Lightest hard center deck for a small cat

As I may have mentioned, I'm about to start building a 20 foot cruising cat. The boat will fold for trailering, so the deck must also fold.

I want to keep it very light, as this is a performance oriented boat. On the other hand, a hard center deck is nice for cruising, as it makes using the center deck as an outdoor/indoor space at anchor much more appealing. It also keeps the ride drier.

The deck is only 5 ft. X 10 ft, in two sections split the long way. So far, my best idea is a wooden skeleton, filled in with a couple inches of styrofoam and faced with glass or maybe glassed doorskin. Does anyone see any problems with this approach? Better ideas?

The above idea appeals to me also because of the amount of flotation it would add.

Ray

 

You could use foam sandwich construction and remove the wooden portion of the construction all together, it would probably be a slightly more expensive option but having recently worked with corecell foam building a deck on a friends boat I've been very impressed with how stiff and light the construction method is. The only downside I could see is you spend a bit more time fairing foam sandwich than your door skin method.

 

Corley, I think you'd be right, if the deck could be bonded into the hulls, but as this is a folder, the deck halves must rest on relatively small projections along hull topsides and center spine. I worry about point loads. The wood frame I'm thinking about is really just a perimeter frame with a few cross members-- shouldn't add too much weight.

 

I imagine a perimeter frame would not add much weight allthough you could still go the composite method and use endgrain balsa at the highly loaded areas and foam in between, the balsa gives compressive strength to support the laminate and the foam is a little lighter. I suppose it all boils down to how light it has to be. A lot of boats I've helped out on have a strong focus on racing so they follow the Ian Johnston tradition of cutting off toothbrush handles to save weight.

 

Ditch the timber but especially the styrofoam

Gday Ray

No worries about point loading around the edge of foam. After you have put the glass on just dig out the core about 2 cm and replace with filler around the edges. If you post some pics I can understand better but it should be no prob in foam.

The great thing about foam is stiffness and lightness. I like ply until it needs stringers or supports and then I am into composite. Corecell, Divinycell, Klegecell - its all the same to me. BUT STAY AWAY FROM STYROFOAM.

This is one of the many stories about stuff ups I have made. I can list them because I have since removed the piece from Kankama. She is pretty good now.

I used the densest EPS I could buy and put it in the dogger roof. It was not good. Even though it had 4mm ply on the bottom and 5 mm on top. It just squished a bit.

It makes no sense to make something you have to redo later. Go down to the various foam suppliers and start talking. I have a thing with the local guy. Strictly cash but good prices - I feel like a drug user talking about his dealer.

Use about 15mm foam with 600gm biax for the top and 400gm (or 600gm) for the bottom - stiff and solid.

About point loading - my engine nacelle pivots and has little ears which limit its movement. The ears touch about 5 cm squared of deck for each one - there are two. The core is honeycomb and it has about 6 layers of glass over it but just doesn't move and the load from the 25hp fourstroke and waves is pretty massive. Go composite Ray - then think about chainplates too

cheers

Phil

 

Catsketcher, when you say Styrofoam are you referring to Dow Corning 2.25 pound density blue closed cell foam (used for bouyancy billets and waterproof insulation), or are you referring to beadboard (the generally white stuff they make cheap coolers and cheap insulation out of)?

 

Phil, thanks. That's very helpful info.

Here's a sketch, though there have been a few changes since I drew it:



The cabin is lower and extends aft a bit more. But the center deck is the same, divided into two sections lengthwise, each about 2.5 feet X 10 feet..

 

The good foam is pretty pricy. Lots of surfboards and sailboards are made using EPS sometimes with better foam laminated to that. since your span is so small, you could do a blend of techniques, ply, foam and some stringers.

I'm thinking of what to do with my cat & the span is more like 53". Cedar on stringers, composite, or ply foam ply?

Dan

 

Deck

FWIW,
Here are a few shots of the middle section of my 3 section bridgedeck and the beefier stringers for the two side sections. These are for my 30 foot cat project. The center deck section is about 44" wide by 11' long. It is supported by the port and starborad deck sections which have similar , but beefier stringers that run fore n aft where the sections mate. The lateral stringers on the center section in the photo are 3 layers of 1/2" corecell with a hollowed out trough on the bottom where I laminated a bunch of 12k carbon tows, before bagging with 20 oz carbon 12k twill.

The picture of the the two stringers shows the ones that run fore n aft, about 11' long. They are made of 4 sections of foam and hollow, comprised of a web, 2 canted sides, a top and base plate, all of them 1/2" corecell. These were laminated after hogging out a trough for tows. You can see the tows being bogged in. Each stringer was then bagged onto the inboard egde of the port and starboard deck sections with a large wrap of 20 oz carbon to give the side sections their own strength fore n aft, as well as strength to support the center section.

I have it all temporarily assembled and it seems really sturdy, yet quite light.


bax

 

Darth - the foam I used was a denser form of the white stuff. A mate has used the extruded polystyrene for a stern extension. Having ditched a few items I am a great believer in using high quality material in boatbuilding. A thin deck (25mm or so) such as the one Ray is building will put high shear loads on the foam. This is why you would need a high quality foam that can handle shear properly. To get the shear loads low enough for the cheaper foams you would have to increase the thickness until the shear load was acceptable.

cheers

Phil

 

Ray, another core material to consider is the resin impregnated kraft paper honeycomb, sandwiched between thin ply such as doorskin, there are several manufacturers,verticell is one i have used. Regardless of what core or skins you choose you will want to rout out the edges and either backfill as Phil suggested or glue in wood strips, i prefer the wood strips as it is lighter and stiffer than filler.
Steve.

 

Phil is right regarding the foam, better to buy any one of the core foams. However,if you go the polystyrene route you would want to use some additional stringers within the panel as well as the wood around the perimeter to take care of the shear loads.
Steve.

 

Phil, you make an interesting point. One of the reasons I'd considered a sort of composite of inexpensive foam, doorskin, and glass is that a foam deck of say 2 inches in thickness would provide a lot of flotation without a lot of weight. It would also, I think, be in a good spot to keep much of the hulls out of the water in the event of a capsize.

I had thought about box beams that curved upward along the top, filled with foam, which would float a capsized cat even higher, but the disadvantage is windage, which is already a concern, due to the necessity of sitting headroom in the cabins.

The rig is from a Nacra 5.2, but at 20 X 12 feet, the new boat ought to be pretty hard to capsize. But it would be dumb not to take the possibility into consideration.

 

That's why I thought the EPS they build surfboards might work well here. Thick is fine for the flotation Ray is looking to get, thick make the sheer loads drop quite a bit, and the span is less than 3'. Would corecell or similar be better? Sure. Surfboard and sailboard manufacturers have gotten around the crap core by using thickness necessary for flotation and also by laminating a tough outer layer on the side to be stepped on--point loads. That could be sheet foam in lesser thicknesses or ply.

Note that this is not a "structural panel" other than needing to be stepped on.

 

Hard deck

Dynamic loads can be considerable even with just a couple of people on a hard deck. In my opinion, it is worth going with a core that has good compressive strength as well as shear strength paired with a good fabric. 1/2" corecell (A500) and 20 oz carbon twill is extremely strong and weighs only 3/4 lbs / sq ft when bagged with epoxy. If your deck area is 11 by 3, that's a 33 sq ft section that will weigh only 25 lbs before adding stringers and finishing. The stringers shouldn't add more than 5 to 10 lbs tops.

Not cheap, but light and durable.
bax