Rebuilding a 14 foot 1960s or 1970s Pigeon Marine fiberglass runabout...

No, I’m not crazy.
The vertical foam sandwich floor in your drawing can be all the structure you’d need under the floor.
The glass laminated between the layers of foam can create an impressively strong stringer, as long as techniques and materials are correctly utilized.

 

You gotta get busy with a chisel. And you'll need to taper grind all the glass and gelcoat on the back of the left bits of transom. I like an oscillating tool with a carbide head to help the chisel work.

The repair can only be done with epoxy because you don't want an ester repair bond to be between you and sinking.

As for the stringers. Tone it down a bit, please. Whether 1 or 3, in all cases, your stringers need to be strong enough on top to bond the sole to.. And a single stringer with foam under and glassed bottom of 1/2" ply would suffice. Personal opinions are not 'crazy', well, usually.

I'd probably go with two stringer in the old locations and glass them heavily, unless you find a datasheet. So much fun, three guys and three opinions on stringers.

 

The picture of the boat with the hammer laying on the floor gave me perspective as to how tiny the boat in question actually is.
That’s how I arrived at one stringer, especially since the deck would be bonded to the foam, and the bottom of the boat can’t help but be stiff with something like 30” per side at the widest point.
Also seats (thwarts) will help stiffen the hull, and might provide additional bracing with a center support.
I’m envisioning a low powered lake boat, but looking back, we don’t know what horsepower or rigors the boat will see. ??

 

Moderator: please consider a title change here...

 

Oups isolofoam forgot the lo...
So here's my setup for the cleanup...
And then
This is the hole that I want to eliminate.
The reason for the foam...

But then again is my foam better in any way than float foam? Cost? Ease of use? The website clearly mention; Not structural... I can punch hard in isolofoam and only make a dent 1/4 inch deep maybe... How about float foam?
Ok... Option #1...
I make a central 1 inch stringer, I got 1/2 inch ply and the keel is longer so I'll double it up... (Transom is triple) I reinforce the old glass stringers with a mat, 1/2 inch ply for the floor, then I pour the float foam in the 2 half and hope it doesn't push my floor up. Thinking of reinforcing the connection from the keel to the transom with the central ply wood going up and attaching to the transom...
Option #2 my little crazy home foam idea with still no magic hold it all together liquid.
Option #3 I don't know but I'm sure y'all can think of something...
It's always good to think outside the box but you need to know what's in it...

 

I'll be using the boat on the fleuve St-Laurent Near Montreal with the original 40hp 2 cycle Johnson motor... To watch the international firework shows... Yes A lot of current and maybe even some rapids... So no I'm not building a slow fishing boat... I want that thing to plane... Needs to be solid... Not sure the HP rating of the original...
Look like it was double punched...

 

Why? I don't understand...

 

So, the foam is only used as a mould and this means you need to make a nice radiused edge and glass it up heavy.

Say you have 2" foam two strips 4" high and 4# density 10' long. This is 2/12•4/12•10 or 0.5 cuft of foam which floats about 60#/cuff or say only 30#, not enough to float my tackle box eh.

Let's say, before you put a sole on, you fill the bilge 3" deep by 60" wide by 8', this is 3/12•60/12•8 or 10 cuft which floats 600 pounds or more if light 2# foam.

Don't lid it before foam. After you cut the foam, best to paint it with epoxy and seal any cracks per our research. Then you still need more foam to float it, but up higher so she doesn't float upside down.

 

Typically, we wouldn't use the language in titles. The moderator will decide. It ain't up to me.

If someone searches on google, in order to find your thread title, they have to search crap 14' fiberglass runabout and it deserves a better title..unless you keyworded better, etc.

Title changes are done, believe it or not, to improve the forum.

Hope I can help you some.

 

You are an active part of my project but Darn my head just overheated a little...
I understand the foam higher now...

But the rest is too technical for me with the # sign the word cuff or cuft...
I'm not sure why we care about floatation of the foam under the floor... It used to be air (+ water and rotten wood) now the boat might be a bit heavier than new but I'm sure it'll stay at that weight a lot longer...

 

Cuff is a typo. My eyesight is getting a little worse near for me at age 55, but not too bad to need glasses. When I tire, my fingers struggle and typing is a bear.

Float foam where you have space, but enough
Or why bother..

 

So, flotation foam is provided for positive buoyancy.

One cubic foot of freshwater is 62 pounds.

One cubic foot of 2 pound marine foam is 2 pounds.

Each cubic foot of 2# density marine foam thus offers 60 pounds of buoyancy. # is shorthand for pounds in medicine

If your boat is made from all materials that sink and weighs 1200 pounds, you need to provide 1200 pounds of flotation to keep her afloat.

This means you need 20 cubic feet of flotation foam.

This kit is 8 cubic feet, so 3 of these placed in the boat will float about 1440 pounds of hull..

TotalBoat Flotation Foam - 2 Part Expanding Polyurethane Marine Pour Foam for Boat Floatation, Insulation and Soundproofing (2 LB Density, 2 Gallon Kit) Amazon.com https://www.amazon.com/dp/B01AAP3ILC/ref=cm_sw_r_cp_api_i_E1RJMKAV6GY2Q3D5SA7W?_encoding=UTF8&psc=1

 

I'm sorry but I don't need flotation. I want to displace the air that used to be under the floor... Displacement is what makes a boat float... Buoyancy is when I go diving under the water not on top... So if I get swamped or break in half yes the foam might help me get back to shore but it will not help it float more... It would actually make it float less, because of the added weight.
Steel ships don't need foam for floatation they use it for sound proofing... Which is also a reason I want to use foam...

Now we're talking...
First the vertical stripes of foam was a cool idea of mine but I don't know how I would measure the curve let alone cut them... But then again I really need to be able to measure/copy the hull shape if I want to make any stringers... Cardboard probably... Now the real question... I've left 2 inch of glass around the floor, but that is gonna stop me from using a single sheath of plywood, I need to glass the floor before putting it in the boat so it doesn't rot again soon, and it won't be flexible, so... Can I remove the 2 inch and "tab" directly on the walls instead? Then I'll be able to just drop the floor in...

 

Don't remove the cleats of the old sole. That is the wise way to repair an old boat. You template the floor and glass the pieces and lay them into a bed of epoxy putty.

 

You don’t have to foam in the floor, stringers can carry it just fine. Flotation works better if it’s higher up in the hull, like under the gunnels.
Foam in the floor only insures that when (if) the boat fills with water, it capsizes, then floats upside down, which is less than ideal for occupants!