Hard top lamination thickness help

[Manuel R Alonso]

Hi, Going to build a hard top for my boat. Something very similar to the attached picture, except it is a little longer. Size will be about 8'-6" x 16'. Would like to support at front windshield and rear aluminum structure similar to picture, except it was a part of a tuna tower previously so the "ladder bars" are in an angle pointing towards the center of the boat and are connected to each other, so it is very strong and will not allow side to side movement. Going to build a one off mold and give it some crown, like 3" higher in the center than the sides. Using epoxy and thinking of 3 layers on 1700 on top, 1/2" of scored 5# pvc foam core, and 2 lavers on 1700 on the bottom. Also going to glass in 3 channels made of 2" PVC pipe cut in half, tabbed in with a layer of 1700 running from front to back, which I would assume would add support, and second as a wire chase. On the 4 sides. I would like to turn down about 2" Will that be strong enough ? I already have the foam and 1700 so would like to work with what I have if possible. Thanks

 

[bajansailor]

This should be a very neat project Manuel.
Can you post a photo (or two) of your boat for reference please?

Re the length of 16' - assuming that the forward aluminium leg of the aft support structure is (say) 4' forward from the aft edge of the roof, then you will still have a 12' long (maybe longer) unsupported span which is quite a lot really.
Although having 3" of camber across a width of 8'6" will certainly help to stiffen it up somewhat.

Re your half round channels made from 2" diameter tube - they will only be 1" deep. It would be more effective (re stiffness) if you used a square or rectangular section instead - but then you have sharp corners to contend with when laminating over it.
Adding a flange around the perimeter of the roof will also give it some extra stiffness, but I am wondering if a depth of 2" will be enough? If it is deeper would you have issues with people bashing their heads on the roof flange when climbing out over the side (eg when coming alongside a dock)?

 

[fallguy]

The layup is a bit off, to be honest. The framing below you have not been real clear about, but 8' wide unsupported is too much for the layup cited.

I am building now a 53" wide by about 65" long hardtop and it is supported by aluminum tubing welded across to each side and it also has the crown, so was able to use the frame as the jig.

Without a framework and at the widths cited; you will need much thicker foam and about the same layup, although the glass would need to be more on the bottom side and you probably want to finish in a uni or hexcel 16-18 oz woven just to make it easy to finish without fairing much.

My top is made from 3 sheets of 6mm foam for half the width. I made a 12mm gutter around the edges because I did not want water flying off the thing, so mine is 30mm of core on the edge. I recommend you make yours much thicker; at a minimum 24mm, but perhaps up to 48mm of core plus the gutters and any turndown. I opted for no turndown on mine..

it will also need high density inserts as typical foams have no holding power

here is before gutters were added...I have to finish the edge and glass the gutters; this does have a 3/4" crown in the metal and foam and 3/4" coosa 20# density inserts for screwholding and bolting horn and lights and flir and radar

 

[fallguy]

Here is the framework under. I think for my spans, a full inch of foam would have worked no 4' crossmembers and we probably could have gotten by with fewer crossmembers as well woth the 3/4" thickness I made.

 

[fallguy]

Also, not a big fan of scrim here as you've shown. I admit I don't know if regular core will make a 3" crown, but scrim is a pain and too thin anyhow.

The better way for you will be to build up probably three layers of 12mm foam. It is a lot of work and you'll have to work fast, but a vee trowel and thickened epoxy with cabosil can be done. You'll need to spot it with screws probably for such a big panel and so I'd do it in two-three steps. One day, edge bond the panels on a flat surface. Next day, clean up the edges with a sander and trowel with a 1/2mm or 1mm trowel and add the next layer. Be warned, a 1mm vee trowel is gonna require a lot of epoxy..use slow epoxy at low temps and keep the epoxy on the board and spread out so it does NOT kick before you get the next panel down. Use some 1.25" construction screws to hold the panels together; those holes fix easy; a void or hump is going to be a nightmare to repair..you could even deep spot a few 1.25" screws on the first panel on the jig and just rip them out of the foam when done

Or, as an alternative, you could be thicker foam and screw it to a jig on the bottom and glass it.

In a built up, then cut in your inserts, or alternatively, you can make raised inserts or pods for hardware..

In a solid, glass the top probably with 2 layers of heavy hexcel woven; then let it cure at least 3 days, preferably a week before flipping it and removing from the jig and glassing the bottom.

I have only done one of these, so wait and see of you get another response or two.

Also, post the way you intend to support the middle of it. If it is only foam; gonna be a problem and probably need 2" thick foam or it'll sag on you I'd say.

ps-avoid edge bonding ahead of time on the length of the width and only edge bond on the radius on the jig; those will not bend well ....okay? If you screw; you can also edge bond on the jig if you are fast

 

[wet feet]

I have no idea why you would want to use epoxy unless your local supply is a lot cheaper than those I have ever found.The boat will almost certainly be polyester and I see no advantage beyond bragging rights to using anything different for a hardtop.I don't ever understand what I believe are American descriptions of layup since most of the world uses grams/sq.metre.It is usually considered good practice to use a balanced laminate on either side of a core and for this application I would guess at 1800 gsm on either side of the foam and given that there is a plan to use a split tube for cable runs,I would include four or five transverse beams made from 2 layers of the same foam.This could be in strips perhaps 3-4 inches wide and with the corners radiused to suit laminating.It wouldn't be deeper than the split tube and would reduce the unsupported span of each element.You might want to include a pad of something a bit tougher where hardware is to be attached as the foam core might crush.If female moulding the part it will be crucial to get a good bond between the outer laminate and the foam core,what will you be doing to ensure such a bond?

 

[bajansailor]

@Manuel R Alonso Google tells me that the zip code in your location is in Florida, so you shouldn't have to worry about the possible effects of snow building up on this roof.
Is it just going to be a roof to keep the rain and sun off, or are you planning on building a flying bridge as well?
If it is just going to be like a large bimini awning, it might be easier to put in some more aluminium structure which would then allow you to have lighter scantlings on the foam cored roof?

 

[fallguy]

I have no idea why you would want to use epoxy unless your local supply is a lot cheaper than those I have ever found.The boat will almost certainly be polyester and I see no advantage beyond bragging rights to using anything different for a hardtop.I don't ever understand what I believe are American descriptions of layup since most of the world uses grams/sq.metre.It is usually considered good practice to use a balanced laminate on either side of a core and for this application I would guess at 1800 gsm on either side of the foam and given that there is a plan to use a split tube for cable runs,I would include four or five transverse beams made from 2 layers of the same foam.This could be in strips perhaps 3-4 inches wide and with the corners radiused to suit laminating.It wouldn't be deeper than the split tube and would reduce the unsupported span of each element.You might want to include a pad of something a bit tougher where hardware is to be attached as the foam core might crush.If female moulding the part it will be crucial to get a good bond between the outer laminate and the foam core,what will you be doing to ensure such a bond?

I like the wider foam core section stiffeners. Bravo. Do those start on the very edge because then you need to build the edge core thickness as well, but the idea has merit once details are settled.

Not following how a for an aft pvc tube works with these as I see them intersecting. A tube could run on the top or tubes could be on the edges, but not cut into the core really without affecting rigidity.

600gsm db is same as 17-18 oz/yd db...sometimes it varies a bit here on weight

and it doesn't need to be epoxy; I always default to it, but he could easily hand laminate with epoxy and not need vac

 

[kapnD]

A full aluminum frame will insure adequate support, and allow a lighter covering to be used.
Large spans of FRP require very heavy layup, as well as some sort of stiffeners.
Here’s how I did mine, first a sturdy frame of 1 1/4” aluminum tubing and 3” channels, covered with Herculite fabric on top and glassed 1/2” ply on sides.
The aluminum structure came in just over 100#, the fabric about 20#, not sure how much the sides weigh.
Turns out the glassed plywood portion was by far the most labor intensive part of the build, in hindsight, should have done the whole thing in aluminum!
The boat stays in the tropics, so insulation is not an issue, but ventilation is very important.

 

[wet feet]

I had a few minutes this evening to roughly model a version of what I attempted to explain earlier.If the intended image file displays correctly you will see the two green longitudinal sections representing the glassed in split tubes for cables.You would add even more stiffness to the structure if you included a 3" unidirectional tape under the final layer of glass surrounding the tubes.Then you could shape a couple of strips of foam to something like the shape shown and add the glass-again a strip of unidirectional tape would be helpful beneath the final ply of glass.The unbroken inner surface of the plastic tube will make threading cables quite easy and the continuous fibres over the top will help the structure.To be clear,you should completely bond the longitudinals before adding the transverse stiffeners.