'76 22 Osprey re-build

[Que Pasa]

Hard to tell by the picture but the layup is   

1. On top is Mat

2. 1/8 inch layer of Bondo

3. Transom Skin

[RickK]

I was starting to think about it maybe being a previous redo - did the floor look like it was original or had been cut?
I don't think they set transoms back then in bonding compound - I haven't seen any of the rebuilds with it here. Did the lower part of the transom look like it was repaired on the outside? Maybe behind some bottom paint?  There was no reason to use "bondo" so I'm at a loss.  If it was original they normally put the transom and the stringers in it while it was still in the mold and everything was wet on wet lamination when possible.  Maybe it was a Monday morning or Friday afternoon hull? 

To get a good bond it must come out.  What are you using on the grinder?  A flap sandpaper wheel?  They will each up glass in a flash.  I used them all the way through my rebuild - 40 grit.  I buy them at Home Depot.

[Que Pasa]

  I ground out the base of the stringer and they are bedded with it also

[Que Pasa]

If you look at this picture you can see that everything was bedded with this stuff.

[CLM65]

The stringers on my flatback were attached to the hull with some type of bonding putty.  I'm certain it was original.  Not sure if it the same as what you have, but I wouldn't be surprised.

[Que Pasa]

Thanks CLM, Yep that is what it is for sure. Today I started to grind it all off, same bedding as my stringers and keel so it is original. Guess that stuff was inexpensive for them cause they sure were liberal with it. I have areas that are 1/4 inch thick, crazy. I think I have about another days worth of grinding as today I got half of it off in 4 1/2 hours.

[RickK]

You're not using a badassed enough tool/wheel if it's taking you that long to grind it out.  Try the flap sander wheel. Hopefully you're using a 7" grinder (Harbor freight - I'm on my 3rd one on this rebuild)

[Que Pasa]

Rick using a 40 grit Diablo flap wheel, once the bondo gets hot it gums up the wheel. Also, being a 1/4 inch thick isn't helping. One more day should get it done.

[Que Pasa]

has anyone left the foam out of the stringers?

[Capt. Bob]

Not that I recall.

You'll want it in place for its structural aspect. The added floation is often sited but it is there to mitigate the forces applied to stringers by the twisting of the hull, which is one of the main reasons for the stringer system.

To stiffen the hull.

Put it in.

[Callyb]

I agree with Capt Bob. You need to put the foam in the original trapezoid stringers. I'm no engineer, but I would think the only way you could even consider getting away with leaving the foam out is to make the stringers really thick, and even then you would have to install a pretty robust bulkhead system for latitudinal support of the main stringers.

In review.... Put foam back in the stringers.     

[CLM65]

Agree with the above, and will add that 4# density foam is recommended for inside the stringers.  The 2# stuff is great for flotation, but it doesn't offer the strength and rigidity of 4#.

[Que Pasa]

Thanks for the advise, it's hard to believe that the foam could add any structural value. My foam has basically disintegrated where it would bond anywhere or anything. I don't think I'm going to use any, haven't decided yet. Got everything ground out on the transom now I'm ready to start glasssing.

[Capt. Bob]

it's hard to believe that the foam could add any structural value.

Hard to visualize maybe because one is looking at structural value as "strength" rather than function. 

Strength wise, you have the fiberglass walls (one on each side, top and bottom). This design works similar to a steel I-beam in concept in that the flex (shear of the side walls) is mitigated by the top and bottom walls acting in opposite directions. Still, the fiberglass walls are only so strong and foam itself is only so strong and their total strength is the sum of both. Still not really that strong. Therefore why add it?

Well by coring (or sandwich if you prefer) the walls of the stringer, you increase its resistance to shear though its overall "strength" increase is minimal as stated above. Add the trapezoidal shape (not really geometrically true but sounds good in marketing speak) aka a poor man's I-beam and you have a light, rigid, easy to manufacture and cost friendly structural element for your boat.

PS.... For all you cut and paste guys, it's been some time since I used the concept of shear and maybe that's the wrong term but hopefully the "idea" is understood.

Good luck. 

[CLM65]

PS.... For all you cut and paste guys, it's been some time since I used the concept of shear and maybe that's the wrong term but hopefully the "idea" is understood.

Good luck.

Hmm, somehow I think that was directed at me .  I think you conveyed that pretty well, CB.