No core floor

[DMAL]

I am working on a small open bow boat. It has less than 50 square foot of floor to replace. I have a layman's understanding of why a cored floor is better, stronger and lighter. I have a couple of fitment issues if the floor is too thick. My question is regarding layers thickness and strength of a solid fiberglass floor. My widest span is about 2 feet between stringers. Can I get enough strength from about 1/4 inch or possibly up to 3/8 of an inch of solid glass alone? I have already built a mold to make this floor in one piece, but I decided I could use some advise first.

 

[Chris1956]

Balsa Core is used to save weight. IMO it's strength in not the factor.

I would recommend dry 1/2" pressure treated CDX plywood. Cover it with at least one layer of 1.5 oz fiberglass mat. The plywood gives the strength and the glass waterproofs it.

Solid fiberglass will be real heavy, as it is much more dense than core or plywood.

 

[Scott Danforth]

if your floor is soft, the stringers and transom long ago rotted

core separates the two layers of fiberglass. the more separation, the stronger the two layers of fiberglass become.

a 5/8" thick section of floor with two layers of .060-.070 " glass and core in between will be stronger and stiffer than solid 5/16" thick piece of fiberglass and a whole lot lighter

 

[DMAL]

Balsa Core is used to save weight. IMO it's strength in not the factor.

I would recommend dry 1/2" pressure treated CDX plywood. Cover it with at least one layer of 1.5 oz fiberglass mat. The plywood gives the strength and the glass waterproofs it.

Solid fiberglass will be real heavy, as it is much more dense than core or plywood.

I don't have room to slide 1/2" plywood under the open bow walk through. I built my stringers a little too tall...also I took rotten wood out with my bare hands...I will not put wood back into it. I was going to use Coosa, but I don't have room. I am aware of the weight issues, but thickness is the deciding factor.

Are you aware of any place that has information on strength and thickness of solid glass?

 

[DMAL]

if your floor is soft, the stringers and transom long ago rotted

core separates the two layers of fiberglass. the more separation, the stronger the two layers of fiberglass become.

a 5/8" thick section of floor with two layers of .060-.070 " glass and core in between will be stronger and stiffer than solid 5/16" thick piece of fiberglass and a whole lot lighter

I have already replaced the stringers and the transom. I am aware that the the core adds strength and reduces weight, but unfortunately I just don't have room for a core. I may have 3/8 of an inch max. I made the transom from solid glass...it was just a 23"x24 pad the with IO cut out. I don't think I could have saved 5 pounds with Coosa. The floor is small also... probably about 45 SQ. FT in total. If I was to use a core with 2 layers of glass on each side Verses just using 8 or 10 layers of glass (if that is enough), I can't imagine adding more than 50 pounds...I have already done some weight reduction to the boat...but I could be wrong. That's why I am asking questions.

Where can I find information on strength and thickness of fiberglass?

Will 3/8 of an inch be strong enough to span 24"?

 

[Chris1956]

If you do not plan to walk on the deck, foam it up flush with the stringers (actually, do this no matter what your plans are), and lay a couple of layers of 1.5 oz mat over it.

If you plan to walk on the deck, I do not understand your design constraint.

 

[DMAL]

If you do not plan to walk on the deck, foam it up flush with the stringers (actually, do this no matter what your plans are), and lay a couple of layers of 1.5 oz mat over it.

If you plan to walk on the deck, I do not understand your design constraint.

I will need to walk on it and attach seats to it. I do plan on foaming outside of the stringers. Inside of the stringers needs to remain open for the ski storage and fuel tank.

My problem is that the open bow has its own fiberglass floor in the walk through area. I need the new floor to slide under the open bow floor. I made my stringers a little too tall.

 

[DMAL]

This entire floor should be about 150 pounds if I figured this right..I took that much water weight out of it and have done several other things that reduced the over all weight to some extent.

I just need to know if it will be strong enough, and I can't find much in the forums about glass thickness and strength.

 

[JASinIL2006]

The stringers on my boat were made with foam cores and thick fiberglass walls. I'd guess the fiberglass was 1/4"-3/8" thick and they are extremely rigid. I'd start by planning on 3/8" thick and then adding a layer or two if you have any bounce/flex. I don't recall ever seeing a guide that addressed thickness of a fiberglass-only laminate deck.

 

[flashback]

I don't know your situation but couldn't you core it as far forward as possible and then run solid glass just for the edge that tucks under the forward section.?

 

[Scott Danforth]

I will need to walk on it and attach seats to it. I do plan on foaming outside of the stringers. Inside of the stringers needs to remain open for the ski storage and fuel tank.

My problem is that the open bow has its own fiberglass floor in the walk through area. I need the new floor to slide under the open bow floor. I made my stringers a little too tall.

pull your cap

 

[Jeff Fro]

x 2, pull the cap.... and trim the stringers down to where the floor will fit between the stringers and the bottom of the cap.

 

[Chris1956]

Gee, if you don't pull the cap, how will you fasten the new deck to the stringers? In boats, if it is not fastened real well, it will move.

My rule of thumb is that if a nail works on land, use a screw on a boat. If a screw works on land, use a bolt and locknut on a boat.

 

[DMAL]

Gee, if you don't pull the cap, how will you fasten the new deck to the stringers? In boats, if it is not fastened real well, it will move.

My rule of thumb is that if a nail works on land, use a screw on a boat. If a screw works on land, use a bolt and locknut on a boat.

It would probably be a little better if I pulled the cap, but the new floor only goes about 6 or 8 inches under the open bow floor. I can fully access the new floor tabbing through the front seats and/or under the dash.

 

[DMAL]

I don't know your situation but couldn't you core it as far forward as possible and then run solid glass just for the edge that tucks under the forward section.?

I could do that...it may cause puddling where the two floors meet.

 

[DMAL]

The stringers on my boat were made with foam cores and thick fiberglass walls. I'd guess the fiberglass was 1/4"-3/8" thick and they are extremely rigid. I'd start by planning on 3/8" thick and then adding a layer or two if you have any bounce/flex. I don't recall ever seeing a guide that addressed thickness of a fiberglass-only laminate deck.

I have been thinking about adding some small ribs in the center section if needed. The outside sections will have foam support and a short span...about 16". the center of the boat between the ribs is about 24" at the back and 20" in the front, so I only have about an 8'x2' section that I am concerned with I don't know if it would be a bad idea to foam between the gas tank and the floor or not. I could get some support from the gas tank. The front has the cut out for the ski storage so it is somewhat less of an issue. The ski storage hole will have some added structure to beef that up a little. Looking at it that way i guess I am down to a 4x2 section above the gas tank that I am concerned about.

 

[todhunter]

Are you only concerned about the thickness of the portion of the floor that runs under the open bow floor? If so, use a router, belt sander, hand planer, or something similar to thin out a piece of 1/2" plywood in the 6-8" where it will tuck under the open bow floor. Use a smear of PB to make a fillet where the thickness transitions then lay glass. Slide in place, fasten, and tab.

Or, pull the cap and cut the stringers down.

 

[DMAL]

Are you only concerned about the thickness of the portion of the floor that runs under the open bow floor? If so, use a router, belt sander, hand planer, or something similar to thin out a piece of 1/2" plywood in the 6-8" where it will tuck under the open bow floor. Use a smear of PB to make a fillet where the thickness transitions then lay glass. Slide in place, fasten, and tab.

Or, pull the cap and cut the stringers down.

The point of this thread is about the strength and thickness of fiberglass with out a core. I should have limited the discussion to that and that only.

 

[todhunter]

True...sorry for the derail. I have no input on a fiberglass only floor. I think you're on the correct path regarding putting stiffening ribs or additional supports to minimize the free span, if you go that route. Good luck.

 

[Bondo]

Can I get enough strength from about 1/4 inch or possibly up to 3/8 of an inch of solid glass alone? I have already built a mold to make this floor in one piece, but I decided I could use some advise first.

The point of this thread is about the strength and thickness of fiberglass with out a core. I should have limited the discussion to that and that only.

Ayuh,.... You need to build in some sorta structure for strength,...

Properly placed cardboard or plastic tubes, cut in half, 'n glassed over creates stiffness,....
Maybe strips of stiff foam, 'n well glassed over,...

Kinda like workin' tin,.... flat sheets are flimsy,... a few bends in it, 'n it'll support thousands of pounds,....