Help - New seats extends over the water

So I need some advice from you guys!

My plan is to build two seats on my aft deck that extends outside the deck and hangs over the water
The aft deck is very narrow and at the moment it doesnt feel safe and the space feels wasted cause not usable. So, I would like to put some seats there. As the deck in so narrow, I try to not fasten anything in the deck because it will steal so valuable space. At the moment my idea is to have the seat to start about 7cm in to the deck

You will see my sketches attached. Haha, ok, these are sketches I have being playing around with, so sorry for all the clutter.

My idea is having a seat that is about 70cm long, bench depth 38c, and back support 37cm.
I want it to be as light as possible, but I cant build in any high tech material like divinycell, corecell etc, cause it's not easily available here in the tropics.

So my plan is to build it out of ply and glass. The seat itself I was planning to use a 15mm plywood, with glass on top of it (20-30oz on each side?).

My biggest concern is how to build the structure that is gonna support it.
One idea is to build "beefier" boxes in L-design with stringers inside. Glassed on the outside. Big radius. Either one on each side of a seat (15cm wide?). Or one wider in the center (35cm?)?

Or should I go with something solid plywood from the seat back support in 30 degree angle down to the hull? And one from the front edge down to the same point where they merge?

I want to build it as safe as possible. Having the support catching as little water/waves as possible.

My hope was to keep the weight down to maximum 10kg.

If anyone has a total different suggestion how to design em Im open for suggestions. But it has to be with material I have access to here. (polyester, epoxy, glass, ply, pink foam etc.)


Edit: Uploaded pics where it will be installed.

 

Whereabouts in the tropics are you located? Your 'Location' mentions Sweden......

It looks like you want to put these seats on the aft deck of the main hull of a trimaran (?)
Can you post a photo or two please showing the area where you want to mount these seats?

 

I don't doubt that the ambition can be achieved within the weight target but this may be affected by the weight of the particular 15mm ply.The doubt that I do have concerns the concept of a seat hung on an effective hinge as the base does not seem to have anything close to enough effective span athwartships.An additional leg,parallel to the outboard leg,would make a lot of difference.Alternatively you might use a pair of diagonal braces from the arms of the seats to the centre of the transom.Have you considered using a foam flange on the edges of the seat portion for longitudinal stiffness?The inboard facing down and the outboard facing up,as this would provide support for both edges of the panel bearing the weight of occupants I would guess that 4cm deep would be enough.

I was a little surprised to see a poster from Sweden using ounces for glass,I would have expected gsm.

 

You might be able to get some ideas from looking at the gallery for Cutwater 30 boats. They have 2 seats, built into the gunwale, that do just what you want to do

C-32 C | Cutwater Boats https://www.cutwaterboats.com/models/model-category/c-32-c/

When folded, the finished position makes the seat part of the gunwale that look just like a padded bolster. You may have to look at other pictures or check google for a video tour of the Cutwater 30 or 32, pretty much the same boat

 

I would run two aluminium poles across the deck at the transom and aft of the cockpit well.
They could be on the deck or through the side wall just below the deck (with reinforcement).

On the poles you can lash your seats. Use your imagination here, turn the pontoon over and cut a seat out of the hull (not really, it just to get you to use your imagination):


You can lash the poles to the deck as well:


In this case, the poles can be easily removed.

Regards AlanX

 

Hehe, Im from Sweden, but located in the tropics But yes, I wanna put seats on the aft deck on the mainhull, that extends over the water.

I didnt really underastand what you ment here
I have been considering having a frame along the edges of the seat to stiffen it ( if needed?)? But I thought 15mm ply with glass of a span of 70cm would be stiff enough?

Yeah, I think I spent to much time outside europe with my boats, therefor so used to oz...but I dont mind "gram per sqm" either

It's an interesting idea! thanks!


I have now updated with photos of my aft deck

 

Thank you for the photos - they help a lot.

Do you want these seats to be used by the helmsman while steering?
I am assuming that they will be aft of the mainsheet track, on the port and starboard sides of the deck hatch?
If so, it looks like the tackles on the running backstays might get in the way a bit?
Or have I got it all totally wrong re my interpretation of what you want to do?

 

Well, thank you for trying to help out

Yes, aft of main sheet track by about 15-20cm. Exactly! One on each side of the deck hatch!
Yeah, it would be nice being able to steer from there as well. The runners wont be interfering with the seats, that is confirmed cause the seats wont go all the way back. There will be about 60cm from the transom edge to where the aft part of the seat ends.

Hope this helps

 

I looked into this a little bit....could be a solution. But I might would recess the beams instead so it goes flat with the deck. There is a solid wood beam that goes along each side of the deck and hull, under the aft deck. About 6cm x 12 cm.

 

In order to clarify my earlier remarks I have added an annotation to one of the images in post #1 and created a very crude CAD model to illustrate the point I was attempting to make.The two black lines show the points at which a load will deflect the proposed seating and it should be apparent.The screenshot shows an additional leg to brace the whole structure,together with flanges to stiffen the seating section.I have omitted the seat back and arms because it took me quite long enough to fumble this far.

 

Thanks for taking you the time!

I might have been vague on how my intended solution was going to be on the "hull side". It is supposed to be bolted to the hull. The the "legs" are about 30-40cm from deck level down at the hull. The seat can not extend over the deck more than 7-10cm to not interfere with the deck space to much,

I was first planning to have legs on the front part of the seat down to deck...but I try to avoid that due the limit deck space.