Fiberglass top

[Dallas_B_]

Iam building a fram/mold for my boat top. After I wax/gel coat (white) my mold, how many coats of fiberglass should I put on? Iam using 6oz mat or cloth. The mole is 13' 4'' with 4'' down drop/over hang by 8' 2'' with 4'' down drop/over hang...........or........14'x8'10'' total. The mat/cloth is 50'' wide. On top in the middle there will be a Furuno Radar, on sides 2 Antenna Mounts and 1 Spot light up front. Some people said to use 2 to 3 coats of fiberglass, and 5 gal of res,does that sound right to you? I was woundering how thick to make it and not be to top heavy. Any input would help.Dallas

[marshmat]

I assume by "mole" you mean mold?
To have a panel this size hold its shape you need either a core, stiffeners, or both. I'd suggest the following layup schedule as a starting point:
(Layer# - Material)
1 - 6oz mat
2 - 6oz cloth
3 - 6oz cloth
4 - 3/4" honeycomb or foam core
5 - 6oz cloth
6 - 6oz cloth
If the top has a frame to it, you can glass or bolt the frame onto this panel structure. If the top is to be self-supporting across its span, add top-hat stiffeners parallel to the 8'2" axis, spanning full width, wherever there is a pair of support posts.
Again, just a suggestion. Others may have a different opinion. And it depends what you're using the boat for, what conditions, what weight/wind load it has to take.

[Dallas_B_]

Thanks for the info....and yes I change the mole to mold :}

Originally Posted by marshmat

I assume by "mole" you mean mold?
To have a panel this size hold its shape you need either a core, stiffeners, or both. I'd suggest the following layup schedule as a starting point:
(Layer# - Material)
1 - 6oz mat
2 - 6oz cloth
3 - 6oz cloth
4 - 3/4" honeycomb or foam core
5 - 6oz cloth
6 - 6oz cloth
If the top has a frame to it, you can glass or bolt the frame onto this panel structure. If the top is to be self-supporting across its span, add top-hat stiffeners parallel to the 8'2" axis, spanning full width, wherever there is a pair of support posts.
Again, just a suggestion. Others may have a different opinion. And it depends what you're using the boat for, what conditions, what weight/wind load it has to take.

[marshmat]

Dallas- in response to your email, here's the specs on that laminate:
Thickness is 1.01" including a 20mil gelcoat.
Weight is about 2.0 lb/sq.ft. (depending on core choice).
Young's modulus is around 0.32 MSI. Ultimate strength is on the order of 1200 in.lb/in. The plate by itself is not rigid enough to make this span, but stiffeners would put it well within safe limits.
I'd strongly recommend VectorLAM (freeware, from Vectorply) for comparing layups - http://www.vectorply.com/lamdesign/vectorlam_sw.htm
As for cores- there's a thousand different architectures, your best bet is to ask your local suppliers what they carry and what they recommend for your project.

On second thought- the mat really isn't helping the strength much. You can probably take it out of that layup schedule if you want, or go to a thinner one. Mat serves mainly to stop the pattern of the cloth from showing through the gelcoat, it doesn't add much strength.

[yokebutt]

Largely concur with Matt, but i would be more inclined to use a one inch core without stiffeners. To save some money you can use balsa instead of foam. I would also recommend using mat between the cloth and core, if you are using polyester. And finally, make some fiberglass tubes and embed them in the core so you have conduits for the wiring.

Yoke.

[marshmat]

Whoops- I assumed this was vacuum-cured and so didn't think of the need for something between cloth and core. Yoke- I love the conduits idea. Would be a much cleaner setup than having wiring on zip-ties.

[yokebutt]

I should add that I would recommend vinylester instead of poly, that way you can probably skip a few layers of mat, the extra cost is minor.

Polyesters are usually not very strong resins, so they often need resin-rich mat between the structural layers to improve bond-strength.

Yoke.

[blared]

I want to do the same project but how do you get the fiberglass to be so shiny?

[marshmat]

Blared- the shiny coating, it's called "gelcoat", sort of a thick and brittle resin that you spray/brush into the mold before you start laying fibreglass. Nasty stuff to work with, and doesn't hold its colour as well as most would like, but it keeps water out and looks nice. Tricky to use, so read up a bit; and very sensitive to mixing ratios, so follow the manufacturer's directions exactly. Applied with a small helping of skill and care, it will last for decades.

[blared]

wow thanks for the info..

[Dallas_B_]

Sir, To build a flat mold, what would you use? It will be a one time mold. As I stated above the top will be 8x16 now. 1/2 3/4 playwood or what? I have been reading the replys but no one has said just how to build a low cost onetime mold or I just over looked it. Is there a web site that showes how to do this? Iam looking at about $ 800.00 + to do this myself, sure don't want to mess it up. Sure with a small loan I could have someone to build it for me, but than again where is the fun of haveing a boat if you have someone do it for you. Iam so ready to get back on and in the water. Its time to go to the Gulf. Dallas

[marshmat]

Dallas, you can get a pretty decent one-use flat mold using 2x6 lumber framing underneath 3/4" to 1" MDF panels. At the seams between panels, Bondo or thickened epoxy can be used to fair the joints. Four panels at 4'x8' would do. I'd get a fifth panel, and cut it into 6" strips that you then put around the edge- giving you a flat surface with a lip around. More Bondo fairs the resulting sharp corner into a smooth curve. Give it a few coats of wax and a teflon release agent and it should work just fine for your purposes. It does need to sit on a level surface (ie. concrete floor) though.