How to build a cockpit with highly compound curves

[Roland Stockham]

I have a very technical challenge. I need to replace the cockpit seats on a canoe stern sailboat. The original cockpit is U shaped following the hull outline and the seats were cut to fit from flat sheets of plywood. It worked OK but would be more comfortable if the seat profile was S shaped. Nice roll over at the front edge, rear slope across the seat base the curving into the seat back. If the seats where straight this would be no problem but forming this shape around the curved back of the cockpit is another matter. Could be done by building a mold but that. Is a lot of work for a one off and would also be heavy if done in solid glass.
Looking for suggestions as to how this can be done using a foam or Bolsa core without a mold?

The alternative is to stay with flat seats then build a slatted seat base to go over it to create the profile but it would be more elegant if it can be done in one piece.

 

[fallguy]

Strips over male stations. Choose your material, okume or foam or cedar. The stations would be made with maybe 12mm plywood or glassed 4# marine foam. You'd need to pre-release some flanges probably or make some clunky cleats for some glue space for the strips.

You'd tab the stations in place; then do the stripping work and the glue cleats.

the s won't allow much, but perhaps some simple opening and drains would allow wet storage

the angle of the U is some challenge, you could make a station at each corner or make an angle, etc

obviously, you have no access to the inside; so your strips need to be strong enough without sheathing after

 

[Blueknarr]

Make a strip of your seat profile longer than the outside perimeter. Glassing only the bottom.
Cut it into wedges. Temporarily install. Fair the seat to your satisfaction. Glass the topside.

The backrest and butt rest could be in either one or two pieces

 

[Will Gilmore]

What is your preferred material? Do you want wood or glass or composite?

Fallguy laid out the basics. I use to work for a stair company. Spiral and curved stairs were always an interesting project.

Frame it out in place, and use thin stock, steam if you need to. Time and patience are the most important components.

Glass would probably be the simplest. Wood isn't that hard and it's cleaner to work with than glass. Build it like a strip planked canoe.

Don't hesitate to use a form if that will give you the best results.

Can you post a picture of the cockpit?

 

[C. Dog]

What about epoxy/glass over shaped polystyrene foam?

 

[wet feet]

How big is the boat and what sort of total depth is under consideration?I would discount polystyrene foam but attaching pieces of thin Airex to a skeletal frame and skinning one face before flipping and skinning the other side might be a solution.

 

[TeddyDiver]

My pick would be 10mm Divinycel. Cut to stripes narrow enough to follow the curve..

 

[wet feet]

Divynicell,Corecell,Airex doesn't really matter but the principle is the same.The size of the boat will have an influence on the severity of the curves needed.

 

[Tops]

What about epoxy/glass over shaped polystyrene foam?

I thought about this too. Shaped EPS foam form left in place, glass and epoxy, a thin core over that (people stand on cockpit seats, needs to be sturdy) and covered with more glass and epoxy...?

PS Pictures of the cockpit as-is would be really helpful

 

[TeddyDiver]

Another method which allows more complex forms. Wood strips layed as base grid, fabric stapled over, plaster to form final "mold". Laminate over or make sandwich laying endgrain wood blocks first and laminate over. Turn over, rip the grid and plaster, brush and another laminate if sandwitch. Can place whatever integrated drink holders you like..

 

[fallguy]

If you give us a picture of the cockpit; we can sort of help better with some crude finger sketches.