Fiberglass Kayak Structure

I am planning to build a 18.5 feet double kayak with fiberglass. The breadth is 27 inches and the depth is 14 inches.

Will it be strong enough if I use 3 layers (– 1 mat – 1 net – 1 mat) in the skin surfaces and 3 bulkheads as the only stiffeners ( the 3 bulkhead adjacent to the 2 cockpits) and without any extra longitudinal or transverse stiffeners?

I have attached a photo from my 3d model in Rhino.

 

There are many sites on the internet that deals with building fibreglass kayaks and can even supply a kit for the build.

With a two person kayak you would want to keep the boat around 50 - 60 pounds.
From your drawing, it appears that the rear seat is quite near the stern. I would go to some of the larger kayak manufacturers' web sites an look at the profile from the top.
Normally the rear of the kayak is narrower than the front, much like a fish, to get better pressure recovery in the aft half. This then lowers the amount of buoyancy that you will achieve say on the basis of the buoyancy per lineal foot.
To me it appears that the stern cockpit is too far back unless the back is extremely wide which is not the best profile for efficiency

I would build a keel that is perhaps 2 inch deep by 3/4 inch horizontal. This looks like an ocean cruising kayak and you will not need quick turning response. Alternatively, a rudder helps in pointing into a wind.

In my formative years, we had a set of plans that gave us frame patterns and the distance between the frames. These were mounted on a 2 x 6 as a temporary keel.
Long 3/4 by 1/2 inch strips were then screwed into the ply frames at about 4/5 inch intervals. Over this frame when complete, we would attach naugahyde and we would have a hull. They were heavy


Later we used a product called Seconite, which is a fabric that was used for planes way back in the day. A very light fabric that we would staple to the kayak snugly then iron it.
The Seconite (which is a trade name I suspect) would then shrink and become extremely tight. We would then use an extremely light cloth and fibreglass it over the seconite, roll it, sand it when it was dry (extremely messy) and give it a coat of marine paint and end up with a pretty nice item.

Alternatively for you, you could develop some frames to be built from plywood, and get some long strips of wood that are about 3/16 by 1inch x the length that you need and staple them onto the frames. This would get you the shape that you want. Fill the cracks, smooth them and you will have a plug that you can use to make a female mold

A lot of people make cedar strip plank kayaks and canoes with these strips. They have a small curve routed into the strips on one narrow side and a convex curve on the other narrow side. Many even overlay a single fabric of fibreglass, use a clear resin and end up with very usable boats

 

Your design is wrong. The seating will make the stern drag.

See the attached positions for a proven double design

 

The aft cockpit is not as backward as it seems in the perspective view, here is the profile view. The aft portion of the hull is fuller so there will be severe trim by stern.

 

The aft section should not be "fuller" in the rear if you are trying to optimize efficiency. Efficiency for this note would be the least amount of effort to propel the boat to say 4-5 knots

You can be "fuller" in the front, and a slimmer less full taper in the back permits more pressure recovery from the energy that you have already "spent" that created a higher pressure area in the front portion of the kayak.

Examples in nature, fish
Examples, man made, 747's, all other airplanes, submarines, sailboats etc and most other items that are trying to reduce drag

"fuller" in the aft section is not the most efficient profile

A symmetrical profile would probably be the best and the cockpits situated to accommodate the weight that the seats will be carrying
Ie if you are 100 kg's and your normal paddling partner is 50 kgs, then the location of the cockpits should be placed so the kayak runs flat

It is an easy task to look at some of the well designed kayaks on the internet, print the picture, scale the position of the rear cockpit and move your position ahead.

 

The more efficient designs have a fine entry and relatively full aft sections. From the profile alone, it is not possible to tell the volume of the sections. You should at least post a plan view. Normally, the rear person is seated more forward than what you show.

 

This software is easy to download and lots of fun to play with. It is for strip planking, but could be used to make a plug for a mold. Software is free, but you can donate. There are also lots of existing plans available on the forum their, last time I looked.

http://www.blueheronkayaks.com/kayak/software/software.htm

KayakFoundry 1.6 Features:
•Stability Curves
•Multiple views for shaping the kayak in Plan, Profile, and Body
•Supports 1, 2, or 3 cockpits with independent or shared shapes
•Auto-positioned cockpit tracks with changes to center of buoyancy
•Resizable/hideable panels
•Waterlines and buttock lines
•Active Section view shows all section dimensions
•Curve shaping by dragging control points
•Rubber-banding for multiple control point selection
•Keyboard arrow keys provide fine adjustments of control point position.
•Real-time calculation and display of all kayak curves and hydrostatic data
•Real-time location markers indicate critical hydrostatic positions
•Optional auto-roundover of deck at bow and stern
•Sliding position line for viewing cross-sectional shape along the length
•Unlimited undo/redo
• Imperial or Metric units
•Bow Left or Bow Right display
•Printed stations - adjusted for user-specified strip thickness (sample form)
•Printed study drawing (sample study drawings)
•Zoom in/out
•Optional grid with user-specified interval
•Exaggerated curves
•A "notes" panel let you record comments and descriptions of the design
•Integrated drag predictions using John Winter's KAPER algorithm
•Form layout canvas to minimize paper usage
•Parameters for the Broze drag prediction spreadsheet