1970 22'2 FLATBACK REBUILD

[LilRichard]

I would have just layed new glass across the top.  How did you glass the old glass to the stringers?

[wysongja]

we wanted to have somthing solid to screw the floor to.  By placing the plywood there it allowed us to pack the stringers tight with foam.

[slippery73]

Seems like a smart idea to me, I like the idea of having the plywood there after the foam is poured. You aren't going to have any dead spots and it should be nice and dense done this way. I want to do the same type thing when my deck goes back in, cut holes in it to pour foam in then plug it from the bottom like you did here. I like the idea of having the foam bond to the bottom of my deck rather than just having my deck lay on top of it. This will produce a type of monocoque structure that disperses loads, vibrations, sounds etc. much better. Thats why the boston whalers have done so well.

[LilRichard]

I would have just layed new glass across the top.  How did you glass the old glass to the stringers?

This is an important part - I think the wood is a good idea too - but really curious as to how you reglassed because there are ways that will work well and others, well... not.

[slippery73]

Seems like he put the pieces back in he cut out, which is easy enough because they have the wood backer plates. Then he said they reglassed, im guessing they probably added some resin to the top of the wood and reinserted the fiberglass cutouts. If he did that he would just need to lay some new glass over the seem he created. I would just lay a couple runners down the length of the stringers and call it a day.

[wysongja]

Because the foam held the marine plywood tight to the lip of the stringers we simply placed the cut out pieces back in place added resin and mat and sealed them in.  It allowed us to pack the foam in really tight and gave us something solid to connect the floor to.

[RickK]

Good idea.

[LilRichard]

So how far did you grind the edges of the glass before adding new glass to the stringers?  How far did you bevel back the edge?  What kind of material did you glass the stringers back together with?

I do not know if you guys are aware of how much stress occurs at these joints, but simply adding a little 1.5 oz "mat" to the joint is not going to cover it.  There is no structural strength in the foam - and stringer "caps" take a beating.  How about some pictures?

[slippery73]

FYI there IS sructural strength in the foam with this kind of structure.

[LilRichard]

Slippery-

How do you figure?  The foam is 2 or 4 lb density - there is ZERO strength in that... sorry.  You need to be in the 20lb density range before you can count on the foam to provide structural support.

[slippery73]

No, any type of foam regardless of density will still add structural dexterity to a sandwiched panel. The higher denisty foams as used in transoms is mainly used for compression strength since bolts go through it and are torqued down. Shear strength is greatly increased and overall structure is much stronger than a hollow core. If it had no structural purpose it would have never been put in the boat. Foam is not more boyant than air, it is less boyant as it has more weight than air. Foam is put into boats because it displaces air and will not let water fill up a hollow space if it contains foam. As for the stringers in these boats they were built as a mold, a hollow air space like a pontoon if you will, if they only added foam for boyancy they could have simply done without it as a hollow air filled chamber is actually more boyant than a foam filled one. What most people dont think about structurally is the fact that the strength on an application like this comes from the actual sandwich construction process not the density of the foam. Its like cold molding for boat building, (ask shine) the strength comes from the sandwich technique not from the plywood itself. This is used in home construction...  S.I.P. panels, bridges, etc. If you dont believe me, take two pieces of cardboard and span them across a small span, now add weight to them till they give way. Now take a thin piece of styrofoam and do the same thing. Once more take same length carboard and sandwich the foam and see how much weight it takes to give. The core construction method yields much higher, tension, torsion, and shear strengths. Anyone that thinks even low density foam isn't structural is misinformed. It can certainly be done without in a lot of instances but it does make for a stronger structure when used. I can go on forever on this but i've got to quit somewhere.

[slippery73]

http://boatdesign.net/articles/foam-core/index.htm

[LilRichard]

I agree that foam adds deflection strength to glass sandwich structures - but the foam itself (at that low of a density) has no strength in its own right - that is a different statement than you made.

I am curious how this gentleman built his stringers back up, because this is a high stress area - and relying on the "strength of the foam" as you suggest - is flawed logic.  

And if you want to check out Shine's work, please explain why he used high density foam for his stringers (if, as you suggest, foam density does not matter except for in transoms)?

Not trying to get into a pissing contest here, but I do not want folks to get mis-information on structural component strength.

[slippery73]

Hold your horses with your pissing match, im not into that kind of thing. However, there is more than one way to skin a cat. Everyone on this forum learns from helping one another, the way he refoamed his stringers was a good idea and I dont think he needed to get interrogated about it. Your said the foam adds no structural strength. I simply informed you and the rest of the forum that it does. It might not be structural by itself but combined with the lamination of glass it is. I must not be the only one with this crazy idea as the manufacturers of these core materials make them in densities as low as 3 lbs/ft. This is not 2 part foam either, this is divinycell CORE material. Boston Whaler must be one of those companies that thinks foam isn't structual too, they're simply chop glass and injected foam. One good thing that was done here is leaving the existing stringers system in place. The trapezoidal stringer system on these boats is a major design element that has proven these boats so reliable. I think its something that most people overlook, especially when rebuilding. Perhaps Shine used carbon fiber on the stringer caps to compensate for the lack of size of the original stringers. Carbon or not, it still doesn't justify rebuilding a stringer system with something that is 1/4 the size of the original system. Thats not what I would consider "over building."  

The trapezoidal stringers on these boats serve a mulititude of purposes, giving the hull its longitudinal strength down the length of the hull, the wide base gives it lateral strength from side to side, and its trapezoidal shape helps with torsional strength or twisting within the hull. All without the use of bulkheads. Simply adding partial bulkheads to a puny stringer system is not adding any more strength to these boats, in fact its probably actually worse. The bulkheads that some people have added in their hulls create what builders would call "hard spots," they are hinge points along the length of the hull that give the hull areas points to flex over. These are not rigid boat hulls, they were built to flex. When one adds hard areas to points that were made to flex you are going to end up having stress cracks later on.

Sorry to hop on my soap box and preach to the choir but I would try to be more helpful to our new members rather than making them think they did something wrong. Everyone has their own means of working and if it makes the builder happy dont second guess them just to be an ass. After all your techniques might not be so great either.

[RickK]

Ok guys, it's time for me to step in and make sure we play nice. Each of you are making valid points and there is certainly no need to start name-calling.

Also, there is no need to derail this thread any more, so you may want to take this debate off-line.