Female Fiberglass Mold - Thoughts?

[Captain Bill]

Originally Posted by sandy daugherty

Assuming there is no non-skid texture on the seats, your best bet is to spray several thin coats of parting compound on the best one, then build the mold directly over it in glass. It you are going to pull more than a couple seats from this mold you should consider using a very hard gel coat. Since the design isn't changing much, its likely the new seats will crack too, so make a few spares. The parting compound is water soluable, so clean up and prep are pretty easy. If the part is small enough, you can bag it in a food vacuum bagger and get very good control of the resin/glass ratio.

It's a fun way to learn about glass work.

Unfortunately the center of the seats has a non-skid pattern that looks like little pyramids. I've pulled some test molds using POP off of the patterned area and it seems to release fine. I saw an internet post that suggested covering the male piece with a thin coat of petroleum jelly as a release agent when making the POP female mold. It seemed to work quite well though I'm concened about possible reactions with the gelcoat when I make my seats. I plan to clean the mold with acetone. I have already purchased some PVA. I haven't built my parts yet but I've purchased Evercoat Marine Premium Gel-Kote. Hopefully that will do.

If all goes well I should only be making one of each seat (yea right). Unfortunately the seats are mirror images so I need two separate molds. I think the old seats cracked because the glass did not wrap around the bottom of the seat. I'm not sure I have explained it clearly. I believe that the molded piece was a hand layup or maybe even chop. I believe that this was allowed to cure. The plywood was placed in the molded piece along with some resin, but no glass wrapped the upper surface of the plywood. The plywood was recessed perhaps an 1/8 inch below the top of the molded piece. A layer of chop was them placed over the plywood up to the top edge of the molded part forming what was basically an unreinforced butt joint. It is this joint that failed. My plan is to allow 4 inches of unresined glass to protrude above the edge of the mold until I have the core set in place then to wrap that glass over the edge of my core material. I will then put two layers of biaxial cloth over the entire bottem before applying a layer of gelcoat. This should result in a 4" lap joint around the entire perimeter instead of a 1/8" butt joint. Hopefully that will distribute the stresses better and prevent the type of cracking I experienced with the current design.

My planned layup schedule is as follows.

1 Layer of Gelcoat, 20 mils.
2 Layers 1.5 oz Fiberglass Mat
1 layer 3/8" Divinycell
1 Layer 1.5 Oz mat Note this layer will be put in place and cured before the core is placed in the mold.
1 layer 3/8" Divinycell
Chopped strand resin mixture to fill any gaps in the corners between the layup and the core.
Wrap at least 4" of contigious mat over the edges of the core.
2 Layers Biaxial cloth
Hand applied layer of gel coat.
Sprayed on PVA to cure layer of gelcoat.

I'ld be happy to hear any comments. To make it clear I plan to bond and trim the layers of divinycell before starting the layup. I'll have all the glass precut and plan to do the layup in one continious operation. I'll probably mix a separate batch of resin for the biaxial cloth.

I'd be happy to have any comments on my layup plan.

[GordMay]

Is there any reason you couldn't/wouldn't fabricate replacement seats entirely from wood or HDPE (StarBoard)?

[Captain Bill]

I thought about teak, but wasn't sure I had the proper shop to do it right. I also figured I'd have to piece it together as I have not been able to locate any 20" wide boards. Honestly I did not look that hard. That said, I have never really done much with teak, but I do know there are issues with gluing it due to the oil content. The seats are only supported at three points and I was not confident about being able to glue up a piece I was confident would not come apart. If I had more experience with it I might have considered it more seriously.

My experience with starboard has been that the surface is pretty slippery, especially when wet. I just was not comfortable using it for something that holds people on moving vessel.

Lastly though it was 25 years ago, I did a lot of work with fiberglass sandwich structures with foam cores. I know how strong they can be and how well they hold up if done correctly. Maybe it was just more in my comfort zone.

[S/V Antares]

The "Pyramid" nonskid was a brain storm by a power boat builder in Indiana about 35 years ago. They used plastic florecent light diffusers with the points sanded down to the desired hight inserted in the mold. You can buy the diffuses at any "Lowes, HD or lighting store for about $2. Many boat builders use this pattern.