14 Feet Kayak Building

I want to build a 14 feet fiberglass kayak, the breadth is 23 inch.
To keep it strong enough and as light as 40 pound of weight what should the sampling (number and type of layers and reinforcement) of the boat?
I have attached my design in Rhino.

Awaiting your kind support.

 

That is a very practical size. Long enough to glide nicely between strokes, and short enough to keep the weight down. Nice beam also. Any wider makes it more difficult to paddle. Wide enough to be easy to self rescue. I would recommend you build in some foam flotation on the inside of the cockpit sides. This will provide enough stability for you to clamber back in, and reduce the amount of water you need to bail out after you do. My kayak is about 16 feet long and 24" wide, but I am over 200 pounds. I think 14 feet by 22" is good for a person in the 140-160 range. Longer kayaks are perhaps better if you have the strength and stamina, and want to take better advantage of wind and waves downwind, but it is harder to build them and keep the weight down, and they are more cumbersome to load and offload of a car roof. I would not go shorter than 14 feet, except perhaps for a child's kayak, in which case you should go proportionally narrower also. Say 12 feet by 18" for 60 to 100 pounds 4-5 feet tall.

40# is a good target weight in fibreglass for a boat of that size. Calculate the square feet. Divide 40# by that. Work out the amount of glass from the resin and glass ratio. Use all cloth, no matt. Use paint instead of gelcoat. That will help keep the weight down. Leave some weight aside, like 10 pounds, and use that for the final reinforcement in the areas where you will need it. Make sure you are at least able to clamber over the aft deck without breaking it. Production boats are overbuilt at 60#, but be prepared to do some repairs if you want to keep it light without using Kevlar or Carbon. Use some wood for stiffeners and for trim. Cheers.

 

Hello Jamie, thanks for your reply. Can you tell me particularly how much layers I should use? And can you tell me a bit more details regarding the reinforcement?

 

I will ask my brother, Fred Kennedy. He would have a better answer than me from both the theoretical and practical standpoint.

 

Thanks Jamie, for your wonderful support.

 

Thanks Pavel,

Here is my brother's reply to my email. I broke it up a bit to make it easier to follow each point. (my comments are in parenthesis where I think you might need further information)

generally speaking for easy hand layup a 10 oz cloth will take 10 oz of resin for each square yard, so 20 oz for 9 square feet (cloth is measured in ounces per square yard here)

and 1.5 oz matt will use 3 oz for each square foot, so 4.5 oz per square foot. (matt is measured in ounces per square foot here, and requires more resin than cloth to achieve saturation)

A litre of gelcoat varies in weight by colour but might weigh up to 3 lbs and cover 30 square feet moulded side, or maybe 45 nonmoulded side...only use where needed, but generally all of exterior (it adds weight and cost but can still be nice on the inside in areas where your body may be in contact with it)

Obviously heavier layup toward bottom and center of kayak and less on deck and especially ends (the aft deck will need to be strong as you will need to clamber over it after a capsize and wet exit)

...you know kayaks better than I do, but I would start with a weight budget under 35 if trying to get under 40. (so this is where I think you should calculate the square footage of your hull and your deck and go from there)

Don't add any interior gelcoat until you are happy with weight strength and stiffness so you can alway prep and add more fiberglass where needed. Don't forget to add airdry if you do add any interior gelcoat. (airdry is something you must add to gelcoat if it is to set properly when in contact with air, as opposed to being against the mold and covered in additional layups)

- Hope this helps. Do you know how many square feet you have for the hull deck?
- You will also need to work out a method for your hull-deck joint, which can add alot of weight, but also some strength.
- Hatches, Bulkheads, Seats can add alot of weight and cost also, but make or break a good design. For a single one-off you might want to work with wood for these parts. For that matter, you might consider a wood deck over a fibreglass hull. Lots of possibilities.
- You might also consider natural fibres and cloths versus glass fibres and cloths, at least for some parts. Hemp is for example. Do some experimenting.

 

Thanks a lot Jamie...

 

"The surface area of the hull is 30 square feet and 15 square feet in the deck."

So working backwards, and somewhat empirically rather than theoretically, you can try allocating as a start about 0.7 pounds per square foot for the hull, and perhaps 0.5 pounds per square foot for the deck. This will total 21 pounds plus 7.5 pounds for 28.5 pounds, leaving some 11.5 pounds for hull-deck joint, re-enforcement, and fitting out.

From Fred's data: (per square foot)
...working from the mold side up:

Hull (per square foot)
Gelcoat = 0.1 pounds
10oz cloth plus resin = 0.14 pounds
1.5 oz matt plus resin = 0.28 pounds
10oz cloth plus resin = 0.14 pounds
============
0.66 pounds per square foot for the hull
so ~20 pounds for 30 square feet

Deck (per square foot)
10oz cloth plus resin = 0.14 pounds
1.5 oz matt plus resin = 0.28 pounds
10oz cloth plus resin = 0.14 pounds
============
0.56 pounds per square foot for the hull
so ~8.5 pounds for 15 square feet

This leaves room for adding reinforcement where you think you will need it. For strength, such as hull deck joints, and under the seat, I would suggest another matt and another cloth, or perhaps just cloth tape. For bulkheads some people use closed cell foam. If you have flat sections on your hull or deck you may need some sort of core to avoid getting too heavy, but I would suggest stringers running lengthwise, so even if you have deflection the boat will still be streamlined. This of it like a skin on frame, but your original fibreglass layup is the skin, and you add the frame after you are done. So make the skin as reasonably light as possible, which is cloth-matt-cloth, and add some wooden stringers, and some frames inside of those, later as needed. For the deck, I suggest doing the original layup without gelcoat. This will keep the weight down, and allow you to stiffen the deck externally without the gelcoat being in the way. When finished, you can use polyurethane paint for the deck, and the inside of the boat where needed. This will weigh less than gelcoat.

Could you post an image of the hull shape? I would like to make some comments on how it might look, and where you might put stringers, frames, bulkheads, etc. Would also like to hear from others on ideas for the notoriously problematic hull-deck joint.

 

Pavel; If light weight is the main consideration, you can do far better with wood. Pound for pound wood is stiffer than fiberglass. If you use 4mm Ocumee you can build a very competent 14 foot kayak under 30 pounds. If you use Meranti the panels will be even stiffer but the weight will creep up a little bit. The ultimate method is strip building with light weight wood like cedar or whatever is readily available in Bangladesh. In either case the wooden boats will be quicker to build, stronger on a weight basis, and can be made very pretty.

If you plan to build a fleet of boats then building a plug, then a mold, is a better option, but not for a one off boat.