Comments on my building methods (hardtop construction)

Hello All and a prayer for all those who have been and/or will be affected by hurricane season 2005.


My posting is in search for guidance, opinions or comments on my plans and methods of building a 11.5 x 12 Divinycell hardtop to replace a pilot house on my sailboat. I will be using 2” H130 Divinycell for the fabrication as do not want it to look too thin in relation to size. The Hardtop will be mounted by ten (five each side) 2” aluminum poles with 6” round wielded top plates. I will be doing most of the work myself less the finishes. I have attached a few renditions of my plan at the following link.

http://pg.photos.yahoo.com/ph/dnsealmax/album?.dir=21e4&.src=ph&store=&prodid=&.done=http%3a//pg.photos.yahoo.com/ph//my_photos

My questions relate to the following:

Female or male mold?
My plan is to make a male curved table (12x12) shape Divinycell material as required (corners, edges, etc) then score the Divinycell and secure to table. Glass the top side of the Hardtop and edges; allow to cure and the flip to complete the underside. I understand that doing it this way creates more finishing work later but it permits me to remove all air pockets by rolling the surfaces out through direct sight.

How much glass and type?
My plan was to place two layers of biaxial weave cloth (weight I don’t know) on top and one greater weight on the bottom.

Mounting methods and fastening systems?
My plan was to insert 8”x 8”x 1.5” hardwood (Mahogany or Oak) into predetermined locations (10 sites) and glass over them. These sections will be even with the bottom of the Divinycell. It is at these points that I will fasten the aluminum standoff plates. My goal is to keep the top side of the hardtop penetration free. If I have to bolt though with aluminum plate on both sides I will, but what size must I make them and how do I prevent the fastening of the bolts to compress the Divinycell glass combination?

Thank you for your assistance and I am so glad I have found this site.

 

Once you have glassed one side and removed from your bending jig the foam will become flat again. It needs to be held in it's curved shape when the second side is glassed and cured.

Maybe glass the underside first while the foam is flat then flip it, over shape your edges, bend and hold while the top side is glassed. This way once cured the bend will stay.

This method is not using any mould so the top side at least will need hand finishing.

130kg foam is a little heavy??

 

Heavy?? I agree....but the price was right

Thanks for the insite. I did not consider nor think think it would bend back since I was shaping mostly by scoring/cutting, but I see your point. I was hoping to get some info as to bending it, but I see that it will not be possible. My concern with your suggestion on "the fast flipping" is with the size of this thing. I really do not think there will be "anything fast" with this project.... smile. In addition I want to insure no air in get's into the glass while I attempt to flip a wet project; If it does create air pockets I won't know till I flip it back and then it will be too late.

As for the weight, yes I agree it will be a bit heavier than necessary, but I want the look to be substancial and the edges rounded. Finally I am told the denser and thicker the stronger it will be. But mostly I'm going this route because the price was right... I paid $620.00 total including dilivery for the 144 sqft of 2" Divinycell... makes a lot of financial sence to me... smile..

The rating on the H130 2" is 8 lbs per square ft.
I figure 24 square feet at the 8 lbs comes to 192 lbs. Not sure what to calculate for the cloth and dried resin; maybe another 150 more which brings me to a possible 350lbs. Not too bad for something this size... I think. comments?

By the way what did you think about the mounting method?


thanks
gil

 

Gil,

What John's saying, (I think) is to let the first skin cure out before flipping, the part will be floppy and bendable until you get the second skin on.

I'll add a couple of my own thoughts too; I'd fair the first skin before jigging it up to do the second skin, it's difficult to fair concave surfaces. I'd also recommend painting the underside with a fine non-skid, (like awlgrip beads) then it'll look really good without having to make it super-fair. I'd also spend more time fairing the underside, because you'll look at it a lot more closely, and if you paint the top with coarse non-skid, It'll hide just about anything you wouldn't trip over.

Can't give you a layup-schedule for the glass, but I'd prefer to have some of the glass running along the shortest distance between supports, at 90 degrees, ie, port to starboard, (if I read your drawing correctly) to give it maximum stiffness. Lastly, a balanced layup is probably the best, but if you do want to make one skin thicker than the other, then do that on the side that sees the most compression load. (the top)

P.S. For fastening to the supports, you could rout and glue in a thick piece of G-10 in the foam, and then tap into it, or, just bond the G-10 to the underside.

Yoke.

 

Fair the skin before jigging it up.. is this a R site..smile

Yoke, understood most of your comments but the fair it up and jigging part? I don't want to get to personal with this thing.

I was thinking about making a cradle to hold it when I invert it to do the bottom (reverse stand). This way it won't stress or warp out of shape. Based on the comments I've recieved I would like to keep it in the mold until I flip it to prevent further stresses. This has me thinking about a light jig/table aparatus but still strong.

gil

 

Yoke's right there, let it cure before flipping. The sheet of foam will probably bend this small amount (6") reasonably easy without scoring the foam so your jig may not need to be very strong, just accurate. I like the idea with G10, wood/plywood inserts may just be trouble in future with water etc.

 

Gil,

I was thinking you'd laminate the first skin with the foam flat, and then installing it on the jig (jigging it, I call it, but please be patient with me, I'm not a native speaker) to get the right curvature, and then you can do the second skin to lock in the shape. Don't forget to edge-glue the foam sheets, so it'll bend fair.

Fairing; since the glass won't be perfectly smooth, you have first sand off the worst bumps and then fill the low spots with either primer or putty, (depends on degree of unfairness) then you sand that flat and smooth. (Duratec primer is great if you're using a styrenated resin-system)

For making the jig/cradle/stand/fixture/mold/thingamajig, I'd set up a few transverse stations, (four or five probably) and then wooden battens lengthwise every few inches to make a grid with the right shape. Consult a book on boatbuilding (like the Gougeon brothers one) for the technique to develop the right shape of the curve. (or maybe someone will be nice enough to post a picture of it)

Yoke.

 

Yoke, hows about vaum forming. You can see my plan at the yahoo photo album listed. My plan goes as follows

Create a male and female mold similar to how you described. Run nine 2x4's lengthwise and cover them with 1/4" plyw that has a mica type smooth finish. The lower mold will have an edge to create an air seal against the top mold once in place. I will use something likt thick double sided tape or foam. I will precut and fit the Divinycell prior to starting. I will also score (1/2" deep) the Divinycell on both sides with a radial hand saw to enable the removal of the air. These air channels will radiate from a central hole (3") through the Divinycell. I will make this hole at a location where I plan on fabricating a hatch later. I want it large enough to hold any resin should it suck too much out. The top mold will have a vacum port that matches the hole once in place. The final seal will occur when all the glass and resin have been applied, this will be done with aluminim foil tape... not duct tape... smile.

The process (I Hope):
1. apply release agent to all contact surfaces
2. layout glass in bottom mold
3. apply resin and roll out air pockets
4. wet bottom side of Divinycell and install
5. apply glass and resin on top half quickly roll out
6. close molds creating chamber
7. apply vacum and allow to cure before removal
8. pray

Notes:
1. The two halves will be held together by straps,
clamps, weights, etc.
2. The edges of the hardtop will be left unfinished to
with air removal. I will finish the edges during 2nd
applications, etc.

Questions?
1. How many inches or lbs of vacum will I require?
2. What does lack of air due to curing time?
3. Will there be any substantial heat build up?
4. What weight and type cloth would you reccomend?
5. If thinner cloth, how many layers should I attempt
to apply during the vacum process?


thanks
gil

http://pg.photos.yahoo.com/ph/dnsealmax/album?.dir=21e4&.src=ph&store=&prodid=&.done=http%3a//pg.photos.yahoo.com/ph//my_photos

 

Gil,

I see what you mean, I'm just a little worried about the things that can go awry, now don't get me wrong, I think it's a method with some merit, but I'd suspect you'd have to make a few junk-parts before you'll have it all figured out. I'll get back to you later tonight.

Yoke.

 

Gil,

Alright, now where were we? Oh yeah, hardtop, okay, perhaps I should give you an outline of how I would approach this, with a few thoughts added on why.

First of all, I'm generally fairly adventurous with trying new tricks, but the more money is riding on it, the more conservative I get. (although some people would beg to differ, fricking luddites) I can tell from your proposed plan that you don't particularly relish the thought of sanding, so I'll try to give you a method to make that possible.

No warranties expressed or implied, this is just how I would do it.

So, at long last, the general idea is to build first the top skin, gelcoated, then build the bottom skin together with the core in one shot, then bond on the top skin to that with core-bond adhesive. (by the way, this assumes polyester resin)

Begin by building a strong-back, fasten it down well to the floor, then add legs to connect the stations to. Cut stations, one set of concave and one set of convex, the convex ones (sticking up) are the more important to get the shape right on. Start by installing the concaves, then run 1X2 battens on about 4 or 5 inch centers. Install the surface, screwed from below, (I used 3/4 inch melamine coated particleboard last time) run the joints crosswise, (no kinks) and cover the joints with scotch-tape. Wax the table, spray gelcoat, laminate the skin, put on the process-stack, (peelply,perforated release ply, breather and bag) and bag it 'til cured.

Remove the skin from the mold and put it aside. Install the convex stations, and replace the battens and surface. Tape the joints, wax again and gelcoat. Saw grooves length-wise in the foam, (so it will lay down nicely, grooves also act as vents) one inch deep, every two inches, do the same thing on the other side, but move them over one inch. Laminate the skin as before with at least one layer of mat as the final piece of glass, wet one side of the core, (squeegee the bubbles) install core onto the wet laminate, assemble the process-stack and bag it.

Bring out the skin you made first, remove the peelply, wet the foam with resin, smear the skin with core-bond putty, put in place, and bag it down.

I wouldn't want to try these layups without at least three people that know what they are doing. Also see one of the recent issues of Professional Boatbuilder for a good article on core-bonding. (proboat.com)

Yoke.

P.S. Any particular measurement I've given needs to be tested for suitability.

P.P.S. You can also install the core with putty on the bottom skin after it's cured.

 

HARDTOP start to finish. March 2005- May 2005

I built a hardtop for my Slickcraft SS 235.

I designed initially to be of MG plywood, but switched to aluminum frame with a glassed in plywood top. 4 posts and a square frame of aluminum witha plywood top glasssed in... It came out great- All of the 2005 season on Lake Michigan was perfect. Not a wiggle.

I picked 2 inch OD schedule 40 pipe and metal fittings that were functional, but unobtrusive. I rotary-polished the pipes with a drill on the pipe and a rag with polishing compound in the otherhand. Its a close to a mirror finish as can ba done on aluminum.

I made MANY drawings, and then made a full size on-the-boat wood mock-up of 1X to finalize the dimensions BEFORE cutting/bending any pipe. I made minor adjustments for fit after the frame was done.

The fittings are designed to hold the pipe with a set-screw, this was unaccaptable due to the motion-stress. I upgraded this by drilling and tapping all the way through the pipe and securing the 1/4-20 SS bolts in with LOK-TITE.
It has a 2.5 inch bow in the crest of the top. The curves around the perimeter give it a softer custom look. Its got beveled edges and a drip edge along the back edge to run water outboard to keep water from the cabin...

I sealed the joint between the pipe and the top with caulk to deter spray, a GW trick I saw on my friends 272 Sailfish... The top is through bolted with fender washers and caulked with 3M5200. It has 3 coats of paint over 1 coat of resin and glass. Its got enough strength, and yet is light too.

I'll sew the 40 mil clear vinyl windshield and side curtains over the winter, but have yet to get the edge web, zippers, and snaps.

I have pictures, but due to a PC crash cannot e-mail or post any yet.