Thick core

[Steve W]

I have a fair sized hardtop to build for a powerboat, it will be about 8 1/2ft x 16ft, it will be molded in a one off female mold using gelcoat,polyester resin and a fairly standard mat and biax layup with probably a balsa core. The hardtop will be supported on an existing radar arch and four ss posts. The only problem is that the arch that dictates the camber does not really have much itself and the four posts are around the perimeter only with nothing in the middle, oh,this thing extends forward from the arch to cover the flybridge area with about 3ft cantilevered aft if the arch.What i would like to do is use a thick core and thin skins but am finding it hard to find core thicker than 1" so i am thinking of going with 2 layers of 3/4" core with just a bedding layer of mat between, i have not seen this done but dont see any reason not to, what do you guys think? More camber would be better, a post in the middle would be great, but i dont have the ok for that yet.

Steve.

 

[Herman]

Stacking cores is not a problem. OK, you have double the work bedding the core, but technically there is nothing against it.

 

[Steve W]

Thanks Herman, thats what i thought, but its good to have someone knowlegable confirm.

Steve.

 

[waikikin]

I have a fair sized hardtop to build for a powerboat, it will be about 8 1/2ft x 16ft, it will be molded in a one off female mold using gelcoat,polyester resin and a fairly standard mat and biax layup with probably a balsa core. The hardtop will be supported on an existing radar arch and four ss posts. The only problem is that the arch that dictates the camber does not really have much itself and the four posts are around the perimeter only with nothing in the middle, oh,this thing extends forward from the arch to cover the flybridge area with about 3ft cantilevered aft if the arch.What i would like to do is use a thick core and thin skins but am finding it hard to find core thicker than 1" so i am thinking of going with 2 layers of 3/4" core with just a bedding layer of mat between, i have not seen this done but dont see any reason not to, what do you guys think? More camber would be better, a post in the middle would be great, but i dont have the ok for that yet.

Steve.

It's nice to mold a return down around the sides, this gives the edge some extra "goodness" & can fasten clears etc, you can mold a nice liner from the outside surface too, sometimes you need to cut a little from the middle to get it in, a nice molded conduit can cover this & good for lighting etc. Try & plan for conduits & inserts for bolting within the outside edges. Jeff.

 

[edmundps]

If your female mould has a sufficiently wide lip around the edges, it would be a good idea to use a vacuum bag to ensure proper adhesion between the layers and drill lots of holes through the core material layer(s). Ideally, I would prefer to use epoxy and something like 15mm Airex foam which would be light, stiff and strong and not involve very much material. But, if this is excluded on grounds of cost, and assuming the roof is not going to be subject to impact loads, my next choice would be epoxy and 25mm styrofoam. Polyester and balsa would be my last choice as the combination will be heavy and not very strong to the extent that its weight will increase the tendency to sag. If you were using a vacuum bag and you wanted a thicker core, you could preform your multi-layer core using a polyurethane adhesive between the layers.

Edmund

 

[Ilan Voyager]

I would not use styrofoam for nothing. Simply not suitable on a boat and not strong enough in shear.
On these sizes and weights a 3/8- 1/2 core is largely enough. A nidacore honeycomb would be suitable for that. No need to make the expense of vacuum tooling on a simple hardtop which only to stand up his own weight and the wind, and it's only 8.5 feet wide. A few sand bags will do the job. The problem with foams and balsa is the absorption of resin that kills the theoretical advantage of weight in such light stressed structures, the nidacore has not this problem, the weights can be controlled
It can be done in polyester with coremat if in the design you make some rigidifiers on the edges and center. But the inside (at the view of the passengers) wont be as neat that with nidacore or sandwich.
You have not interest to make a too rigid hard top or it will crack because the hull under in not absolutely rigid. It's a well known structural problem, so you have advantage to some light flexing and fixations that allows a bit of movement to dissipate the stresses.

Finally you're just making a hard top not a hull or a war tank...

 

[edmundps]

It is true that Styrofoam should not be used with polyester resin. Used with epoxy in sandwich panels that are not subject to impact loadings, it works fine. All composite panels benefit from being vacuum-bagged. No tooling is required providing there is something around the perimeter of the panel to which the bag can be sealed. The tacky tape used for sealing is only 12mm wide.

Edmund

 

[Ilan Voyager]

The problem with polystirene is shear resistance, far too low, and it's crucial in a sandwich (I'm naval engineer with 30 years of experience in sandwiches and honeycombs...). Vacuum needs suitable pump and it's not cheap. Furthermore on a simple hard top I do not see the need of epoxy, vacuum (and the expenses involved as peel ply, bleeder and so on) I would keep it MISS, simple, stupid and cheap. It's just a "sheet" of material to protect the passengers from the sun and rain.
The design of where to put the material to get enough strength is far more important.

 

[Steve W]

This is not the first hardtop i have done, the last one i built was for an outbound 43 but was quite a bit smaller and i was free to put a decent camber in it, (3/4" to the foot if i remember right) i used a 3/4" balsa core, matched the boats gelcoat color and turned the edge down so the visual thickness was 2" and we could hide the channel for the roll up sides, i did run conduit in the core for lights and for the wiring for the solar panels and also molded in cutouts and bosses across the back edge to accept a 1" ss tube which we bent in 2 planes as a hand hold, it was quite tricky as it required parts of the mold to come apart with the part because of both concave and convex radii, i also molded in a recess for a fixed lexan window for viewing the sails and windex,it was very sucessful and looks like it came with the boat which was the goal. However, this one is bigger and may be restricted to the camber of the radar arch which will support it about 2-3ft in from the aft edge, i would like more camber and will push for it.
Edmund, for a female molded part polyester or vinylester makes much more sense to me, i can have gelcoat mixed that is a perfect match to the boat and not require a tie coat,i personally believe that one can achieve a better resin/glass ratio (in a hand layup) with poly, it just wets out better than epoxy, it is much easier and faster to work through the process without having to wait forever for epoxy to cure and not having to deal with blush if you miss your time windows (due to slow cure) Epoxy and styrofoam would not work at all in a female mold situation, while epoxy does bond to styrofoam after a fashion the shear strength is very poor and the foam itself is weak, as i explained, this top has less camber and less supports than i would like to see which is why i am looking at using a thicker than usual core and when you go very thick with the core i feel you also need a core that is not going to shear within itself, if i were using foam i would probably want to go a little more dense than the usual H80 although if i were to laminate 2 layers, maybe not, anyway this is why i will probably use balsa. This top does not need to take a lot of impact so it doesnt need thick skins but it does need to be able to support a couple of women sunbathing, not jumping around, i am going to try to talk the owner into a post off the steering console,that would do more than anything to stiffen it.

Steve.

 

[Ilan Voyager]

This is not the first hardtop i have done, the last one i built was for an outbound 43 but was quite a bit smaller and i was free to put a decent camber in it, (3/4" to the foot if i remember right) i used a 3/4" balsa core, matched the boats gelcoat color and turned the edge down so the visual thickness was 2" and we could hide the channel for the roll up sides, i did run conduit in the core for lights and for the wiring for the solar panels and also molded in cutouts and bosses across the back edge to accept a 1" ss tube which we bent in 2 planes as a hand hold, it was quite tricky as it required parts of the mold to come apart with the part because of both concave and convex radii, i also molded in a recess for a fixed lexan window for viewing the sails and windex,it was very sucessful and looks like it came with the boat which was the goal. However, this one is bigger and may be restricted to the camber of the radar arch which will support it about 2-3ft in from the aft edge, i would like more camber and will push for it.
Edmund, for a female molded part polyester or vinylester makes much more sense to me, i can have gelcoat mixed that is a perfect match to the boat and not require a tie coat,i personally believe that one can achieve a better resin/glass ratio (in a hand layup) with poly, it just wets out better than epoxy, it is much easier and faster to work through the process without having to wait forever for epoxy to cure and not having to deal with blush if you miss your time windows (due to slow cure) Epoxy and styrofoam would not work at all in a female mold situation, while epoxy does bond to styrofoam after a fashion the shear strength is very poor and the foam itself is weak, as i explained, this top has less camber and less supports than i would like to see which is why i am looking at using a thicker than usual core and when you go very thick with the core i feel you also need a core that is not going to shear within itself, if i were using foam i would probably want to go a little more dense than the usual H80 although if i were to laminate 2 layers, maybe not, anyway this is why i will probably use balsa. This top does not need to take a lot of impact so it doesnt need thick skins but it does need to be able to support a couple of women sunbathing, not jumping around, i am going to try to talk the owner into a post off the steering console,that would do more than anything to stiffen it.

Steve.

That was a pretty complicated one...with lot of requisites.
Look at the Nidacore site, it's good stuff that I know since ages and the guys are very helpful in structural problems.
Nidacore can be cambered and it's very very resilient and strong (viscoelastic is the word). Very easy to work and well priced. Look at the site, you'll get all the infos. In this case go to a 1.5 even 2 thick one with very thin skins. You can put all the wiring inside in a tube...Oh yes a post in the"middle" would be very helpful...
Allow some flexing that helps to avoid cracking, the disease of hard tops on flexy hulls.

 

[Steve W]

Yeah, Ilan, sorry, i forgot to mention in the first post that his wife wants to sunbath up there so it needs more stiffness than otherwise. I had thought of Nidacore and have samples but it did not seem like it would be easy to lay into the camber with the remay on both sides, it seems that it would require bagging which is no big deal, but a lot more dicking around than just laying in the balsa. Lets just say i can increase the camber to 3/4"/ft with a spacer on the arch, would the 1.5" nidacore bend into the mold without too much difficulty, i dont mind bagging it if need be. What do you think a good laminate would be? Im thinking gelcoat, 3/4 oz mat skincoat, 3/4oz mat, light uni fore and aft, uni across, uni fore and aft. 3/4 oz bedding layer, 1.5" nidacore,3/4 oz mat, uni across, uni fore and aft uni fore and aft using vinylester instead of poly, then drill and inject the cells to form a pad where items will be fastened, with vinyl/milled glass fiber.

Steve.

Steve.

 

[edmundps]

Sorry. I had thought this was a matter of a simple cover. If there is the possibility of two women sunbathing on the roof then I agree that the shear resistance of Styrofoam would be insufficient. I'm still not all that thrilled by the use of lots of balsa, though, which doesn't have all that much shear strength either, especially if you're using something like Contourcore, unless you drape it both ways over a barrel and bung resin between all the little blocks. However, there's nothing to stop you having a multi-layer core with 12.5mm balsa on the top doing the impact resistance and a couple of layers of pvc foam underneath. The more layers you've got, the more likely the panel is to retain its camber unsupported. It would also reduce the amount of glassfibre needed in the skins.

I quite like the idea of Nidacore, depending on the price, but you'd have to find a way of avoiding filling all the pockets with resin - especially if vacuum bagging.

Edmund

 

[Steve W]

The samples of Nidacore i have, have remay fused to both sides which i would think would prevent the cells from filling with resin and of course give you your bond strength, it seems to me that selectivley filling cells would be an easy way to create solid areas under hardware. Im going to explore Nidacore further. Otherwise while balsa is the heaviest of the cores its easier for me to get and has excellent shear strength.

Steve

 

[brian eiland]

I think you may be over engineering the situation to some degree. Why does this roof structure need a core with big shear strength? What big shear forces is it going to see? It does need to be 'stiff' enough to resist bending, and strong enough in compression to hold those people waking and lying on it.

PP honeycomb is plenty strong for that. You are up in Minn, somewhat near Mich,...why don't you consider Plascore
http://www.plascore.com/honeycomb-thermoplastic.php

......likely cheaper than Nidacore, now that 3M bought them
.....and they have a ready made 'infusion grade' PP core.

I plan on spec-ing the deck and whole superstructure of a 40' coastal trawler out of this PP honeycomb material.....and building those pieces on a 'table' like this KSS method I outline here:
http://www.trawlerforum.com/forums/showpost.php?p=175507&postcount=109

..... this guy built a whole vessel of it.
http://www.buildacat.com/lyra1.html