replacing bottom-need opinions

Hello, First post here so i appreciate any help I get.
I have a 14'X4' flat-bottomed duck boat made out of 1/4" plywood with about 1/8" of mat on the outside and top side decks for waterproofing. The interior is just plywood. A few months back I found a submerged pipe and needless to say the pipe won. It ripped a 5"X4' hole in the bottom of the boat. Duck season wasn't over yet so I just quickly repaired it with an outside patch to finish the season. Well the next season is upon us and I am finally getting started replacing the bottom. When ripping out the old bottom, I found that the plywood had delaminated from the polyester fiberglass so I ripped it out completely. Now I am left with about 1/8" of fiberglass mat as the bottom of the hull and the interior floor. I am now looking at options for repairing/strengthening the bottom. The floor was never quite strong enough before and the engine was nearly impossible to line up. On the plane, the engine would bounce around endlessly. Because this is a duck boat and weight is an issue, I would like to finish the bottom without any ribs while minimizing weight (if I have ribs, then I need a false bottom, that means more weight). I called a distributor in Florida and talked to a tech guy.
He recommended finishing the inside by:
1-laying a couple layers of mat on bottom.
2-bonding a 5/8" foam board to the mat (while the mat was still wet)-H80 5# density
3-Top with 1708 biaxial cloth

My questions are::?:
1-Will this be strong enough for my 4' bottom if the boat planes out?(Will my bottom bounce around like it did before with the plywood?)
2-should I maybe use a cloth first instead of the 2 layers of mat to bond the board to the bottom?
3-Does this system sound appropriate? If not what would you recommend?
4-What type of finish will the 1708 leave and should I top it with something else?(I don't need it to be like glass inside but I would like it easy to clean)
5-Since the foam board is not strong enough to bolt the strut and engine mounts through, I will need to cut out and use a filler board through the center(10"X6'). I was thinking maybe just layer a bunch of fiberglass together to the thickness of the foam board and then top with 1708 when I do the rest of the floor. Any suggestions here? Maybe use a wood board instead of the layers of fiberglass?
Sorry for the kind of long post, but I wanted to explain as much as possible and once again thanks for any advice I get.

 

The cored laminate the tech guy recommends seems light to me, but I'm not sure what speeds you'll be moving at, nor what boat we're talking about.

I'd reinforce the exterior mat with 1708 first (mat down), next a layer of mat, then use the core material, then a top layer of 1708 (mat down) with a top coat of mat to make finishing off easier.

Yes, in the areas where you need to attach stuff, then use solid laminate or bond in some wood. Since polyester doesn't stick well to wood, consider solid laminate in these locations (it's what most manufactures do).

If you used epoxy for the repair, I'd use a whole different laminate schedule. I'd use a layer of 9 ounce biax against the original mat, then some core or plywood, then another layer of biax (two layers of biax on both sides, if using a foam core), on the inside and call it done (after fairing). The advantage of epoxy is the much stronger layup and easier use, plus it sticks to wood extremely well, so you don't need expensive core materials. You don't need (nor is it desirable) mat in an epoxy laminate. The draw back (there's always something) is epoxy costs a little more then polyester.

I'd use epoxy, because it's easier to work with, considerably stronger and you can do the whole thing with one type of fabric over a core material. Polyester will do the job too.

 

Thanks man.
The boat planes out and goes max about 22 knots. It is a completely flat-bottomed and light-weight boat made to go through just about anything. I have passed over mudflats without a millimeter of water. (Sometimes the ducks are really hard to get to). As I mentioned before, weight is an issue but at the same time the bottom was never strong enough before and I would eat up shaft bearings after only a few outings because of engine alignment issues. I really want to make it strong enough and will sacrifice weight to do so.
Thanks for pointing out that the mat side has to bond with the core material. I think I would have gone that way with it but it would have probably been a "game time" decision as I would not have thought of that till it was time. Got to maximize the surface area.
When you say that the cored laminate seems "light", do you mean that I should go with a higher density? Or that the entire system seems light and that you are recommending the substitution of the 1708 as the first step to make it all stronger?
Both the repair and the original outer layer are polyester. I have sanded the inside of the patch flat. The outside patch will be sanded flat after refinishing the inside. I don't want to say money is no option but a difference in total cost from maybe $300 to $400 is ok. But if it doubles the total cost from $300 to maybe $600 then I might just go with the polyester again. If the epoxy is stronger, can it be combined with the polyester already in place?

Also, the tech guy says that the cored material comes in a 4'X7' sheet. This matters because the entire area to be fixed is approx 45"X10'. If it is a 7' sheet I will come up about 1' short once I add in the cut outs for the solid laminate. I probably know the answer to this already but since I will come up a little short, could I just cut it into 6 sections spaced about 2" apart and fill in the gaps with composite to make up the extra foot. (They don't give away that core material.) I know I probably should just shell out the extra $140 for another sheet but I will use so little of it and probably never use the rest. What a waste.

 

I was also doing some local shopping and research and found a core material called Nida-core, a honeycombed core material. Has anyone used this stuff. It's half the cost of the H-80 so I'm a little skeptical. They do build work boats (commercial fishing, etc...) there so they probably do know what they are talking. They has some samples for me to look at but it is about 45 min away and I'm working nights now so I will not be able to get there for another week. Just looking at all options. Maybe I'm way off course here but its something I'll look at if no one has any objections. Thanks again for letting me pick your brains.

 

Now you know why they used plywood. Plywood is a structural material, the core isn't. You save on laminate material with plywood, where you need laminate thickness with the core, just to make it work. Then there's the cost. You could save a fair bit of money by using plywood. More then enough to compensate for the additional cost of epoxy. For the price of one sheet of core material, you could have the finest marine grade plywood available, more then enough for the job. If you elected to use a slightly lower quality plywood, then you'd save even more, not to mention have less goo and 'glass work to perform.

Use 3/8" Okoume plywood. Buy the BS 1088 grade from a marine lumber retailer. It's thicker, lighter and probably stronger then what was there. It's not cheap, but it's cheaper then core material. If the price of this bugs you, then step down to a BS 1088 sheet of Meranti plywood. It's about 5 pounds heaver then Okoume per sheet, but it's 30 - 40% cheaper in most places.

Nida-Core and Pro-Core are probably cheaper then what you're looking at, but they still are more costly then plywood, especially when you count the extra goo and 'glass it takes to get a panel as stiff as plywood.