Composite deck insert

I have a flats boat that has a large open cockpit.
I really enjoy having the open space. I want to build a deck insert that would fill in the area and make the cockpit flush with the rest of the cap. While this would still be removable when wanted.
My plan is to make it out of 1/2 H80 divinycell. I would like to utilize carbon fiber. With a 90 degree flange around the edge where I could clamp or bolt it to the existing deck flange. I can add small stringers to the underside running side to side as needed. It is roughly 5’ wide by 7’ long.

I would like this to be strong enough that I could walk on it. 170-180 lbs Not multiple people. Not additional weight, gear etc. I would like to use carbon fiber using vinylester resin in a hand lay up situation. I do not have the equipment to vacuum bag. I am quite confident in my ability’s and using fiberglass. I have done many floors with divinycell and 1708 with success. These were on top of stringers with 24” centers. I would like this deck to be “free floating”.

My main question is carbon fiber lamination schedule to support the weight with 1/2 divinycell. What cloth? One layer? Two layers? With or without supports on the underside.

thank you for the input

 

Welcome to the Forum Dugan.

I presume that this means without any vertical supports (pillars) under the deck?
If you have say 2 supports on the centreline of the boat, that will reduce the unsupported panel size considerably, and hence require less materials to provide the strength required.
The maximum unsupported span longitudinally would then be 7'/3 = 2' 4", and the maximum unsupported span transversely would be 5'/2 = 2' 6".

How often would you want to remove this deck?

 

I have seen a number of removable deck hatches that had foam beams on the underside,which were laminated over to add stiffness and not too much weight.With a longitudinal beam of perhaps 2 1/2" depth that terminates at a deep flange at the aft end,you would have a good start.You could then add some shallower beams athwartships to sub-divide the unsupported areas.For optimal stiffness I would cover the beams with one laminate of the chosen material with the fibres at +/- 45 degrees,a strip of unidirectional along the beams and another ply of woven over the entire beam and 3 inches or so onto the underside of the sandwich deck.At mid span on the aft end you might consider a folding leg that drops down and it could be secured in position with a door bolt of marine grade material.I would have doubts about a single ply of any woven material being very resistant to having things dropped on it,but I don't know what choices you have from your supplier.

Presumably,without vacuum bagging,you will be holding the foam to the required camber and laminating one side,then inverting it to laminate the other side.It might be a little challenging to create an accurate flange to fit the sides to the existing deck but no doubt ways can be found.

 

Thank you for the helpful reply’s.

It would potentially be installed for some short periods of time then removed for normal usage.

Yes free floating as in no drop down legs or supports that would contact the cockpit deck. I understand the immense benefits but would like to avoid the potential wear spots of support leg contacting the deck.

I was planning to have a large flange and the rear of the part that would match the current cockpit flange just as you mention Wet Feet.
I believe 2.5/3” is close to actual. I would need to measure.

The additional foam supports that would be laminated in could be as large or smaller in height.

 

What is the reason for carbon fiber? A small panel like that will have an insignifican weight difference between a carbon fiber vs a fiberglass laminate. 1/2" core will be too thin unless you add stiffeners or supports under. A couple of removable legs in the center would allow you to make a lighter insert.

 

I would like to keep it lighter if possible. I understand there would not be a large weight difference between carbon and glass. There would be stiffeners on the underside. Adding support legs are not an option.

 

For light and simple, I'd cut my H80 to fit and then glue a 4" or thicker piece of XPS foam on the underside set back 2" or 3" from the outside edge and bevelled +/- 45 degrees inside and then laminate some biaxial or triaxial on top and biaxial on the bottom. Only downside it would need epoxy instead of vinylester.

 

So can anyone suggest a carbon fiber to use with the 1/2” H 80 core?

Thanks

 

It would not be preferred because of the elongation mismatch between resin/glass/core. The CF is very low elongation to break, so a heavy point load could crack the cf before anything else in the matrix. CF is 1-2% elong and glass is about 8%, iirc.

If, for example, you have a 3’x3’ area; this is one yard.

On my boat, I used db1200x2 layers each side for a very strong deck on same core or 12mm. Say you used two layers of 5oz cf..

A one yard area hand laid glass would be 64oz resin and glass or 128 ounces or 8 pounds. For a lower quality CF laminate, say 10 oz, you are talking 40 ounces or call it 3 pounds. Sounds attractive, but the CF fibers cannot elongate more than 1%. So the layup needs to be enough to stop that.

And so 10 oz may be too light and you may end up going to 15 or 20. But the reason you don’t get a reply is cf and glass are not really comparable.

The only way to be sure is via testing, and knowing the cf is not allowed to bend as much as the glass. My hunch is 10 oz of cf might be a bit light, so then you move up to 15 or 20 and now you are sliding into 63-80% of the weight of glass for a poorer product.

For my hatches, I actually used a 20 pound density core versus light foam. This was to reduce bending and then I only applied 12oz glass to each side. There is a weight penalty, but my hatches are like 2.4 square feet on average or about 1/4 yd. If you went to a better core with higher strength and rigidity; you could use say 5oz of CF and not worry about fiber failure. Then to compare to mine, you are running 20oz total versus 48 for mine or about 42% weight reduction, but a much heavier core.

 

When they use CF in say the America’s Cup or high speed sailing vessels; they actually anticipate a poorer laminate; so keep that in mind. You want hatches to be pretty durable, even able to install screws and hardware, so a screw holding core may be preferred. The work involved in decoring for hardware is also extra effort for a weaker hatch..etc. As others have stated, the weight savings here is not necessarily a high value target.

 

The missing factor is the weight that may be imposed on the new panel.A laminate that would keep rain or spray out of the well could be quite light but if a 300lb man carrying an anchor is going to be standing in the middle at sea,a more substantial laminate will have to be in place.Then you may need to anticipate what will happen if the anchor,or some other object with sharp corners is dropped...

 

The insert would never need to support more than 200lbs. Even this is excessive.
If something is dropped on it and damaged that is to be expected.

This is basically going to be used as a race fairing.

 

Have you considered making a small test piece-say 18 inches square and seeing how it behaves?

 

Well, everything changes to make it walkable.

An ultralight walkable 5’x7’ surface is something of an oxymoron. Pretending CF is magic cloth is an error. The deck is going to move under load. When it does, you want a cloth that will elongate a bit; the CF will fail at about 1% movement. Over your 7’ span or 84”, 1% deflection is not measured at 7’, but under the load area. So say a foot is 3” wide; the maximum deflection is about 1-2, call is 1.5 percent of 3” or 0.045” or about a piece of paper. Compare to typ glass of 8% or 0.24”. See the problem? The CF seems nifty, but it cannot function as an ultralight component when walked on.

In order to walk on it; it needs to not move more than about 1/4” for fiberglass skins or it will crack.

So, for 5’x7’ using 12mm core is a bit light.

I’m not a composites engineer, but I made a walkable roof with a lower deflection rating than what you would need. Most engineers would spec something pretty thick for walking 5x7.

So, you need stiffeners. You already said 90s on the edges. Is this to make a temporary connection with metal? You cannot put metal fasteners thru h80 and then apply a vertical load; the holes will crush, so they need to be premade. The way to do it is determine fastening points and then overbore them and resin fill. I’d overbore a minimum of 2x to 3x fastener dim. Then do the fills and use a temp something for the approximate center of the bolt to avoid exotherm of the putty.

The structure needs stiffeners. They will prevent the movement from exceeding 1/4”. On my roof, I used 2 3/4” wide redwood stiffeners 3 pieces 1/2” thick over a jig for the shape I wanted. They are only 11” on center, so for 5’ wide; you’d need like 4. There is a lighter method or two. I also built corecell 12mm i beams. These were about 2” wide by about 2” high. Make 2 pieces of H80 2” wide and one piece about 1” wide. Laminate all the sides with something like db1200. You can laminate a panel and all the sides and rip these on a saw as well. Anyhow, assemble with hotglue the three strips with the 1” width in between and then fillet the inside and each join and lay a piece of db1200 inside. The piece will be about 2.5” long and pickup the web and the top of each; do the other sides in a second go. This short web with 4 layers of glass gets very strong and walkable and is quite light. You could try to build the stiffeners with CF because they won’t be point loaded like the deck above. I probably would not risk it, but 5 oz cf and then a 5 ounce piece over would be pretty strong. If you had a vac setup; a tophat over a mould could be lighter.

Anyhow, about 4 of those spanners 7’ long should do it.

Then bond the big panel by prewetting the core edges about 30 minutes and then bonding them together with thickened epoxy resin. Or VE I suppose. After bonding them together; you can sand the bondlines or try to glass it. If it were me, I’d put the join above a stiffener.

I’d glass the bottom with db1200 and the top with 10oz woven. Because the glass is not 5” wide you will end up with a fairing ridge. If you have an electric planer; you can accomodate the ridge with a groove. I suppose you could butt the glass above a stiffener as well, but it is not generally done in any boatwork. But you could here.

Make sure to use a fillet at the intersections of the 90s to the cockpit coaming edges. This is done by hotgluing the flanges in place and then flipping it over for putty bonding; the hotglue stays in place so use little spots like tack welding.

Without formal weight calcs or vac work, I can’t drive it down much lighter.

if anyone wants to critique my layup, feel free

 

Went with 2x layers of a 6k weave on top and thicker single 12k plain on the bottom... because they were surplus drops on the shelf. Haven't had a chance to test as the waters kinda hard here for a few more months. The rationale was double the tops to help with single point perforation from gravel in my shoe tread.

Hard to say if it would be right for you with your weight requirements. I'll probably add more stiffeners as I'm well north of 180, it seems OK for the wife but she's fair bit under 180.