Changing dimensions or outfitting

Latest updated Tuesday, December 06, 2011, 18 comments

(see also FAQ)

One of the most important advantages with home building is the possibility to customize the craft to your liking - be it looks, outfitting or sizing.

But still, I am a little ambivalent about alterations. On one hand, of course you should use the opportunity to build the craft of your dreams. Otherwise you will miss the most rewarding part of building. Many excellent ideas have also originated from uninhibited experimenting.On the other hand, there is doubts. I have spent hours tweaking the performance of the kayaks and canoes, calculating balance, stability, maneuverability, efficiency etc and even more building and testing the prototype to optimize each craft to their intended use and user. To improve upon that you will need a solid understanding of the consequences of a dimensional change – and if you are in that position, why not design your own kayak from scratch? It is a very rewarding experience to see the theory come alive in the shop and on the water.

It is also about esthetics. I spend hours fine-tuning lines to create a harmonious relation between lines, surfaces and curves. It makes me sad seeing a kayak where someone has uncritically copied a line or detail from another kind of kayak that s/he happened to fancy, and achieved a disharmony that sticks out like a sore thumb. It also grieves me that there is an ugly kayak out there, built from my plans, and knowing that someone spent many hours on it, achieving what to other looks crude and amateurish.

The same applies here: if you know what you are doing, go ahead – if not: perhaps best to be a little conservative...

What then is OK to change?

Length

The length is easy to adjust. Up to approx 10% adjustment is normally no problem. Just change the distance between molds, and the distance between the stern and the aft edge of the cockpit rim (the same coefficient on both). You must keep the same c/c distance for all molds, otherwise you disrupt the lines and may even cause unforeseen effects on performance. No stretching midships or such ideas! Most plans have 12 molds, meaning that the craft gets 13 cm longer or shorter for every cm mold distance change. Do not forget to stretch or shorten the stems accordingly.

Reason to change the length may be that you are short and light – a Nomad shortened to Kavat-length may give a adequate volume. If you are tall and heavy the other way around may produce a suitable craft – a Kavat stretched to Nomad-length.

Beam

Do not change the beam, without redrawing the entire station plan. It is very easy get erratic lines that may affect the performance – or just lose the harmony of lines...

Dimensional changes

Linear changes in a photo copier are generally a bad idea, since the different qualities are not linear. If you fx enlarge the plans by 200% (a stupid idea of course) you end up with a kayak that is 1.4 times faster but needs 8 times more effort to keep at that speed and will be 32 times harder to maneuver (see the table below). It is easy to lose control over the performance – and often better to design a new kayak from scratch.

Deck height

The deck height is relative simple to change if you think you will need more volume for knees and feet. I like my decks a little lower to improve kayak control. A couple of kayaks have therefore an alternative deck line with more space. But don not change without thinking it through – there are benefits with a tight cockpit, that may not be self evident from looking at the plans.

Decorating

Some builders decorate their kayaks with different wood species. I leave no suggestions on the plans for this practice, but on the internet there are many inspirational sites.

The cockpit

Of course you build the cockpit and rim to your liking. On the plans is a standard suggestion, suitable for the type of kayak and most paddlers, but also advice on how to adjust the dimensions. Change if you like – but not without a good reason.

Seat and backrest

These items are also easily customized. If you are an experienced paddler you will know exactly on what you like to sit and what kind of backrest (if any) you prefer. Again, what is on the plans is a standard solution, suiting the craft and most paddlers. If you have no previous experience, you can safely follow this instruction.

Deck layout and outfitting

Deck layout and outfitting leave a lot of room for imagination and creativity. On the plans are suggestions that will work in most cases: bulkheads and hatches, rudder or skeg, deck hardware, lines etc. These can be followed or used as starting point for your own creativity. If you are pondering changes far from the consensus, you may send a mail – someone else may have tried something similar...

Practical consequences

The table below shows what happens if you make linear dimensional changes. It is obvious that photo copier alterations may create surprises.

If you change the dimensions by a factor x, this is what happens:

• The speed potential changes by the square root of x
• The beam, loa and height changes by x
• The wetted surface changes by x²
• The displacement changes by x³
• The resistance changes by x³
• The stability changes by x⁴
• The inertia changes by x⁵

Contemplate for a simple demonstration a kayak linearly enlarged to the double size (which of course would be a very stupid idea). It would go 1.4 times faster, be 2 times longer, wider and higher, get 4 times the wetted surface (and thus friction), be 8 times heavier (if loaded to the design waterline), take 8 times the effort to keep at speed, be 16 times more stable and be 32 times harder to accelerate, stop and turn.