Gunwales, Keel Strips, Central Spine, Keel out of Foam Sandwich instead of PlyTimber?

Hello,

so I am in the process (the beginning rather) of building a Woods Design Quattro 14 beach catamaran. I am new to boat building and have a few questions on the materials. I talked to the designer and got information on how to use the foam sandwich method with the plans, the only thing I am not sure about are the gunwales, keel strips, the central spines and the keel.
The plans ask for 2"x1" for the central spine and for the keel strips its 1"x1" lightweight softwood, like sitka spruce or yellow cedar. Does it make any sense to replace the wood for those parts with a foam sandwich too or would this make the whole catamaran unsound because of structural reasons?
The keel itself is normally done with two times 12mm plywood glued together to make it 24mm, would it be ok to use 2x15mm Divinycell as a replacement?
Here is a link of a drawing of the Quattro 14 for reference: https://www.sailingcatamarans.com/ASSETS/quattro14.gif

Thank you in advance for any help!
- eam

 

Welcome to the Forum Eam.

Your link does not work - at least it does not work for me.
Here is a link to the Woods Quattro 14 -
Sailing Catamarans - Quattro 14ft single trapeze beach catamaran http://sailingcatamarans.com/index.php/designs-2/46-beach-cats-and-dinghies/6-quattro-14

Richard would be the best person to ask re your questions above - he posts on here occasionally.

 

Thank you bajansailor, you are right with asking Richard, i just figured its weekend and I don't want to annoy him over every small thing and try a forum instead.

 

The keel of 2 x 12mm ply would be good to be replaced by foam for sure , you save a lot of weight and when you hit something or beach it it's not a big deal .

Replace the 2 12mm ply with 2 12mm foam glued together with thickened epoxy and coat with 400g bias 45/45 (with Richards blessing or adjustments ) tape the keels to the Hulls with strips of same 400g bias glass with lots on Hulls and on keels ,say 3" , not sure of metric ....

The other strips can also be replaced the same way ,Richard will inform on weight of cloth to lay on keel /both sides to stiffen in lieu of timber .

Substituting foam for 4mm ply ,sounds like RW has already informed you on the specs for this area ,(9mm foam & 400g bias both sides?)

The foam & glass will be heavier than the 4mm ply & epoxy with light glass sheathing one would think ,more work by far ....but you get a foam Beach Cat !

 

Stager the Keel strips , one short & one longer overlapping onto keel and hull.

Most likely the keel strips to replace timber will be 2 x400g bias glass 6" wide staggered or less both sides ,full length .

 

thank you too for your help, jim.

for future reference in case anyone else ever has these question ill add the info i got from richard here:

- for the gunwales: L-shape made out of glass, 25mm x25mm x 3mm the length of the hull
- central stringer: "You shouldnt need the central deck stringer. Instead you can fit a central glass stringer say 25mm deep (maybe over paper rope or cardboard tube cut in half) or more of that L strip if you make the gunwales that way.
But i would try dry fitting the deck before adding the stringer just to see if you are happy with the curve without it. You would probably only need the stringer between the beams"
- keel: 15mm foam and glass over 3 x 400g overlap along the bottom edge so they get doubled. Glass to hull 3 x 400g say 70mm wide strips
- for the keel strip its a bit tricky to get it stiff enough to help join the panels in a fair line initially - i might just use some wood here in the end as its non structural, or maybe i can figure out how to make glass or glass/foam stiff enough by experimenting a bit