makes.
The way I see it now sealed up cpes wood stringers and epoxy glass in, swelling is not going to happen (this is dependent on how well its done in the rebuild )
But if your going back with polly then a gap would be good to let any swelling over time have somewhere to go if your using wood stringers.
From a UK hobbist

daddyo wrote: Interesting thread for someone like me who has a stringer job in his future. So are the stringers more to support the engine and floor, the hull, or both?

It's a weight and stress distribution "system". The stringers and hull work together.

Manny, if your boat has spent most of its life in salt water you may find the stringers are good just some rot on the floor side where the rain has been,they will be wet and all fixings will be bad but salt water soaked wood will not rot like fresh water soaked stringers.

Here is a pic of mine over 30 years old but used in salty water

8122pbrainard wrote: Roger, You never cease to amaze me!! Man, you are more OCD than I am!! But, I also feel you may have more free time available than I do!!

Pete when I have one of my boats finished and it comes somewhere close to your CC I might just match you on the OCD top table   

Manny wrote: I wish mine were like yours, at least port side. Primary Stringer port side is completly rotten, is coming out in flackes. Secondary was not that bad, but not close to yours. If salt water doesn't rot wood, it was fresh water from washing the boat and rain. Starboard side doesn't seems to be that bad. I will find out this weekend. Hopefully, I may use starboard stringer as template. Did you set (bond) the stringers to the hull with epoxy and cloth?

Manny sorry to hear you have no luck with your old stringers, its mostly cold here so wet wood tends to stay wet............I guess it’s much hotter in your neck of the woods, as for bonding stringers to the hull I can’t help you on wood as I used Glass fibre sheet and fabbed up mine, as at the time it’s all I could find here ,Joe and Tim have used Cooser Board as a core in there builds, which is much better than my way as the weight is much less.
Joe showed me some at GL so I can see what I miss out on living in the UK

The way I see it using wood the key thing is to seal it up well with CPES if you can leave the wood out in the sun to warm up but keep turning it to warm all round then coat with CPES it will help to penetrate deeper, as for bonding the underside of stringers to the hull it’s your call........ to me the stringers are not that wide and epoxy mat ,cloth , and most fillers all give epoxy a bit more flex but also seal the hull stopping water hydroscopic into that area keeping it all nice and dry, each side will be flared then bonded with cloth build up again it’s your call if you won’t to add biax too. The sides will be in as one to the hull so the gap of 1’1/4 on the main and ¾ on secondary is small along with a thick hulled small boat everything works out, yes if you pin point load the hull in some way a problem may occur but it will on a unbonded stringer boat too just down to luck.

If you won’t to try something different and bed the stringers in you could use 3m 5200 but I think it’s Polly based so will not bond that well to CPES/epoxy or bed using 3m 2216 B/A flexible epoxy, whatever way you go it will be superior to the way it has been for most of its life.

Keep us updated on your build, Pictures please lots to read up on here and share your findings with us.

This is how thick a 77 SN hull is around the drain hole,13.25mm
Cheers Roger.

Hi Manny, yes its a real pain for us living outside the USA getting supplies at good rates,may be worth dropping a line to US composites to see how much to buy and ship everything you need,but as always need to check how much import tax your Gov adds on, here its + 20% on personal import.
If they don't ship I have used IPSthey will pick up and ship to you and can give you a US post address so you can buy from number of suppliers they will pack them all up at there hub then ship to you,and you fill out the shipping doc's online and send to the supplier to print off and stick on the box
But if Poly is your only way you need to seal your wood with something other than CEPS as its not a good poly to epoxy I would look at a Pre-Cat or AC sealer for Poly.
As for setting the stringers in look at the old glass where it meets the hull once you have ground off the gel as one of the guys said it needs to be like a honey colour, some here have left the bottom 1/4 inch on the hull to give you channels to lay the new into or just leave tabs at each end and 1 or 2 along the stringers to line up the new then as you say brace the tops so there upright some weight on them to set is good but don't go overboard with it,we do like to see new things used so far its been kitchen sink to beer packs

I knew sanding down would be a messy task, but wiith 90+ F temperature, I was exhausted and it was only the port secondary stringer. After all that labor, I'm re-thinking usimg epoxy, not poly. I ordered the cloth to US Composites, so I will call them for resin quote.
My doubt with poly is if after encapsulating the wood with cloth and resin, does humidity builts inside or it just absorbs water. If the problem is that absorbs water, does a final coating of epoxy over the poly solve the problem.

In this forum, there is a materials list which says: 5 gallon 3:1(medium) + another 5 galon of 4:1(fast), total 10 gallon. Does the stringer rebuild, including the floor, takes 10 gallon? Why not using resin 2:1 (slow)?

Greg, thanks for your tech tip. I did just that. Also, I did a jig. I will look for those nice notch and reliable jigs.