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Building new sea kayak

Submitted by Neno on Wed, 06/05/2019 - 03:40

Dear all,

I'm new member to this forum so allow me to first introduce myself.

My name is Neno and i live in Croatia (European union) which has great potential for kayaking with many rivers, lakes and beautiful Adriatic see.

I don't have extensive kayaking experience (<1000km). Two years ago i bought rotomolded SOT (RTM Tempo which is basically copy of famous Ocean Kayaks Scupper Pro). With it i paddled few rivers and lakes and did several circumnavigations of Croatian islands. Last year we did 150km touring/exploration circumnavigation of island Pag which we documented in short video https://www.youtube.com/watch?v=s4QTviZAhdk .

In out local kayaking club i paddled several sit-in kayaks (small Prijon Capri; then some unnamed very long and very very tippy seakayak, etc.) so i started to notice differences between designs.

Recently i came to idea to build fiberglass kayak myself. Then make mold and several more fiberglass copies which would be great start for growth of our paddling group.

That lead me to need for creating prototype so i started to research related topics on net and forums (including this one).

Many suggested trying KayakFoundry software which i did so i came with plan for my new kayak.

First let me say what were my guidelines (sorted by priority):

1.     Security – kayak that is stable enough for begginer/advanced kayaker. I am 194cm high and have 80-85kg so i can not go with too narrow kayak since it will be too tippy for me (especially with round hull which i would like to have)

2.     Speed – this closely relates to security so that we can make certain distance as fast as possible

3.     Efficiency – since we like to go on multi day/week tours i would like to have kayak with minimal drag to save energy

4.     Tracking – for our expeditions keeping track and using most energy in forward movement is much much more important than maneouvrability since we will not go rock hopping or surfing

5.     Cargo capacity – it must be able to carry at least 25kg of gear (10L water, 6kg tents, 5kn food, rest clothes and accessories)

6.     Less rocker – since most of our paddling is in normal weather conditions with small waves and also to minimise weathercocking

Shape wise i tried to imitate design of RTM Tempo which i really like (long waterline, plumb bow, etc.) but made some changes like changing shape from fishform to swede.

So this is some general dana regarding design and i would be gratefull if i could get feedback from experienced kayakers and kayak builders … eg.: is this kayak going to tick all boxes mentioned earlier? Should i go with even more rounded hull/chine? How to further decrease drag?

These are some specs from my plan:

 Principal Dimensions
    Overall Length 500 cm
    Waterline Length 493.2 cm
    Overall Beam 58.8 cm
    Waterline Beam 54.6 cm
    Draft 11.6 cm
    Entry 4.7 cm
    Exit 497.9 cm
    Half Angle of Entry 6.9°
    Half Angle of Exit 11.5°
  Cockpit
    Shape C
    Paddler Weight 79.83 kg
    Aft edge to CG 35.5 cm
    Length 90 cm
    Width 45.8 cm
    Fore Edge Height 29.68 cm
    Aft Edge Height 22.5 cm
    Aft Edge Position 304.5 cm
  Capacity
    Finished Boat Weight 20.41 kg
    Cargo Weight 24.95 kg
    Paddler Weight(s) 79.83 kg
    Target Displacement 125.2 kg
      Target Volume 0.125 m³
    Design Displacement 124.5 kg
      Design Volume 0.125 m³
    1" Sinkage 43.15 kg
      Sinkage Volume 0.0432 m³
    Enclosed Volume 0.354 m³
  Shape Constraints
    Bow Tip Radius 15 mm
    Stern Tip Radius 15 mm
    Minimum Keel Width 0 mm
  Areas
    Wetted surface Area 2.09 m²
    Waterplane Area 1.65 m²
    Lateral Plane Area 0.523 m²
    Maximum CS Area 440 cm²
  Centroids
    Center of Buoyancy 269 cm
    Center of Flotation 271.9 cm
    Center of Lateral Area 257.7 cm
  Coefficients
    Cwp 0.614
    Clp 0.914
    Cp 0.574
    Cx 0.713
    Cb 0.399
  Active Section
    Position 450 cm
    Overall Height 21.4 cm
    Deck Height 10.5 cm
    Sheer Height 5.8 cm
    Sheer Width 25.1 cm
    Waterline Width 19.1 cm
    Draft 10.9 cm
    Rocker 7 mm
    Underwater Area 99.7 cm²
    Cf 0.474

 Btw, is there any option to attach plan file from KayakFoundry to this message? If not i will add later drag and stabilitz chart and picture of design.

Thank you all ! :)

Br,

Neno

JohnAbercrombie

Wed, 06/05/2019 - 19:32

Your boat looks to me like it will be 'tippy' (little primary stability) but I'm not a designer.

If you are developing a new design, one idea would be to make your prototype over a wood 'core' (strip or plywood) with foam on the outside with only a thin layer of fiberglass. Then you could change the shape easily after testing in different conditions, test some more, etc...I think this is what the Broze brothers did when they designed some of the Mariner kayaks..

 

I've built a few kayaks but never had the courage (??) to design my own. I buy plans or copy known boats; it's safer!

JohnAbercrombie

Wed, 06/05/2019 - 19:41

Most sea kayakers I know (not K1 or surfski racers) don't paddle faster than 4 knots, so concentrating on optimizing the boat for the lower part of the speed range could be useful.

Have you studied other designs?

Why did you decide to design your own boat?

Are you planning to fit a rudder or skeg? Without one of those things, the tracking will not be good. That is because of the relative positions of the CLA and LCB.

Have you posted the design on the Blue Heron Kayaks website where you downloaded Kayak Foundry? There are a lot of people there who have used the program to design their own kayaks and are willing to help others refine their design. You could post the yak file here, but unless people have the Kayak Foundry program, they won't be able to open it.

There are a number of other things I think should be changed on your design.  I will wait to see if it appears on the Blue Heron Kayaks forum.  I would rather work with you there.

thank you for your comments.

regarding stability... there are many kayaks with similar width and hull shape which people consider very stable. of course, there must be some compromise regarding primary vs. secondary stability... eg. this RTM Tempo that i paddle it has great primary stability (as many SOTs due to higher  seating position and center of gravity) but it can not be easily leaned in waves.

regarding designing kayak from scratch... i believe that building kayak is great acomplishment and designing it is another thing that adds to overall feeling. What is there to loose? ... some time and money, but if you like the process it doesn't really matter ;)

br

This is great input regarding tracking being dependent on CLA & LCB positions. What is proposed solution for that within given design?

I tried to register on Blue Heron Kayaks forum but there is some error/bug in process so i contacted Administrator. I hope it will be solved soon so i can ask for advice there as well.

br

One reason I started designing and building my own kayaks is that I weigh 60 kg. I built a lot of fibreglass kayaks years ago and decided that when I retired and had more time, I would tackle the more labour intensive approach of building a strip built kayak.

The plans I bought resulted in an excellent kayak which I really enjoyed paddling.  However, there were circumstances where it was obvious that I was too light for the kayak.  It wasn't a big problem, but it was obvious that I was too light for the design.

When my wife paddled an old touring kayak that I had built in the 1970's, she had real problems because it was quite windy and she was far too light for the kayak. So I designed and built a kayak optimised for someone her weight. She loved it.

So I realised that I could design kayaks for someone my weight. Since then I have built 17 kayaks and canoes of my own design.  They include several recreational sea kayaks, K1s,  a TC1,  a C1, a couple of child's kayaks, and a SOF.  Every one of them has behaved and performed just as I had hoped and as the software predicted.

There are all sorts of reasons to design and build your own kayaks. There is an enormous amount of satisfaction to be had in paddling something that you designed and built yourself. If you are significantly lighter than the average paddler, there is a good chance the kayak will behave better than many of the commercially available kayaks.

I did a lot of research before I committed to building a kayak of my own design.

Strip built kayaks are probably the most labour intensive way of creating a kayak, but the finished product is something special. Kayaks I have built have won awards at woodworking shows and I have won gold medals in state and national competitions (in my age group) paddling kayaks and canoes I designed myself, so it is possible and as I said, very satisfying.