Flatback Fireboat Rebuild

[seabob4]

Looking good and nice to see you got those boys out there doing some work.    

A little $$ goes a long way...  

Craig, that glass thickness is pretty typical for stringers, even today's smooth gel stringer grids.  One has to look at the weight that additional layers of glass throw into the mix when designing a boat.  Pick up a 36" X 18" piece of 3610, then pick up that same piece wetted out...BIG difference.  Now multiply that many times over, and you are talking a lot of weight.  I'm not saying that builders would use just the bare minimum to maintain strength and integrity, but the weight factor is always there...

[CLM65]

Looking good and nice to see you got those boys out there doing some work.    

It's good to see something in their hands besides XBox controllers!

[CLM65]

Looking good and nice to see you got those boys out there doing some work.    

A little $$ goes a long way...  

Craig, that glass thickness is pretty typical for stringers, even today's smooth gel stringer grids.  One has to look at the weight that additional layers of glass throw into the mix when designing a boat.  Pick up a 36" X 18" piece of 3610, then pick up that same piece wetted out...BIG difference.  Now multiply that many times over, and you are talking a lot of weight.  I'm not saying that builders would use just the bare minimum to maintain strength and integrity, but the weight factor is always there...

Good point Bob.  Hopefully the weight of additional glass will be offset by the removal of the foam outside the stringers and the lighter weight composites.

[Blue Agave]

Rick built, but they are more like 1/16"-1/8" thick.  There is no way I would trust them to hold up the deck without the structural reinforcement that foam adds.  I will need to beef them up after adding the new foam, which will be 4 lb density.

After 40 plus years those stringers didn't have a problem supporting the original floor and load.  

[CLM65]

Rick built, but they are more like 1/16"-1/8" thick.  There is no way I would trust them to hold up the deck without the structural reinforcement that foam adds.  I will need to beef them up after adding the new foam, which will be 4 lb density.

After 40 plus years those stringers didn't have a problem supporting the original floor and load.  

True, but they were also sandwiched between foam both inside and out.  That added a bit of lateral support to the glass, so there was nowhere for it to go.  By not adding foam back in on the outside of the stringers, the potential may exist for them to buckle/blow out.  Increasing their thickness and/or adding bulkheads for lateral support may be the ticket.  Maybe I am overthinking this, but I hope to err on the side of conservatism.

[gran398]

Craig, I like the comment about "willing" helpers  

Re the strength....this is pretty interesting to think on.

The stringers system is basically an inverted corrugated panel. If I understand it correctly, this large panel was then adhered to the hull skin.

Question: When you look through the tops of the stringers...are they open at the bottom? That is, are you looking at the hull itself....or the stringer system "liner" so to speak?

If you are looking at the hull itself....ie open to the hull...you could tab each stringer internally before adding the foam back in, therefore locking down the stringers to the hull.

If there is a bottom "liner" at the base of each...then some glassed nida/etc. bulkheads as you mention may be a consideration.

She's looking good  

[CLM65]

Thanks Scott.  The auger bit I use has about 1-1/2" of shaft below the blade, so I haven't actually removed the foam at the bottom of the stringer yet, so I'm speculating here...but I believe I will see the inside of the hull when I get all the foam out.  Then I plan to run a 8" - 12" strip of 1708 or 1808 along the inside of the stringer-to-hull joint for the length of the stringers (both sides).  This will hopefully, as you said, lock the stringers to the hull.  After foaming the stringers, I will add 2-3 layers of 1708, extending partially down the outsides of the stringers.  Then I'm thinking of using NidaCore bulkheads every 4' to coincide with the joints in the decking.  This will add some reinforcement to what I believe will be the weak spots in the decking, while providing some lateral support to the stringers.

These are just my thoughts - I'm certainly open to recommendations and suggestions!

[gran398]

That makes good sense. One plus is where the gas tank coffin goes....you'll have lateral strength....and you could extend the bulkheads over from there.

[seabob4]

Craig,
I wouldn't think there would be a possibility of the stringer forms "blowing" out as long as the foam injected into them has a chance to escape during the cure process...hence the use of "foam holes" in stringer grids.  A good "foam gun" guy will pretty much, based on experience and knowledge of the product, get a stringer grid filled with very little escaping out the foam holes yet still leaving very few, and very small, voided areas (no foam).  The foam used pretty much throughout the industry, BASF 2-part isocyanate, is some pretty strong stuff when it comes up against it's limits and isn't done kicking off.  Will bust out a stringer side for sure...

Once the foam has kicked off completely, it's not gonna grow, even with water intrusion and hence saturation.  It's just gonna get heavier per given volume...

[CLM65]

Bob,

I'm not too concerned with blowing them out due to foam expansion.  I'm more concerned with the weight of the console, t-top or tower, etc.  That weight is trying to compress the stringers beneath them.  When the entire area below the deck was foamed in, the fiberglass sides of the stringers were restrained from moving.  They couldn't move inward, and they couldn't move outward.  Now take away the foam outside the stringers.  I'm postulating that if enough force is applied downward on the deck, such as may occur after coming off a big wake, the stringers may compress and the fiberglass sides may bow out and rupture.  I know, a bit extreme, and I'm not taking much credit for the foam inside the stringers.  But it is a theoretical problem that is easily solved with bulkheads as discussed above.  Overkill?  Maybe.  But it will give me some peace of mind.  I'm sleeping better already!

Now, if someone can think of any problem I may be creating by doing this, please let me know!

[Capt. Bob]

Now, if someone can think of any problem I may be creating by doing this, please let me know!

Extra work and,

Peace of mind

[gran398]

Was looking back through your pics. Reckon the open rectangular area is where the optional viewing windows went.

To me, a lot of where and how on bulkheads would be determined by where the tank coffin goes. For kicks and giggles tomorrow will go back through the FB builds. Wonder if any of our FB owners would have made any changes on tank placement if they could do it again?

[gran398]

Here is Ashley's

[seabob4]

A bit of "build theory" here, but...

We used athwrartships bulkheads at PL, but basically in the desire/need to reduce the size of unsupported panels...not in regards to laterally stabilizing the stringers.  Unsupported panels will distort over time, so the key is to minimize their size.

Now, let's look at the concept of uni-body construction.  Once the deck (or liner) is bonded to the stringers, the entire structure, hull, stringers, liner, works as one.  Forces applied to any given area (within reason) are distributed throughout the structure.  The force exerted downward on the cockpit sole is first applied to the stringer tops, migrates down the stringer sides, then migrates into the hull...the whole time growing weaker and weaker as the migration occurs.  The force is not borne alone by the stringer tops, or even the entire stringers themselves.  A good bit of it is even absorbed and dispersed by the cockpit sole itself.

Analogous to t/bolting a t/top.  1/4" bolts with 1/4" flat washers will do the job of securing the top to the boat, but allow too much force to be absorbed by the sole as there is minimal area in which to distribute the load.  1/4" fender washers improve the distribution.  Backing plates GREATLY improve the distribution.

Like Fernando stated, those trapezoidal shaped stringers served her well for 40 years.  Duplicate them with today's materials, and you'll be long gone before this boat ever has any problems...    

[Callyb]

I kinda feel like I'm at this point but...

I agree with Bob 100%. My plan to to build the stringers to 1/8" thickness just like the original design. I feel like you are looking at the stringers in terms of the entire structure, when in reality the are only part of it. Maybe think of it this way, when a house is built it isn't just the framing that is the entire structure, the sheathing that goes over it contributes to the strength.

What I'm saying is, the stringers alone aren't the only structure. It is only part of the boat, and the entire "hull" (the hull, stringers, frames, sole, casting deck, flotation foam, transom core) of the boat is a number of structural components adding to the overall strength of the boat.

And as far as reasons not to build them up to a larger thickness (in my opinion) is;

1. A lot of extra weight
2. A lot of extra money
3. A lot of extra work
4. No real return for any of the above
5. A heavier boat means more fuel to push her

Not trying to be negative, I'll support which ever way you go.

I just wanted to offer my point of view on things.