kayak dinghy under 5kg

I'm looking into making a lightweight tender for my trimaran. Hoping to make something under 5kg total weight, and looking for advice/ideas.

Generally speaking I want a kayak. I can buy a paddle that weighs 1kg, which leaves 4 for the craft itself.

I am thinking something 4 meters long long, using lets say 9x 1/2 inch carbon fiber tubes and carbon/foam bulkheads all lashed together using dyneema. Is it better to use different diameters?

The tubes weigh about total 1kg. Then the bulkheads lets say I have 5 of them, this weigh total 1.5kg, another 0.5kg for the bow and stern. My plan was to use UHMWPE fabric skin. Is this reasonable? Maybe difficult to sew? Anyway, using 100 gsm cloth is going to be probably 0.5kg for the total skin, then something like 0.5kg for the seat and combing.

I am hoping to make this a bit wider, say 80cm beam for stability and weight carrying. Can it work? any suggested designs? I do not intend for it to fold easily.

 

Probably lightest as I know will be skin on carbon frame . Using fishing rods as tubes will be most cheap.

 

K1 Olympic Warrior 2024 - Plastex Boats
That one is really on the heavy side.

 

Mylar - Wikipedia

 

You did not rule out inflatable boats, and there are some that are pretty rugged used for packing in to White water. Do it yourself pack rafts has some lightweight kits,

Here's a general link that will give you some stats,

The Best Packrafts of 2025 https://gearjunkie.com/boats-water/packrafts#best-tandem-packraft

DIY Packraft Overview - DIY Packraft https://www.diypackraft.com/diy-packraft-overview/

 

If I remember the name correctly, Platt (Pratt?) Monfort had several very innovative skin on frame ultralight designs. He used wood and aircraft fabric with CF tow. I had thought about replicating one of his dinghy designs in CF tube with a dyneema skin to make a crazily light dinghy. I went as far as sourcing suitable tube and skin fabric but went a different way.
I had one of the smallest designs, a pram dinghy, but the builder was about 4kg over the projected ~ 15kg design weight. It rowed beautifully but wasn't really light enough for me to easily lift onto my boat or carry solo. After tearing the bottom of my inflatable on a hidden snag, I changed my mind....
None of his designs are as light as 5kg, but should give you a good idea of what will be required. The business was sold after he died and the new owner has documented various builds on YT. Geodesic areolite boats.
There are also traditional lashed SoF kayak designs. Adapting one of these with CF tube and an ultralight skin should be easy enough.
I will follow this with interest to see how you do. And as far as packrafts are concerned, while light I don't think they'd be much use as a tender. Paddling one any distance in a wind would be pretty tedious.
For me, as long as the boat would be easy for one person to pick up and move around, a little extra weight won't matter but YMMV.

 

Years ago I tried to build a 10# sit on top kayak. Frame was like a stick and tissue model plane; minimal ply frames and 3/8" sq stringers covered with winter window shrink film. Target weight shifted to 5 kg when it became apparent I wouldn't make the 10# target. Actual finished weight weight was ~13#.
Boat was named the "Leastie Beastie" and was a transparent marvel until the uv rapidly destroyed the window film.

 

well I was thinking 1/2 inch or 12mm carbon tubes with 0.5mm wall thickness, but is this reasonable/unreasonable? Any designs? If I had a better idea for this I could come up with the right design, even if based on a wooden frame.

As for inflatable, I've looked, and these are both heavier (although TPU inflatable is very light) but more importantly not as efficient to row.

It seems UHMWPE fabric is quite durable, especially if I can get TPU coated for waterproofness. The hard part is finding it, and probably sewing it.

 

The alpaca rafts make the weight cut. Cant imagine anything but an inflatable making the weight.

 

Best bet for fabric is one of the specialty shops that cater to the uber lightweight tent crowd.

 

Any fabric that is light enough is also very susceptible to small punctures and pin holes.

The off the shelf backpacking inflatables can be found at well under your target weight. Buy one, it will end the obsessing over something that has already been perfected to the level of your needs.

 

Tube frame under 3-cord tension, compressed fore and aft, with shrink skin.

Kind of ridiculous weight expectations and what for if you weigh 80kg?

 

Dyneema fabric is not hard to sew but I think you'll find it quite hard to get the fabric tight. Most designs use heat shrink fabric to achieve that. Dyneema doesn't shrink like that and doesn't take heat well.

 

I have used polyester before to make kayaks with success. I essentially lace it up like a shoe to make it tight by sewing like a bolt rope that runs down the middle of the top of the kayak, with a flap over that to cover it. I dont weigh 80kg i weigh about 60kg.

The reason to keep this light is my boat I dont want to weigh it down much, this particular trimaran is very weight sensitive.

 

PC: A One man Kayak as a tender wouldn't have much space for carrying things, but you may not need it depending on what you are doing?

"As for inflatable, I've looked, and these are both heavier (although TPU inflatable is very light) but more importantly not as efficient to row."

I found 1.5 lb weight for the lightest inflatable pack boats from two different companies, and many more below the 5 lb range that were considerably more rugged, some were self baling, helps with waves or rough conditions. Maybe any kayak shape ones could somehow be fitted with a CF frame to make rowing more efficient. Skin and frame designs have the same vulnerabilities to puncture or tear, without as much advantage of bouncing off or sliding/flexing at their frame contact areas when coming in contact with potentially hazardous bottoms, rocks, etc. Inflatables rarely have catastrophic failure where they sink instantly, because of Trapped air pockets away from the puncture or a second bladder that provides at least some flotation to cling on to. Another possible inflatable advantage is they could remain collapsed occupying less space than a loaf of bread on your mother boat, giving more space, and inflated quickly when needed in one or two minutes with a 2 oz self-powered battery pump. Some inflatables have capability to adjust pressure with a mouth tube on the fly, for ergonomic comfort or cold water conditions. That's compared to SOF which remains deployed occupying space, and catching wind on board or resistance from towing the weight of any water that splashes in, and Maybe windage if being towed.

Most Inflatables probably have less weight below the surface which would be a disadvantage going into wind, but a possible advantage when rowing with the wind. I wouldn't expect either lightweight design to have an inertia advantage to help with coasting between rowing power strokes.

But of course, I do not know your specific setup and applications. So please kindly disregard the above commentary if it is not helpful for your particular application!