# Need Help with Physics Project on Kayaking

Discussion in 'Boat Design' started by kayakingfiend24, Apr 2, 2009.

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### kayakingfiend24New Member

I am working on a Physics project on Buoyancy/flotation, and i am wanting to make a draft on making a kayak, and how to calculate everything i need to make sure it floats and floats with weight in it. I was hoping someone out there could help me and maybe explain what they say a little. Also, i will be using these calculations later to actually make a kayak. Thank you.

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### LucklessSenior Member

Why kind of Physics project? You mean like a mathematical simulation to prove that it should float in theory?

Closed or open deck Kayak? (I've seen small craft marketed as deckless kayaks,... but isn't that really just a funny looking canoe then?) Open hatch or sealed skirting?

What kind of water are you doing the calculations for? Calm still water, or waves/rapids?

Are you doing this by hand, or can you write software to do the math for you? (If you want to get into light weight sims and quick 'small scale' number crunching, I suggest looking up Python. It is easy to learn and works great as an advanced calculator, but has issues with speed when you get into far larger problems. Something all science students should spend a summer learning if they're new to programming.)

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### ancient kayakeraka Terry Haines

Kayak Design 101

If there was a choice of subject, you could have picked something simpler, like rocket science!

If you were just wanting to design a kayak I would suggest you download a copy of suitable free software such as FreeShip or DeftShip, create a hull design and note the hydrostatic values that the software can provide, such as displacement and metracentric height for a given draft.

However, for a Physics project perhaps more is required. For an arbitrary hull shape this is not a trivial problem. One approach could be to create a spreadsheet but that might be too time consuming. Alternatively, assuming a simple mathematical shape for the hull, such as a semi-elliptic cross-section with a semi-elliptic side projection would allow you to set up simple mathematical expressions for buoyancy and heeling moment, that might be good enough for the purpose of the exercise.

Your own weight and CoG can then be used to establish if the boat will float and remain upright. The CoG of an average seated human is surprisingly low, you can determine that from a human engineering reference, it is about 8" if I recall correctly, plus the seat height of course; a metacentric height of more than that should in theory keep it upright: I like to have a minimum of 16" for kayak which allows bent knees for comfort.

For a real design that is to be built and entrusted with a human life, there is more. There are issues such as primary and secondary stability: a flat-bottomed boat has lots of the former and not much of the latter, a semi-circular hull has a lot of secondary stability and most hulls fall in between. Secondary stability is what puts the brakes on when a hull heels well off center. It seems intuitively obvious that a broad flat hull would be the most stable but that is a simplification; it also has the greatest buoyancy so it rides high if not heavily loaded which raises the CoG. Narrow boats like kayaks get a great deal of their stability by keeping the weight low in the water. In a flat-bottomed kayak you would feel stable until you started to tip it to one side at which point it would feel like it was going to dump you out; a flat bottomed boat has to be way wider than a kayak before it has enough stability to feel secure.

The hull can and should be modified from the mathematical "ideal" before building to address practical concerns, for example more lateral area is needed at bow and stern to get adequate tracking, otherwise the boat will swing side to side in response to paddling forces.

There are lots of kayak types that have been developed by various northern communities for specialised purposes and different sea types, and some modern plastic kayaks are of the sit-on-top variety. There are many books ... however, from the nature of your enquiry I assume the Physics is the important part and styling and application of secondary importance.

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### kayakingfiend24New Member

To Luckless. its for a closed kayak skirted, and more for whitewater kayaking. Thanks

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### ancient kayakeraka Terry Haines

Speaking only for myself, I would hesitate to design and build a whitewater kayak. It's a demanding environment for both boat and paddler, and quite capable of seriously damaging both. In a flat water boat on a lake or quiet river, if your boat expells you or expires under you at least there's a good chance you won't get your head smartly rapped on a rock.

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### rwatsonSenior Member

I built three canoes without plans or calculations when I was a teenager, they all floated. All I had to know was that 1 cubic metre displaces 1000 kilos, and that the canoe would be wide enough for my bum.

Just look up some photos of typical kyaks for a rule of thumb, and go for it. Wear a helmet and a life jacket and start small.

The other thing I learned, is that a badly performing canoe hull is a great incentive to build another better one real soon.

Now - the big question - how good at fibreglassing and woodwork are you?

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