'76 170 Fuel Coffin–Stringer Enclosure

[boatnamesue]

1976 170

Does anyone know the structural layout along the bow/starboard sides of the fuel coffin?  I'm certain there is a narrow support down the center underneath, running from the coffin compartment aft to stern.  The stringers are lateral to the coffin.  What I don't know is if the coffin liner is snug up against the stringers or if there is space between. 

I did the ole knuckle stud finder test you'd do on a wall, listening for the audible pitch change from open space to solid space.  It definitely sounded hollow in the middle 1/3 section of liner and as you get closer to the bottom of the side wall the pitch changes to solid.  The first pic below is of my fuel coffin.  Red line indicates the area where I hear solid, yellow hollow.  The second pic is not my boat, found it on here.  The red lines indicate where the obvious structural support is for the coffin.  But this boat was undergoing restoration, so I don't know if the lateral structural support was added or is original.


 

[Brendanpd28]

You can find some pics on here of the stringer layouts for your 170. I'm not positive but I don't believe there are any additional support under the coffin. Just  a glassed in keel stringer that runs from stern to bow. I believe this was for structure as it wouldn't really assist in floatation in any way. Sorry for the bad pics but maybe they'll help you see what I mean. First one is looking through the aft pie plate towards the bow. Second is looking through a pie plate in the right casting deck locker back towards the stern. You can see the underside of the coffin somewhat.

[boatnamesue]

You can find some pics on here of the stringer layouts for your 170. I'm not positive but I don't believe there are any additional support under the coffin. Just  a glassed in keel stringer that runs from stern to bow. I believe this was for structure as it wouldn't really assist in floatation in any way. Sorry for the bad pics but maybe they'll help you see what I mean. First one is looking through the aft pie plate towards the bow. Second is looking through a pie plate in the right casting deck locker back towards the stern. You can see the underside of the coffin somewhat.

Not bad pics at all considering the exposure, or lack thereof.  Thanks this helps.  I'm leaning towards securing tank using welded tank tabs, mounting blocks bonded to coffin and lag screws.  But not knowing how much space is in between the coffin liner and stringers has me wondering...big difference between 1/4" of space and 2-1/4".  I could drill through the liner with a 3/32 bit, then use it to measure distance.

[boatnamesue]

Update:

My fabricator started work on the new tank yesterday.  Should be finished early next week.  Here's what I'm still battling...how to secure the tank.  Below is a cross-section sketch of the fuel coffin liner and the adjacent stringers. 

I had my mind made up to weld tabs on tank and fixate these to a pvc mounting block, the block either glassed in or bonded.  I planned on overdrilling the stringer, fill with epoxy, then drill the tab screws into the hardened epoxy.  But the problem I forsee is the liner does not rest against the stringers.  The stringers and the liner have completely different angles from their base, as you can see from my sketch.  The only contact point the stringer and liner make runs the length of the liner, bottom corner.   

Unless I were to cut out large sections of the liner to allow me access to stringers, which I'm not going to do, there is no way for me to have access to the stringers to fill the overdrilled holes with epoxy. 

I took a 3/32 drill bit and drilled through the liner, in a location where tabs would line up.  Once I drilled through the thin liner I used the drill bit to measure the amount of space between liner and stringer.  About an 1" to 1-1/4". 






Another option would be to use Moeller's hold down strap, pictured below.  I like this method.  This is how AQ  originally secured tanks, 2 aluminum straps.  But if I were to use these straps I'm left with the same problem as described above...no solid structure for the anchor screws to bury in.





I'm open to using foam now that I've read more about methods to keep the foam from creating air pockets.  But filling the space between liner and tank with foam will only prevent the tank from moving laterally.  I would still be left with figuring out how prevent the tank from elevating.  I also don't have the option to glass in a 2x4, the width of tank coffin, because there will only be a 1/2" space between the top of the fuel inlet hose and underside of fuel tank access panel.   

Maybe im overthinking all this, it's possible with my analytical my mind.  Just trying to make the best install possible with the structural limitations im working with.

Thoughts and suggestions welcomed.